Cranial anatomy of the spade-headed amphisbaenian

Cranial Anatomy of the Spade-Headed AmphisbaenianDiplometopon zarudnyi (Squamata, Amphisbaenia)Based on High-Resolution X-ray Computed TomographyJessica Anderson Maisano,1* Maureen Kearney,2 and Timothy Rowe1

1Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 787122Department of Zoology, Field Museum of Natural History, Chicago, Illinois 60605

ABSTRACT The skull of the trogonophid amphisbae-nian Diplometopon zarudnyi is described from high-resolution X-ray computed tomographic (HRXCT) imageryof a whole museum specimen preserved in ETOH. Theskull was digitally resliced and disarticulated into indi-vidual elements, producing novel visualizations that allowdetailed morphological analysis of its three-dimensionallycomplex structure. The prefrontal and jugal are absent inDiplometopon. The septomaxilla is present but hiddenentirely from superficial view. In contrast to previousstudies, we recognize a splenial fused to the compoundbone of the mandible and a squamosal fused to the otic–occipital complex. Comparison of Diplometopon to the twoother amphisbaenians previously described in comparabledetail, Rhineura hatcherii and Amphisbaena alba, revealsa mosaic of cranial similarities and differences. BothDiplometopon and Rhineura exhibit a craniofacial angu-lation and expanded rostral blade related to use of thehead as a digging tool, but the detailed architecture ofthese features is quite different. Additionally, whereas thesnout of Rhineura exhibits a high degree of sculpturingand sensory innervation, this is not the case in Diplometo-pon. Unlike in Rhineura and Amphisbaena, the cranialelements of Diplometopon do not exhibit an extensive de-gree of overlap or complex interlocking sutures; instead,most of the cranial elements lie in loose apposition to eachother. The degree to which this mosaic of features reflectsfunctional demands, shared ancestry, and/or convergenceis unclear in the absence of a stable hypothesis of amphis-baenian phylogeny. J. Morphol. 267:70–102, 2006.© 2005 Wiley-Liss, Inc.

KEY WORDS: Squamata; Amphisbaenia; Trogonophidae;Diplometopon zarudnyi; cranial anatomy; computed to-mography

Amphisbaenians (“worm lizards”) are a poorlyknown group of fossorial squamate reptiles, nearlyall of which are limbless. Their secretive habits inthe wild make them difficult to collect and, as aresult, museum specimens generally are rare andtoo precious for traditional, destructive anatomicalstudies. Worm lizards exhibit dramatic cranial mod-ifications related to their head-first burrowing be-havior in the soils they inhabit. Four basic cranialmorphotypes (“shovel-headed,” “round-headed,”“keel-headed,” and “spade-headed”) are recognized

(Gans, 1974), and the monophyly of these morpho-types is the subject of renewed study from a phylo-genetic perspective (Kearney, 2003). A recent molec-ular study (Kearney and Stuart, 2004) suggests thatsome amphisbaenian cranial morphotypes may haveevolved convergently among geographically distinct,independent lineages, providing the impetus to ex-amine the skull anatomy of these forms in moredetail.

A bone-by-bone study of the “round-headed” am-phisbaenid, Amphisbaena alba, was conducted byMontero and Gans (1999). Kearney et al. (2005)provided the first detailed cranial description of anamphisbaenian based on high-resolution X-ray com-puted tomographic (HRXCT) data. The descriptionof that species, Rhineura hatcherii, revealed numer-ous new details of skull construction in a “shovel-headed” rhineurid. The purpose of the present studyis to build on these works, providing a detailed cra-nial description of the “spade-headed” morphotypeseen in trogonophids. The use of nondestructiveHRXCT techniques is especially well suited to am-phisbaenians due to the three-dimensional complex-ity of their skulls. Ultimately, we aim to compareand contrast all four amphisbaenian cranial mor-photypes using HRXCT to assess the evolutionarymodifications of each cranial shape/digging mode,and to fill in knowledge gaps in amphisbaenian cra-nial anatomy relative to other squamates.

Diplometopon zarudnyi (hereafter Diplometopon)is monospecific and occurs in arid habitats in west-ern Iran, southern Iraq, Kuwait, Oman, and north-ern Saudi Arabia (Gans, 1978). It inhabits aeoliansand deposits, but is known to emerge onto the sur-

Contract grant sponsor: National Science Foundation (NSF); Con-tract grant numbers: EF-0334961 (to M.K.); IIS-0208675 (to T.R.).

*Correspondence to: Jessica Maisano, Jackson School of Geo-sciences, The University of Texas at Austin, Austin, TX 78712.E-mail: [email protected]

Published online 28 October 2005 inWiley InterScience (www.interscience.wiley.com)DOI: 10.1002/jmor.10388

JOURNAL OF MORPHOLOGY 267:70–102 (2006)

© 2005 WILEY-LISS, INC.

face to breed. All trogonophids use a stereotyped,oscillating burrowing behavior in which the head isrotated about the long axis of the body in one direc-tion, then reversed to the other direction, therebyloosening soil from the end of the tunnel (Gans,1974). Trogonophid monophyly is well corroboratedby several synapomorphies, including a strongcraniofacial angulation, a highly modified pectoralgirdle, acrodont dentition, and a short tail:body ratio(Gans, 1960; Kearney, 2003).

The cranial anatomy of Diplometopon has beendescribed by Gans (1960), and in somewhat moredetail by El-Assy and Al-Nassar (1976), using clas-sical techniques. We use HRXCT data to describethe skull in a comprehensive bone-by-bone ap-proach, to provide both a thorough cranial descrip-tion of a trogonophid amphisbaenian and a generalmorphological characterization of the “spade-headed” morphotype. We offer preliminary compar-isons with Rhineura hatcherii (hereafter Rhineura)and Amphisbaena alba (hereafter Amphisbaena),the only other amphisbaenians currently known incomparable detail. We also present additional char-acter data for comparison among amphisbaeniansgenerally, with the hope that these will contributetoward greater resolution of amphisbaenian phylog-eny in future studies.

MATERIALS AND METHODS

This description is based primarily on a single HRXCT-scannedspecimen of Diplometopon (Field Museum of Natural History,FMNH 64429) collected from Qatif Oasis, Saudi Arabia. Whilestudying this primary specimen, we also compared it to skeletalspecimens (FMNH 206115, 74007) and a cleared-and-stainedpreparation (National Museum of Natural History, USNM141833). Given the level of detail of the following description, andthe availability of HRXCT data for only one specimen, someaspects of the internal structure of the skull may be subject toindividual variation not recognized here.

The specimen was scanned at the High-Resolution X-ray CTFacility at The University of Texas at Austin using the followingparameters. A FeinFocus microfocal X-ray source operating at180 kV and 0.133 mA with no X-ray prefilter was employed. Anempty container wedge was used. Slice thickness corresponded to2 lines in a CCD image intensifier imaging system, with a source-to-object distance of 41 mm. For each slice, 1,000 views weretaken with five samples per view. The field of image reconstruc-tion was 9.5 mm and an image reconstruction offset of 6,200 wasused with a reconstruction scale of 1,600.

The dataset consists of 459 HRXCT slices taken along anoblique axis of the skull from the tip of the snout to the occiput.The scan axis was offset �20° from orthogonal due to a stiff tilt ofthe specimen’s head relative to its body. Each slice image wasgathered at 1024 � 1024 pixels resolution, resulting in an in-plane resolution of 9 �m per pixel. Each slice represents a thick-ness and spacing of 192 �m, resulting in no overlap betweenslices. The dataset was digitally resliced along the true orthogo-nal axes of the skull—transverse (�coronal, front to back), frontal(�horizontal, top to bottom), and sagittal (left to right)—usingVoxBlast� (Vaytek, Fairfield, IA). Figure 1 illustrates the relativeposition of the labeled HRXCT slices shown in Figures 2–4. Allreferences to particular slice numbers refer to the orthogonallyresliced dataset, not the original scan dataset.

Individual elements of the skull were digitally isolated by handby tracing sutures (which are readily apparent) in the original

scan images using Adobe PhotoShop� (San Jose, CA), then ren-dered in three dimensions using VGStudio MAX� 1.2 (VolumeGraphics, Heidelberg, Germany). All elements were rotated to thetrue orthogonal axes of the skull prior to rendering. For pairedelements, the left element was isolated except in the case of themandible, where sutures were more distinct on the right side.Right mandibular elements were mirrored to maintain consis-tency in spatial orientation in the figures. The cranial bones andmandible are described separately.

The skull as a whole (Fig. 5) is described first, followed by eachindividual element (Figs. 6–25). The HRXCT slices spanned byeach element are noted at the beginning of the description of thatelement, and selected labeled HRXCT slices (Figs. 1–4) illustratethe internal anatomy of the skull in transverse (Tra), frontal(Fro), and sagittal (Sag) slice planes. Slice numbers are refer-enced throughout the description.

An interactive, web-deliverable version of the HRXCT datasetkeyed to the slice numbers cited below, as well as animations ofall isolated bones, can be viewed at http://www.digimorph.org/specimens/Diplometopon_zarudnyi, and the original full-resolutionHRXCT data are available from the authors.

RESULTSOverview of Skull (Fig. 5)

The skull of Diplometopon is short and broad incomparison to the elongated skulls seen in manyamphisbaenians. It measures 8.2 mm from the tip ofthe snout to the posterior tip of the occipital condyle,and 4.8 mm at its widest expanse, at the level of theexternal opening of the jugular recess. The skulldisplays the typical trogonophid condition (Kearney,2003), in which the spatulate snout projects sharply

Fig. 1. Diplometopon zarudnyi (FMNH 64429). Diagramshowing approximate relative positions and orientations of la-beled HRXCT slices shown in Figures 2–4.

71CRANIAL ANATOMY OF D. ZARUDNYI

Journal of Morphology DOI 10.1002/jmor

Figure 2

72 J.A. MAISANO ET AL.


anterior to the mouth and significantly overhangsthe mandible (Fig. 5A). Other amphisbaenians havea pointed or rounded snout, but in the “spade-headed” trogonophids the snout forms a broad,squared-off wedge (Fig. 5E) whose leading edge isoriented horizontally and protrudes beyond the bor-ders of the nares.

For descriptive purposes, the skull can be dividedinto facial and cranial segments, with a strongcraniofacial deflection angle delimiting the two (Fig.5B). Whereas the facial segment in Diplometoponsuperficially bears a striking resemblance toRhineura (Kearney et al., 2005) (and a number ofother amphisbaenians [Kearney, 2003]), its under-lying structure and the anatomical relations amongits parts are quite different. The facial segment iscomposed of the premaxilla, septomaxilla, maxilla,nasal, frontal, and the anterior portion of the pari-etal and palate. Gans (1960) found no external evi-dence of the septomaxilla and questioned its pres-ence in Diplometopon, but this element wasidentified by El-Assy and Al-Nassar (1976). TheHRXCT data confirm its presence, wholly enclosedwithin the nasal chamber, lying lateral to the pre-maxillary nasal process, dorsal to the vomer andmaxillary palatal process, and ventral to the nasal.There is no prefrontal, jugal, or lacrimal.

The median premaxilla is dorsoventrally flattenedto form the sharp leading edge or rostral blade of the“spade snout” and covers the full width of the lead-ing edge of the snout (Fig. 5C). This blade is but-tressed laterally on each side by the nasals andmaxillae, and medially by several palatal elementsthat make up a median strut of complex structure.This median strut includes contributions from thepremaxilla, septomaxilla, maxilla, vomer, and pala-tine. Together, these bones may be in a position totransmit rostral loads to the braincase via the ba-sipterygoid processes.

The dorsal and lateral surfaces of the facial seg-ment intersect to form a distinct angle known as thecanthus rostralis, which includes contributions fromthe nasal, maxilla, frontal, and parietal. This edgeinclines medially as it continues posterodorsally tomeet its counterpart at the parietal apical process(Fig. 5C). In life, the entire surface of the face be-tween the canthi rostrali is covered by thickened,heavily keratinized scales (Kearney, 2003). Theseform a sharp edge at the canthus rostralis, which isused to scrape soil from the sides of the tunnel wallduring burrowing.

Together, the premaxilla, maxilla, and nasal formthe border around the external naris. As in other

Fig. 2. Diplometopon zarudnyi (FMNH 64429). Selected transverse (Tra) HRXCT slices. A: Tra 075, B: Tra 105, C: Tra 175, D: Tra212, E: Tra 282, F: Tra 333, G: Tra 375, H: Tra 391. Scale bar � 1 mm. aiaf, anterior inferior alveolar foramen; appr, alar process ofprootic; avsc, anterior vertical semicircular canal; bh, basihyal; bpp, basipterygoid process; c, columella; cb1, first ceratobranchial; cc,cranial cavity; chv, choanal vault; co, coronoid; cpb, compound bone; dn, dentary; dnc, dentary canal; dnt, dentary tooth; ec,ectopterygoid; enc, endolymphatic canal; exc, extracolumella; f, frontal; fo, fenestra ovale; for, foramen; Gf, Gasserian foramen; hc,hyoid cornu; hsc, horizontal semicircular canal; icf, internal carotid foramen; jr, jugular recess; lr, lagenar recess; m, maxilla; marst,medial aperture of recessus scalae tympani; Mc, Meckelian canal; mf, mental foramen; mt, maxillary tooth; n, nasal; nc, nasalchamber; nf, nutritive foramen; ooc, otic–occipital complex; orb, orbit; os, orbitosphenoid; p, parietal; pal, palatine; palf, palatineforamen; pas, processus ascendens of supraoccipital; pbs, parabasisphenoid; pc, pulp cavity; pd, perilymphatic duct; pm, premaxilla;pop, paroccipital process; pt, pterygoid; pVf, posterior Vidian foramen; pvsc, posterior vertical semicircular canal; q, quadrate; rcc,recessus crus communis; rst, recessus scalae tympani; sc, sagittal crest; sds, subdental shelf; sltu, sella turcica; sm, septomaxilla; sp,splenial; sq, squamosal; stat, statolithic mass; v, vomer; vb, vestibule; Vc, Vidian canal; vlp, ventrolateral process; voc, vomeronasalchamber; X, “element X.”



“spade-headed” amphisbaenians (Kearney, 2003),the naris in Diplometopon opens ventrolaterally(Fig. 5B), in contrast to the anterolateral openingfound in many “round-headed” amphisbaenians

(Montero and Gans, 1999) or the ventral openingfound in “shovel-headed” rhineurids (Kearney, 2003;Kearney et al., 2005). Behind the external naris, thelateral wall of the facial portion of the skull is

Fig. 3. Diplometopon zarudnyi(FMNH 64429). Selected sagittal(Sag) HRXCT slices. A: Sag 062,B: Sag 090, C: Sag 121. Scale bar �1 mm. aar, anterior ampullary re-cess; app, apical process; at, atlas;aVf, anterior Vidian foramen; avsc,anterior vertical semicircular ca-nal; bh, basihyal; bpp, basiptery-goid process; c, columella; cb1, firstceratobranchial; cc, cranial cavity;chv, choanal vault; cpb, compoundbone; crpr, crista prootica; dc, den-tal canal; dn, dentary; dnc, dentarycanal; dnt, dentary tooth; drf, dor-sal rostral foramen; ec, ectoptery-goid; en, external naris; f, frontal;fm, foramen magnum; for, foramen;fvo, fenestra vomeronasalis; glf,glenoid fossa; hc, hyoid cornu; iaf,inferior alveolar foramen; jr, jugu-lar recess; lr, lagenar recess; m,maxilla; Mc, Meckelian canal; mf,mental foramen; n, nasal; nc, nasalchamber; nf, nutritive foramen; occ,occipital condyle; occr, occipitalcrest; ooc, otic–occipital complex;os, orbitosphenoid; p, parietal; paf,posterior auditory foramen; pal,palatine; par, posterior ampullaryrecess; pas, processus ascendens ofsupraoccipital; pbs, parabasisphe-noid; pc, pulp cavity; pd, perilym-phatic duct; pm, premaxilla; pmt,premaxillary tooth; pt, pterygoid;pvsc, posterior vertical semicircularcanal; q, quadrate; rcc, recessuscrus communis; sc, sagittal crest;sltu, sella turcica; sm, septomaxilla;sp, splenial; sq, squamosal; stat,statolithic mass; v, vomer; vb, ves-tibule; Vc, Vidian canal; vf, vagusforamen; vlp, ventrolateral process;voc, vomeronasal chamber; X, “ele-ment X.”



formed by the maxilla along with a small contribu-tion from the nasal. However, in contrast to mostsquamates, the maxilla contributes little directly tothe lateral wall of the nasal chamber. Instead, the

nasal forms both the dorsal and lateral walls of thenasal chamber, while the septomaxilla lines its me-dial and ventral walls anteriorly, and the maxillaforms its floor posteriorly.

Fig. 4. Diplometopon zarudnyi(FMNH 64429). Selected frontal(Fro) HRXCT slices. A: Fro 107,B: Fro 132, C: Fro 178. Scale bar �1 mm. ac, accessory cusp; appr, alarprocess of prootic; at, atlas; avsc,anterior vertical semicircular canal;c, columella; cc, cranial cavity; chv,choanal vault; co, coronoid; cpb,compound bone; crpr, cristaprootica; dn, dentary; dnt, dentarytooth; drf, dorsal rostral foramen;ec, ectopterygoid; en, external naris;enc, endolymphatic canal; eth, eth-moid passageway; exc, extracolu-mella; f, frontal; ff, facial foramen;fm, foramen magnum; for, foramen;Gf, Gasserian foramen; jr, jugularrecess; lf, labial foramen; m, max-illa; mt, maxillary tooth; mxc, max-illary canal; n, nasal; nc, nasalchamber; occ, occipital condyle; ocr,occipital recess; ooc, otic–occipitalcomplex; orb, orbit; os, orbitosphe-noid; p, parietal; pal, palatine; par,posterior ampullary recess; pbs,parabasisphenoid; pc, pulp cavity;pd, perilymphatic duct; pm, pre-maxilla; pmt, premaxillary tooth;pt, pterygoid; pvsc, posterior verti-cal semicircular canal; q, quadrate;rc, rostral canal; sltu, sella turcica;sm, septomaxilla; sq, squamosal;stat, statolithic mass; v, vomer; vb,vestibule; Vc, Vidian canal; vf, va-gus foramen; voc, vomeronasalchamber; vof, vomerine foramen.



Figure 5


The nasals are completely separated from eachother by the premaxillary nasal process, which ex-tends posteriorly to contact the frontals (Gans, 1960;Kearney, 2003). The premaxilla is inscribed by deepbilateral grooves connecting foramina that commu-nicate with the nasal cavity. The nasal extends pos-teriorly over the frontal as a sharp lappet thattapers to a point at the level of the posterodorsallimit of the premaxillary nasal process (Fig. 5C). Thenasal fits into a V-shaped depression in the anteriormargin of the adjacent frontal.

The eye in Diplometopon is reduced but still func-tional (Gans, 1960). The orbit faces laterally and isroofed by the supraorbital ridge of the frontal dorsalplate (Fig. 5B). The orbital wall is formed anteriorlyby the maxillary orbital process, while the maxilla,ectopterygoid, and palatine contribute to the orbitalfloor, and the frontal descending process forms themedial orbital wall. The orbital rim is incompleteposteriorly.

The palate is surrounded on three sides by theupper dentition, there being no palatal teeth (Fig.5D). There are three teeth in the premaxilla andthree acrodont teeth in each maxilla, which are metby six acrodont teeth in each dentary. The palate isformed by the premaxilla, maxilla, pterygoid, ec-topterygoid, palatine, and vomer. There is no palatalvacuity, and the pyriform recess is narrow androofed by a long parabasisphenoid cultriform pro-cess. The fenestra vomeronasalis is bounded by aslight contribution from the premaxilla anteriorly,the maxilla laterally, and the vomer on all othersides. This passageway communicates with a volu-minous vomeronasal (Jacobson’s) chamber. Poste-rior to the fenestra vomeronasalis the choana isbounded by the vomerine horizontal wing medially,the maxillary palatal shelf laterally, and the pala-tine posteriorly (Fig. 5D). The choana is separatedfrom the fenestra vomeronasalis by contact betweenthe vomerine horizontal wing and the maxillary pal-atal shelf. The premaxilla, septomaxilla, nasal, andvomer all contribute to a complex internasal sep-tum, which divides the nasal chamber into right and

left passageways from the external naris posteriorlyto the level of the first maxillary tooth.

In Diplometopon, as in other amphisbaenians(Zangerl, 1944; Montero and Gans, 1999), the cra-nial segment of the mature skull mainly comprisesthe parietal and a single continuous element knownas the otic–occipital complex (Fig. 5B). The latter iscomposed of bones that develop from distinct ossifi-cation centers and that generally remain separatethroughout at least a portion of postnatal ontogenyin other squamates. These include the median su-praoccipital and basioccipital and the paired exoc-cipital and prootic (Montero et al., 1999). InDiplometopon, as in Rhineura (Kearney et al., 2005),these bones are also coossified with the basicranialaxis (parabasisphenoid) and squamosal.

The roof of the cranial segment is formed anteri-orly by the parietal (Fig. 5C). An anterior triangularportion of the parietal is deflected onto the facialplane of the skull. Just posterior to this, a sagittalcrest runs posteriorly over the dorsal surface of theparietal, and continues along the otic–occipital com-plex to the back of the skull. The cranial segment iswidest across the otic region, at the level of theparoccipital processes, and tapers anteriorly to itsnarrowest constriction at its junction with the facialsegment. The otic capsule is proportionally enor-mous. The foramen magnum is tall and weakly tri-angular and roofed by the occipital crest (Fig. 5F).The occipital condyle is U-shaped and strongly de-veloped as compared to most other squamates.

The otic–occipital complex is flanked by the tem-poral area, which is unenclosed in contrast to thecondition in most squamates. A squamosal is notevident in external view and was not discussed byGans (1960). However, the HRXCT data show thepresence of a squamosal in Diplometopon (Fig. 5B),its dorsal edge fused along most of its length to theotic–occipital complex. A coossified squamosal wasalso identified in Rhineura (Kearney et al., 2005). Incomparison to other squamates the squamosal inDiplometopon is displaced ventrally, extending tothe level of the columellar shaft. The quadrate is ashort, robust element (Fig. 5A). Its broad proximalcephalic condyle articulates loosely with the oticcapsule and squamosal. The long axis of the quad-rate is oriented anteromedially, ending in a mandib-ular condyle that forms a saddle-shaped articulationwith the mandible. The extracolumella is greatlyenlarged and fully mineralized into a triangularplate (Fig. 5A). It articulates proximally with thecolumellar shaft and extends distally to partiallycover the lateral aspect of the quadrate and mandi-ble (Fig. 5E).

The mandible (Fig. 5A) is a robust compound ele-ment that is formed almost entirely by the dentary,as in trogonophids generally (Gans, 1960; Kearney,2003). The mandibular teeth occlude entirely medialto the maxillary teeth in a one-to-one correspon-dence; such occlusion is highly unusual in squa-

Fig. 5. Diplometopon zarudnyi (FMNH 64429). Three-dimensional digital reconstruction of specimen with flesh ren-dered transparent in (A) lateral, (B) lateral with mandible re-moved, (C) dorsal, (D) ventral with mandible removed, (E)anterior, and (F) posterior views. Anterior to left in A–D. Scalebar � 1 mm. app, apical process of parietal; appr, alar process ofprootic; bpp, basipterygoid process; c, columella; ch, choana; co,coronoid; cpb, compound bone; dn, dentary; ec, ectopterygoid; en,external naris; exc, extracolumella; f, frontal; fm, foramen mag-num; fvo, fenestra vomeronasalis; Gf, Gasserian foramen; h, hyo-branchium; m, maxilla; n, nasal; occ, occipital condyle; occr, oc-cipital crest; ooc, otic–occipital complex; os, orbitosphenoid; p,parietal; pal, palatine; pbs, parabasisphenoid; pm, premaxilla;pop, paroccipital process; pt, pterygoid; q, quadrate; sc, sagittalcrest; sq, squamosal; v, vomer; vlp, ventrolateral process; X, “el-ement X.”



mates (Schwenk, 2000). The dentaries are unfusedwhere they meet at the anterior midline (Fig. 5E),and the anteroventral corner of each dentary ex-tends ventrally to a pronounced point (Fig. 5A). Be-hind the tooth row the mandible bears a tall coro-noid process in a mechanical configuration that mostlikely affords a powerful bite (Gans, 1960). There isno retroarticular process; the posterior margin of themandible is the glenoid fossa.

Individual Elements of the SkullPremaxilla (Fig. 6; Tra 002–114). As in all other

amphisbaenians, the premaxilla in Diplometopon isan azygous element with complex relations to thesurrounding bones. Externally, it contacts the max-illae ventrolaterally, the vomers posteroventrally,

the nasals dorsolaterally, and the frontals pos-terodorsally. Contact between the premaxilla andfrontals distinguishes Diplometopon from Rhineura(Kearney et al., 2005) and Amphisbaena (Monteroand Gans, 1999); this is a condition that uniquelydiagnoses trogonophid amphisbaenians (Kearney,2003). Internally, the premaxilla is contacted by thevomers dorsally and the maxillae and septomaxillaedorsolaterally. Inside the nasal chamber the pre-maxillary nasal process separates the septomaxillaeanteriorly (Tra 065), then lies dorsal to them poste-riorly (Fig. 2B).

For comparative purposes, we describe the pre-maxilla as being organized around its alveolar plate(�basal plate of Montero and Gans, 1999). InDiplometopon, as in Rhineura (Kearney et al., 2005),the alveolar plate supports three tooth positions.This is in contrast to the alveolar plate in Amphis-baena, which holds seven teeth (Montero and Gans,1999). Viewed from below, the alveolar plate is rect-angular and its teeth are arranged in a shallowV-shaped pattern (Fig. 6C). The most anterior pre-maxillary tooth is by far the largest, a conditioncharacteristic of amphisbaenians generally (Gans,1978; Estes et al., 1988; Kearney, 2003). The smallerteeth on either side are more posteriorly placed andmore posteriorly inclined. The dorsolateral surfacesof the alveolar plate lie in apposition with the ven-tromedial surfaces of the maxillary premaxillaryprocesses (Tra 045). This simple overlap is alsopresent in Amphisbaena (Montero and Gans, 1999)but contrasts with the more elaborately intertongue-ing articulation between these bones in Rhineura(Kearney et al., 2005).

The alveolar plate projects forward, then expandsinto its distinctive rostral process. This structure,which forms the entire rostral blade of the “spadesnout” in Diplometopon, is absent in most amphis-baenians (e.g., Amphisbaena; Montero and Gans,1999), but a closely comparable structure is devel-oped in a few others (e.g., Rhineura; Kearney et al.,2005). Current phylogenetic hypotheses (Kearney,2003; Kearney and Stuart, 2004) suggest that thisdistinctive rostral blade evolved convergently inrhineurid and trogonophid amphisbaenians.

The nasal process rises dorsally from the rostralprocess (Fig. 6D). The nasal process is approxi-mately the same width throughout its length, as itruns posteriorly between the nasals to the pointwhere it contacts the frontals at their midline (Tra084). The nasal process is completely exposed on thesurface of the skull in Diplometopon, whereas thenasals obscure it from view partially in Amphis-baena (Montero and Gans, 1999) and completely inRhineura (Kearney et al., 2005). The anterodorsalsurface of the premaxilla is inscribed by a pair ofgrooves that begin shallowly on the rostral process,deepen as they extend posteriorly along the nasalprocess, then shallow again before they terminate atapproximately the midpoint of the nasal process.

Fig. 6. Premaxilla in (A) lateral, (B) dorsal, (C) ventral, (D)anterior, and (E) posterior views. Anterior to left in A–C. Scalebar � 1 mm. alvp, alveolar plate; drf, dorsal rostral foramen; en,external naris; np, nasal process; pmt, premaxillary teeth; pp,palatal process; rb, rostral blade; rp, rostral process; vrf, ventralrostral foramen.



Each groove houses a dorsal rostral foramen, whichis subdivided on the left side of this specimen. Theposterior end of the nasal process fits into a depres-sion on the anteromedial corner of each frontal (Fig.2B), where it abruptly tapers to a point.

In most squamates the alveolar and nasal pro-cesses make substantial contributions to the narialborder, while the superior surface of the alveolarplate forms the floor of the naris (Oelrich, 1956); thiscondition persists in Amphisbaena (Montero andGans, 1999). However, in many “spade-headed” am-phisbaenians, including Diplometopon, the externalnaris is displaced such that it opens from the ven-trolateral surface of the rostrum and has no floor(Fig. 5B). The premaxillary rostral process contrib-utes to the narial border, but most of the bordercircumference is built by the maxillary rostral spineand the nasal (Fig. 4B). When viewed from below,the premaxilla can be seen to widely separate theexternal nares on the ventral face of the rostrum(Fig. 6C), but the intervention by the maxillary ros-tral spine almost completely separates the premax-illa from the narial border.

The premaxilla continues onto the palate as a pairof moderately developed, divergent palatal pro-cesses (�lateral process of Montero and Gans, 1999)that extend to the level of the first maxillary tooth(Tra 082). The palatal process lies ventrolateral tothe vomer and lines the inferior surface of the max-illary palatal shelf (Fig. 2A). The premaxillary pal-atal process is absent or weakly developed in manyamphisbaenians, but is extremely well developed inrhineurids (Berman, 1976; Kearney, 2003; Kearneyet al., 2005).

The premaxilla is penetrated by a rostral canal(�longitudinal canal of Montero and Gans, 1999)that opens from the medial wall of the nasal cham-ber via the septomaxillary apical foramen (Tra 062).The canal passes anteriorly for a short distance be-fore penetrating the premaxilla at the juncture ofthe nasal process and alveolar plate (Tra 039). Thesecanals then emerge as paired dorsal rostral foram-ina on the anterodorsal surface of the premaxillajust dorsal to the alveolar plate (Fig. 4B), and aspaired ventral rostral foramina on the ventral sur-face of the alveolar plate at its junction with therostral process (Fig. 6C). The ventral rostral foram-ina are also present in Rhineura (Kearney et al.,2005) but not in Amphisbaena (Montero and Gans,1999). The rostral canal likely transmits cutaneousbranches of the medial ethmoidal nerve, an anteriorextension of the maxillary nerve (V2), and a rostralbranch of the maxillary artery (Oelrich, 1956). Threedental canals penetrate ventrally into the premax-illa from the floor of the rostral canals (Tra 036),each supplying a premaxillary tooth locus (Fig. 3C).

Septomaxilla (Fig. 7; Tra 057–116). The sep-tomaxilla is a paired element in Diplometopon thatlies entirely within the nasal chamber, where it con-tacts the premaxilla, maxilla, vomer, and nasal. The

septomaxilla is of complex shape, comprising a ver-tical lamina that contributes to the internasal sep-tum and roofs the vomeronasal chamber, and a hor-izontal lamina that spreads to form a partial floor tothe nasal chamber (Figs. 2A, 7A). A median processprojects anteriorly from the dorsal edge of the ver-tical lamina, turning ventrally to abut its counter-part at the midline (where they are separated by acartilaginous nasal septum), forming a highlyvaulted vomeronasal chamber (Figs. 2A, 7E). Thejuncture of the median process with the vertical

Fig. 7. Left septomaxilla in (A) lateral, (B) medial, (C) dorsal,(D) oblique dorsal, (E) ventral, (F) anterior, and (G) posteriorviews. Scale bar � 1 mm. af, apical foramen; hl, horizontal lam-ina; mpvl, median process of vertical lamina; rs, rostral spine; vl,vertical lamina; voc, vomeronasal chamber; 1, ridge at juncture ofmedian process and vertical lamina; 2, ridge along dorsal edge ofmedian process.



lamina is marked by a pronounced anteroposteriorridge (Fig. 7C). A smaller ridge runs along the me-dian process for most of its length (Fig. 7E), contact-ing the premaxillary nasal process (Fig. 2A). Themedian process of the vertical lamina gradually di-minishes in size posteriorly.

The vertical lamina narrows anteriorly to a shortrostral spine (Fig. 7A) that extends to roughly themidpoint of the naris (Fig. 4A) and contributes to themedial wall of the internarial girder. The septomax-illary rostral spine is also present in Rhineura(Kearney et al., 2005) but not in Amphisbaena(Montero and Gans, 1999). The rostral spine alsocontributes to the lateral wall of the short, pairedethmoid passageway, a median longitudinal channelaffording passageway from the nasal chamber to thealveoli and the surface of the face. Its opening is theapical foramen (�longitudinal passage of Monteroand Gans, 1999) (Tra 062) (Fig. 7D), from which itpasses for a short distance along the internarialgirder before penetrating the premaxilla as the ros-tral canal (Tra 039). The septomaxilla forms thelateral wall of the ethmoid passageway while thenasal and premaxilla form its roof, the premaxillaforms its medial wall, and the vomer forms its floor.The ethmoid passageway conveys premaxillary andethmoid branches of the trigeminal nerve to thepremaxillary dentition and cutaneous tissues of therostrum, respectively.

The vertical lamina bends sharply at its base (Fig.7E), where it spreads laterally onto the floor of thenasal passage as the horizontal lamina (�lateralprocess of Montero and Gans, 1999) (Fig. 2A). Thehorizontal lamina lies dorsal to the maxillary pala-tal process, tapering posteriorly and terminating atthe level of the vomerine horizontal wing.

The septomaxilla in Diplometopon is much morecomplex than that in Amphisbaena (Montero andGans, 1999). It is similar to the septomaxilla inRhineura (Kearney et al., 2005) in being divided intodistinct vertical and horizontal laminae and in itsgeneral relations to the nasal septum and vomero-nasal chamber. However, the septomaxilla inDiplometopon is comparatively foreshortened. As aresult, the apical foramen and ethmoid passagewaylie largely dorsal to the vomeronasal chamber,whereas in Rhineura both structures lie anterior tothe vomeronasal chamber.

Maxilla (Fig. 8; Tra 035–178). As in amphisbae-nians generally, the paired maxilla in Diplometoponis roughly triangular in lateral view (Fig. 8A) androughly L-shaped in cross-section (Fig. 8E). It makescontact with the premaxilla, septomaxilla, vomer,palatine, ectopterygoid, nasal, and frontal. The fa-cial process forms most of the lateral surface of thefacial segment of the skull (Fig. 8A), while the pal-atal process contributes primarily to the anterolat-eral palatal roof (Fig. 8C). The two processes meet atan approximately right angle along the alveolarmargin, which supports three teeth. A short, broad

rostral process projects anteromedially from the al-veolar margin (Fig. 8C), and a short orbital processprojects posteriorly from the other end of the palatalprocess beneath the orbit, where it contributes to theorbital floor.

The facial process (�lateral plate of Montero andGans, 1999) expands over the side of the snout tocontact the nasal anteriorly (Fig. 2A) and the frontaldorsally (Fig. 2B). It is taller than wide inDiplometopon, whereas the reverse is true in Am-phisbaena (Montero and Gans, 1999) and Rhineura(Kearney et al., 2005). A small rostral spine projectsanteriorly from the alveolar margin (Fig. 8C) whereit buttresses the ventrolateral edge of the nasal (Fig.3A). This spine most likely corresponds to the moreintricate maxillary nasal process of Rhineura (Kear-ney et al., 2005). An elongate, bifurcate frontal pro-cess (Kritzinger, 1946) juts from the superior edge of

Fig. 8. Left maxilla in (A) lateral, (B) medial, (C) dorsal, (D)ventral, (E) anterior, and (F) posterior views. Scale bar � 1 mm.alvm, alveolar margin; en, external naris; fap, facial process; fp,frontal process; lf, labial foramen; mt, maxillary tooth; nf, nutri-tive foramen; op, orbital process; pp, palatal process; rp, rostralprocess; saf, superior alveolar foramen; 1, spine ventral to nasal-maxillary suture; 2, margin of choana; 3, area of overlap withpremaxillary palatal process; 4, finger that contacts vomer hori-zontal wing; 5, notches that receive anteriorly projecting fingersof frontal; 6, area of maxilla-ectopterygoid contact.



the maxillary facial process and extends posterodor-sally almost to the frontoparietal suture (Tra 141).The anterior prong wedges between two fingers ofthe frontal (Fig. 2B), while the posterior prong lieslateral to the frontal, where it forms the anterodor-sal margin of the orbit (e.g., Tra 129). A bifurcatefrontal process is characteristic of trogonophids(Gans, 1960). In both Rhineura (Kearney et al.,2005) and Amphisbaena (Montero and Gans, 1999),the frontal process is unforked and abutted posteri-orly by the prefrontal, which is absent in Diplometo-pon.

The maxillary canal runs longitudinally throughthe maxilla just above the tooth row (Tra 086–140)(Fig. 4B). It conveys the superior alveolar nerve(Oelrich, 1956), a sensory branch of the trigeminalmaxillary nerve that passes from the orbit into themaxillary canal through the superior alveolar fora-men (Tra 134) (Fig. 8C). As it courses through themaxilla, the maxillary canal gives off three groups ofsubsidiary canals, which supply innervation to theteeth, surface of the snout, and inner surface of thenasal passage. Three narrow dental canals pene-trate the maxilla ventrally (e.g., Tra 121) where theysupply a small nerve fiber to each maxillary toothposition. Two large labial foramina pass laterallyonto the face above the teeth (Figs. 4B, 8A), convey-ing cutaneous branches of the superior alveolarnerve and the maxillary artery (Oelrich, 1956) to thesuperficial tissues of the face. Larger numbers oflabial foramina occur in Amphisbaena (Montero andGans, 1999) and especially in Rhineura (Kearney etal., 2005). The third group of subsidiary passagespenetrates the maxilla ventromedially above theteeth (Fig. 8A) as a series of nutritive foramina thatcommunicate to the tissues lining the roof of themouth. Lastly, at its anterior terminus, the maxil-lary canal exits into the nasal chamber through theanterior inferior alveolar foramen (Fig. 2B).Branches of the maxillary artery and superior alve-olar nerve pass through this foramen, supplying cu-taneous tissues of the snout (Oelrich, 1956) andtransmitting the alveolar nerve toward the sep-tomaxillary apical foramen and premaxillary rostralcanal.

The palatal process (Fig. 8C) extends medially asa horizontal shelf forming a floor beneath the nasalpassage (Fig. 2A). It is overlapped medially by theseptomaxilla and contacts the premaxillary palatalprocess anteroventrally. It approaches but does notcontact the palatine posterodorsally. The medialedge of the palatal process contacts the vomerinerostral process. Behind this contact, it forms thelateral border of the fenestra vomeronasalis (Fig.3B). Further posteriorly, along the anterior half ofthe vomerine horizontal wing, the palatal processforms the anterior and lateral margins of the choana(Fig. 8D). Its anterior contribution to this margin isformed by a posteromedially projecting finger of themaxillary palatal process (Tra 111) that contacts the

vomerine horizontal wing. This finger also contactsthe ventral edge of the septomaxillary vertical lam-ina. The rostral process (�premaxillary process ofMontero and Gans, 1999) (Fig. 8D) extends antero-medially to lie along the dorsolateral surface of thepremaxillary alveolar plate. Its dorsomedial edgecontacts the vomerine rostral process (e.g., Tra 059).

The junction of the facial and palatal processesmarks the maxillary alveolar margin. Three acr-odont teeth occupy this margin, the first (and larg-est) near the anterior margin of the facial processand the last just anterior to the orbit (Fig. 8A). Thisreduced number of maxillary teeth contrasts withthe six in Rhineura (Kearney et al., 2005) and thefive in Amphisbaena (Montero and Gans, 1999). Theteeth are fused at their bases by extensive bone ofattachment, and several give the impression of hav-ing small accessory cusps (Fig. 4C). The crowns areslightly inclined posteriorly (Fig. 8A).

The short orbital process lies in simple articula-tion with the anterolateral surface of the ectoptery-goid (Figs. 2C, 8B), extending posteriorly to the levelof the frontoparietal suture. It is much shorter thanthe orbital process in Rhineura (Kearney et al.,2005), which is clasped by a bifurcate ectopterygoidmaxillary process. Amphisbaena lacks a maxillaryorbital process (Montero and Gans, 1999). BecauseDiplometopon lacks a prefrontal, the anterior andventral margins of the orbit are formed entirely bythe maxilla (Fig. 4A,B), distinguishing it from thecondition in Rhineura (Kearney et al., 2005) andAmphisbaena (Montero and Gans, 1999).

Nasal (Fig. 9; Tra 018–113). The nasal is apaired, trough-shaped element that contacts thepremaxilla, maxilla, and frontal; it approaches butdoes not contact the septomaxilla. The nasal formsthe dorsal border of the external naris and the roofover the nasal chamber (Fig. 9D). It bears four pro-cesses: an anterior rostral process, a lateral facialprocess, a medial premaxillary process, and a poste-rior frontal process.

The short, broad rostral process forms the an-terodorsal corner of the external naris (Figs. 4A, 9A).The facial process (�facial lamina of Rhineura,Kearney et al., 2005) extends posteroventrally toform the lateral wall of the nasal chamber (Fig. 2A).It lines the medial surface of the maxillary facialprocess; the extent of overlap by the maxilla is de-limited by a ridge on the nasal (Fig. 9A). Its bluntventral tip abuts the rostral spine on the maxillaryfacial process (Figs. 3A, 9E). The facial process ex-hibits the same topological relationships inRhineura, except that its ventral tip is received by amore elaborate maxillary nasal process, and it ispenetrated by three small foramina that communi-cate between the nasal chamber and the externalsurface of the snout (Kearney et al., 2005). Amphis-baena lacks a facial process; the lateral edge of thenasal simply abuts the dorsal edge of the maxilla(Montero and Gans, 1999).



Medially, a premaxillary process (�medial sheetof Montero and Gans, 1999) projects ventromediallyto line the ventrolateral surface of the premaxillarynasal process (Fig. 2A); the extent of overlap by thepremaxilla is delimited by a ridge on the nasal (Fig.9C). This is similar to the condition in Amphisbaena(Montero and Gans, 1999), whereas in Rhineura thepremaxillary process clasps the premaxillary nasalprocess, thereby obscuring it from view in the artic-ulated skull (Kearney et al., 2005). The nasal pre-maxillary process in Diplometopon approaches butdoes not meet the septomaxillary vertical lamina,unlike in Amphisbaena (Montero and Gans, 1999)and Rhineura (Kearney et al., 2005).

The frontal process (Montero and Gans, 1999)projects from the posterodorsal edge of the nasal(Fig. 9C) and inserts between two anteriorly project-ing fingers of the frontal (Fig. 2B), as in Rhineura(Kearney et al., 2005) and Amphisbaena (Montero

and Gans, 1999). The posterodorsal corner of thenasal bears a depression that receives the tip of themiddle forward projecting finger of the frontal (Fig.9C). The frontal process extends posteriorly to thelevel of the distal tip of the premaxillary nasal pro-cess, terminating bluntly without overlapping thefrontal. At its base, its lateral edge is notched (Fig.9E); the middle forward-projecting finger of the fron-tal closes this notch to form a foramen that commu-nicates with the nasal chamber (Tra 092). This fo-ramen also occurs in other trogonophids (Gans,1960). A similar foramen is formed by the nasal andfrontal in Amphisbaena; however, it communicateswith a complex series of foramina that open onto thedorsal surface of the nasal (Montero and Gans,1999), not into the nasal chamber. No foramina pen-etrate the nasal in Diplometopon. The condition inRhineura is unknown owing to postmortem erosionof the nasals in the single specimen that has beenstudied with HRXCT (Kearney et al., 2005).

Vomer (Fig. 10; Tra 051–202). The vomer is apaired element that forms the floor of the vomero-nasal chamber. Its general configuration resemblesboth Rhineura (Kearney et al., 2005) and Amphis-baena (Montero and Gans, 1960) in that it contrib-

Fig. 9. Left nasal in (A) lateral, (B) medial, (C) dorsal, (D)ventral, (E) anterior, and (F) posterior views. Scale bar � 1 mm.en, external naris; fap, facial process; fp, frontal process; nc, nasalchamber; pmxp, premaxillary process; rp, rostral process; 1, shelfoverlain by premaxillary nasal process; 2, shelf overlain by max-illary facial process; 3, notch closed by projecting finger of frontalto form foramen; 4, depression receiving middle forward project-ing finger of frontal; 5, tip abutting spine on maxillary facialprocess.

Fig. 10. Left vomer in (A) lateral, (B) medial, (C) dorsal, (D)ventral, (E) anterior, and (F) posterior views. Scale bar � 1 mm.chp, choanal process; fvo, fenestra vomeronasalis; hw, horizontalwing; map, median articular plane; rp, rostral process; vof,vomerine foramen; 1, groove that receives palatine vomerine pro-cess.



utes to the internarial girder, the roof of the palate,and the borders of the choana. The vomer is con-tacted medially by its counterpart, anteriorly by thepremaxilla, septomaxilla, and maxilla, laterally bythe maxilla, dorsally by the septomaxilla, and pos-terodorsally by the palatine.

A rostral process (�anterior process of Monteroand Gans, 1999) (Fig. 10C) projects anteriorly, con-tacting the maxillary palatal process (Tra 065) andthe septomaxillary vertical lamina (Tra 069). In con-trast to Rhineura (Kearney et al., 2005) and Am-phisbaena (Montero and Gans, 1999), its anteriorextremity is deeply bifurcated where it clasps thepremaxillary alveolar plate (Fig. 3C). The rostralprocess is triangular in cross-section and forms arobust strut extending from the midpoint of the na-ris to approximately the midpoint of the vomerona-sal chamber (Sag 091). The rostral process abuts itscounterpart posteriorly along a deep, flat, medianarticular plane (Figs. 2A, 10B), forming the majorstructural element along the ventral midline of thesnout. The vomers tie together projections of thepremaxilla, septomaxillae, and maxillae into a com-plex internarial girder that appears to brace therostral blade, via the roof of the palate, against thebasicranial axis. The rostral process abuts the max-illary palatal process, overlies the premaxillary pal-atal process, and underlies the septomaxillary ver-tical lamina (Fig. 2A), forming the anteromedialmargin of the fenestra vomeronasalis (Fig. 10C).The rostral process also forms the floor of the ante-rior portion of the vomeronasal chamber (Fig. 2A).

The vomer expands laterally to form a broad hor-izontal wing (�lateral wing of Montero and Gans,1999), which lies posterior to the rostral process atthe approximate midpoint of the vomer (Fig. 10C).The horizontal wing contributes simultaneously tothe roof of the palate, the floor of the vomeronasalchamber, and the posterior border of the fenestravomeronasalis. Its lateral extremity contacts themaxillary palatal process and the septomaxillaryvertical lamina (Fig. 2B). The horizontal wingprojects anteriorly such that its anterior edge isdeeply incised by the fenestra vomeronasalis (Fig.4B). A similar but less extreme condition in seen inAmphisbaena (Montero and Gans, 1999), whereas inRhineura (Kearney et al., 2005) the anterior edge ofthe horizontal wing slopes posterolaterally to form amore slit-like fenestra vomeronasalis. A ridge di-verges laterally along the dorsal surface of the hor-izontal wing, marking the edge of a groove thatreceives the vomerine process of the palatine (Fig.10C). A foramen pierces each vomer at the midpointof the horizontal wing (Figs. 4B, 10D), exiting dor-sally just lateral to the groove receiving the palatinevomerine process. This foramen, which is alsopresent in Rhineura (Kearney et al., 2005) and Am-phisbaena (Montero and Gans, 1999), transmitsbranches of the anterior palatine nerve and inferiornasal artery (Oelrich, 1956).

The right and left vomers remain in contact alongthe midline via a flat median articular plane (Fig.2B) for approximately three-quarters of their length(Tra 073–169), until a very narrow pyriform recessintervenes between them. The vomer graduallytapers posteriorly into a slender choanal process(�posterior process of Montero and Gans, 1999)(Fig. 10C) that courses along the midline of thepalatal roof, terminating in a thin point wedgingshallowly between its counterpart and the ventro-medial edge of the palatine (Fig. 2C).

Palatine (Fig. 11; Tra 111–266). The palatine isa paired element forming the roof of the choanalvault and portions of the floor of the orbit. It isinclined anteriorly to parallel the angulation of theskull (Fig. 11A). It contacts the vomer anteroven-trally, the maxilla anterolaterally, the ectopterygoidlaterally, and the pterygoid posteroventrally. Theright and left palatines closely approach each other

Fig. 11. Left palatine in (A) lateral, (B) medial, (C) dorsal, (D)ventral, (E) anterior, and (F) posterior views. Scale bar � 1 mm.chv, choanal vault; mp, maxillary process; ol, orbital lamina; palf,palatine foramen; ptp, pterygoid process; vp, vomerine process; 1,ridge delimiting area of overlap by pterygoid; 2, notches formingforamina in articulation with ectopterygoid; 3, area of articula-tion with ectopterygoid.



anteriorly and are separated posteriorly by a verynarrow pyriform recess (Fig. 4B). The palatine bearsa vomerine process anteromedially, a maxillary pro-cess anterolaterally, and a pterygoid process poste-riorly. In its general relations the palatine resem-bles that of Rhineura (Kearney et al., 2005) andAmphisbaena (Montero and Gans, 1999); however,the precise geometries of its processes differ amongthese taxa.

The vomerine process (�parasagittal plate ofMontero and Gans, 1999) (Fig. 11A) is formed by aslender finger that projects anteromedially into agroove on the dorsal surface of the vomer, medial tothe vomerine horizontal wing (Tra 119). The maxil-lary process (�lateral process of Montero and Gans,1999) projects only a short distance forward (Fig.11C), where it contacts the maxillary palatal processand the ectopterygoid (Fig. 4B). In contrast toRhineura (Kearney et al., 2005) and Amphisbaena(Montero and Gans, 1999), the maxillary process isweakly developed in Diplometopon. In Amphisbaenathe maxillary process extends far laterally, whereasin Rhineura it extends far anteriorly. The pterygoidprocess (�posterior process of Montero and Gans,1999) projects posteriorly (Fig. 11C), where it over-lies the dorsal surface of the pterygoid transverseprocess (Figs. 2D, 3B); this area of overlap is delim-ited by a ridge on the palatine (Fig. 11D). A similarrelationship is seen in Amphisbaena (Montero andGans, 1999), whereas in Rhineura (Kearney et al.,2005) the pterygoid process is also weakly forked toreceive the pterygoid.

Above the roof of the choanal vault, the superiorsurface of the palatine approaches the overlyingparabasisphenoid and frontal descending process.However, unlike in Rhineura (Kearney et al., 2005)and Amphisbaena (Montero and Gans, 1999), it doesnot contact either (Figs. 2C, 4B). An orbital laminaprojects ventrolaterally from its superior face (Fig.11A) to contact the ectopterygoid via a flat surfacethat extends along most of the length of the orbitalfloor (Figs. 2C, 4B). The orbital lamina is incised bytwo shallow notches approximately halfway backalong this contact (Tra 174, 191). The notches areclosed by the medial surface of the ectopterygoid toform two small foramina that communicate betweenthe palate and the orbit (Fig. 5D). A single openingwas described previously in this area in Diplometo-pon (Gans, 1960; El-Assy and Al-Nassar, 1976) andin Amphisbaena (Montero and Gans, 1999); the lat-ter authors questioned whether it might represent areduced suborbital fenestra.

The inferior surface of the palatine (Fig. 11D) issmooth and highly arched along much of its length,forming the bony roof of the choanal vault (Fig. 2C)and constituting the posterodorsal border of the cho-ana (Jollie, 1960). The vault is tallest anteriorly,becoming shallower posteriorly until the palatineeventually flattens out, ending in its contact with

the pterygoid (Fig. 11A). In life, the choanal vault isclosed ventrally by connective tissue (Oelrich, 1956).

The palatine is pierced by a single foramen, whichpenetrates the orbital lamina (Fig. 11A) near theanterior end of the articulation of the palatine withthe pterygoid (Fig. 2D). It affords communicationbetween the choanal vault and the temporal fossa,and carries the intermediate palatine branch of thefacial nerve (VII) (Oelrich, 1956). There is no pala-tine foramen in Rhineura (Kearney et al., 2005) orAmphisbaena (Montero and Gans, 1999).

Ectopterygoid (Fig. 12; Tra 147–200). InDiplometopon the ectopterygoid is a paired elementthat lies between the maxilla and palatine along thelateral edge of the palate, where it also contributesto the floor of the orbit. The ectopterygoid contactsthe maxilla along the anterior half of its length, andthe palatine orbital lamina along its entire length. Itgradually tapers to a point posteriorly. Its ventro-lateral surface closely parallels the dorsomedial sur-face of the pterygoid palatine process (Tra 194).

The ectopterygoid is short, thick, and roughly tri-angular in cross section (Fig. 2C). A weak maxillaryprocess (Fig. 12E) extends dorsolaterally to line themaxillary orbital process (Figs. 2C, 4B), and an evenweaker palatine process (Fig. 12C) extends mediallyto meet the ventrolateral edge of the palatine orbitallamina (Tra 152). The simple morphology and artic-ulations of the ectopterygoid in Diplometopon con-trast sharply with those in Rhineura (Kearney et al.,2005) and Amphisbaena (Montero and Gans, 1999).In the latter taxa, the ectopterygoid bears an elon-gated, forked maxillary process that clasps the max-

Fig. 12. Left ectopterygoid in (A) lateral, (B) medial, (C) dor-sal, (D) ventral, (E) anterior, and (F) posterior views. Scale bar �1 mm. mp, maxillary process; palp, palatine process; 1, surface ofarticulation with maxilla; 2, surface of articulation with palatine;3, surface exposed in orbital floor.



illary facial process and a robust forked pterygoidprocess that clasps the pterygoid transverse process.

Pterygoid (Fig. 13; Tra 187–327). The pterygoidis a paired element lying on either side of the mid-line of the ventral surface of the skull. In Diplometo-pon it articulates with the palatine anteriorly andwith the parabasisphenoid and quadrate posteri-orly. The pterygoids form the posteriormost portionof the palate. Each pterygoid is elongate and slightlyconcave dorsally. The parasagittal axis of the ptery-goid is inclined anteriorly to parallel the sagittalcrest of the skull, and emphasizes the angulation ofthe facial segment against the cranial segment (Fig.13A).

The body of the pterygoid (�pterygoid plate ofMontero and Gans, 1999) is a thin triangular platethat lies against the ventral surface of the palatinepterygoid process (Figs. 3B, 4C). Unlike in Rhineura

(Kearney et al., 2005) and Amphisbaena (Monteroand Gans, 1999), there is no anteromedial processthat extends from the body to meet the vomer orpalatine, respectively. Posteriorly, the body of thepterygoid lies in tight communication with the ba-sipterygoid process (Fig. 2E) in a contact marked bya facet on the dorsomedial margin of the pterygoid(Fig. 13F).

The transverse process (�anterolateral process ofMontero and Gans, 1999) (Fig. 13A) projects antero-laterally to lie along the edge of the palatine orbitallamina (Fig. 2D). The ectopterygoid briefly inter-venes between the two anteriorly (Tra 194). Becauseof the close association of these three elements, thesuborbital fenestra is closed (Fig. 5D; but see above).The transverse process exhibits more complex rela-tionships in Amphisbaena (Montero and Gans,1999), where it is clasped by the ectopterygoid ptery-goid process, and especially in Rhineura (Kearney etal., 2005), where it interfingers with the ectoptery-goid, maxilla, and palatine.

The quadrate process (�posterior process ofMontero and Gans, 1999) (Fig. 13C) extends postero-laterally, forming a floor beneath an extracranialspace known as the cavum epiptericum (Tra 265),which lodges the trigeminal (Gasserian) ganglion(Oelrich, 1956). The quadrate process turns later-ally, closely approaching the mandibular compoundbone (Fig. 2E) before abutting the quadrate (Fig.3A). The area of contact with the quadrate is markedby a facet on the posterolateral surface of the quad-rate process (Fig. 13A). The quadrate process con-tinues further posteriorly, terminating in a blunt tipnear the columellar shaft. The quadrate process ex-hibits similar relationships in Rhineura (Kearney etal., 2005) and Amphisbaena (Montero and Gans,1999), but is not as distinctly developed in the latter.

Frontal (Fig. 14; Tra 082–207). The frontal is acomplex, paired element in Diplometopon. It com-prises two main divisions, the dorsal plate and thedescending process, each with its own form andfunction. A similar configuration occurs in Rhineura(Kearney et al., 2005) and Amphisbaena (Monteroand Gans, 1999). The superficial surface forms theleading surface or dorsal plate of the “spade snout”(Fig. 5E). The descending process lies beneath thedorsal plate and comprises two distinct laminae in-volved in forming the medial wall of the deeplysunken orbit and a tube that completely encirclesthe forebrain. The frontal is contacted by the pre-maxilla, nasal, and maxilla anteriorly, its counter-part medially, the orbitosphenoid ventrally and me-dially, and the parietal dorsally and posteriorly.

The dorsal plate (Fig. 14C) contacts the premax-illa, maxilla, nasal, and parietal. It forms the middleportion of the facial aspect of the skull, while alsocontributing to the roofs over the nasal chamber andthe cranial cavity. The dorsal plate overhangs theorbit via a supraorbital ridge (Fig. 14A) that formsthe orbital roof and contributes to the dorsal portion

Fig. 13. Left pterygoid in (A) lateral, (B) medial, (C) dorsal,(D) ventral, (E) anterior, and (F) posterior views. Scale bar � 1mm. bptaf, basipterygoid articular facet; qaf, quadrate articularfacet; qp, quadrate process; tp, transverse process; 1, area ofcontact with palatine pterygoid process.



of the orbital rim. The dorsal plate is gently convex,somewhat inclined toward the rostrum, and widestanterior to the orbit. The left and right frontals abutin a relatively straight suture along the medial sur-face of the dorsal plate (Fig. 2C), whereas they in-

terdigitate in Amphisbaena (Montero and Gans,1999) and Rhineura (Kearney et al., 2005).

Three fingers project anteriorly from the dorsalplate (Figs. 2B, 14C). The most medial finger, whichis the broadest, wedges between the premaxillarynasal process and the nasal frontal process. Thenext most medial finger wedges between the nasalfrontal process and the maxillary frontal process.The most lateral finger inserts into the bifurcatemaxillary frontal process. The last is absent in Am-phisbaena (Montero and Gans, 1999) and Rhineura(Kearney et al., 2005), both of which lack the bifur-cate maxillary frontal process to receive it.

The posterior margin of the dorsal plate is alsocomplex (Fig. 14C), exhibiting four major posteriorlyprojecting fingers that interdigitate with corre-sponding fingers on the anterior margin of the pari-etal (Fig. 5C). The dorsal plate also slightly overlapsthe parietal (Fig. 3A), and this area of overlap isdelimited by a shelf on the anterior margin of theparietal. This is also the case in Rhineura (Kearneyet al., 2005), whereas in Amphisbaena (Montero andGans, 1999) there is no overlap of the parietal by thefrontal.

The dorsal plate lines the medial surface of themaxillary frontal process anterolaterally (e.g., Tra130). The supraorbital ridge projects from the lat-eral margin of the dorsal plate posterior to the max-illary frontal process (Tra 148). It is almost imme-diately excluded from the dorsal margin of the orbitby an anteriorly projecting finger of the parietal (Tra153). There is no finger-like supraorbital process inDiplometopon, unlike in Amphisbaena (�lateralprocess of Montero and Gans, 1999) and Rhineura(Kearney et al., 2005).

The surface of the dorsal plate is relatively smooth(Fig. 14C). It is pierced near its lateral edge by twocommunicating foramina that mark the endpoints ofa frontal canal (Tra 132–155). A branch of the tri-geminal ophthalmic nerve penetrates the dorsome-dial wall of the orbit through the superior orbitalforamen (Tra 157) (Fig. 14A) and feeds into thisfrontal canal. The condition in Rhineura is similar(Kearney et al., 2005), except that many more com-municating foramina open into the surface of thedorsal plate. Amphisbaena is strikingly different

Fig. 14. Left frontal in (A) lateral, (B) medial, (C) dorsal, (D)ventral, (E) anterior, and (F) posterior views. Scale bar � 1 mm.cc, cranial cavity; dpl, dorsal plate; dpml, descending processmedial lamina; dpvl, descending process vertical lamina; fcf, fron-tal communicating foramen; sof, superior orbital foramen; sor,supraorbital ridge; 1, notch that receives tip of parabasisphenoidorbitosphenoid process; 2, frontal-frontal contact (dorsal plates);3, frontal-frontal contact (medial laminae of descending process-es); 4, finger inserting between premaxilla and nasal; 5, fingerinserting between nasal and maxilla; 6, finger inserting intomaxillary frontal process; 7, fingers that articulate with parietal;8, area underlain by orbitosphenoid; 9, area overlain by premax-illary nasal process.



(Montero and Gans, 1999) in that the frontal iscompletely unperforated and lacks a frontal canal.

The descending process (�ventral process andventrolateral process of Montero and Gans, 1999)lies beneath the dorsal plate (Fig. 14A). It contactsthe orbitosphenoid, parabasisphenoid, and parietal,but unlike in Rhineura (Kearney et al., 2005) andAmphisbaena (Montero and Gans, 1999), it does notcontact the palatine. The descending process is or-ganized around a vertical lamina of bone that de-scends ventromedially from the dorsal plate towardthe floor of the orbit and braincase (Figs. 2C, 3A).The vertical lamina is inclined posteriorly and grad-ually turns to form an anteroposteriorly broadenedmedial lamina (Fig. 14A). The medial lamina exhib-its a notch on its posterior margin (Fig. 14F) thatreceives the anterior tip of the parabasisphenoidorbitosphenoid process (Fig. 3A). As in Amphisbaena(Montero and Gans, 1999), the descending processlacks a temporal wing. The condition in Rhineura(Kearney et al., 2005) is markedly different, with apronounced temporal wing projecting posteriorly toform part of the lateral wall of the braincase.

The vertical lamina forms the medial wall of theorbit as it descends from beneath the dorsal plate(Tra 156). It is penetrated near its junction with thedorsal plate by the superior orbital foramen (Tra158) (Fig. 14A), which transmits cutaneous innerva-tion and vasculature to the posterior surface of thedorsal plate. This is also the case in Rhineura (Kear-ney et al., 2005).

The vertical lamina also forms the long lateralwall of the anterior part of the cranial cavity (Figs.3B, 14B). The base of this wall curves inward as themedial lamina, which projects to contact its counter-part along the ventral midline, thereby forming afloor beneath the cranial cavity (Fig. 2C). Thus, thefrontals form a complete and robust bony tube thatencircles the forebrain, an unusual condition that ispresent throughout amphisbaenians (Zangerl, 1944;Vanzolini, 1951) and in some other squamates suchas snakes. No perforations in this tubular forebrainhousing are apparent in Diplometopon. Addition-ally, the cranial cavity is open anteriorly (Fig. 14E)via the olfactory fenestra (�frontal fenestra ofMontero and Gans, 1999), which provides a largepassageway for the neuronal axons of the olfactoryepithelium of the nasal chamber.

The medial laminae are underlain posteriorly bythe orbitosphenoid anteromedial process (Tra 158)(Fig. 3C). The orbitosphenoid anterolateral pro-cesses begin to line the lateral walls of the cranialcavity further posteriorly, at the level of the widestpoint of the supraorbital ridge (Tra 170) (Fig. 2C).The orbitosphenoid anteromedial and anterolateralprocesses merge near the level of the frontoparietalsuture (Tra 180), at which point the cranial cavity islined by the orbitosphenoid laterally, the frontaldescending processes ventrally, and the parietaldorsally. The descending processes terminate just

posterior to the anterior tips of the parabasisphe-noid orbitosphenoid processes (Tra 207), leaving theparietal, parabasisphenoid, and orbitosphenoid toclose the cranial cavity (Fig. 2D). These relation-ships are quite different in Rhineura (Kearney et al.,2005), where paired orbitosphenoids lie dorsal to themedial laminae, completely enclosed within thebraincase.

Parietal (Fig. 15; Tra 152–419). The parietal isunpaired in Diplometopon, but it may have devel-oped from paired ossification centers that fusedearly in ontogeny as in Amphisbaena darwini(Montero et al., 1999). The parietal is a domed ele-ment that is constricted at the narrowest point ofthe skull, just behind the frontoparietal suture (Fig.5C). The anterodorsal surface of the parietal contrib-utes a large triangular portion to the deflected facialsegment of the skull. The apical process (Figs. 3C,15B) forms the tallest prominence of the skull, at thepoint of deflection between the facial and cranialsegments. A midline sagittal crest arises just behindthe apical process and extends posteriorly along theentire length of the bone. Its development inDiplometopon and Rhineura (Kearney et al., 2005) isweak relative to Amphisbaena (Montero and Gans,1999), where the sagittal crest reaches a height al-most one-third the total height of the parietal. As inother amphisbaenians, there is no pineal foramen.The parietal forms most of the roof and anterolateralwalls of the cranial cavity.

The parietal contacts the frontal anterodorsally ina complex suture (Fig. 5C). Three major lateral fin-gers (e.g., Tra 173) and several minor medial fingers(e.g., Tra 189) project anteriorly from the parietal tointerdigitate with corresponding fingers on the pos-terior margins of the frontal dorsal plates. The an-terolateral process of the parietal comprises thethree lateral fingers (Figs. 2C, 15A). The anteriormargin of the parietal is overlain slightly by poste-rior projections of the frontal dorsal plates (Fig.3A,B), resulting in a weakly developed shelf (Fig.15B).

The temporal lamina (�lateral plate of Monteroand Gans, 1999) (Fig. 15A) descends from the skullroof to form much of the external surface of theanterior braincase (Figs. 2D, 3A). Its ventral edgecontacts the orbitosphenoid anteroventrally (Fig.3A), the parabasisphenoid ventrally (Fig. 4B), andthe alar process (�anterolateral process of Monteroand Gans, 1999) posteriorly (Fig. 4A). The temporallamina contributes to the anterodorsal margin of theGasserian foramen (Fig. 4B). This is also the case inAmphisbaena (Montero and Gans, 1999), whereas inRhineura (Kearney et al., 2005) it is excluded fromthis margin by the prootic alar process. These areasof contact are delimited by shallow shelves on theventral and posterolateral surfaces of the temporallamina (Fig. 15A,C).

The parietal broadly overlies the roof of the ves-tibular chamber of the otic–occipital complex (Fig.



2F). As in amphisbaenians generally, there is noposttemporal fenestra and no supratemporal pro-cess of the parietal. A channel on the posteroventralsurface of the parietal (Fig. 15C) receives the su-praoccipital ascending process (Fig. 2G), which ex-tends anteriorly to approximately the midpoint ofthe otic capsule (Fig. 3B). The parietal dividessmoothly just anterior to the origin of the supraoc-

cipital ascending process into bilateral lappets(�posterior plates of Montero and Gans, 1999) thatoverlie the supraoccipital (Figs. 2H, 3A, 15B). Theposterior margins of the lappets are bluntly rounded(Fig. 15B) and lie well anterior to the supraoccipitaloccipital crest (Fig. 5C).

The central vault of the internal surface of theparietal is relatively smooth (Fig. 15C), except for ashallow sulcus reflecting the position of the sagittalcrest (Fig. 2E). A pronounced ridge marks the pointwhere the parietal begins to overlie the otic–occipital complex (Fig. 15C). Bilateral depressionsimmediately posterior to this ridge mark the dorsal-most point of the anterior verticular semicircularcanals (Figs. 2F, 3A).

Otic–occipital complex and squamosal (Figs.2–4, 16; Tra 193–441). In Diplometopon, the sepa-rate embryonic ossifications of the otic and occipitalunits become conjoined above and below the fora-men magnum, creating the fused otic–occipital com-plex. As in Rhineura (Kearney et al., 2005), theparabasisphenoid and squamosal are also coosifiedwith the otic–occipital complex. These bones arefigured and described as a single unit below. Forconvenience of more general comparisons amongsquamates, the constituent parts are then describedregion-by-region. Lastly, the internal morphology ofthe endocranium and otic capsule is summarizedfrom front to back as viewed in successive HRXCTslices (Figs. 2–4).

External Anatomy of Otic–Occipital Complex (Fig.16). In lateral view (Fig. 16A), the otic–occipitalcomplex inclines from its shallowest anterior pointat the cultriform process tip (Tra 193) to its deepestposterior point (Tra 356), where “Element X”(Zangerl, 1944) caps the ventrolateral process. Theparabasiphenoid forms the anteriormost and me-dian portion of the basicranium (Fig. 16E), while thecranial cavity is bounded laterally by the otic com-plex and dorsally and posteriorly by the occipitalcomplex. The jugular recess is largely obscured fromview, as in Amphisbaena (Montero and Gans, 1999),whereas it is fully exposed laterally in Rhineura(Kearney et al., 2005).

In posterior view (Fig. 16B), the paroccipital pro-cess marks the widest point of the otic–occipitalcomplex, where the horizontal semicircular canalreaches its greatest lateral inflection. The cranialcavity is restricted dorsally by the medial inflectionof the otic capsules. The margins of the foramenmagnum are formed by the occipital complex dor-sally and ventrally and the otic capsules laterally.The border of the foramen magnum is roughly tri-angular in shape, with the occipital crest risingprominently above it. The occipital condyle isU-shaped, expanding upwards around the ventralportion of the foramen magnum. This differs fromthe dumbbell-shaped condyle in Rhineura (Kearneyet al., 2005) and the bicipital condyle in Amphis-baena (Montero and Gans, 1999). These differences

Fig. 15. Parietal in (A) lateral, (B) dorsal, (C) ventral, (D)anterior, and (E) posterior views. Anterior to left in A–C. Scalebar � 1 mm. alp, anterolateral process; app, apical process; cc,cranial cavity; oocl, otic–occipital lappet; sc, sagittal crest; tl,temporal lamina; 1, fingers inserting into notches on posteriormargin of frontal dorsal plate; 2, areas of overlap by posteriorfingers of frontal; 3, area of overlap by prootic alar process; 4, areaof overlap by orbitosphenoid; 5, area of overlap by parabasisphe-noid; 6, ridge delimiting overlap of otic–occipital complex; 7,channel receiving supraoccipital ascending process; 8, sulcus re-flecting sagittal crest; 9, depression marking dorsalmost point ofanterior verticular semicircular canal.



Fig. 16. Otic–occipital complex and squamosal in (A) lateral, (B) posterior, (C) anterior, (D) dorsal, and (E) ventral views. Anteriorup in D,E. Scale bar � 1 mm. appr, alar process of prootic; aVf, anterior Vidian foramen; avsc, anterior vertical semicircular canal;bpp, basipterygoid process; cc, cranial cavity; cp, cultriform process of parabasisphenoid; crpr, crista prootica; ff, facial foramen; fm,foramen magnum; for, foramen; Gf, Gasserian foramen; hf, hypoglossal foramen; hsc, horizontal semicircular canal; icf, internalcarotid foramen; jr, jugular recess; occ, occipital condyle; occr, occipital crest; ocr, occipital recess; osp, orbitosphenoid process ofparabasisphenoid; pas, processus ascendens of supraoccipital; pbs, parabasisphenoid; pop, paroccipital process; pVf, posterior Vidianforamen; sc, sagittal crest; sltu, sella turcica; sq, squamosal; vf, vagus foramen; vlp, ventrolateral process; X, “element X”; 1, area ofoverlap by parietal lappet; 2, depression receiving parietal descending finger.


presumably relate to different head postures duringstereotyped digging behaviors among “spade-headed,” “shovel-headed,” and “round-headed” am-phisbaenians (Gans, 1974). The occipital recesshouses four foramina: the vagus foramen dorsally;two hypoglossal foramina medially; and another fo-ramen laterally that communicates with the peri-lymphatic duct.

In anterior view (Fig. 16C), the otic–occipital com-plex is roughly oval. Its dorsal surface rises to anapex that is formed by the supraoccipital processusascendens and the dorsalmost limit of the anteriorverticular semicircular canal. The cranial cavity, al-though open at the olfactory fenestra, is mostly ob-scured from view by the dorsally inclined parabasi-sphenoid. The anterior Vidian foramen lies at themedial corner of the basipterygoid process (Tra 279),which is much more protuberant than in Rhineura(Kearney et al., 2005) or Amphisbaena (Montero andGans, 1999). The facial foramen penetrates the otic–occipital complex anteromedial to the cristaprootica. The jugular recess is partially obscured inthis view by the pronounced crista prootica mediallyand the squamosal laterally.

In dorsal view (Fig. 16D), the prootic alar pro-cesses extend anteriorly to the level of the sellaturcica. The otic capsules extend dorsomedially toform the beginnings of the endocranial roof. Theotic–occipital complex is roofed posteriorly by thesupraoccipital, which completely obscures the occip-ital condyle. This differs from the condition inRhineura (Kearney et al., 2005) and Amphisbaena(Montero and Gans, 1999), where the occipital con-dyle is visible in dorsal view. The sagittal crest,which is continued forward in the articulated spec-imen as the sagittal crest of the parietal, projectsalong the dorsal midline of the supraoccipital. Amoderately well-developed occipital crest runs alongthe posterodorsal margin of the supraoccipital. Thiscrest is also prominent in Amphisbaena (Monteroand Gans, 1999), but apparently absent in Rhineura(Kearney et al., 2005). The supraoccipital processusascendens, which extends from the anterior end ofthe sagittal crest, fits into a median channel on theparietal. A shallow depression on either side of theprocessus ascendens receives a small descendingfinger of the parietal (Tra 402). This relationship isalso present in Rhineura (Kearney et al., 2005) butnot in Amphisbaena (Montero and Gans, 1999). Thedorsomedial margin of the otic capsule exhibits ashallow excavation that marks the area of overlapby each parietal lappet. The otic capsules are smoothand bulbous and form most of the dorsal surface ofthe otic–occipital complex. The internal carotid fo-ramen pierces the parabasisphenoid at the postero-lateral edge of the sella turcica. Anterolateral tothis, a small foramen pierces the parabasisphenoidorbitosphenoid process.

In ventral view (Fig. 16E), the otic–occipital com-plex is roughly triangular, with the anterior tip of

the cultriform process forming the apex of the trian-gle and the otic capsules forming its corners. Theprootic alar process extends anteriorly lateral to theparabasisphenoid, arching above the Gasserian fo-ramen that is visible between these two elements.The pronounced basipterygoid process is located atthe level of the Gasserian foramen and is orientedanterolaterally. This is also its orientation in Am-phisbaena (Montero and Gans, 1999), whereas inRhineura it faces directly anteriorly (Kearney et al.,2005). The ventrolateral process, capped by “Ele-ment X,” lies further posteriorly. The voluminousotic capsules lie behind the basipterygoid processes,and the occipital complex closes the cranial cavityposteriorly.

Parabasisphenoid Complex. In Diplometopon, theparasphenoid and basisphenoid are coosified to formthe parabasisphenoid, which in turn is partially co-ossified with the basioccipital. Thus, the boundariesbetween these elements can only be estimated. Theparabasisphenoid exhibits three anterior processes(Figs. 2D, 4B, 16E): a median cultriform process anddorsolateral orbitosphenoid processes. The cultri-form process extends over approximately the ante-rior third of the otic–occipital complex (Fig. 16E).The basipterygoid process lies at one-third the dis-tance between the tip of the cultriform process andthe occipital condyle, and the protuberant ventrolat-eral process lies approximately midway between thebasipterygoid process and the occipital condyle.

The anterior tip of the cultriform process insertsinto the space formed between the palatines and thebody of the orbitosphenoid (Figs. 2D, 4B). This isalso the case in Amphisbaena (Montero and Gans,1999), whereas in Rhineura (Kearney et al., 2005) itinserts into the space between the frontal descend-ing processes, palatines, and vomers. The cultriformprocess broadens posteriorly to the base of the ba-sipterygoid process (Tra 269) (Fig. 16E).

The orbitosphenoid process lies dorsal and lateralto the cultriform process (Fig. 16C), inserting into ashallow depression on the dorsal surface of the or-bitosphenoid (Fig. 2D). The orbitosphenoid processflares posterolaterally to form the anterior margin ofthe Gasserian foramen (Fig. 4B). Near its widestpoint, the orbitosphenoid process is pierced at mid-body by a small foramen (Fig. 16D) that also pene-trates the underlying orbitosphenoid (Tra 252). Theorbitosphenoid process contacts the parietal tempo-ral lamina laterally just anterior to the Gasserianforamen (Tra 248), thereby excluding the orbito-sphenoid from the braincase wall in this region.

The anterior floor of the cranial cavity is formedby the dorsal surface of the parabasisphenoid (Fig.16D). The sella turcica, which lodges the pituitarygland, is a shallow basin in the dorsal surface of theparabasisphenoid at the level of the basipterygoidprocess (Fig. 2E). The internal carotid foramenopens into the cranial cavity through the floor of thesella turcica.



The basipterygoid process protrudes sharply be-neath the Gasserian foramen (Figs. 2E, 16E), ex-tending anterolaterally to articulate with the ptery-goid. It is relatively constricted at the base andflares distally into a broad, flat, anteroventrally ori-ented articular surface (Fig. 2E). The ventrolateralprocess swells behind the basipterygoid process(Figs. 3B, 16C). It is capped by “Element X”(Zangerl, 1944), the homology of which is problem-atic (Lakjer, 1927; Zangerl, 1944; Vanzolini, 1951;Kesteven, 1957; Jollie, 1960; Gans, 1960, 1978). Inthis specimen of Diplometopon, “Element X” ismostly coosified with the ventrolateral process, al-though sutures are readily apparent anteriorly (Fig.16C). It is composed of spongy bone, as is the case inRhineura (Kearney et al., 2005) and Amphisbaena(Montero and Gans, 1999).

Otic Complex. The embryonic otic capsule, whichossifies as separate prootic and opisthotic bones inother squamates, coosifies in amphisbaenians alongwith the rudimentary squamosal (when present)and “Element X” to form a single fused otic complex(Zangerl, 1944; Kesteven, 1957; Montero et al.,1999). The quadrate and columella articulate withthe otic complex.

The otic complex is conjoined to the occipital com-plex above and below the foramen magnum (Fig.16B). The supraoccipital forms the roof of the cranialcavity and unites the right and left otic capsulesdorsally. The exoccipital is confluent with the pos-teroventral wall of the otic capsule ventrolateral tothe foramen magnum.

The short prootic alar process contributes to thelateral closure of the braincase (Figs. 4B, 16A). Itsanterior tip lies ventral to the parietal temporallamina (Fig. 4B), whereas posteriorly it overlaps theparietal temporal lamina laterally (Fig. 2E); thisarea of overlap is delimited by a ridge on the ven-trolateral surface of the parietal temporal lamina.The ventral margin of the alar process forms thedorsal and posterior borders of the Gasserian fora-men (Fig. 4B). This is also the case in Amphisbaena(Montero and Gans, 1999), whereas in Rhineura(Kearney et al., 2005) it also forms the anteriorborder of the Gasserian foramen. The combined rootof the trigeminal nerve (V) passes from the cranialcavity through this opening into the cavum epipteri-cum (e.g., Tra 267). There it balloons into the Gas-serian (trigeminal) ganglion and divides into theophthalmic (V1), maxillary (V2), and mandibular(V3) branches of the trigeminal nerve (Oelrich,1956).

The otic capsule is a voluminous spherical cham-ber that dominates the back of the skull (Fig. 16D).A large jugular recess is excavated into the capsulebeneath the horizontal semicircular canal (Figs. 2G,16A), opening laterally. The dorsal edge of the jug-ular recess is bounded by the paroccipital process(Fig. 2G), its anterior edge is formed by the cristaprootica (Fig. 16A), and its posterior edge is formed

by the crista tuberalis. Its external aspect is par-tially bounded by the squamosal (Fig. 2G).

The facial foramen pierces the prootic just ante-rior to the crista prootica (Figs. 4C, 16C) where thetrunk of cranial nerve VII exits the cranial wall. Theposterior Vidian foramen occurs at the junction ofthe crista prootica and the ventrolateral process(Figs. 2F, 16A), then opens into the Vidian canal(�parabasal canal of Montero and Gans, 1999) (Fig.2E). The canal conveys the internal carotid arteryand palatine branch of the facial nerve (VII) longi-tudinally through the parabasisphenoid until open-ing beneath the sella turcica (deBeer, 1937; Oelrich,1956; Jollie, 1960). The canal continues its anteriorcourse and emerges into the cavum epiptericumthrough the anterior Vidian foramen, which is lo-cated at the base of the basipterygoid process (Figs.3A, 16C). The fenestra ovale opens ventrolaterallyinto the jugular recess slightly medial to the paroc-cipital process (Fig. 2G). The footplate of the colu-mella is enormous and completely fills the fenestraovale.

The occipital recess is an excavation between thecrista tuberalis and the occipital condyle (Figs. 4C,16B). Four foramina open into the occipital recess.The most ventral foramen pierces the anterolateralwall of the occipital recess (Fig. 3A) and communi-cates with the perilymphatic duct (Tra 398). Theother three foramina are clustered at the dorsalapex of the occipital recess (Fig. 16B). The vagusforamen is dorsalmost (Fig. 4C) and transmits theglossopharyngeal (IX) and vagus (X) nerve trunksand the internal jugular vein. Two smaller hypoglos-sal foramina, occurring just below the vagus fora-men (Tra 408) (Fig. 16B), transmit branches of thehypoglossal nerve (XII). The vagus and hypoglossalforamina follow direct routes from the cranial cavityas they pierce the cranial wall. In Amphisbaena, theoccipital recess apparently houses only one largeforamen identified as the vagus (Montero and Gans,1999). One foramen is also present in Rhineura;however, the HRXCT data show it to be a combinedopening for distinct vagus and hypoglossal passages(Kearney et al., 2005).

Occipital Complex. The supraoccipital, paired ex-occipital, and basioccipital coosify to form the occip-ital complex in amphisbaenians (Montero et al.,1999). As in other reptiles, the basioccipital contrib-utes to the floor of the braincase, the exoccipital toits walls, and the supraoccipital to its roof.

In Diplometopon, the basioccipital portion of theoccipital complex forms the most posterior portion ofthe braincase floor between the otic capsules as wellas the median portion of the occipital condyle. Thedorsal surface of the basioccipital is smooth andforms an arched floor beneath the hindbrain (Fig.3C). The supraoccipital portion of the occipital com-plex forms the roof of the foramen magnum, theposterior part of the braincase, and the dorsal por-tion of the otic complex. The foramen magnum opens



posteriorly and is not elevated (Fig. 3C). Lookinginto the foramen magnum (Fig. 16B), the dorsalportion of the cranial cavity is largely filled by theprojecting dorsomedial walls of the otic capsules.

Squamosal. The squamosal is coossified with theotic–occipital complex in Diplometopon (Fig. 16A)such that its margins are discernable only in section(e.g., Tra 339) and, even then, only anteriorly. Itforms a roughly rectangular tab that extends ven-trally from the posteroventral corner of the otic cap-sule (Fig. 16A), covering the columellar footplateposteriorly (Fig. 2G) and terminating at the level ofthe midpoint of the columellar shaft (Sag 037). Thesquamosal contributes to the posteroventral portionof the articular surface for the quadrate cephaliccondyle (Fig. 4C). The squamosal is globular proxi-mally, but dramatically thins and elongates into aplatelike structure distally (Fig. 2G).

The position of the squamosal is strikingly dis-placed ventrally in Diplometopon relative to the typ-ical squamate condition. A squamosal was not iden-tified in Diplometopon by Gans (1960); however, hefigured a similar platelike structure partially cover-ing the columellar footplate in several trogonophidspecies. El-Assy and Al-Nassar (1976) identified atopologically equivalent element in Diplometopon asthe supratemporal; however, its significant contri-bution to the quadrate articulation suggests that itis the squamosal. A coossified squamosal was iden-tified in Rhineura (Kearney et al., 2005) but not inAmphisbaena (Montero and Gans, 1999).

Internal Anatomy of Otic–Occipital Complex (Figs.2–4). The internal morphology of the endocraniumand the otic capsule are described, in turn, fromanterior to posterior with reference to representa-tive HRXCT sections.

Anteriorly (Tra 158), the cranial cavity is roughlyheart-shaped and almost completely enclosed by thefrontals, with a small contribution from the orbito-sphenoid anteromedial process. More posteriorly(Fig. 2C), the orbitosphenoid anterolateral processesbegin to form the lateral walls of the cranial cavity.They eventually meet the orbitosphenoid anterome-dial process, thereby excluding the frontal descend-ing processes from the lateral wall (Tra 183). At thispoint, the parietal anterolateral processes begin toexclude the frontal from the dorsal margin of thecranial cavity. At the posterior limit of the frontaldescending processes (Tra 205), the parabasisphe-noid orbitosphenoid processes begin to contribute tothe floor of the cranial cavity and the parietal formsits dorsal and lateral walls (Fig. 2D). Eventually, theparabasisphenoid orbitosphenoid processes contactthe parietal temporal laminae (Tra 227), therebyexcluding the orbitosphenoid from all but the ven-tromedial tip of the cranial cavity (Fig. 4B). Furtherposteriorly (Tra 233), the parabasisphenoid orbito-sphenoid and cultriform processes converge and theorbitosphenoid posteromedial process terminates

(Tra 242) (Fig. 3C). At this point the shape of thecranial cavity is circular.

The Gasserian foramen opens between the pari-etal temporal lamina and parabasisphenoid (Tra255). The prootic alar process begins to contribute tothe dorsal border of the Gasserian foramen at ap-proximately the same level that the orbitosphenoidposterolateral process terminates (Tra 265) (Fig.4B). Just posterior to this (Tra 277), the anteriorVidian foramina pierce the parabasisphenoid at theproximal corners of the basipterygoid processes (Fig.3A). The internal carotid canal opens into the floor ofthe cranial cavity (Fig. 2E) just anterior to the clo-sure of the Gasserian foramen (Tra 288). Furtherposteriorly, the prootic meets the parabasisphenoidto close the Gasserian foramen (Tra 297) (Fig. 4C).

Expansion of the otic capsules begins to constrictthe cranial cavity posterior to the level of the Gas-serian foramen (Tra 313) (Fig. 4B). The facial foram-ina exit the endocranial cavity near the anterodorsalmargin of the crista prootica posterior to the ba-sipterygoid processes (Tra 318) (Fig. 4C). The ante-rior auditory foramina pierce the endocranial cavityfrom the vestibules of the otic capsules (Tra 324).The Vidian canals exit through the posterior Vidianforamina at the suture of the parabasisphenoid,prootic, and “Element X” (Tra 330) (Fig. 2F). Poste-rior to this, the cranial cavity becomes further con-stricted dorsally due to great enlargement of thevestibules (Tra 345). The posterior auditory foram-ina enter the cranial cavity from the ventromedialcorners of the vestibules (Tra 350) (Fig. 3B). Con-striction of the cranial cavity reaches its greatestdevelopment at the widest point of the otic–occipitalcomplex (Fig. 2G), at which point the endolymphaticcanals enter the cranial cavity from the dorsomedialwalls of the vestibules (Fig. 4A).

Near the level of the posterior margin of thefenestrae ovali (Fig. 2H), the recessus scalae tym-pani mark the entrance of the perilymphatic ductsinto the ventrolateral corners of the cranial cavity.Further posteriorly (Tra 406), the vagus foraminaexit the cranial cavity (Fig. 3B) and travel ventro-laterally to open into the occipital recesses (Fig. 4C).The hypoglossal foramina do the same slightly fur-ther posteriorly (Tra 410).

Moving back to the anterior end of the otic com-plex, the anterior vertical semicircular canal opensat a level near the midpoint of the Gasserian fora-men (Tra 285). It emerges from the anterior ampul-lary recess and abruptly arcs posterodorsally (Fig.4B) to eventually meet the recessus crus communis(Figs. 2H, 3B). The vestibule opens just posterior tothe anterior ampullary recess (Tra 301) (Fig. 3A).The statolithic mass then appears and the horizon-tal semicircular canal emerges laterally from theanterior ampullary recess (Tra 314). The statolithicmass is an ovoid calcified body that expands poste-riorly to fill roughly half of the vestibule (Fig. 4B).Further posteriorly, the anterior auditory foramen



exits the vestibule (Tra 324) and the perilymphaticduct differentiates from the ventral margin of thevestibule (Tra 328) (Fig. 2F). This duct runs foralmost the entire length of the otic capsule, eventu-ally opening into the dorsal aspect of the recessusscalae tympani (Tra 385) (Fig. 2G). Throughout itslength the perilymphatic duct is open to the lagenarrecess, which begins near the level of the anteriormargin of the fenestra ovale (Tra 341).

The fenestra ovale begins to open at the level ofthe origin of the ventrolateral processes (Tra 339).The posterior auditory foramen then opens into thecranial cavity from the vestibule (Fig. 3B). Continu-ing posteriorly, the perilymphatic duct opens intothe recessus scalae tympani (Tra 385) and the ante-rior vertical semicircular canal opens into the reces-sus crus communis (Figs. 2H, 3B). The recessusscalae tympani then opens into the cranial cavity viathe medial aperture (Fig. 2H) and the posterior ver-tical semicircular canal emerges from the recessuscrus communis (Tra 394) (Fig. 3B).

The fenestra ovale closes and the lagenar recessends just anterior to the occipital recess (Tra 401).The horizontal and posterior semicircular canals en-ter the posterior ampullary recess at the level of thevagus and hypoglossal canals (Figs. 3A, 4C).

Orbitosphenoid (Fig. 17; Tra 150–267). Theorbitosphenoids are fused in many amphisbaenians(�tabulosphenoid of Montero and Gans, 1999),greatly enlarged, and possibly augmented by a der-mal component (Zangerl, 1944; Bellairs and Gans,1983; Montero et al., 1999). In Diplometopon, as inAmphisbaena (Montero and Gans, 1999), the orbito-sphenoid is platelike and lies in close appositionwith the frontal descending processes. In Rhineura(Kearney et al., 2005) the paired orbitosphenoids liedorsal to the frontal descending processes and arehidden from external view. The orbitosphenoid inDiplometopon is butterfly-shaped (Fig. 17B), withthe body paralleling the angulation of the cranialsegment and the wings more steeply inclined ante-riorly (Fig. 17A). It contacts the frontals anterodor-sally and dorsolaterally, the parietal dorsally, andthe parabasisphenoid posterodorsally and postero-ventrally.

The orbitosphenoid exhibits three anterior pro-cesses (Fig. 17B). The anteromedial process insertsinto the space between the palatines and the frontaldescending processes (Figs. 2C, 3C); its ventral sur-face is marked by a ridge that extends posteriorlyapproximately half the length of the orbitosphenoid(Fig. 17C). The anterolateral processes line the me-dial surfaces of the frontal descending processes(Fig. 2C), and this area of overlap is delimited by aridge on the ventral surface of the anterolateralprocess (Fig. 17A). The anterolateral processes meetthe anteromedial process (Fig. 2D) to fully enclosethe posterior edges of the frontal descending pro-cesses (Fig. 4A). A pronounced ridge on the dorso-medial surface of the orbitosphenoid delimits the

area of overlap of the frontal (Fig. 17B). The orbito-sphenoid in Amphisbaena (Montero and Gans, 1999)exhibits only anterolateral processes that line theventral aspect of the frontal descending processes,and is completely excluded from the walls of thecranial cavity by the frontals.

The orbitosphenoid in Diplometopon also exhibitsthree posterior processes (Fig. 17B). The posterome-dial process is clasped by the parabasisphenoid or-bitosphenoid processes (Tra 233) (Fig. 3B); its sur-face exhibits a dorsomedial ridge that extendsanteriorly half the length of the orbitosphenoid (Fig.17B). The posterolateral processes line the ventro-lateral surfaces of the parabasisphenoid orbitosphe-

Fig. 17. Orbitosphenoid in (A) lateral, (B) dorsal, (C) ventral,(D) anterior, and (E) posterior views. Anterior to left in A–C.Scale bar � 1 mm. alp, anterolateral process; amp, anteromedialprocess; dmr, dorsomedial ridge; for, foramen; of, optic foramen;plp, posterolateral process; pmp, posteromedial process; vmr,ventromedial ridge; 1, ridge delimiting area of overlap by frontaldescending process; 2, ridge delimiting area of overlap of frontaldescending process; 3, ridge delimiting area of overlap of paraba-sisphenoid orbitosphenoid process; 4, area of contact with parietaltemporal lamina.



noid processes (Fig. 4B); the extent of this contact isdelimited by a ridge on the ventral surfaces of thelatter and on the dorsal surfaces of the former (Fig.17B). The lateral edge of the orbitosphenoid contactsthe ventral edge of the parietal temporal lamina(Fig. 2D), and a longitudinal shelf marks this con-tact (Fig. 17B). The orbitosphenoid contributes tothe closure of the ventrolateral braincase wall (Figs.2D, 4A) before terminating at the level of the mid-point of the Gasserian foramen. The orbitosphenoidin Amphisbaena (Montero and Gans, 1999) lacks aposteromedial process; this margin of the orbito-sphenoid interdigitates with the parabasisphenoid.

The orbitosphenoid is pierced by two sets of later-ally paired foramina. The optic foramen (Figs. 2D,17A) occurs at midbody and opens into the cranialcavity, providing passage for the optic nerve. It hasa similar position in Amphisbaena (Montero andGans, 1999) but is absent in Rhineura (Kearney etal., 2005). A smaller foramen, centered in the orbito-sphenoid posterolateral process (Tra 238) (Fig. 17C),also pierces the overlying parabasisphenoid to enterthe cranial cavity.

Quadrate (Fig. 18; Tra 278–359). The quadratein Diplometopon is short and stout, its lateral aspectobscured by the extracolumella distally. The shaft isnarrower than the slightly expanded distal mandib-ular condyle and the greatly expanded proximal ce-phalic condyle (Fig. 18A). The shaft is less dramat-ically constricted in Rhineura (Kearney et al., 2005)and Amphisbaena (Montero and Gans, 1999). Thequadrate in Diplometopon reclines at an angle ofalmost 45° relative to the long axis of the skull,similar to the orientation in Rhineura (Kearney etal., 2005), but steeper than that in Amphisbaena(30°; Montero and Gans, 1999). The pterygoid quad-rate process in Diplometopon extends posteroven-trally to lie in apposition with the medial surface ofthe mandibular condyle (Fig. 3A). The mandibularcondyle gently clasps the pterygoid quadrate processin Amphisbaena (Montero and Gans, 1999), whereasin Rhineura (Kearney et al., 2005) the reverse is true.

The posteroventral margin of the quadrate inDiplometopon is deeply concave (Fig. 18A) to allowpassage of the stout columellar shaft from the jugu-lar recess to the extracolumella. The columellarshaft is comparatively more slender in Amphisbaena(Montero and Gans, 1999) and Rhineura (Kearney etal., 2005), and its passage from the jugular recess isaccommodated by a notch on the relatively straightposterior aspect of the quadrate shaft. Each quad-rate in Diplometopon exhibits one or more irregu-larly placed nutritive foramina (Fig. 18C,F), as inRhineura (Kearney et al., 2005) and Amphisbaena(Montero and Gans, 1999). Amphisbaena also pos-sesses a relatively large lateral foramen (Monteroand Gans, 1999) that penetrates the cephalic con-dyle lateromedially.

The cephalic condyle in Diplometopon articulatessimply with the squamosal/paroccipital process pos-

terodorsally (Figs. 2F, 4C), as in Rhineura (Kearneyet al., 2005). In Amphisbaena (Montero and Gans,1999), the cephalic condyle is augmented by an ex-tension (lateral plate) that wraps around the paroc-cipital process, creating an almost ball-and-socketjoint. In Diplometopon, the mandibular condyleforms a saddle-shaped articulation with the mandib-ular glenoid fossa anteroventrally (Figs. 2E, 3A).The two condyles are oriented perpendicular to eachother (Fig. 18E), with the long axis of the cephaliccondyle positioned vertically and that of the man-dibular condyle positioned horizontally. The dorsalcrest is a vertical ridge that runs along the antero-lateral edge of the quadrate between the two con-dyles. The cephalic condyle is anteroposteriorly ex-panded, with a pronounced posteroventral process(Fig. 18A) that extends over the dorsal surface of thecolumellar shaft to abut the platelike squamosalventral process (Fig. 4C).

Columella (Fig. 19; Tra 315–400) and extra-columella (Fig. 20; Tra 243–322). The columella inDiplometopon consists of a short shaft that expandsproximally into a basal footplate. An enlarged, min-

Fig. 18. Left quadrate in (A) lateral, (B) medial, (C) dorsal,(D) ventral, (E) anterior, and (F) posterior views. Scale bar � 1mm. cec, cephalic condyle; dcr, dorsal crest; mac, mandibularcondyle; nf, nutritive foramen; pvp, posteroventral process; 1,surface of articulation with pterygoid.



eralized, fan-shaped extracolumella extends anteri-orly from the distal tip of the columella almost to thelevel of the coronoid process (Fig. 5A).

The basal footplate is proportionally huge, subcir-cular (Fig. 19A), and laterally inclined (Fig. 19E),fitting into the deeply recessed fenestra ovale (Fig.2G). Its external surface is smooth (Fig. 19A). Thefootplate expands both anteriorly and medially toform an anterior process (Fig. 19C) that lacks ananalog in Amphisbaena (Montero and Gans, 1999)and Rhineura (Kearney et al., 2005). This anteriorprocess clasps the crista prootica (Figs. 3A, 4C), andthis contact is delimited by a trough running alongits dorsomedial aspect (Fig. 19B). The medial sur-face of the footplate is deeply concave, unlike inRhineura (Kearney et al., 2005) and Amphisbaena(contra Montero and Gans, 1999). The center of themedial surface of the footplate is pierced by a fora-men (Tra 352) that does not occur in Rhineura(Kearney et al., 2005) or Amphisbaena (Montero andGans, 1999).

A short, robust shaft arises from the lateral face ofthe columellar footplate anteroventral to its center(Fig. 19A). It is roughly circular in cross-section, asin Rhineura (Kearney et al., 2005), whereas in Am-phisbaena (Montero and Gans, 1999) it is dorsoven-trally compressed. In Diplometopon, the columellarshaft extends anterolaterally under the quadrate

cephalic condyle (Fig. 2F), terminating in a roundedknob (Fig. 19E) that attaches to the extracolumella.

In amphisbaenians, the extracolumella may beabsent, present and cartilaginous, or present andpartially or fully mineralized (Kearney, 2003). Inmost trogonophid amphisbaenians the extracolu-mella is enlarged, platelike, and fully mineralized(Gans, 1960), which is the case in Diplometopon. Theextracolumella in Amphisbaena is mineralized, slen-der, and rod-like, extending anteriorly just past theapex of the coronoid process (Montero and Gans,1999). This element was not preserved in Rhineurahatcherii (Kearney et al., 2005), but it is described asshort and mineralized in the extant R. floridana(Gans, 1978).

The extracolumella in Diplometopon is dorsoven-trally expanded anteriorly (Fig. 20A). Its medial sur-face is gently concave (Fig. 20E) and lies in closeapposition with the quadrate mandibular condyle(Fig. 2E). The extracolumella terminates posteriorlyin a flat surface (Fig. 20F) that articulates with thecolumellar shaft (Tra 321).

Overview of Mandible (Fig. 21)

The lower jaw in Diplometopon consists of tworami joined anteriorly in a symphyseal articulationand articulating posteriorly with the quadrates.Fig. 19. Left columella in (A) lateral, (B) medial, (C) dorsal,

(D) ventral, (E) anterior, and (F) posterior views. Scale bar � 1mm. ap, anterior process; fpl, footplate; sh, shaft; 1, trough re-ceiving crista prootica; 2, extracolumellar attachment surface.

Fig. 20. Left extracolumella in (A) lateral, (B) medial, (C)dorsal, (D) ventral, (E) anterior, and (F) posterior views. Scalebar � 1 mm. 1, columellar attachment surface.



Each ramus curves anteromedially to meet its oppo-site in a deep, flat symphyseal surface (Tra 092)(Fig. 21B). A shallow, posteriorly facing, C-shapedglenoid fossa (Figs. 3A, 21F) receives the quadrate

mandibular condyle, which has an anterolateral ori-entation. The postdentary elements are extremelyreduced, and there is no retroarticular process.

The mandible exhibits two mental foramina nearits anterior end (Tra 098, 112) (Fig. 3C) and one atmidbody (Tra 184) (Figs. 3A, 21A). The Meckeliancanal is closed along most of its length, its wallsformed by the dentary labially and the splenial lin-gually (Fig. 2C). The canal is open at its anterior end(Tra 146) (Fig. 21A), and then briefly opens dorsally(Tra 202–235) (Figs. 2D, 21C) before exiting via theanterior mylohyoid foramen (Tra 263) (Fig. 21B). Adentary canal runs through the dentary below thetooth row (Fig. 2B), exiting through the inferior al-veolar foramen at the point of dorsal exposure of theMeckelian canal (Figs. 3A, 21B). The posterior endof the mandible is pierced by two foramina (Tra 255,264) (Fig. 21A), one of which may be the posteriormandibular foramen (see Compound bone, below).

In most amphisbaenians, the postdentary ele-ments exhibit significant fusion, resulting in a com-pound postdentary bone. Gans (1960) interpretedthe mandible of Diplometopon to comprise dentary,coronoid, angular, surangular, prearticular, and ar-ticular. Only three discrete mandibular elementscan be identified confidently in the specimen studiedhere: dentary, coronoid, and compound bone (Fig.21A). Contra Gans (1960), we interpret the anteriorprojection of the compound bone to represent a coosi-fied splenial (Fig. 21B) based on its lingual exposureand closure of the Meckelian canal (Fig. 2C). It is notpossible to determine what other elements arepresent in the compound bone without developmen-tal studies.

The mandible is short (approximately half thetotal length of the skull) and very deep (approxi-mately half the maximum height of the skull), witha pronounced coronoid process that extends dorsallyto the level of the orbit (Fig. 5A). The mandibularcoronoid process is formed by the coronoid, with amajor contribution from the dentary coronoid pro-cess and a minor contribution from the compoundbone (Figs. 4B, 21A).

Individual Elements of MandibleDentary (Fig. 22; Tra 054–278). The dentary in

Diplometopon bears six acrodont teeth along its dor-sal margin (Fig. 22A), in contrast to the seven sub-pleurodont teeth in Rhineura (Kearney et al., 2005)and the eight in Amphisbaena (Montero and Gans,1999). The first tooth is the largest in Diplometopon,with tooth size generally diminishing towards theback of the tooth row. The bases of the teeth arebulbous, hollow (Fig. 4C) and fused to each other,and some give the impression of having small acces-sory cusps (Fig. 22A). A small subdental shelf ex-tends medial to the tooth row just posterior to thefirst tooth (Figs. 2C, 22C). Nutritive foramina are

Fig. 21. Right mandible (inverted) in (A) labial, (B) lingual,(C) dorsal, (D) ventral, (E) anterior, and (F) posterior views. Scalebar � 1 mm. amf, anterior mylohyoid foramen; co, coronoid; cop,coronoid process; cpb, compound bone; dn, dentary; for, foramen;glf, glenoid fossa; iaf, inferior alveolar foramen; Mc, Meckeliancanal; mf, mental foramen; nf, nutritive foramen; sp, splenial;sym, symphyseal articular surface.



present along the subdental shelf at the base of eachtooth (Figs. 3B, 22B).

A small dentary canal (Fig. 2B,C) runs the lengthof the dentary, transmitting the inferior alveolarnerve and artery. It communicates with the three

mental foramina on the labial surface of the dentary(Figs. 3A, 22A) and gives off a nutritive foramen tothe base of each tooth (Fig. 3B). It exits posteriorlythrough the inferior alveolar foramen at the level ofthe anterior edge of the coronoid process (Figs. 3A,22F). The Meckelian canal is expressed as a poste-riorly widening groove on the lower aspect of thelingual surface of the dentary (Fig. 22B). In thearticulated mandible, the canal is open anteriorly,originating at the level of the third tooth (Tra 145).It is closed lingually halfway between the third andfourth tooth (Tra 150) by the splenial portion of thecompound bone (Figs. 2C, 3B). Posteriorly, the canalis open dorsally (Figs. 2D, 3A, 21C) from the level ofthe inferior alveolar foramen (Tra 201) to the level ofthe apex of the coronoid bone (Tra 238). This open-ing appears to correspond to the “central foramen” ofMontero and Gans (1999).

The symphyseal surface at the anterior end of thedentary is vertically oriented and slightly broaderdorsally than ventrally (Figs. 2B, 22B). It is sharplypointed ventrally, more so than in Amphisbaena(Montero and Gans, 1999) and Rhineura (Kearney etal., 2005). The ventral margin of the dentary isgently concave posteriorly to the level of the fourthtooth, then straight thereafter (Fig. 22A).

A pronounced coronoid process (�posterodorsalprocess of Montero and Gans, 1999) projects dorsallyfrom just behind the last dentary tooth (Fig. 22A).The dentary coronoid process is mediolaterally com-pressed (Figs. 2D, 22F) and lies in apposition withthe coronoid and compound bones medially (Fig. 4B)and the coronoid dorsally (Tra 224). This is also thecase in Amphisbaena (Montero and Gans, 1999),whereas in Rhineura (Kearney et al., 2005) the den-tary coronoid process is relatively tiny and is claspedby the coronoid dorsally.

The posterior margin of the dentary in Diplometo-pon lacks distinct processes. It slopes steeply fromthe coronoid process (Fig. 22A) to lie in simple ap-position with the lateral surface of the compoundbone (Fig. 4C), and this contact is delimited by aridge on the posterior margin of the latter. The pos-terior margin of the dentary in Amphisbaena(Montero and Gans, 1999) exhibits a posteroventralprocess that articulates with the compound bonedorsally. In Rhineura (Kearney et al., 2005), an ar-ticular process lies along the labial surface of thecompound bone, and a ventral process clasps theventral margins of the splenial and compoundbones.

Coronoid (Fig. 23; Tra 203–264). The coronoidin Diplometopon is a simple, small element thatforms the dorsalmost edge of the mandibular coro-noid process (Figs. 4A, 5A). The coronoid in amphis-baenians may be extensively overlapped by a largedentary coronoid process, slightly overlapped by asmall process, or not overlapped at all (Kearney,2003); the first is the condition in trogonophids.

Fig. 22. Right dentary (inverted) in (A) labial, (B) lingual, (C)dorsal, (D) ventral, (E) anterior, and (F) posterior views. Scalebar � 1 mm. ac, accessory cusp; corp, coronoid process; dnt,dentary tooth; iaf, inferior alveolar foramen; Mc, Meckelian ca-nal; mf, mental foramen; nf, nutritive foramen; sds, subdentalshelf; sym, symphyseal articular surface; 1, area of overlap bycoronoid and compound bones; 2, area of overlap by splenial.



The coronoid is an inverted parabola in lateralview (Fig. 23A) with the long axis anteriorly in-clined. It is strongly mediolaterally compressed witha thickened dorsal edge (Figs. 2D, 23E). It lies inapposition to the medial surface of the dentary coro-noid process and the compound bone (Tra 234) (Fig.4B), and curves laterally above these elements toform the dorsal and posterior edge of the mandibu-lar coronoid process. The coronoid extends ventrally(Fig. 23B) almost to the level of the inferior alveolarforamen (Fig. 2D), thinning significantly towards itsventral margin. The coronoid is much more complexin Rhineura (Kearney et al., 2005), with anteriorprocesses that clasp the dentary coronoid processand a posterior process that lines the lingual surfaceof the compound bone to the anterior margin of theposterior mandibular foramen. In Amphisbaena(Montero and Gans, 1999), the coronoid brieflyclasps the dorsal margin of the compound bone andextends posteriorly, as in Rhineura. In all three taxathe coronoid covers most of the lingual surface of themandibular coronoid process.

Compound bone (Fig. 24; Tra 148–300). Thecompound postdentary bone presumably incorpo-rates the fused supraangular, articular, prearticu-lar, and splenial in most amphisbaenians (Montero

Fig. 24. Right compound bone (inverted) in (A) labial, (B)lingual, (C) dorsal, (D) ventral, (E) anterior, and (F) posteriorviews. Scale bar � 1 mm. amf, anterior mylohyoid foramen; arp,articular process; corp, coronoid process; for, foramen; glf, glenoidfossa; Mc, Meckelian canal; sp, splenial; 1, area of overlap bycoronoid; 2, area of overlap by dentary; 3, ventral projections.

Fig. 23. Right coronoid bone (inverted) in (A) labial, (B) lin-gual, (C) dorsal, (D) ventral, (E) anterior, and (F) posterior views.Scale bar � 1 mm. 1, area of overlap by dentary and compoundbone.



and Gans, 1999; Montero et al., 1999). Most of theseelements are indistinguishable in Diplometopon dueto fusion. However, based on the topological rela-tionships discussed above, we interpret the anteriorprojection of the compound bone to be the coosifiedsplenial (Fig. 24A).

The compound bone is the most posterior compo-nent of the mandible. It is a triradiate element (Fig.24A), exhibiting an anterior projection (the coossi-fied splenial), a dorsal coronoid process, and a pos-terior articular process. The compound bone con-tacts the dentary, coronoid, quadrate, and pterygoid.

The splenial is a thin, flattened bony projectionlying along a groove in the dentary and located en-tirely on the medial surface of the mandible (Fig.2C,D). It forms the lingual wall of the Meckeliancanal (Fig. 24E). It is exposed along its entire length,extending anteriorly and tapering to a point justpast the midpoint of the fourth dentary tooth (Tra150). The posterior extent of the splenial cannot bedetermined due to its coosification with the com-pound bone. However, a small foramen that we in-terpret to be a reduced anterior mylohyoid foramen(Fig. 24B) occurs at the level of the anterior edge ofthe coronoid process (Tra 265). This suggests thatthe splenial extends posteriorly at least this dis-tance into the compound bone. The anterior mylohy-oid foramen transmits the anterior mylohyoid nerve(Oelrich, 1956).

The bladelike coronoid process extends anterodor-sally (Fig. 24A) to contribute to the mandibular coro-noid process (Fig. 4B). In the articulated mandible,its superior portion is exposed in lateral view (Fig.21A), but obscured in medial view by the coronoid(Fig. 21B). Its inferior portion is largely obscured,sandwiched between the dentary coronoid processlabially and the coronoid lingually (Fig. 4B). A smallportion is visible ventral to the coronoid medially(Fig. 24B). A deep sulcus at the base of the coronoidprocess represents the open portion of the Meckeliancanal (Fig. 24B,C). The coronoid process in Amphis-baena (Montero and Gans, 1999) is fully exposedlabially, but mostly obscured lingually by the coro-noid. The compound bone in Rhineura (Kearney etal., 2005) lacks a coronoid process.

Two foramina pierce the compound bone near thebase of the coronoid process (Fig. 24A). The ventralone (Tra 264) is slightly larger than the dorsal one(Tra 258), and both follow a direct path from thelabial to the lingual surface. These foramina com-municate with the Meckelian canal, which is open inthis region (Fig. 3A). Based on their position andpathway, one of them probably represents the pos-terior mandibular foramen.

The articular process comprises a shallow,C-shaped glenoid fossa (Figs. 3A, 24A) that is ori-ented posterolaterally (Fig. 24F) to receive the an-teromedially oriented quadrate mandibular condyle(Fig. 2E). The anteromedial portion of the articularprocess loosely contacts the pterygoid quadrate pro-

cess, whereas the former tightly clasps the latter inAmphisbaena (Montero and Gans, 1999) andRhineura (Kearney et al., 2005). The articular pro-cess extends ventrally into a short, robust knob (Fig.24D), the ventral surface of which exhibits two smallprojections. There is no retroarticular process, as inAmphisbaena (Montero and Gans, 1999) but notRhineura (Kearney et al., 2005). Numerous nutri-tive canals occur within the articular process (Fig.2E) in all three taxa.

Hyobranchial Apparatus (Fig. 25; Tra 217–391)

The hyobranchial apparatus consists of cartilagi-nous and mineralized elements that serve as theskeletal support for the tongue and throat. Althoughthe hyobranchium of Diplometopon has been figuredpreviously (Gans, 1960; El-Assy and Al-Nassar,1976), for completeness we include a descriptionhere as it is part of the head skeleton. Only thoseportions of the hyobranchium that are mineralizedcan be seen in HRXCT data, thus we augment our

Fig. 25. Hyobranchial apparatus in (A) ventral view withskull showing natural position, edges outlined for clarity, (B)anterior, (C) dorsal, and (D) lateral views. Scale bars � 1 mm. bh,basihyal; cb1, first ceratobranchial; cb2, second ceratobranchial;hc, hyoid cornu; lp, lingual process; 1, area expanded by cartilag-inous paddle-shaped tip; 2, area extended by cartilaginous firstepibranchial.



description with examination of a cleared-and-stained specimen. The hyobranchium in Amphis-baena was not described by Montero and Gans(1999), and was not preserved in Rhineura (Kearneyet al., 2005).

The hyobranchium in Diplometopon lies along theventral aspect of the skull, extending from the pyri-form recess anteriorly to approximately the occipitalcondyle posteriorly (Fig. 25A). At its widest point itextends just past the jugular recess. The hyobran-chium as a whole is gently inclined anteriorly (Fig.25D), such that its anterior half is obscured by themandible in the articulated skull in lateral view(Fig. 5A). The hyobranchium comprises the basi-hyal, lingual process, first and second ceratobranchi-als, hyoid cornua (anterior processes), and first epi-branchials (Fig. 25C). The first epibranchials andthe connection between the hyoid cornua and thebasihyal are not visible in the HRXCT data becausethey lack mineralization.

The basihyal is the central element of the hyo-branchium. It is an inverted U-shaped elementwhose center lies between the basipterygoid pro-cesses and the ventrolateral processes of the otic–occipital complex (Fig. 25A). The posterior arms ofthe basihyal flare slightly at their distal ends.This element is completely mineralized in matureDiplometopon (Fig. 25C). The lingual processarises from the anterior apex of the basihyal,gradually tapering to a point at the level of thepyriform recess (Fig. 25A). It is mostly mineral-ized and there is no visible discontinuity betweenit and the basihyal. The second ceratobranchialsarise from the posterior arms of the basihyal (Fig.25C). These extend posteriorly almost to the pos-terior margin of the ventrolateral processes (Fig.25A), gradually tapering to a point. The secondceratobranchials are moderately well-mineralizedand exhibit no visible discontinuity with the basi-hyal (Fig. 25C).

Two elements articulate with each posterior armof the basihyal (Fig. 25A). The hyoid cornu extendsanterolaterally to closely approach the posteroven-tral edge of the mandible. It is a mineralized rodproximally, while distally it expands into a cartilag-inous, flattened, paddle-shaped tip (Fig. 25C) nearthe level of the apex of the basihyal. The first cera-tobranchial extends posterolaterally from the poste-rior arm of the basihyal to the posterior portion ofthe jugular recess (Fig. 25A). It exhibits a distinctdorsolateral bend at its midpoint (Fig. 25B,D) and isthicker proximally than distally (Fig. 25C). As insquamates generally, the first ceratobranchial ishollow (Fig. 2F) and is the only hyobranchial ele-ment to truly ossify. The cartilaginous first epi-branchial arises from the distal tip of the first cera-tobranchial (Fig. 25C) and extends to theposterolateral edge of the skull.

DISCUSSION

Historically, amphisbaenian taxonomy reflectedmultiple rationales, including morphology, function,and biogeography (e.g., Vanzolini, 1951; Gans, 1974,1978). The emergence of phylogenetic systematicsinitiated a search for shared derived characters asthe basis for inferring phylogeny. Recent phyloge-netic studies (Kearney, 2003; Kearney and Stuart,2004) support the recognition of five monophyleticamphisbaenian lineages that differ somewhat incontent and diagnosis from the groupings derivedfrom pre-phylogenetic methods. Resolution of phylo-genetic relationships remains somewhat poor, how-ever, partially due to the incongruence betweenmorphology-based and molecular-based approaches.Additionally, at least part of the problem in resolv-ing the relationships among these five lineages islack of morphological data and concomitant difficul-ties in assessing homology for phylogenetic charac-ter coding.

Those amphisbaenian species that have now beendescribed in detail (Amphisbaena alba, Rhineurahatcherii, and Diplometopon zarudnyi) begin to al-low a more refined assessment of the homology ofstructures. For example, both Diplometopon andRhineura have a strong craniofacial angulation anda rostral digging blade that are related to use of thehead in burrowing, but these features are built indifferent ways. In Diplometopon, the facial segmentof the skull is foreshortened and flattened, forming astraight line from its point of deflection to the tip ofthe snout, whereas in Rhineura, the facial segmentis elongated and its surface is smoothly rounded.Elongated nasals and an elongated premaxillary na-sal process contribute to approximately half thelength of the facial segment of the skull inDiplometopon, while in Rhineura the nasals are rel-atively short and elongated frontals comprise atleast half of the facial segment. Additionally,Rhineura exhibits an extraordinary degree of sculp-turing and perforating canals on the facial bones,indicating a high degree of sensory innervation;these structures are mostly absent in Diplometopon.In Rhineura, a well-developed maxillary nasal pro-cess projects anteriorly to clasp the nasal rostralprocess. The former is reduced to a miniscule spinein Diplometopon, thus there is no contribution fromthe maxilla to the rostral blade. These comparisonsdo not support hypotheses of homology relative tothe spatulate heads in these taxa.

Discovery of cranial elements not detectable be-fore the availability of HRXCT data will also affectfuture phylogenetic studies. In the morphologicaldata matrix of Kearney (2003), several charactersrelated to internal cranial morphology were scoredas “?” for Diplometopon due to the inability to ob-serve them clearly on scarce museum specimens.These characters can now be scored based on theHRXCT data provided here. For example, the dis-



covery of the splenial and squamosal in Diplometo-pon and Rhineura will change character codingswhich may, in turn, affect phylogenetic hypotheses.

These studies also reveal a major distinction be-tween the closure of the anterior braincase inRhineura, Diplometopon, and Amphisbaena. Alarge, platelike orbitosphenoid forming the antero-lateral wall of the braincase has traditionally beenconsidered a uniquely diagnostic character for am-phisbaenians (Gans, 1978; Bellairs and Gans, 1983).This element is thought to represent the fusion ofprimitively paired orbitosphenoids augmented by adermal component, and this is the condition in boththe “spade-headed” Diplometopon and the “round-headed” Amphisbaena. However, in the “shovel-headed” Rhineura, the orbitosphenoids are small,paired elements that are completely enclosed withinthe braincase; the anterolateral braincase wall isclosed instead by a large temporal wing of the fron-tal (Kearney et al., 2005). A similar condition report-edly occurs in the “shovel-headed” amphisbaenidMonopeltis capensis (Kritzinger, 1946). Thus, theclosure of the braincase is achieved differently indifferent amphisbaenian lineages. Whether the con-ditions in Rhineura and Monopeltis are homologous,and whether the single orbitosphenoid seen in Am-phisbaena and Diplometopon represents the primi-tive or derived condition for amphisbaenians, re-mains unclear in the absence of a stablephylogenetic hypothesis. However, observationssuch as these will aid in homology assessment andthe accurate construction and coding of phylogeneticcharacters in future analyses.

Presumably, cranial similarities and differencesamong species reflect not just phylogenetic affini-ties, but also functional considerations. As Gans(1960) explained, the form of the facial skeleton inamphisbaenians may be influenced by different sub-strate types, different feeding modes, the loss orreduction of the eyes, and other functional and de-velopmental forces. The “shovel-headed” Rhineurafloridana burrows by driving its head down to pen-etrate the soil with its snout, then lifting it up tocompress loosened soil particles onto the top of itsburrow (Gans, 1974). The “round-headed” Amphis-baena is less obligately fossorial, and is consideredto be a generalized burrower that forces its headthrough the soil in repeated forward pushes (Gans,1974). The “spade-headed” Diplometopon burrows inmuch looser aeolian sands, using an oscillatorymovement in which the head is rotated about thelong axis of the body in alternate directions, therebyloosening soil from the tunnel walls as it progressesforward (Gans, 1974). The forces exerted on theskull during this type of burrowing are doubtlessquite different from those exerted on the skulls ofAmphisbaena or Rhineura, and this may explain theabsence of extensive overlap of elements and inter-locking sutures in Diplometopon.

It remains unclear the degree to which groupingsof “round-headed,” “shovel-headed,” “spade-headed,”and “keel-headed” forms reflect functional versushistorical influences (Gans, 1978; Kearney, 2003). Arecent molecular phylogenetic analysis offered evi-dence that limb reduction and loss has occurredmore than once in amphisbaenian history (Kearneyand Stuart, 2004). It also implied, as have virtuallyall previous analyses, that convergent evolution af-fected the head skeleton as well, but it is far moredifficult at present to ascertain which particular cra-nial features are convergent. For example, if themolecular tree (Kearney and Stuart, 2004) provesmore accurate than the morphological tree (Kear-ney, 2003), then many of the distinctions in facialarchitecture of the “spade-headed” Diplometoponmay be transformations of relatively plesiomorphicconditions found in “shovel-headed” or “round-headed” forms. Although our present view of am-phisbaenian phylogenetic history is incomplete,there is little doubt that convergent evolution hasoccurred throughout the skeleton. Until a stablephylogenetic hypothesis is recovered, it will be diffi-cult to separate historical and functional influences.Toward this goal, ongoing detailed anatomical stud-ies such as the present one will aid in the develop-ment of a more complete and accurate understand-ing of amphisbaenian cranial morphology.

ACKNOWLEDGMENTS

Scanning was performed by Richard Ketcham. Wethank Olivier Rieppel and Kurt Schwenk forthoughtful reviews.

LITERATURE CITED

Bellairs Ad’A, Gans C. 1983. A reinterpretation of the amphis-baenian orbitosphenoid. Nature (Lond) 302:243–244.

Berman DS. 1976. A new amphisbaenian (Reptilia: Amphisbae-nia) from the Oligocene-Miocene John Day Formation, Oregon.J Paleontol 50:165–174.

deBeer GR. 1937. The development of the vertebrate skull. Ox-ford: Oxford University Press.

El-Assy YS, Al-Nassar NA. 1976. Morphological study of thecranial osteology of the amphisbaenian Diplometopon zarudnyi.J Univ Kuwait (Sci) 3:113–141.

Estes R, de Queiroz K, Gauthier J. 1988. Phylogenetic relation-ships within Squamata. In: Estes R, Pregill G, editors. Phylo-genetic relationships of the lizard families: essays commemo-rating Charles L. Camp. Stanford, CA: Stanford UniversityPress. p 119–281.

Gans C. 1960. Studies on amphisbaenids (Amphisbaenia, Rep-tilia). 1. A taxonomic revision of the Trogonophinae and a func-tional interpretation of the amphisbaenid adaptive pattern.Bull Am Mus Nat Hist 119:129–204.

Gans C. 1974. Biomechanics. An approach to vertebrate biology.Philadelphia: JP Lippincott.

Gans C. 1978. The characteristics and affinities of the Amphis-baenia. Trans Zool Soc Lond 34:347–416.

Jollie MT. 1960. The head skeleton of the lizard. Acta Zool 41:1–64.

Kearney M. 2003. Systematics and evolution of the Amphisbaenia(Reptilia: Squamata) based on morphological evidence fromfossil and living forms. Herp Monogr 17:1–75.



Kearney M, Stuart B. 2004. Repeated evolution of limblessnessand digging heads revealed by DNA from old bones. Proc R SocLond 271:1677–1683.

Kearney M, Maisano JA, Rowe T. 2005. Cranial anatomy of theextinct amphisbaenian Rhineura hatcherii (Squamata, Am-phisbaenia) based on high-resolution x-ray computed tomogra-phy. J Morphol 264:1–33.

Kesteven L. 1957. Notes on the skull and cephalic muscles of theAmphisbaenia. Proc Linn Soc New South Wales 82:109–116.

Kritzinger CC. 1946. The cranial anatomy and kinesis of theSouth African amphisbaenid Monopeltis capensis Smith. S AfrJ Sci 42:175–204.

Lakjer T. 1927. Studien uber die Gaumenregion bei Sauriern imVergleich mit Anamniern und primitiven Sauropsiden. ZoolJahrb (Anat) 49:57–356.

Montero R, Gans C. 1999. The head skeleton of Amphisbaenaalba Linnaeus. Ann Carn Mus 68:15–80.

Montero R, Gans C, Lions M. 1999. Embryonic development ofthe skeleton of Amphisbaena darwini heterozonata (Squamata:Amphisbaenidae). J Morphol 239:1–25.

Oelrich TM. 1956. The anatomy of the head of Ctenosaura pecti-nata (Iguanidae). Misc Publ Mus Zool Univ Mich 94:1–122.

Schwenk K. 2000. Feeding in lepidosaurs. In: Schwenk K, editor.Feeding: form, function and evolution in tetrapod vertebrates.San Diego, CA: Academic Press. p 175–291.

Vanzolini PE. 1951. Evolution, adaptation and distribution of theamphisbaenid lizards (Sauria: Amphisbaenidae). Ph.D. Thesis,Harvard University, Cambridge, MA.

Zangerl R. 1944. Contributions to the osteology of the skull of theAmphisbaenidae. Am Mid Nat 31:417–454.



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Cranial anatomy of the spade-headed amphisbaenian