Rieppel, 2000

Zoological Journal of the Linnean Society (2000), 130: 635–659. With 10 figures

doi:10.1006/zjls.2000.0232, available online at http://www.idealibrary.com on

Paraplacodus and the phylogeny of thePlacodontia (Reptilia: Sauropterygia)

OLIVIER RIEPPEL FMLS

Department of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, IL60605–2496, U.S.A.

Received October 1999; accepted for publication February 2000

The skeletal anatomy of Paraplacodus broilii Peyer from the Grenzbitumen-horizon (Anisian–Ladinian boundary) of Monte San Giorgio (Switzerland) is described and compared withthat of other placodonts. Paraplacodus is found to share a number of potential synapomorphieswith Placodus which could potentially corroborate the monophyly of the Placodontoidea, butPlacodus also shares an number of potential synapomorphies with the armored placodonts(Cyamodontoidea) which are absent in Paraplacodus. Parsimony analysis rejects the monophylyof the Placodontoidea, and places Paraplacodus at the root of the placodont tree, as the sister-taxon of all the other representatives of the clade. This correlates with a configuration ofthe temporal region of the skull that suggest the loss of the lower temporal arch in a diapsidskull. The loss of the lower temporal arch is therefore recognized as a sauropterygiansynapomorphy, and might even be a lepidosauromorph synapomorphy.

2000 The Linnean Society of London

CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . 635Material . . . . . . . . . . . . . . . . . . . . . . . . 637Systematic palaeontology . . . . . . . . . . . . . . . . . . 638

Paraplacodus broilii Peyer . . . . . . . . . . . . . . . . . 638Cladistic analysis . . . . . . . . . . . . . . . . . . . . . 650Discussion . . . . . . . . . . . . . . . . . . . . . . . 656Acknowledgements . . . . . . . . . . . . . . . . . . . . 658References . . . . . . . . . . . . . . . . . . . . . . . 658

INTRODUCTION

The genus Paraplacodus from the Grenzbitumen-horizon (Anisian–Ladinian boundary)of Monte San Giorgio, Switzerland (southern Alpine Triassic), has been claimed tobe of special importance for the understanding of placodont phylogeny, as it wasthought to represent the most ‘primitive’ representative of the group (Peyer & Kuhn-Schnyder, 1955). First described by Peyer (1931a, b, 1935), the taxon was knownfrom incomplete and disarticulated material only. A complete and articulatedskeleton, collected in 1936, was cursorily described by Kuhn-Schnyder (1942). An

6350024–4082/00/120635+25 $35.00 2000 The Linnean Society of London

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Figure 1. The skull of Paraplacodus broilii Peyer (BSP 1953 XV 5). Scale bar=20 mm.

isolated skull, and indeed the best preserved skull available for the genus (Fig. 1)was commented upon by Zanon (1989), and discussed, as well as figured, by Pinna(1989) and Rieppel (1995). An isolated tooth plate from the middle to upper Anisian(Formazione di Braies) of the Val Pusteria (southern Alps, northern Italy) wasreferred to Paraplacodus by Nosotti (1986), although the specimen is not diagnostic.An isolated dorsal rib with indications of a broadened uncinate process, from theupper Ladinian of Henarejos (Cuenca, Spain), was referred to Paraplacodus by Pinna(1990). Isolated teeth very similar to those of Paraplacodus have been reported fromthe lower Muschelkalk (Anisian) of southwest Germany (Hagdorn & Rieppel, 1999).Very fragmentary material from the Anisian of Transylvania, Romania ( Jurczak,1976), and from the upper Anisian and/or lower Ladinian of Israel (Haas, 1975)has also been referred to Paraplacodus, but all this material is not diagnostic, even atthe genus level.

In his study of the dentition of placodonts, Mazin (1989) recognized a basaldichotomy among placodonts, reflecting the Placodontoidea and Cyamodontoidearespectively of Peyer & Kuhn-Schnyder (1955; see also Mazin & Pinna, 1993). ThePlacodontoidea include the genera Paraplacodus with no osteoderms, and Placoduswith a single row of osteoderms along the dorsal midline of the body. TheCyamodontoidea include a variety of placodonts characterized by the developmentof a turtle-like dermal armor composed of interdigitating or fused osteoderms. Thisphylogeny of placodonts was recently confirmed by cladistic analysis (Rieppel &Zanon, 1997), yet it removes Paraplacodus from its position as sister-taxon of all otherplacodonts, as would be expected if Paraplacodus were, indeed, the most ‘primitive’member of the clade (Zanon, 1989). It is the purpose of this paper to summarizeall available anatomical information on the anatomy of Paraplacodus, and to analyseits interrelationships within the Placodontia.

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Figure 2. The skull of Paraplacodus broilii Peyer (PIMUZ T4775). Scale bar=20 mm.

MATERIAL

The first fragments of the skull of Paraplacodus to be found were a disarticulatedbut beautifully preserved premaxilla (PIMUZ T2806), and a fragmentary palate(PIMUZ T4776) showing the palatal dentition, and a fragmentary dentary (PIMUZT4777; Peyer, 1931b). A second specimen, an incomplete and disarticulated skeleton,was the object of a preliminary note (Peyer, 1931a) in which the name Paraplacodusbroilii was first published, designating the specimen as holotype for its species (PIMUZT4773). Later, the holotype was described in greater detail (Peyer, 1935), togetherwith a third incomplete and disarticulated skeleton (PIMUZ T4774), the ‘specimenB’ of Peyer (1935). The complete and articulated skeleton (PIMUZ T4775), describedby Kuhn-Schnyder (1942), has a strongly compressed and poorly preserved skull(Fig. 2). The description of the cranial anatomy of Paraplacodus given below isprimarily based on the skull from Munich (Fig. 1; BSP 1953 XV 5; a cast is keptat the Field Museum, FMNH PR 2102), preserved in left lateral view, in combinationwith information obtained from the specimens PIMUZ T4773 and T4774.

Most information on the postcranial skeleton is obtained from the specimenPIMUZ T4775, supplemented by an incomplete yet articulated postcranial skeleton(PIMUZ T4827). Additional information comes from the specimens described byPeyer (1935), including the holotype.

Institutional acronyms are: BMNH, British Museum (Natural History); BSP,Bayerische Staatssammlung fur Palaontologie und historische Geologie, Munich;FMNH, Field Museum of Natural History, Chicago; PIMUZ, PalaontologischesInstitut und Museum der Universitat, Zurich.

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SYSTEMATIC PALAEONTOLOGY

Sauropterygia Owen, 1860Placodontia Cope, 1871

Paraplacodontidae Peyer & Kuhn-Schnyder, 1955Paraplacodus Peyer, 1931a

Diagnosis. Three long, pointed and strongly procumbent teeth on each premaxilla;seven hemispheric maxillary teeth; dermatocranial cheek region strongly excavated,jugal L-shaped; quadratojugal absent; postfrontal extending far anteriorly alongdorsal margin of orbit; two elongated, pointed and strongly procumbent anteriordentary teeth followed by seven hemispheric teeth; low coronoid process with thecoronoid exposed in lateral view; dorsal ribs with distinct, fan-shaped and overlappinguncinate processes; ilium with narrow and tall dorsal process; osteoderms absent.

Paraplacodus broilii Peyer, 1931a

Holotype. PIMUZ T4773; incomplete skeleton.

Stratum typicum and locus typicus. Grenzbitumen-horizon, Anisian–Ladinian boundary,Middle Triassic; Valporina, Monte San Giorgio, Kanton Tessin, Switzerland.

Diagnosis. Same as for genus, of which this is the only known species.

Morphological description.The skull of Paraplacodus (Fig. 3) is relatively high and narrow, as is that of

Placodus, unlike the dorsoventrally depressed and broadened skull of cyamodontoids.Paraplacodus therefore groups with Placodus in Meyer’s (1863) ‘Macrocephali.’ Asnoted by Zanon (1989), and Pinna (1989), Paraplacodus shows a more pronouncedventral emargination of the cheek region than any other placodont, which togetherwith the narrow, L-shaped jugal, corroborates diapsid affinities of the Placodontia.

The premaxillaries of Paraplacodus form an anteriorly projecting rostrum, distinctlyset off from the remainder of the preorbital skull by a step visible in lateral view. Asimilar step, at the height of the external naris, is also observed in Placodus.Each premaxilla of Paraplacodus carries three elongated, slightly curved, stronglyprocumbent, and pointed teeth, of which the anteriormost one is the longest. Thepresence of a rostral constriction, a synapomorphy of sauropterygians in general,remains equivocal for Paraplacodus for reasons of incomplete and/or distortedpreservation. However, the palate of specimen T4773, preserved in ventral view,shows a slight constriction of the lateral margin of the maxilla at the level of theinternal naris, which may indicate the presence of a rostral constriction.

The left external naris is distinct in the specimen BSP 1953 XV 5. It is an almostcircular opening, as high (7.5 mm) as it is wide (7.6 mm), which differs from Placodus,where the external naris is much higher than it is wide (Rieppel, 1995). Thepremaxilla forms the anterior margin of the external naris. The premaxillary-maxillary suture is distinct, as it trends from the anteroventral margin of the externalnaris in an anterolateral direction towards the margin of the upper jaw. The areaabove and behind the external naris has been subject to severe crushing which


Figure 3. The skull of Paraplacodus broilii Peyer (BSP 1953 XV 5). Scale bar=20 mm. Abbreviations:an, angular, c, coronoid; d, dentary; f, frontal; j, jugal; m, maxilla; n, nasal; p, parietal; pm, premaxilla;po, postorbital; pof, postfrontal; prf, prefrontal; q, quadrate; sa, surangular, sq, squamosal.

renders the identification of separate cranial elements difficult. The maxilla formsa relatively high ascending process between the external naris and the orbit (alsoseen in the disarticulated maxilla of specimen PIMUZ T4774). It defines theposteroventral and posterior margin of the external naris, and broadly enters theanteroventral margin of the orbit. Dorsally, it narrows to a point as it enters betweenthe nasal and the prefrontal.

Anterodorsal to the external naris, an anteroventral projection of the nasal canbe identified, overlapping the premaxilla. Forming the dorsal margin of the externalnaris, the nasal extends posterodorsally on the prefacial skull, but extensive damageobscures details of its relation with the frontal and prefrontal respectively. It remains

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unknown whether the nasals were fused in Paraplacodus, as they are in Placodus, butfrom the general proportions of the skull it can be inferred that the nasals arerelatively large elements in Paraplacodus, as they also are in Placodus.

At about mid-height of the anterior margin of the orbit, the ventral tip of theprefrontal is distinct. The posterior end of the prefrontal is well exposed at theanterodorsal corner of the orbit. It appears bifurcated, and must have embracedthe anterior tip of the postfrontal in the undistorted skull. The postfrontal is readilyidentified as an element located dorsal and posterodorsal to the orbit. With aposteroventrally directed process, it defines the posterodorsal margin of the orbit.An anterior process extends far anteriorly along the dorsal margin of the orbit,defining most of its dorsal margin, and most probably contacting the prefrontal atthe anterodorsal corner of the orbit. In Placodus, prefrontal and postfrontal alsocontact each other dorsal to the orbit, but more posteriorly, at about the midpointof the longitudinal diameter of the orbit. The anterior extent of the postfrontalalong the dorsal margin of the orbit is an autapomorphy of Paraplacodus.

A deep concavity in the posterior margin of the postfrontal, slightly displaced inthe specimen BSP 1953 XV 5, must have defined the anterior margin of the uppertemporal fossa. As in Placodus, but unlike in cyamodontoids (except for Henodus, whichobliterated the upper temporal fenestra), the upper temporal fossa of Paraplacodus isnot very much larger than the orbit. In the specimen BSP 1953 XV 5, the longitudinaldiameter of the (left) upper temporal fossa is 40.5 mm, that of the (left) orbit is31.9 mm. The postorbital forms most of the posterior margin of the orbit, while thejugal is located at the posteroventral corner of the orbit. In Paraplacodus, the jugalforms an L-shaped element, with an anterior process lining the posteroventralmargin of the orbit, and a dorsal process which enters into the postorbital arch.Unlike in Placodus, the anterior process of the jugal does not extend to a level infront of the anterior margin of the orbit. According to Zanon (1989) the jugal ofParaplacodus enters only minimally into the temporal arch, whereas Pinna (1989)shows a contact of the jugal and quadratojugal in the ventral margin of the temporalarch. My own observations (Rieppel, 1995, fig. 46) concur with Zanon’s (1989) inthat the dorsal tip of the jugal can be identified in the anterodorsal corner of thecheek emargination (Fig. 3). Other sutures are more difficult to identify in thetemporal arch, especially in the heavily varnished original of specimen BSP 1953XV 5, but a cast of that specimen (FMNH PR 2102) reveals more structural detail.

A posteroventral process of the squamosal is readily identified as it caps thecephalic condyle of the quadrate. The participation of the squamosal in the formationof the posterior and posterolateral margin of the upper temporal fossa again isdistinct, and so is the squamosal–postorbital contact somewhat behind the midpointof the dorsal margin of the temporal arch. From this point, the irregular postorbital–squamosal suture, although partially obscured by breakage, can be followed acrossthe temporal arch trending anteroventrally before curving backwards in its lowermost part. The squamosal thus extends across the entire height of the temporalarch, and a quadratojugal is absent (as is also indicated in a drawing of the temporalregion of Paraplacodus by the late Robert Zanon). Paraplacodus shares the absence ofa quadratojugal with Placodus (Rieppel, 1995), but a quadratojugal is retained incyamodontoids (Rieppel, 1995, fig. 22, and work in progress). The anteroventral tipof the squamosal remains separated from the posterodorsal tip of the jugal inParaplacodus (Fig. 3), which corroborates Zanon’s (1989) conclusion that the contactof the jugal with the squamosal is secondary where it occurs, as for example in


Figure 4. The palatines of Paraplacodus broilii Peyer (holotype, PIMUZ T4773) in ventral view. Theanterior ends of the palatines (to the left) define the posterior margin of the confluent internal nares.Scale bar=10 mm.

Placodus. The deep emargination of the temporal region, the L-shaped jugal whichdoes not contact the squamosal, as well as the absence of a quadratojugal in contactwith the jugal, postorbital, and squamosal (as postulated by Pinna, 1989), confirmthe diapsid status of Paraplacodus.

The diapsid status of Paraplacodus is further corroborated by the lateral exposureof the quadrate below and in front of the squamosal, revealing its weakly concaveposterior margin. In front of and deep to the shaft of the quadrate, an expandedanteromedial flange of the quadrate is exposed, which meets the quadrate ramus ofthe pterygoid in an overlapping contact, as is also the case in Placodus. Severecompression of the skull does not allow to identify any more details of this overlap,nor is it possible to ascertain whether palatoquadrate cartilage persisted in the adultParaplacodus as it does in other placodonts (Rieppel, in press).

The anterior part of the palate of Paraplacodus is known from the holotype (PIMUZT4773) and from the second specimen (PIMUZ T4774) described and illustratedby Peyer (1935). Each maxilla carries seven subspherical teeth, separated from thepremaxillary teeth by a distinct diastema. The labial side of the tooth crown maybe drawn out into a weakly expressed pointed tip, particularly in the anteriormaxillary teeth. The medial surface of the tooth crown forms a lingually slopingshoulder. Each palatine carries a minimum of four subspherical teeth with a blunttooth crown. The most salient character of the anterior palate of Paraplacodus arethe confluent internal nares, a character shared with Placodus.

Inspection of the holotype (PIMUZ T4773) revealed well preserved and articulatedanterior parts of both palatines (Fig. 4). These elements become rather narrowanteriorly, and the anterior margin of both palatines combine to form the gentlycurved posterior margin of the confluent internal nares. There is no indication of any

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breakage of possible anteromedial processes of the palatines, or of a palatine–vomercontact, which would have indicated an original separation of the internal nares.Nor are there any indications of facets on the palatines for the reception of theposterior tips of the vomers. The presence of confluent internal nares can thus besafely assumed for Paraplacodus (Peyer, 1935).

The mandible is well exposed in lateral view in the specimens PIMUZ T4774and BSP 1953 XV 5, whereas that of the holotype is broken. Sutures are verydifficult, in most cases even impossible, to identify. In general, the mandible ofParaplacodus is relatively deep and massive (Fig. 3). The mandibular symphysis carriesfour elongated, sigmoidally curved, strongly procumbent and pointed teeth, two oneach dentary, which match the premaxillary teeth in shape and arrangement. Behinda distinct diastema, each dentary carries a total of seven subspherical crushingteeth which match the maxillary teeth in their morphology. Peyer’s (1935, fig. 1)reconstruction of the lower jaw, molded by the preparator Chr. Strunz, shows anelongated mandibular symphysis which remains narrower than the premaxillaryrostrum and which shows no indication of a rostral constriction. It also shows theposterior dentary teeth to bite against the groove defined by the maxillary teethlaterally, and the palatine teeth medially. Although this seems to represent afunctional arrangement, it should be borne in mind than no articulated lower jawis known for Paraplacodus.

Paraplacodus differs from other placodonts, and in particular from Placodus, by amuch less developed coronoid process, which also reflects a lesser degree ofdurophagy. And whereas in Placodus the coronoid process of the dentary obscuresthe coronoid in lateral view, the coronoid of Paraplacodus is exposed laterally behindthe coronoid process of the dentary. A series of mental foramina opens below thedentary teeth on the lateral surface of the dentary. The delineation of the surangular,angular, prearticular and splenial remains obscure for Paraplacodus, but the retro-articular process can be seen to form a much more massive structure (Fig. 3) thanthe slender retroarticular process characteristic of Placodus.

Vertebral counts were established by Kuhn-Schnyder (1942) for the only articulatedspecimen available (PIMUZ T4775). The number of cervical vertebrae cannot beprecisely established. Six cervicals can be identified, an incomplete number. Theseare followed by 21 dorsal vertebrae, three sacrals, and 54 caudals. The vertebralcentra are slightly constricted ventrally and laterally, and their terminal articularsurfaces are distinctly amphicoelous, but as far as can be determined, not notochordal.Short and ‘knobby’ dia- and parapophyses are distinct on an anterior cervicalcentrum of specimen BSP 1953 XV 5, exposed in lateral view. Other cervicalvertebrae are less well exposed, and do not allow the identification of relevantmorphological detail. In several specimens, the vertebral centra have separated fromthe neural arch, and expose the sutural facets for the pedicels of the neural arch.With respect to this character, Paraplacodus resembles the eosauropterygians moreclosely than Placodus. In the latter genus, the centrum forms low ridges which definethe lateral margins of the neurocentral canal, and which receive the neural archpedicels. In Paraplacodus, both cervical and dorsal centra carry flat facets for theneural arch pedicels which also show some very weak broadening reminiscent ofthe cruciform or ‘butterfly shaped’ structure characteristic of eosauropterygians. Theneural canal is slightly constricted at the middle of the centrum.

Within the dorsal region, the transverse processes of the neural arches are


Figure 5. Postcranial elements of Paraplacodus broilii Peyer (holotype, PIMUZ T4773). A, isolated dorsalneural arch; B, isolated dorsal rib. Scale bars=20 mm.

elongated and distally expanded (Fig. 5A), more distinctly so than in Placodus. InParaplacodus, the length of the transverse processes increases from the anterior dorsalregion towards its middle section, behind which it decreases again. In the anteriordorsal region of specimen T4775, the width across both transverse processes is44.5 mm, in the middle section the width is 60.1 mm, in the posterior dorsal regionit is 57.2 mm. The same trend is observed in specimen PIMUZ T4827. Compressionof the material does not allow the identification of details of the orientation of thepre- and postzygapophyses (Peyer, 1935; Kuhn-Schnyder, 1942), which in Placodusshow a changing inclination in an antero-posterior trend. However, Peyer (1935)established the presence of accessory, hyposphene–hypantrum articulations betweenthe dorsal vertebrae on the holotype, which was confirmed by personal inspection.An isolated dorsal neural arch with a total height of 32.4 mm, and a total width of64 mm, shows the high (9.8 mm) and narrow (3.1 mm) neural canal characteristicfor placodonts. A deep and wide trough, undivided by a vertical internal septum,lies at the base of the neural spine, above the neural canal and below theprezygapophyses, forming the hypantrum. Another isolated dorsal neural arch,exposed in posterior view, shows a crushed hyposphere projecting from the base ofthe neural spine, below the postzygapophyses. Specimen PIMUZ T4774 showsagain an isolated dorsal neural arch in anterior view, which is somewhat betterprepared and preserved the distinct ridges which run from below the prezygapophysisto the neural arch pedicels, defining the lateral margins of the hypantrum. Theseridges lack the anteriorly projecting lappets that are present on an enigmatic vertebrafrom the Muschelkalk of Makhtesh Ramon (Rieppel, Mazin & Tchernov, 1999, fig.54C), which was tentatively referred to Paraplacodus by Haas (1975).

As shown by specimen PIMUZ T4775, the neural spines are relatively low yetelongated and fairly thick sagittal blades which are of trapezoidal shape in lateralview throughout the cervical and dorsal region. Their height increases somewhattowards the middle section of the dorsal region, and slightly decreases again towardsthe sacrum. The uniform appearance of the neural spines changes in the caudalregion (Fig. 6). Their height increases abruptly in the third and succeeding caudalvertebrae, which caused the tail to rotate relative to the sacrum as the carcassbecame embedded in sediment. As a consequence of that rotation, the tail is exposedin lateral view, whereas the dorsal and sacral region are exposed in dorsal view. Inthe anterior caudal region (Fig. 6A), the neural spines are higher than they are long,and they slant slightly posteriorly. In the middle section of the tail (Fig. 6B), the

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Figure 6. Caudal vertebrae of Paraplacodus broilii Peyer (PIMUZ T4775). A, anterior tail section; B,middle tail section; C, distal tail section. Scale bars=20 mm.

neural spines have an elongate and low base which quickly narrows to a slenderdorsal process which is slightly curved in an anterior direction. Further back, theslender dorsal process which forms the neural spine becomes progressively bentbackwards, running parallel to the vertebral centrum at the tip of the tail (Fig. 6C).

The first chevron is carried by the fourth caudal vertebra. As described by Kuhn-Schnyder (1942), the anterior haemapophyses articulate with the posteroventralaspect of their respective centra. In the middle section of the tail, the chevronarticulation bridges the contact between two successive vertebral centra, while theyarticulate with the anteroventral aspect of the centra in the posterior part of thetail. The distal end of the haemapophyses is distinctly broadened in Paraplacodus. Inthe anterior part of the tail, each chevron expands into a semicircular distal platewith a straight ventral margin (Fig. 6A). In the middle section of the tail, the ventralmargin of the semicircular plate becomes distinctly concave, such that the chevronappears to expand into short and curved anteroventral and posteroventral processes(Fig. 6B). Further back still, the chevrons expand distally into short and slenderanteroventral and posteroventral processes which are aligned along the longitudinalaxis of the centrum (Fig. 6C). In general, the neural and haemal arches contributeto a lateral flattening of the tail, especially in its anterior and middle portion, which


Figure 7. The pectoral girdle of Paraplacodus broilii Peyer. A, left clavicle (PIMUZ T4775); B, rightcoracoid (PIMUZ T4827). Scale bars=20 mm.

indicates the support of subaqueous locomotion through lateral undulation of thetail.

In Placodus, the first cervical rib is associated with the axis; no specimen ofParaplacodus is well enough preserved to allow the unequivocal identification of thefirst cervical rib and its associated vertebra. In specimen BSP 1953 XV 5, theanteriormost cervical ribs are located immediately behind the quadrate. As in allsauropterygians, the cervical ribs are double headed, and they carry a free endinganterior process.

The dorsal ribs of Paraplacodus are highly characteristic (Fig. 5B). They expandinto a distinct uncinate process along their posterior margin. This expansion startsclose to the proximal articular head of the rib, and becomes broader distally. Itterminates with an irregularly notched and ridged ventral margin at about twothirds of the length of the rib. If articulated, the uncinate processes of succeedingribs tightly overlap. As described by Kuhn-Schnyder (1942), the 14th dorsal vertebrastill carries a well developed uncinate process, which is strongly reduced on the 15thdorsal rib, and absent on the 16th rib. The three sacral ribs differ from the caudalribs by their distinct distal expansion.

Several specimens of Paraplacodus show disarticulated clavicles. This is a generallyslender, strongly curved or boomerang-shaped element (Fig. 7A). Its shanks, whichenclose an angle of approximately 75°, taper at both ends. Specimen PIMUZ T4774in particular shows a close similarity of the clavicle to the same element of Placodus,with a posterior lamellar expansion of the anteromedial process partially filling thespace between the two shanks (Peyer, 1935). This broadening of the anteromedialprocess of the clavicle cannot be identified on specimen PIMUZ T4775 (Fig. 7A).Specimen BSP 1953 XV 5 preserves a curved, elongated element located betweenthe skull and the right scapula, which may represent the right clavicle. If correctlyidentified, this clavicle appears much less angulated than that observed in otherspecimens but rather evenly curved, although this may be an artifact of preservationas the element is broken across cervical vertebrae. It also shows a relativelybroad termination of the anteromedial process, whereas the posterolateral process,

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Figure 8. The appendicular skeleton of Paraplacodus broilii Peyer. A, right humerus (PIMUZ T4775);B, left zeugopodium and carpus (PIMUZ T4775); C, right humerus (holotype, PIMUZ T4773); D,right femur (holotype, PIMUZ T4773). Scale bars=20 mm.

contacting the scapula, appears more slender and tapering. There seems to be somepotential for variation of the shape of the clavicle in Paraplacodus. The interclavicleof Paraplacodus remains unknown.

Specimen BSP 1953 XV 5 shows the beautifully preserved left scapula in lateralview. Its ventral margin overlaps the distal end of the posterodorsal process of theclavicle, which indicates that the clavicle was applied against the medial surface ofthe scapula, as was described and illustrated by Peyer (1935), and as is also the casein Placodus and other sauropterygians. However, the shape of the scapula differsfrom that of Placodus, as was also noted by Peyer (1935) and Kuhn-Schnyder (1942),in that the ventrally expanding glenoidal portion is distinctly set off from theposterodorsally extending dorsal blade. The two components of the scapula areseparated by a constricted neck region, which together with the posterodorsalorientation of the dorsal blade results in a deeply concave posterior margin of thescapula. The coracoid of Paraplacodus again differs from that of Placodus in beingmore elongate (Fig. 7B). The best preserved coracoid is shown by the specimenPIMUZ T4827 (Fig. 7B). It is 1.5 times as broad as it is long. Its anterior marginis sigmoidally curved, its posterior margin slightly concave in the proximal part.The lateral (glenoidal) margin is weakly concave, the medial (symphyseal) marginis evenly curved. A weak indentation on its lateral (glenoidal) margin indicates thepassage of the supracoracoid nerve between the scapula and the coronoid.

The humerus is well exposed in several specimens (Fig. 8A, C) and most distinctlydifferentiated in the specimen PIMUZ T4775 (Fig. 8A). The humerus is strongly


curved, more strongly so than in Placodus, but its distal expansion is equally prominent.The preaxial margin of the humerus of Placodus is rather straight, while its deeplyconcave postaxial margin still results in the appearance of a generally curvedhumerus (Rieppel, 1995, fig. 42). In Paraplacodus, the anterior margin of the humerusis convex, its posterior margin concave, more like in other sauropterygians. Thedeltopectoral crest is well developed. Dorsoventral compression of the bone mayhave obscured the ectepicondylar groove, or else it was weakly developed. Theentepicondylar foramen is absent. The left radius and ulna are again well exposedin the same specimen (Fig. 8B). The radius has a curved appearance due to adistinct angulation of the proximal part of its preaxial margin. Its postaxial marginis evenly concave. The ulna is a rather straight element, somewhat more lightlybuilt than the radius, with a biconcave diaphysis. Together, the radius and ulnaenclose a distinct spatium interosseum. Between the distal ends of the radius andulna, and the proximal ends of the metacarpals, a total of four ossifications arepreserved. Three of these are closely associated, and exposed in dorsal view (Fig.8D). The largest element must represent the intermedium. It carries a distinctconcavity on its proximal margin, indicating the passage of the perforating arteryproximal to the intermedium, between the distal ends of the radius and ulna. Nextto the intermedium lies the ulnare of intermediate size, which in turn articulateswith the smallest of the three ossifications, the fourth distal carpal. The fourthossification preserved in the general area of the carpus is not associated with theother three elements, and lies perpendicular to the bedding plane, exposing itsarticular margin. It could represent a fourth carpal ossification (third distal carpal)or, less likely, a much displaced phalanx. With three, perhaps even four carpalossifications, Paraplacodus differs from Placodus for which a single carpal ossificationhas been described by Drevermann (1933), perhaps an incomplete number. Themetacarpals and phalanges are incompletely preserved, but still allow the minimalreconstruction of the plesiomorphic phalangeal formula 2–3–4–5–3 for the manus(Kuhn-Schnyder, 1942).

The ilium of Paraplacodus is an autapomorphic structure quite different from thatof other sauropterygians (Fig. 9B). It forms an essentially vertical strut with concavelateral margins. The ventral portion is expanded to from the shallow dorsal half ofthe acetabulum. This acetabular portion is separated from a dorsal process by adistinctly constricted neck. The dorsal process expands dorsally, but not as muchas to reach the width of the ventral acetabular portion, and it ends in an obliquelysloping dorsal margin. The pubis is well preserved in the specimen PIMUZ T4827(Fig. 8C). It is an elongated element with weakly concave anterior and posteriormargins. Its ‘waisted’ appearance resembles the pubis of other sauropterygians moreclosely than the shorter, more rounded element characteristic of Placodus. But as inPlacodus, the slit-like obturator foramen remains open in the adult. The ischium, isagain of typical sauropterygian structure with a narrow lateral (acetabular) head,and a wide medial (symphyseal) expansion which results in distinctly concave anteriorand posterior margins (Fig. 9A). By comparison to Placodus, both the pubis andischium are less expanded to form plate-like elements, which results in a much moredistinct thyroid fenestra in Paraplacodus, the plesiomorphic condition. A comparisonof Paraplacodus with Placodus and other sauropterygians shows that the thyroid fenestrais secondarily reduced in Placodus (Sues, 1987).

Both femora are well preserved in specimen PIMUZ T4775. The element is morelightly built and somewhat shorter than the humerus (Fig. 8D). It is weakly curved,

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Figure 9. The pelvic girdle of Paraplacodus broilii Peyer. A, left ischium and femur (PIMUZ T4775); B,ilium (holotype, PIMUZ T4773); C, left pubis (PIMUZ T4827). Scale bars: A=20 mm, B=10 mm,C=20 mm.

and shows a well developed internal trochanter distinctly set off from the proximalarticular surface by an intertrochanteric fossa (Fig. 9A), a morphology which isclosely comparable to the femur of Placodus (Rieppel, 1995, fig. 42B). Tibia andfibula are relatively long and slender elements. The tibia is somewhat longer relativeto the femur than is the radius relative to the humerus. The tibia has a relativelystraight preaxial margin and a concave postaxial margin. The diaphysis of the fibulais biconcave. Together, the tibia and fibula enclose a distinct spatium interosseum.

The right and the left hind-limbs lie parallel to each other to the left side of theaxial skeleton. The left and the right tarsus thus come to lie in juxtaposition to oneanother. Combined, the two tarsi preserve three tarsal ossifications. The leftcalcaneum is in articulation with the left fibula, leaving the right tarsus with anastragalus and a calcaneum in a slightly shifted position relative to the right tibiaand fibula. A fourth distal tarsal cannot be identified, and appears to have been


absent (not ossified). If this interpretation is correct, Paraplacodus would share withPlacodus the presence of only two tarsal ossifications. The metacarpals of the left footare incompletely preserved, along with a single phalanx. The phalangeal formulafor the pes of Paraplacodus therefore remains unknown.

Both Placodus and Paraplacodus show gastral ribs, composed of five elements each,and in both taxa, the two most lateral elements on either side of the gastral rib arestrongly angulated, enclosing an angle of almost 90° (Rieppel & Zanon, 1997). InPlacodus, a row of osteoderms is aligned along the dorsal midline of the body, cappingthe tips of the neural arches. Dorsal osteoderms are absent in Paraplacodus (specimenPIMUZ T4827).

ComparisonsParaplacodus shares with Placodus some important characters of the cranial and

postcranial skeleton, such as the high and narrow skull, the large nasals, the relativesize of the upper temporal fossa the confluent internal nares, the presence ofaccessory intervertebral (hyposphene–hypantrum) articulations, the broad distalexpansion of the humerus, the distinct internal trochanter on the femur, and thestrongly angulated lateral gastral rib elements. However, it also shows importantdifferences from Placodus, such as the low coronoid process, exposing the coronoidin lateral view, the low neural spines, the nature of the neurocentral suture, theshape of the chondral elements in the pectoral and pelvic girdles, and the absenceof osteoderms (other than the gastralia). It would therefore seem to be important tore-assess the monophyly of the Placodontoidea, which are to include Paraplacodusand Placodus, as opposed to a hypothesis which would place Placodus closer tocyamodontoids than Paraplacodus, and thus place Paraplacodus as the sister-taxon ofall other known placodonts.

Unfortunately, the postcranial skeleton of cyamodontoids remains very poorlyknown, mostly for preservational reasons. Articulated material of cyamodontoidplacodonts (Cyamodus: Pinna, 1980, 1992; Psephoderma: Pinna & Nosotti, 1989) comesfrom deposits which yielded severely dorsoventrally compressed specimens, andmany details of postcranial structures are obscured by the extensive dermal armour.However, some characters can be identified. Given their extensive armour, cy-amodontoid dorsal vertebrae tend to carry low neural spines, but their transverseprocesses are much enlarged and curved, sometimes even fused with the dorsal ribs(Psephoderma: Pinna & Nosotti, 1989), an autapomorphy of the group. Details of theintervertebral articulations remain unknown. According to Pinna & Nosotti (1989),the chevrons articulate with the anteroventral aspect of their respective centrum inPsephoderma, as is also the case in the posterior part of the tail in Paraplacodus. Distalcaudal vertebrae of Placodus did not carry any chevrons; more anterior caudalvertebrae show articular facets for the chevrons on their posteroventral aspect(Drevermann, 1933), as is also the case in Paraplacodus.

In limb girdle morphology, cyamodontoids more closely resemble Placodus thanParaplacodus in some characters. Probably again correlated with the development ofa carapace, the scapula is reduced by comparison to placodontoids. In Cyamodus,the scapula is still rather large, and its contours, although ill preserved, resemblethose of the scapula of Paraplacodus more closely than that of Placodus, in that it is alittle more slender and its posterior margin more deeply concave (Pinna, 1980). ForPsephoderma, it is described as a simple vertical strut with a limited ventral glenoid

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expansion (Pinna & Nosotti, 1989). In both genera, however, the coracoid is arounded plate of bone with an open notch for the coracoid foramen, very muchlike the coracoid in Placodus, and unlike the more elongate and slender coracoid ofParaplacodus. The ilium is very incompletely known for Cyamodus (Pinna, 1980), butin Psephoderma, the ilium expands dorsal to its acetabular portion into a short andstubby, dorsally somewhat expanding blade (Pinna & Nosotti, 1989). A similarmorphology is indicated for the ilium of Placochelys ( Jaekel, 1907). In cyamodontoids,however, the ventral pelvic elements, pubis and ischium, are expanded roundedplates, the pubis with an open obturator foramen, closely resembling the pubis andischium of Placodus. Given the morphology of the ventral pelvic element, the thyroidfenestra is much reduced in cyamodontoids, as it also is in Placodus.

The stylopodial and zeugopodial elements of the front- and hind-limbs arerelatively somewhat shorter in cyamodontoids as compared to placodontoids. Thehumerus is well known in three-dimensional preservation for Placochelys ( Jaekel,1907), and reasonably well known for other genera (Pinna, 1980; Pinna & Nosotti,1989), and it resembles the placodontoid humerus in its pronounced distal expansion,the presence of an ectepicondylar groove, and the absence of an entepicondylarforamen. However, as in Placodus, the humerus of cyamodontoids is much lessdistinctly curved than it is in Paraplacodus. In cyamodontoids and in Placodus, thepreaxial margin of the humerus is rather straight, whereas it is distinctly convex inParaplacodus. As far as they are known, radius, ulna, and the manus do not offerdistinctive similarities or differences between placodontoids and cyamodontoids.

The femur of Placochelys is again well known in three-dimensional preservation,and although the original material described by Jaekel (1907) is lost, casts are keptat the Natural History Museum London (BMNH R 4070, 4074). The femur showsa reduction of the internal trochanter as compared to Placodus and Paraplacodus,which results in a femur morphology more closely comparable to eosauropterygians(e.g. Simosaurus: Rieppel, 1994, fig. 63A). The badly crushed material of Psephoderma(Pinna & Nosotti, 1989) conveys the impression of a sturdier femur, but this maybe an artifact of preservation. There is no indication that the internal trochanterwas as well developed as it is in placodontoids. As far as they are known, tibia,fibula, and the pes do not offer distinctive similarities or differences betweenplacodontoids and cyamodontoids.

Gastral ribs are present in cyamodontoids, but there is no indication of a strongangulation of the lateral elements. Cyamodontoids share with Placodus the presenceof osteoderms, but these are developed into a carapace and, in some taxa, into aseparate tail shield and plastron.

CLADISTIC ANALYSIS

A recent review of the cranial anatomy, phylogeny, and historical pal-aeobiogeography of the Cyamodontoidea resulted in a data matrix for 67 charactersfor the analysis of placodont interrelationships (Rieppel, 2000). In order to test themonophyly of the Placodontoidea, this data matrix was amended with 11 additionalcharacters which result from the description and discussion above (characters 52through 62). Since this data matrix was designed to test primarily cyamodontoidinterrelationships, but is now used to test placodont interrelationships in general,


T 1. Data matrix for the analysis of the phylogenetic interrelationships of placodonts. See text forcharacter definitions

12345 67891 11111 11112 22222 22223 333330 12345 67890 12345 67890 12345

Pachypleurosaurs 00001 00000 00100 1?000 00000 00000 000??1 1 1

Simosaurus 01001 10000 01010 1???0 1??00 00000 000??Nothosaurus 01101 001?0 00000 1?100 10000 00000 000??

1 1 11Placodus 10001 00000 00000 11000 00000 00000 11111

1Paraplacodus 00001 0?0?? ?0??0 1???0 0??0? 0??00 01001Cyamodus 20000 01110 11011 ?0110 11011 00100 21210

1 2 1 1 1 2Henodus 202?0 10110 10?1? ??110 0?0?1 1?1?? ??43?Macroplacus 200?1 10110 000?? 12111 0?011 0010? ?1220

1Placochelys 21110 01010 00111 11111 01021 01111 ??120

1Protenodontosaurus 20011 00010 00102 0211? 0?021 01110 11320

1Psephoderma 2110? 1??20 00002 1111? 0?0?1 011?1 ??220

33334 44444 44445 55555 55556 66666 6667890 12345 67890 12345 67890 12345 67

Pachypleurosaurs ?0000 11100 0?000 00000 00000 10000 00Simosaurus ?0000 01100 0?000 00000 00000 10000 00Nothosaurus ?0000 01100 0?000 00000 00000 10000 00

1Placodus 00001 00000 00000 00120 10111 01111 11Paraplacodus 0??01 ????? ??001 00010 10000 0111? ?1

1Cyamodus 00000 20101 10012 21121 ?1011 10111 11Henodus 1?000 ?1001 10102 21111 ????1 101?0 11Macroplacus 11100 101?? ????? 211?? ????? ??1?1 11Placochelys 01110 11010 01112 2112? ????1 101?1 11Protenodontosaurus 01100 10000 110?? ?11?? ????? ??1?1 11

2Psephoderma 11110 11010 00112 11121 ??111 101?1 11

2

and is being rooted on sister-taxa of placodonts (i.e. eosauropterygians, includingpachypleurosaurs and the genera Simosaurus and Nothosaurus), placodont syn-apomorphies were also added to the data matrix (characters 63 through 67, fromRieppel & Zanon, 1997). Furthermore, the characters of the species of Cyamodus arecombined and coded at the generic level, which renders some characters withrelevance to the reconstruction of cyamodontoid interrelationships uninformative.These characters are deleted from the list of character definitions given below. Forpostcranial characters, Cyamodus was coded using C. hildegardis. The characterdefinitions are the following (the data matrix is given in Table 1):

(1) Osteoderms absent (0), osteoderms present (1), carapace present (2).(2) Dividing the total length of the skull by the total height of the skull yields a

ratio smaller (0), or larger (1) than 3.

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(3) Rostrum relatively short and broad (0), or narrow and distinctly elongated(1), or spatulate (2).

(4) The ventral surface of the premaxilla is level with the ventral surface of themaxilla (0), or the rostrum is distinctly downturned (1) (Merck, 1997).

(5) The premaxilla extends backwards for more (0), or less (1) than half of thelength of the ventral margin of the external naris (Merck, 1997).

(6) Nasals in contact along midline of skull (0), or separated from one anotherby large posterior (nasal) processes of the premaxilla.

(7) Anterior end of maxilla does not (0), or does (1) expand medially to formmost of the dermal floor of the external naris.

(8) The anterior tip of jugal does (0), or does not (1) extend anteriorly along theventral margin of the orbit beyond the midpoint of the longitudinal diameterof the orbit.

(9) Pineal foramen placed in centre of skull table (0), displaced anteriorly on skulltable (1), or is displaced anteriorly with frontal entering its anterior margin(2).

(10) Anterolateral processes of frontals well developed (0), or reduced (1).(11) Parietal without (0), or with (1) distinct anterolateral processes embraced by

postfrontal and frontal.(12) Frontals do not (0), or do (1) reach posteriorly beyond the level of the anterior

margin of the upper temporal fossa.(13) Partietal skull table constricted in its posterior part, i.e. with concave lateral

margins (0), or square, i.e. with straight lateral margins in its posterior part(1).

(14) Posterolateral margin of postfrontal weakly concave and evenly curved (0), ordeeply concave and angulated (1).

(15) Postfrontal enters upper temporal fossa (0), or is excluded from upper temporalfossa by a narrow (1), or broad (2) contact of the postorbital with the parietal.

(16) Postorbital extends along lateral margin of temporal fossa to a level in frontof or at the midpoint of the longitudinal diameter of the upper temporal fossa(0), or further back (1).

(17) The vertical part of the suture separating the maxilla from the jugal is locatedbehind the level of the posterior margin of the orbit (0), behind the level ofthe midpoint of the longitudinal diameter of the orbit but in front of theposterior margin of the latter (1), or at the level of the midpoint of thelongitudinal diameter of the orbit (2). The jugal in placodonts may form aslender, tapering anterior process which lines the ventral margin of the orbit(character 8 above). This process is distinctly set off from the vertical suturewhich separates the jugal from the maxilla over the greater part of its height,which is the character addressed here.

(18) The dorsal process of epipterygoid is narrow (0), or broad (1).(19) The base of epipterygoid is sutured predominantly to the pterygoid (0), or to

the palatine (1).(20) The postorbital does not (0) or does (1) form a medioventral process which

abuts against the lateral surface of the epipterygoid at the posterodorsal marginof the foramen interorbitale.

(21) The longitudinal diameter of the upper temporal fossa is less than twice (0),or at least twice (1) the longitudinal diameter of the orbit (in the adult).


(22) The epipterygoid does not (0), or does (1) form a posterior dorsal processwhich contacts the squamosal at anterodorsal corner of posttemporal fossa.

(23) The epipterygoid is always fully ossified in the adult (0), or may be incompletelyossified in the adult (1). The incompletely ossified epipterygoid occurs in somecyamodontoids. If present, the epipterygoid appears as a bipartite element,the two parts separated by a cleft that must have been filled with cartilage inthe adult (Nosotti & Pinna, 1998)

(24) The (neomorph) otic process of the squamosal is absent (0), extends to themidpoint of the ventral margin of the posttemporal fossa (1), or extendsbeyond the level of the medial margin of the posttemporal fossa (2) (in lateralview of the skull).

(25) A palatoquadrate cartilage recess is absent (0), or present (1).(26) A basiorbital furrow is absent (0), or present (1).(27) The palatine does not (0), or does (1) contact the quadrate along the lateral

margin of the palatoquadrate cartilage recess.(28) The pteroccipital foramen is absent (0), or present (1).(29) The prootic is not (0), or is (1) exposed in occipital view of the skull.(30) Premaxillary teeth are present (0), or absent (1).(31) Anterior premaxillary and dentary teeth pointed (0), chisel-shaped (1), or

bulbous with anterior transverse ridge (2).(32) A diastema separating premaxillary and maxillary teeth is absent (0), or present

(1).(33) Seven or more (0), five to three (1), two (2), one (3), or no (4) maxillary teeth

(tooth).(34) More than three (0), three (1), two (2) or one (3) pair(s) of palatine teeth.(35) Anterior palatal tooth plate(s) small and rounded (0), or transversely enlarged

(1). The anteriormost palatine tooth plates are small and have a crown witha circular circumference (rounded crown) in cyamodontoids. By contrast, theanterior palatine tooth plates in Paraplacodus and Placodus are larger relative tothe posterior palatine tooth plates, and have a irregularly rhomboidal shape,i.e. a crown with a transverse diameter exceeding the longitudinal diameter.

(36) The ratio of the longitudinal to the transverse diameter of the posteriorpalatine tooth plate less (0), or equal/more (1) than 1.4 (in the adult).

(37) Maxilla without (0), or with (1) anterior process extending into rostrum inventral view.

(38) Ventral surface of rostrum flat (0), or concave (1).(39) Ventral surface of rostrum without (0), or with distinct grooves leading up to

internal nares (1).(40) Internal nares separated (0) or confluent (1).(41) Ectopterygoid present (0) or absent; if absent, palatine extends laterally at the

anterior margin of the subtemporal fossa to meet the jugal (1), or jugal extendsmedially to meet the palatine (2).

(42) The ratio of the length of palatal exposure of pterygoid relative to length ofpalatine is less (0), or more (1) than 0.3.

(43) The posttemporal fossae are relatively large (0), or reduced (1) due to expansionof occipital exposure of parietal, squamosal, and opisthotic.

(44) The squamosal buttress against which abuts the distal tip of the paroccipitalprocess is absent (0), or present (1).

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(45) The posteroventral tubercle is absent (0), or present (1) at the distal tip of theparoccipital process.

(46) The exoccipitals do not (0), or do (1) meet above occipital condyle (above thebasioccipital).

(47) The basioccipital tuber and the ventral opisthotic flange remain separate (0),or meet each other (1) ventral to passage of internal carotid.

(48) Anterior tip of dentary dentigerous (0), or edentulous (1) (Merck, 1997).(49) The coronoid remains well separated from lower margin of the mandible (0),

or closely approaches the lower margin of mandible (1).(50) The retroarticular process is long and slender (0), or robust (1), or short and

sloping (2). The retroarticular process of Placodus is elongate and slender, i.e.distinctly set off from the remaining part of the lower jaw, which results in adeep concavity in the posteroventral margin of the mandible (0). In Paraplacodus,the retroarticular process is equally elongate with a horizontal dorsal margin,but it is less distinctly set off from the remaining lower jaw, and hence deeperand more robust (1). The retroarticular process of cyamodontoids is relativelyshorter, stubby, and has a dorsal surface which slopes in a posteroventraldirection (2).

(51) Tubercular osteoderms, secondarily fused to the underlying bone, are absent(0), or present along the posterior margin of the upper temporal fossa only(1), or present on lateral surface of posterior part of temporal arch also (2).

(52) Quadratojugal present (0), or absent (1).(53) Jugal–squamosal contact absent (0), or present (1).(54) Coronoid process absent (0), distinct but low (1), or very high (2).(55) Number of dorsal vertebrae: 19 or more (0), 15 or less (1).(56) Hyposphene–hypantrum articulation absent (0), or present (1).(57) Chevrons articulating with posteroventral (0), or anteroventral (1) aspect of

centrum.(58) Coracoid elongated, with more or less concave anterior and posterior margins

(0), or rounded plate of bone (1)(59) Thyroid fenestra large (0), or reduced by expansion of pubis and ischium to

form rounded plates of bone (1).(60) Preaxial margin of humerus curved (0), or rather straight (1).(61) Internal trochanter distinctly set off from proximal end of femur by inter-

trochanteric fossa (0), or intertrochanteric fossa much reduced, absent (1).(62) Lateral gastral ribs without (0), or with (1) distinct angulation.(63) Crushing tooth plates absent (0), or present (1).(64) Diastema between symphyseal and posterior dentary teeth absent (0), or

present (1).(65) Palatines separated by pterygoids (0), or meeting in medial suture (1).(66) Pterygoids longer (0), or shorter (1), than palatines.(67) Median gastral rib element angulated (0), or straight (1).

A total of four analyses were performed using the software package PAUP version3.1.1. developed by David L. Swofford (Swofford, 1990; Swofford & Begle, 1993).All searches were done implementing the branch-and-bound search option, anduninformative characters were always ignored. For two analyses, all multistatecharacters were unordered, and these were rooted alternatively on an all-0-ancestor,and on successive sister-groups of the Placodontoidea, i.e. the eosauropterygian taxa


pach

ypleu

rosa

urs

Simos

auru

s

Nothos

auru

s

Parap

laco

dus

Placo

dus

Henod

us

Cyam

odus

Mac

ropl

acus

Proten

odon

tosau

rus

Placo

chely

s

Pseph

oder

ma

Figure 10. The phylogenetic interrelationships of placodonts, showing the paraphyly of Placodontoidea(Paraplacodus plus Placodus). For further discussion, see text.

Pachypleurosauria, Simosaurus, and Nothosaurus. In two further analyses, the multistatecharacters 1, 35, and 36 were ordered. This procedure implies the following a priorihypotheses of character evolution. Ordering character 1 implies the absence ofosteoderms to be plesiomorphic for placodonts, and the appearance of a single rowof osteoderms in Placodus to precede the evolution of a carapace in cyamodontoids.Ordering the characters 33 and 34 assumes a regular trend in the reduction of themaxillary and palatine dentition.

Deleting the eosauropterygian taxa from the analysis, and rooting the search onan all-0-ancestor (characters 12, 21, 32, 56, 63, 64, 66, and 67 uninformative andhence ignored) yielded a single most parsimonious tree with a Tree-Length (TL) of111 steps, a Consistency Index (CI) of 0.775, and a Retention Index of 0.667. ThePlacodontoidea was found to be paraphyletic, with Placodus more closely related tothe Cyamodontoidea than Paraplacodus (Fig. 10). Deleting the all-0-ancestor, androoting the analysis on the three eosauropterygian taxa (character 16 uninformativeand hence ignored), yielded a single most parsimonious tree with a TL of 135 steps,a CI of 0.733, and a RI of 0.7. The hierarchy of the tree was the same: (Paraplacodus(Placodus, Cyamodontoidea). With all multistate characters unordered, Paraplacodus,Placodus fall and the monophyletic Cyamodontoidea form a basal trichotomy in atree only one step longer than the most parsimonious solution. The clade includingPlacodus and the Cyamodontoidea therefore has a decay index of 1.

The second set of analyses paralleled the first two searches, but with the characters1, 33 and 34 ordered. Rooting the analysis on the all-0-ancestor (characters 12, 21,32, 56, 63, 64, 66, and 67 uninformative and hence ignored), yielded a single mostparsimonious tree again, with a TL of 112, a CI of 0.768, and a RI of 0.683. Thetree topology was the same as in the previous two searches (Fig. 10). Rooting thetree of the selected eosauropterygian taxa (character 16 uninformative and hence

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ignored) yielded again one single most parsimonious tree, with a TL of 136 steps,a CI of 0.728, and a RI of 0.715. The tree topology remained the same again:(Paraplacodus (Placodus, Cyamodontoidea). With three multistate characters ordered,Paraplacodus, Placodus and the monophyletic Cyamodontoidea form a basal trichotomyin a tree three steps longer. Ordering multistate characters 1, 33, and 34 thereforeincreased to decay index for the clade including Placodus and the Cyamodontoideato 3.

As is clear from the description given above, Paraplacodus shares with Placodus anumber of potential synapomorphies which are absent in cyamodontoids, such asthe high skull, large nasals, the loss of a quadratojugal, the confluent internal nares,the hyposphene–hypantrum articulation (coded unknown for cyamodontoids), andthe strongly angulated lateral gastral rib elements (Rieppel & Zanon, 1997). Placoduson the other hand shares with cyamodontoids a number of potential synapomorphieswhich are absent in Paraplacodus, such as the secondary squamosal-jugal contact, thereduction of the maxillary teeth to five or fewer, the tall cornoid process, the roundedcoracoid, the rather straight preaxial margin of the humerus, the reduction of thethyroid fenestra, and osteoderms. These character conflicts may explain the lowdecay indices, in spite of the fact that the monophyly of the clade including Placodusand the cyamodontoids is supported by several unambiguous synapomorphies (witha consistency index of 1).

With the tree rooted on the three eosauropterygian taxa, all multistate charactersunordered, and DELTRAN character optimization implemented, Placodus was foundto share with the Cyamodontoidea the following synapomorphies (unambiguoussynapomorphies, with a ci=1, are denoted with an asterisk): ∗31(1), premaxillaryteeth chisel-shaped; 42(0), short palatal exposure of the pterygoid (coded unknownfor Paraplacodus); ∗53(1), secondary jugal–squamosal contact present; 54(2), tallcoronoid process (this would be an unambiguous synapomorphy were it not reversedin the highly derived genus Henodus); ∗59(1), reduction of the thyroid fenestra byexpansion of pubis and ischium; ∗60(1), preaxial margin of humerus rather straight;65(1), palatines meet in ventromedial suture (coded unknown for Paraplacodus,reversed in Henodus), ∗66(1), pterygoids shorter than palatines (coded unknown forParaplacodus).

Ordering the multistate characters 1, 33, and 34 adds these to the list ofsynapomorphies shared by Placodus and cyamodontoids: ∗1(1) osteoderms present(this character is questionably optimized, as the potential to have dermal ossificationsmay well be plesiomorphic in placodonts with respect to sister-taxa of the Sau-ropterygia, and lost in Paraplacodus); 33(1), five or less maxillary teeth; ∗34(1), threeor less palatine teeth.

DISCUSSION

The configuration of the temporal region in placodonts has to the present dayremained a matter of contention (summarized in Pinna, 1989, and Rieppel, 1995).The significance of the configuration of the temporal region has also played animportant role in the discussion of placodont relationships among reptiles in general(Kuhn-Schnyder, 1980). The presence of a contact between jugal and quadratojugal,and their juxtaposition to the postorbital and squamosal respectively, has been cited


as evidence that placodonts never had a lower temporal fossa, and hence could notbe nested within diapsids (Kuhn-Schnyder, 1980; Pinna, 1989). Alternatively, thepresence of an L-shaped jugal, which remains separate from the squamosal, andthe deep embayment of the cheek region in Paraplacodus has been thought to indicatea diapsid status of placodonts (Zanon, 1989). Zanon (1989) also pointed out thatthe limited dorsal and posterior extent of the jugal in Paraplacodus indicates that thecontact of the jugal with the squamosal is secondary in those placodonts where itoccurs (also supported by Mazin, 1982). Following this line of reasoning, placodontssuch as Placodus would have secondarily roofed over the cheek region, which seemsalso to be reflected in the broad and fan-shaped contours of the jugal, and in thecovering of the quadrate in lateral view by dermal bone (Rieppel, 1995). Thesecondary expansion of dermal elements in the temporal region of the skull is carriedto its extreme in Henodus among all placodonts, a taxon which obliterates the uppertemporal fossa (Rieppel, in press).

The temporal region of Placodus has been subject of various interpretations. Broili(1912) figured a postorbital and a much expanded jugal which meet an even moreexpanded squamosal in the formation of the broad temporal arch. Sues (1987)concurred with Broili’s (1912) identification of postorbital and jugal, but separateda narrow dorsal squamosal, defining the posterolateral margin of the upper temporalfossa, from a broad quadratojugal, which would cover the quadrate in lateral view.Pinna (1989) shows a small squamosal which is restricted to the posterior margin ofthe upper temporal fossa, while the quadratojugal would have much expanded,forming the entire posterior part of the temporal arch, and entering the posteriorlateral margin of the upper temporal fossa. Rieppel (1995) noted that there is notone skull of Placodus kept in public repositories which allows the unequivocaldelineation of a quadratojugal from a squamosal, and accepted Broili’s (1912)conclusion that a quadratojugal is absent (or fused with the squamosal?) in Placodus.The interpretation of the temporal region of the skull of Paraplacodus, with an L-shaped jugal, and a temporal bar formed by the postorbital and squamosal only,indicates that it shares with Placodus the lack of a quadratojugal. Furthermore, theconfiguration of the temporal region of Paraplacodus suggests the loss of a lowertemporal arch, and a secondary closure of the cheek region in other placodonts,including Placodus, which results in a secondary jugal–squamosal contact.

This hypothesis conflicts with an alternative view, which would interpret thereduction of the dermal covering of the cheek region in Paraplacodus as the result ofventral emargination. In order to test these two hypotheses, the coding for Placodus(used as paradigm for placodonts in general) in a more inclusive data matrix (Rieppel,1998) was alternated between the presence of a lower temporal fossa with reducedlower temporal arch versus the absence of the lower temporal fossa. The data matrixof Rieppel (1998, character 27) codes the lower temporal fenestra as absent (0),present and closed ventrally (1), present and open ventrally (2). This multistatecharacter was ordered, because the loss of a lower temporal arch logically requiresthe prior presence of a lower temporal fenestra. Using Paraplacodus as the relativelymost plesiomorphic placodont, characterized by the loss of the lower temporal arch(code 2 for Placodus, paradigmatic taxon for placodonts) results in a tree which isone step shorter than the assumption that Paraplacodus and with it placodonts ingeneral would be characterized by the ventral emargination of the cheek region(code 0 for Placodus, i.e. absence of the lower temporal fenestra). The assumption ofa loss of the lower temporal arch in Paraplacodus, and hence in placodonts in general,

O. RIEPPEL658

is therefore somewhat more parsimonious, and is in accordance not only with theconfiguration of the temporal region in Paraplacodus, but also with its position as thesister-taxon of all other known placodonts (Peyer & Kuhn-Schnyder, 1955).

As both, eosauropterygians ( Jaekel, 1910; Kuhn-Schnyder, 1967) as well asplacodonts are characterized by the loss of the lower temporal arch, the latter infact becomes a synapomorphy of Sauropterygia. However, since the Younginiformesare removed from the Sauria (Laurin, 1991; Laurin & Reisz, 1995), and the presenceof a complete lower temporal arch is secondary in those rhynchocephalians whereit occurs (Whiteside, 1986), it seems possible that the loss of the lower temporalarch is a synapomorphy of the Lepidosauromorpha with the Sauropterygia nestedat the base of this clade (Rieppel, 1994,1998; but see Merck, 1997, for an alternativeview).

ACKNOWLEDGEMENTS

I thank H. Rieber and H. Furrer from the Palaeontological Institute and Museumof the University of Zurich for granted me access to the material of Paraplacodus intheir care. The skull of the specimen BSP 1953 XV 5 was studied when it was onloan to G. Pinna, Museo Civico di Storia Naturale, Milano. A cast of the skull wasmade by the late R. Zanon, and deposited in the Field Museum by J. Hopson. JohnMerck allowed me to cite his unpublished thesis. N.C. Fraser read an earlier draftof the manuscript, offering much helpful advice and criticism. To all of thesecolleagues go my sincere thanks. This study was supported by NSF grants DEB-9419675 and DEB-9815235.

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