New Chondrichthyan Spines from the Late Paleozoic of Oklahoma

New Chondrichthyan Spines from the Late Paleozoic of OklahomaAuthor(s): Jiri ZidekReviewed work(s):Source: Journal of Paleontology, Vol. 52, No. 5 (Sep., 1978), pp. 1070-1078Published by: SEPM Society for Sedimentary GeologyStable URL: http://www.jstor.org/stable/1303852 .Accessed: 16/06/2012 08:19

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JOURNAL OF PALEONTOLOGY, V. 52, NO. 5, P. 1070-1078, 3 TEXT-FIGS., SEPTEMBER 1978

NEW CHONDRICHTHYAN SPINES FROM THE LATE PALEOZOIC OF OKLAHOMA

JIRI ZIDEK The University of Oklahoma, Norman, Oklahoma 73019

ABSTRACT.-Platyacanthus Fritsch is revived from synonymy of Anodontacanthus Davis. Anodon- tacanthus americanus Hussakof is reassigned to Platyacanthus. Neither Anodontacanthus nor Platy- acanthus can be regarded as possibly synonymous with Xenacanthus and/or Orthacanthus. Anodon- tacanthus belemnoideus n. sp. and a xenacanthodiid(?) spine with a bulbous base are described from the Oscar Formation (Upper Pennsylvanian) of southern Oklahoma, and Platyacanthus avirostratus n. sp. from the Garber Formation (Lower Permian) of southwestern Oklahoma. The assignment of the spine with the bulbous base to the Xenacanthodii is equivocal due to the fact that the hitherto known xenacanthodiid spines possess unexpanded bases with wide-open terminations of the pulp cavities, whereas in the spine in question the base is expanded and the pulp cavity opens through vascular foramina only.

INTRODUCTION

A NUMBER of detached spines, none of them assignable with certainty to any of the known chondrichthyan orders, have been collected by Larry C. Simpson, former graduate student at The University of Oklahoma, from the Lower Permian Garber Formation of the Northeast Frederick site in southwestern Oklahoma (Simpson, 1976); and by Allen A. Graffham, Geological Enterprises, Ardmore, Oklahoma, from the uppermost Pennsylvanian Oscar For- mation at the Waurika #1 site, (Olson, 1967) in southern Oklahoma (Text-fig. 1). With the exception of one equivocal specimen with a bulbous base, from the Waurika #1 site, the spines belong to the organ genera Anodonta- canthus Davis and Platyacanthus Fritsch, however, they cannot be assigned to the hitherto known species. All the specimens described in this paper are deposited in the Sto- vall Museum of Science and History at The University of Oklahoma (OUSM).

The faunal assemblages and lithologies of the Waurika #1 and Northeast Frederick sites were studied by Olson (1967) and Simpson (1976), respectively, who interpreted the de- positional environments as lacustrine, probably on the coastal plain level. These two au- thors also provide complete faunal lists for the sites. The Waurika #1 site was regarded as Permian by Olson (1967); however, it is approximately 50 feet below the Ryan Sand- stone, which forms the base of the Wellington Formation. The base of the Ryan Sandstone is considered by the Oklahoma Geological Sur- Copyright ( 1978, The Society of Economic I Paleontologists and Mineralogists

vey to be the base of the Permian, and, consequently, the Waurika deposits are in the upper part of the Oscar Formation, Gearyan Series, Upper Pennsylvanian (cf. Zidek, 1972, p. 174, and further references therein).

I would like to thank Allen A. Graffham for generously donating his finds to the Stovall Museum; Roy Davis, Chief Draftsman for the Oklahoma Geological Survey, for preparation of Text-fig. 1; and Donald Baird of Princeton University for information on the ichthyodor- ulite Bulbocanthus Bryant.

SYSTEMATIC PALEONTOLOGY

CHONDRICHTHYES Incertae Sedis Organ Genus ANODONTACANTHUS

Davis, 1881

Type species.-A. acutus Davis, 1881, p. 428, P1. 22, fig. 10.

Generic discussion.-Davis (1881, p. 427) implied that the internal structure of Anodon- tacanthus spines is similar to that of xenacath- odiid spines. He further stated that: "Their general form is also similar to that of the Pleu- racanths: the internal cavity is terminal at the basal extremity; and the walls become thinner near the base in comparison with the remain- der of the spine; the method of implantation in the body of the fish must also have been the same. The great distinguishing feature between the two rests in the absence in this genus of any form of denticulation." In 1892 (p. 747) he characterized Anodontacanthus as follows: "The spines included in this genus are straight, more or less tapering to a point. Internal cav-

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NEW PALEOZOIC CHONDRICHTHYAN SPINES

10 5 ?0 0 * Fossil locality I I I I'*-- III I

KILOMETERS TEXT-FIG. 1-Index map of southern-southeastern Oklahoma, showing Waurika #1 and Northeast Fred-

erick sites.

ity large, terminating at the base without posterior extension of the opening. Distinguished from Pleuracanthus by the absence of denticles."

The revised generic diagnosis of Anodonta- canthus by Hussakof (1911, p. 162) contains a mixture of characters of both Anodontacan- thus and Platyacanthus. This diagnosis is discussed below, following the description of Platyacanthus avirostratus n. sp.

ANODONTACANTHUS BELEMNOIDEUS n. sp. Text-fig. 2A

Holotype.-OUSM 00450; no other material.

Type locality.- Waurika #1 site of Olson (1967, p. 17), SW/4, NW/4 Sec. 36, T4S, R9W, Jefferson County, southern Oklahoma.

Stratigraphic occurrence. -Approximately 50 feet below the Ryan Sandstone, Oscar For- mation, Gearyan Series, Upper Pennsylva- nian.

Etymology.-Named for its resemblance to the belemnoid rostrum.

Diagnosis.-Straight spine about 75 mm in total length; length:width = 6.2:1; tapering most rapid in distal fifth; cross section elliptical proximally and circular in distal half; pulp cavity occupies about 75% of cross-sectional

area near base; posterior median groove is restricted to proximal third of spine's length; entire surface is longitudinally striated.

Description.-A straight spine 69 mm in preserved length, maximum 12 mm broad, and maximum 9 mm thick antero-posteriorly. The proximal end is missing; the spine may have been about 75 mm in total length, from which the length:width ratio can be estimated at 6.2:1. The tapering is most rapid in the distal fifth. The spine reaches its maximum diameter 15-20 mm above the base and from there it narrows proximad. The posterior face bears a median groove that extends throughout the proximal third of the spine. The cross section is elliptical proximally and gradually changes to circular in the distal half. As only a single specimen is available, no sectioning has been done. The extent of the pulp cavity therefore remains unknown except at the proximal end, where it occupied about 75% of the cross-sectional area. The cavity is centered so that the thickness of the spine's walls is approximately the same anteriorly and posteriorly. The entire surface is longitudinally striated.

The posterior median groove definitely is a true morphological feature of the spine and not an artifact. Its presence cannot be explained as the result of crushing because the specimen

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B C

\ /

TEXT-FIG. 2-Anodontacanthus and Platyacanthus spines in anterior (left) and posterior (right) views. A, Anodontacanthus belemnoideus n. sp., OUSM 00450 (holotype), Oscar Formation, Gearyan Series, Upper Pennsylvanian, Waurika #1 site of Olson (1967), Jefferson County, southern Oklahoma. B, Platyacanthus avirostratus n. sp., OUSM 00521A (holotype), with distal part (dashed) restored from OUSM 0052 1B. Upper part of Garber Formation, Lower Permian, Northeast Frederick site of Simpson (1976), Tillman County, southwestern Oklahoma. C, Platyacanthus ventricosus Fritsch, Kounov Beds, Stephanian B (=Upper Pennsylvanian), Kounov, Kladno-Rakovnik coalfield, central Bohemia, Czech- oslovakia. Drawn from Fritsch's type no. 13, deposited in National Museum of Czechoslovakia, Prague (cf. Fritsch, 1889, p. 113, P1. 86, fig. 5). pgr-posterior median groove, plr-(postero-) lateral ridges shown by lines alongside posterior views.

is perfectly 3-dimensionally preserved, and because both the lining of the pulp cavity in the area underlying the groove, as well as the ornamentation of longitudinal striae extending into the groove (as in xenacanthodiid spines), show no evidence of crushing or any other dis- tortion.

Discussion.-The Anodontacanthus spines so far described are A. acutus, A. obtusus, and A. fastigiatus from the Upper Carboniferous of the British Islands (Davis, 1881). Glikman (1964, p. 212) listed this genus as occurring also in the Permian deposits of the European part of the Soviet Union, but I have searched in vain for an actual description of such an occurrence. Hussakof (1911, p. 162) described Anodontacanthus americanus from either the uppermost Pennsylvanian or the Lower Perm-

ian of Texas and placed Platyacanthus ventricosus Fritsch, 1889, from the Upper Carbon- iferous of Bohemia, Czechoslovakia, in synonymy of Anodontacanthus. In my opinion, the reverse is the case, and these two species are therefore further discussed below, under the generic designation Platyacanthus. Traquair (1888) regarded A. fastigiatus Davis as a Pleuracanthus spine deprived of its denticulation by abrasion, and his conclusion was followed by Woodward (1889, p. 6, 8), Davis (1892, p. 748), Hussakof (1911, p. 162), and Romer (1942, p. 227). Romer went so far as to suggest that Anodontacanthus americanus (regarded as Platyacanthus in this paper) cer- tainly, and all the other Anodontacanthus material probably, belong to Pleuracanthus (nom. praeocc., cf. Neave, 1940, p. 816). This was

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,


a gross oversimplification of the matter, however, as Romer himself apparently later recognized (Romer, 1966, p. 349). Some of the Scottish specimens of Traquair (1888) un- doubtedly are worn xenacanthodiid spines, and examples of this sort are known from other areas as well (for instance, Brachiacanthus semiplanus Fritsch, 1889, p. 113, P1. 83, fig. 10). As for A. fastigiatus, however, the synonymy suggested by Traquair (without him, or anybody else, re-examining the fossil) is ob- jectionable on the following grounds: this species is based on only the distal half of a spine that shows a great deal of similarity with A. belemnoideus and that does not seem to have been subjected to abrasion any stronger than is that apparent in A. belemnoideus (Davis, 1881, P1. 22, fig. 12; and Text-fig. 2A of this paper). There is no wayA. belemnoideus could possibly be regarded as belonging to Xenacan- thus (="Pleuracanthus") or Orthacanthus as these genera are today understood, and consequently such a generic assignment is objec- tionable for A. fastigiatus as well. It should be emphasized, however, that in spite of the similarity between the two species they cannot be declared conspecific because of their differing cross sections. Rather than elaborating on this point, the type of A. fastigiatus is herein regarded as indeterminate to species because of its incompleteness. The other two species of Davis, A. acutus and A.obtusus, both lack the proximal ends, and A. obtusus lacks the apex as well. From Davis' illustrations (1881, P1. 22, fig. 11; 1892, P1. 73, fig. 26) it appears that A. obtusus does not terminate in as blunt an apex as restored by him; nevertheless, the two spines differ enough in their cross sections, in the extent of their pulp cavities, and subse- quently also in the thickness of their walls, to warrant species distinction. In A. acutus the cross section is circular, in A. obtusus it is elliptical, whereas in A. belemnoideus it changes from elliptical proximally to circular distally. The pulp cavity occupies about 50% of the cross-sectional area in the proximal part of A. acutus, only about 30% in A. obtusus, and as much as 75% in A. belemnoideus. Anodonta- canthus acutus has the posterior median groove present throughout the length of the spine; in A. obtusus the extent of the groove is equivocal, but it definitely is present in at least a part of the proximal half of the spine; in A. belemnoideus the groove is restricted to

the proximal third of the spine's length. With the possible exception of A. fastigiatus (=An- odontacanthus sp. indet.) the species of Davis have higher length:width ratios than A. belemnoideus.

Organ Genus PLATYACANTHUS Fritsch, 1889

Anodontacanthus partim, Hussakof, 1911, p. 162.

Type species.-Platyacanthus ventricosus Fritsch, 1889, p. 113, P1. 86, fig. 5.

Generic discussion.-Fritsch (1889) did not provide a generic diagnosis for Platyacanthus, and, therefore, his brief description of the only species, P. ventricosus, is here translated from German: "This unique spine is 12 cm long and 13 mm wide in the middle. It is flattened, broadest in the middle, and on the whole pro- tuberant. The cross section is lens shaped in the distal third (Fig. 5d), with the upper surface flattened and the lower surface rounded like an arch. The pulp cavity is moderately large and six branching canals project forward from it into the bulk of the spine. In the proximal half the pulp cavity is very large. The surface of the spine is adorned with fine longitudinal furrows which contain small depres- sions."

Platyacanthus ventricosus is based on a single spine from the Kounov Beds, Stephanian B (=Upper Pennsylvanian), at Kounov, the Kladno-Rakovnik coalfield, central Bohemia, Czechoslovakia. Hussakof (1911, p. 162, P1. 26, fig. 5) named a partial spine from either the uppermost Pennsylvanian (Wichita Group) or Lower Permian (lower part of the Clear Fork Group) of Texas Anodontacanthus americanus and placed Platyacanthus in synonymy of Anodontacanthus. However, A. americanus possesses features characteristic of Platyacan- thus, and Hussakofs (1911, p. 162) revised diagnosis of Anodontacanthus contains a mixture of characters of both Anodontacanthus and Platyacanthus. The genus Platyacanthus Fritsch is thus revived and A. americanus is reassigned to it. The generic diagnosis of Platyacanthus is formulated as follows: Ce- phalic spines with lateral-to-posterolateral ridges in the middle third of the length and with a gentle posterior curvature of the distal third; length:width ratio known to vary be- twen 4.6:1 and 9:1; cross section more or less flattened antero-posteriorly near the base and

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gradually changing to circular near the tip; outline convex proximally and concave further distad, resulting in a somewhat bulbous shape of the proximal part; the posterior wall thinner than the anterior one near the base; the opening of the pulp cavity circular (known from one species only); posterior median groove may be present proximally in some species; surface longitudinally striated or pitted, or both.

The designation of the Platyacanthus spines as cephalic rests (as in case of Anodontacan- thus, cf. Davis, 1881, p. 427) on their resemblance to the xenacanthodiid spines in general, and on the antero-posterior flattening and the abbreviated basal opening of the pulp cavity in particular.

PLAT,YACANTHUS AVIROSTRATUS n. sp. Text-fig. 2B

Holotype.-OUSM 00521A, a distally incomplete spine.

Paratypes.-OUSM 00521B-J, nine fragmentary spines, all from the type locality and stratum.

Type locality.-Northeast Frederick site of Simpson (1976), Sec. 21, T1S, R16W, Tillman County, southwestern Oklahoma.

Stratigraphic occurrence.-Upper part of the Garber Formation, Lower Permian.

Etymology.-Named for its resemblance to a bird's bill-avis, rostrum (Lat.).

Diagnosis.-Stout spines around 40 mm in length; length:width ratio 4.6:1; cross section depresse ovatus proximally, semioval, with flat posterior face in middle third, and circular distally; no posterior median groove; opening of pulp cavity circular; surface longitudinally striated to variable extent.

Description-The holotype is 26 mm in preserved length, maximum 9 mm broad, and maximum 6 mm thick antero-posteriorly. Its distal portion is missing, but from OUSM 00521B, a distal part of spine, the total length may be estimated at about 42 mm. The length:width ratio thus is 4.6:1. From the OUSM 00521B it is also apparent that the spine was gently arched posteriorly in its distal third. The maximum diameter is situated 8- 10 mm above the base. The base is rounded and posteriorly bears a circular opening of the pulp cavity. The pulp cavity occupies about 50% of the cross-sectional area at the base and extends throughout the length of the spine.

The posterior wall is only half as thick proximally as the anterior one, and the pulp cavity becomes centered only further distad. There is no indication of a posterior median groove. In the proximal third of the length the cross section may be characterized as depresse ovatus, with the anterior face more profoundly convex than the posterior one; in the middle third the anterior face remains strongly convex, whereas the posterior face becomes al- most flat and meets the lateral walls at an an- gle, resulting in a paired posterolateral ridge; in the distal third the cross section is circular. With the exception of the area adjacent to the base, the surface is covered with longitudinal striae, some of which anastomose toward the tip. The anterior face is densely striated in all the specimens, whereas the posterior face shows some variation in this respect. The area around the base lacks the shiny orthodentine coating and evidently was not exposed in life.

Discussion.-P. americanus lacks the base as well as the distal fourth. The length:width ratio can thus be only roughly estimated, but it is clear that it was quite high, perhaps about 9:1. A posterior median groove is present in the proximal third, and there are lateral (proximally) to posterolateral (distally) ridges in the middle third of the spine's length. The pulp cavity is not quite centered proximally, resulting in a thinner posterior wall (cf. Hussakof, 1911, P1. 26, fig. 5b). The entire surface is longitudinally striated. P. ventricosus is nearly complete lengthwise, but it has the proximal portion of the posterior face crushed; it cannot be established, therefore, whether a posterior median groove was present, and also the shape of the opening of the pulp cavity cannot be determined. The length : width ratio is 8.3:1. As Fritsch's illustration (1889, PI. 86, fig. 5) is not entirely realistic, particularly in regard to the cross section and the apex, the holotype is here redrawn (Text-fig. 2C). It is slightly dis- torted (bent laterad and perhaps also somewhat flattened), but it clearly shows a posterior curvature of the distal part, of an extent and degree similar to the curvature encoun- tered in P. avirostratus. The apex is not as pointed as shown by Fritsch, and the lateral (proximally) to posterolateral (distally) ridges extend throughout the middle third of the spine's length. Near the base the posterior wall is thinner than the anterior one. The anterior surface is nearly smooth, with but a few lon-

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gitudinal striae and, under higher magnifica- tion, with minute pits. The entire posterior surface is covered with long, longitudinal striae that, nevertheless, interrupt and contain minute pits.

Platyacanthus avirostratus shares with P. americanus and P. ventricosus the presence of the (postero-) lateral ridges, the antero-posteriorly flattened cross section and, subsequent- ly, the somewhat bulbous shape of the proximal part, and the proximally thinner posterior wall; with P. ventricosus it shares also the presence of the posterior curvature of the distal part. In P. americanus the curvature cannot be verified. On the other hand, it differs from P. americanus and P. ventricosus in having a substantially lower length:width ratio (only 4.6:1), and from P. americanus also in the absence of the posterior median groove. In P. ventricosus the groove cannot be verified.

REMARKS ON ANODONTACANTHUS AND PLATYACANTHUS

Hussakof (1911, p. 162) revised the diagnosis of Anodontacanthus as follows: "1. Spines straight, or very gently arched, as viewed from the side. 2. No denticles along posterior margin. 3. Lateral faces ornamented either with incised lines which are discretefor some distance and then anastomose or end abruptly; or with lines and pittings arranged so as to give a reticulated appearance. 4. Pulp- cavity circular in cross-section, completely enclosed, large (occupying about one-third the width of the cross-section in the proximal part of the spine) and gradually diminishing in diameter distalward." Of this diagnosis, the characters printed in italics (mine) do not oc- cur in any of the Anodontacanthus spines described by Davis (1881; 1892), and Hussakof evidently included them in order to be able to fit his new species americanus in the genus Anodontacanthus. In regard to the fourth point of Hussakof's diagnosis, the pulp cavity cannot be unequivocally defined as "completely enclosed." According to Davis (1892, p. 747), the pulp cavity opens at the base "without posterior extension of the opening," but in view of the absence of the proximal parts in all his spines the statement must be regarded as conjectural. In Anodontacanthus the opening of the pulp cavity has never been found preserved, and in Platyacanthus its shape is known only in P. avirostratus n. sp. (Text-fig.

2B). In P. avirostratus the opening is circular, but this species lacks the posterior median groove. If it is taken into consideration that the posterior median groove is present in An- odontacanthus, Platyacanthus americanus, and possibly also in P. ventricosus, then the openings of their pulp cavities, although quite brief, are better visualized as having been somewhat prolonged distad, i.e. terminating further up on the posterior surfaces of these spines.

It may be seen from the preceding descriptions that although Anodontacanthus and Platyacanthus both lack denticle rows, they differ in just about every other aspect of their morphology. However, they are known only as ichthyodorulites, and it might be argued by some workers that the differences are not sig- nificant enough to warrant generic distinction. A comparison of the spines in question with those of the two recognized xenacanthodiid genera, Xenacanthus and Orthacanthus, may be found elucidating in this respect. From the above discussion of Anodontacanthus it is apparent that this organ genus was considered by a number of workers so similar, externally as well as internally, to the xenacanthodiid cephalic spines that they actually regarded it as xenacanthodiid-either Xenacanthus (a synonymy brought about by Traquair's work) or Anodontacanthus sensu lato (including Platyacanthus). Although the evidence pres- ently available does not in my opinion fully warrant taxonomic assignment to the Xena- canthodii, future discoveries may well prove it correct. At any rate, for the Xenacanthodii not only spines but entire fish are known, and there can thus be no doubt about the existence of two well defined genera, Xenacanthus and Orthacanthus.

The Xenacanthus spines are always straight, whereas in the majority of Orthacanthus species they curve gently posteriorly through most of the length. The length:width ratio varies between 12:1 and 20:1 in both genera. In Or- thacanthus species they curve gently posteriorly through most of the length. In Ortha- canthus the cross section is oval-to-circular and in some spines the distal portion may be higher than wide. In Xenacanthus the cross section is flattened antero-posteriorly and has the tendency to becoming lenticular. The pulp cavity is present throughout the length of the spines and its cross section follows that of the

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spines; in some Xenacanthus spines it is dis- tinctly lenticular. In both genera, a paired row of proximad pointing denticles is present lat- erally, posterolaterally, or posteriorly in the distal one-third to one-half of the spines. The denticles of the left and right rows may form pairs or may alternate in either genus. In re- lation to the shape of the cross section, the denticle rows of the Orthacanthus spines are situated on the posterior surface, and consequently are closely spaced, and the denticles face posteriorly. In Xenacanthus, on the other hand, the denticle rows are situated on the lateral walls, which results in wider spacing of the rows and in the denticles' facing either laterad or only slightly posterolaterad. In ad- dition to the denticle rows, the ornamentation consists of longitudinal striae and/or minute pits in either genus. In Xenacanthus a median groove is invariably present in the proximal half, or in some species nearly throughout the length, of the posterior surface. In Orthacan- thus the posterior median groove is less prominent, and it may be altogether absent in some species. The pulp cavity opens at the very end of the base, which is rounded in both genera. In Orthacanthus the opening is circular or oval and causes no excavation in the posterior surface of the base (cf. Fritsch, 1889, P1. 86, fig. 1; Davis, 1892, P1. 72, fig. 2). InXenacanthus, due to the antero-posteriorly flattened cross section and the prominent posterior median groove, the opening has the shape of a distad pointing triangle with oblique base. In some species the triangle is low, and therefore causes but a slight excavation in the posterior face (cf. Fritsch, 1890, P1. 91, fig. 2); in other species it is of a more substantial height and may cause an excavation amounting to as much as one fifth of the total spine's length (cf. Davis, 1892, P1. 72, fig. 12).

In summary, the criteria for distinguishing Xenacanthus from Orthacanthus are: 1) straightness versus posterior curvature, 2) cross section, 3) positioning of the denticle rows and the subsequent orientation of the denticles, and 4) shape of the opening of the pulp cavity. In comparison to that, the distinction between Anodontacanthus and Platy- acanthus is based on: 1) straightness versus posterior curvature, 2) cross section, 3) absence versus presence of the (postero-) lateral ridges, and 4) overall form (outline). 1) is not always valid for Orthacanthus. 2) and 3) are

interrelated in both the sets, because the shape of the cross section determines the position of the denticle rows as well as the presence or absence of the (postero-) lateral ridges, or con- versely. The shape of the opening of the pulp cavity is known for only one Platyacanthus species and remains unknown for all the An- odontacanthus species. Therefore, it cannot be used as a criterion, and is substituted for by the overall form 4). The overall form (outline) can be used for distinguishing Anodontacan- thus from Xenacanthus and/or Orthacanthus only if the length:width ratio is taken into ac- count, because in all these genera the tapering toward the tip results in either linear or slightly convex outline (in anterior and/or posterior views). In Platyacanthus, however, the tapering is uneven due to the longitudinally changing cross section (and the subsequent presence of the ridges), which in anterior and/or posterior views results in a characteristic, proximally convex and further distad concave outline.

It is thus apparent that the Anodontacan- thus and Platyacanthus spines are at least as distinct from one another (and with regard to the third criterion even more distinct) as are those of Xenacanthus and Orthacanthus, and that neither Anodontacanthus nor Platyacan- thus can be regarded as possibly synonymous with Xenacanthus and/or Orthacanthus. This is evidenced by the total lack of denticulation in both Anodontacanthus and Platyacanthus spines which show only a minor degree of abrasion; by the impossibility of comparing the (postero-) lateral ridges of Platyacanthus to worn-off denticle rows in Xenacanthus and/ or Orthacanthus spines because the ridges show no separate denticle bases, and, more- over, are located in the wrong place-in the middle third of the spine's length instead of in the distal third or half; by the differing length:width ratios; and also by the different extent of curvature of the Platyacanthus and Orthacanthus spines.

A CEPHALIC SPINE WITH A BULBOUS BASE

Text-fig. 3 Material studied.-OUSM 00430, a distally

incomplete spine. Locality.-Waurika #1 site of Olson (1967),

SW1/4 NW/4 Sec. 36, T4S, R9W, Jefferson County, southern Oklahoma.

Stratigraphic occurrence.-Approximately

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D

A I lcm B tpc TEXT-FIG. 3-OUSM 00430, a cephalic spine with a bulbous base, possibly of xenacanthodiid derivation.

Oscar Formation, Gearyan Series, Upper Pennsylvanian, Waurika #1 site of Olson (1967), Jefferson County, southern Oklahoma. A, anterior view. B, posterior view. C, lateral view of base. D, trans- verse section through area indicated in B. pc-pulp cavity, vf-vascular foramina, tpc-terminal cap- ping of pulp cavity.

50 feet below the Ryan Sandstone, Oscar For- mation, Gearyan Series, Upper Pennsylva- nian.

Description.-The specimen is 56 mm long and comprises the proximal one-half to two- thirds of a spine, the base of which is expanded into an anteriorly projecting bulb. The base was found fractured and its parts were ce- mented together with a water-soluble glue. The glue has been dissolved in order to gain access to the pulp cavity and to remove the smallest fragment (Text-fig. 3B) for sectioning; the parts have since been reassembled.

An oval depression in the posterior surface of the base corresponds precisely to the position and extent of the pulp cavity. However, instead of representing the cavity's basal opening, the depression is covered with the same hard tissue as the rest of the base and contains two principal, plus several accessory, vascular foramina. It is likely that the depression is the result of collapsing of the thin, terminal cap- ping of the pulp cavity during fossilization. Above the base, the exserted part preserved is

straight and is ornamented with fine, closely spaced longitudinal striae and minute pits. Its cross section is nearly circular at the distal end and elliptical (antero-posteriorly flattened) in the proximity of the base. The cross section of the pulp cavity is circular throughout the specimen's length. The cavity takes about 30% of the cross-sectional area both proximally and distally and is situated somewhat off the cen- ter, toward the posterior surface. There is no posterior median groove and no indication of denticulation of any sort, although denticle rows may have been present in the missing, distal part of the spine. No sectioning of the part above the base has been done, but ex- amination of the polished distal end shows the internal structure to be indistinguishable from that of the xenacanthodiid spines. The sec- tioned fragment of the base (Text-fig. 3D), although small, comprises the entire thickness of the posterior wall cut transversely from the surface down to the lining of the pulp cavity. The majority of the vascular canals are small and are separated by extensive intervascular

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JIRI ZIDEK

trabecles containing bone cell-spaces. The concentric lamellae lining the vascular canals are pierced by dentinal tubes and are, therefore, denteons (0rvig, 1967, p. 57). The tissue thus is a relatively compact osteodentine that in the outer third of its thickness exhibits a laminar arrangement of long and narrow trabecles alligned parallel with the surface and enclosing canals of flattened cross sections.

Discussion.-Should only the exserted part be found, it would doubtless be identified as a xenacanthodiid spine fragment that due to the shape of its cross section and the absence of a posterior median groove would be tenta- tively assignable to the genus Orthacanthus, and there would be no reason for placing such a fragmentary specimen on the record. The presence of the bulbous base, however, makes this spine highly unusual and its taxonomic assignment equivocal. To my knowledge, a similarly expanded base has heretofore been recorded only for the Lower Devonian ich- thyodorulite Bulbocanthus, which, however, is in all other respects quite different from the spine in question, and whose chondrichthyan derivation is highly doubtful (Bryant, 1932, p. 253, P1. 6, fig. 2; D. Baird, written comm., February, 1977). Bulges of hard tissue at the bases of unquestionably chondrichthyan spines are thus known only from the xenacanthodiid sharks (Fritsch, 1889, P1. 85, fig. 20; 1890, Text-fig. 213, P1. 97; Davis, 1892, P1. 72, fig. 1), but in all the instances these consist of ir- regularly shaped chunks of calcified cartilage with clearly apparent prisms. Consequently, these bulges do not form integral parts of the spines but merely surround their bases, which are unexpanded and house wide open terminations of the pulp cavities. In contrast, in the spine in question the basal termination of the pulp cavity is capped and is perforated by vascular foramina, and the basal expansion is an integral part of the spine. The equivocal aspect of the taxonomic assignment of the spine to the Xenacanthodii is not due to any diffi- culty in placing its base in contact with the braincase, but rather due to the fact that the hitherto recorded xenacanthodiid spines possess unexpanded bases with wide open terminations of the pulp cavities, and are therefore in discord with the one in question.

REFERENCES

Bryant, W. L. 1932. Lower Devonian fishes of Bear Tooth Butte, Wyoming. Am. Philos. Soc. Proc. 71:225-254.

Davis, J. W. 1881. On Anodontacanthus, a new genus of fossil fishes from the Coal Measures; with descriptions of three new species. Geol. Soc. Lond. Q. J. 37:427-429.

. 1892. On the fossil fish-remains of the Coal Measures of the British Islands, pt. 1, Pleura- canthidae. Roy. Dublin Soc. Sci. Trans. (ser. 2) 4:703-748.

Fritsch, A. 1889. Fauna der Gaskohle und der Kalksteine der Permformation Bohmens, v. 2, no. 4. F. Rivnac, Prag, p. 93-114.

.1890. Fauna der Gaskohle und der Kalk- steine der Permformation B6hmens, v. 3, no. 1. F. Rivnac, Prag, p. 3-48.

Glikman, L. S. 1964. Subclass Elasmobranchii, p. 196-237. In Yu. A. Orlov (ed.), Fundamentals of Paleontology, v. 10, Agnatha, Pisces. Acad. Sci. USSR, Publ. House "Nauka," Moscow. (In Russian)

Hussakof, L. 1911. The Permian fishes of North America, p. 155-175. In E. C. Case, Revision of the Amphibia and Pisces of North America. Car- negie Inst. Wash. Publ. 146.

Neave, S. A. 1940. Nomenclator zoologicus, v. 3, M-P. Richard Clay & Co., Bungay, Suffolk, p. 1-1065.

Olson, E. C. 1967. Early Permian vertebrates of Oklahoma. Okla. Geol. Surv. Circ. 74:1-111.

0rvig, T. 1967. Phylogeny of tooth tissues: evo- lution of some calcified tissues in early vertebrates, p. 45-110. In A. E. W. Miles (ed.), Struc- tural and Chemical Organization of Teeth, v. 1. Academic Press, New York and London.

Romer, A. S. 1942. Notes on certain Paleozoic fishes. Am. J. Sci. 240:216-228.

.1966. Vertebrate paleontology, 3d edition. Univ. Chicago Press, p. 1-468.

Simpson, L. C. 1976. Paleontology of the Garber Formation (Lower Permian), Tillman County, Oklahoma. Unpubl. M. S. thesis, Univ. Okla., p. 1-216.

Traquair, R. H. 1888. Further notes on Carbon- iferous Selachii. Geol. Mag. (ser. 3) 5:101-104.

Woodward, A. S. 1889. Catalogue of the fossil fishes in the British Museum (Natural History), pt. 1. Taylor & Francis, London, p. 1-474.

Zidek, J. 1972. Oklahoma paleoichtyology, pt. 1, a review and commentary. Okla. Geol. Notes 32:171-187.

MANUSCRIPT RECEIVED JUNE 2, 1977 REVISED MANUSCRIPT RECEIVED NOVEMBER 3, 1977

The School of Geology and Geophysics, The University of Oklahoma contributed $200 in sup- port of this article.

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New Chondrichthyan Spines from the Late Paleozoic of Oklahoma