Cobelodus aculeatus

Maisey, John G., 2007, The Braincase In Paleozoic Symmoriiform And Cladoselachian Sharks, Bulletin of the American Museum of Natural History 2007 (307), pp. 1-122 : 63-76

COBELODUS ACULEATUS ( Cope, 1894)

MATERIAL EXAMINED: Holotype FMNH UF 576, partial skeleton with most of chondrocranium, jaws, pectoral girdle, and anterior part of vertebral column, plus X-rays (prepared by R. Zangerl); Court Creek, NE of Knoxville, Illinois; Mecca Quarry Shale, Liverpool cyclothem, Des Moines series, Westphalian Upper C, Pennsylvanian. Collected by F. R. Jelliffe, described by Cope (1894) and by Zangerl and Case (1976); FMNH PF3090, anterior portion of articulated skeleton in calcareous concretion, with partially exposed three-dimensional chondrocranium, silicone endocast of chondrocranium, plus X-rays; Hesler Quarry, Wabash Township, Parke County, Indiana (field number HQ 77); probably from the Velpen Limestone above the Mecca Quarry Shale, Westphalian (uncertain stratigraphic level), Pennsylvanian; FMNH PF7324, flattened chondrocranium associated with parts of vertebral column; FMNH PF7345, flattened chondrocranium and visceral arch elements; FMNH PF8011, gastric residue mass with chondrocranium and teeth, plus ‘‘Smoothon’’ peels and X-rays of all specimens; City Wide Rock and Excavation Co. (Hansen Quarry, Quarry 6), on route 370 between Papillion and Bellevue, Sarpy County, Nebraska; Wea Shale, Westerville formation, Kansas City group, Missourian, Westphalian D, Pennsylvanian; FMNH PF7347, excellent partially disarticulated skeleton (male individual according to Zangerl and Case, 1976), plus ‘‘Smoothon’’ peel and X-rays of chondrocranium; Logan Quarry, Parke County, Indiana ( Zangerl and Richardson, 1963); Logan Quarry Shale, Lower Wiley cyclothem (Staunton formation), Des Moines Series, Westphalian Upper C, Pennsylvanian; FMNH PF7472, isolated chondrocranium plus ‘‘Smoothon’’ peel and X-ray; City Wide Rock and Excavation Co. (Iske quarry), River Road from Offutt Airbase, La Platte, Sarpy County, Nebraska; Wea Shale, Westerville formation, Kansas City group, Missourian, Westphalian D, Pennsylvanian. ‘‘Smoothon’’ peels were coated with black graphite and whitened with ammonium chloride to improve resolution.

GENERAL REMARKS: The genus Cobelodus was erected by Zangerl (1973) to distinguish Styptobasis aculeata Cope, 1894 from the type species S. knightiana Cope, 1891 (for an account of the systematic history, see Zangerl and Case, 1976: 108). The genus was differentiated from other symmoriiforms chiefly on the basis of its distinctive dentition with numerous rows of minute teeth, and supposedly by unusual fanglike upper teeth with very small bases and simple pulp cavities. However, these ‘‘upper teeth’’ are not found associated with the palatoquadrates, and more likely represent dermal denticles from the head (M. Ginter, personal commun., 2006) Cobelodus aculeatus is the type species of the genus and is known principally from the Pennsylvanian (Westphalian and Stephanian) black shales of Illinois and Nebraska, although a partial skeleton from the Wild Cow Formation (Madera Group, late Missourian or early Virgilian) of New Mexico has been referred to the same species ( Zidek, 1992; see below).

The braincase of Cobelodus aculeatus was described by Zangerl and Case (1976), based mainly on numerous compression fossils which were studied from X-rays and silicone peels, supplemented by data provided by a single three-dimensional endocast preserved in a concretion. Unfortunately, the cranial anatomy described in that work is so much at variance with the present findings in ‘‘ Cobelodus ’’ that a revision is clearly necessary (particularly since Zangerl firmly believed that these fossils were congeneric; various personal communications with the author). The material collected by him and now deposited in the Field Museum (including his extensive archive of X-ray plates) was therefore examined in order to compare the cranial morphology of C. aculeatus and ‘‘ Cobelodus ’’. The material listed above represents only the most informative specimens forming the basis of the present description.

One of Zangerl’s specimens ( FMNH PF 3090) is uniquely preserved inside a concretion and includes parts of an uncrushed neurocranium, whose endocast has been partially excavated ( Zangerl and Case, 1976: figs. 7–10). That specimen is more readily compared with the ‘‘ Cobelodus ’’ braincase than the almost flat compression fossils.

CONCRETION SPECIMEN: According to Zangerl and Case (1976: 120), the three- dimensional concretion specimen FMNH PF 3090 was discovered only after they had reconstructed the braincase of Cobelodus aculeatus from stereo X-rays of compression fossils. Although they attempted to incorporate new information provided by that specimen into their reconstruction, it is clear in hindsight that they made two crucial errors in their final version: first, they assumed that the braincase was morphologically platybasic (as in all elasmobranchs known at that time); and second, they concluded that the three-dimensional endocast had been split open along the

Fig. 36 .

sagittal plane and did not show any left-right symmetrical features ( Zangerl and Case, 1976: 125). In consequence, they effectively tried to fit a quart into a pint pot by reconstructing the endocast between the orbits and by doubling the number of structures, especially in the otic region (fig. 37). In addition, some features were misidentified in the original description, and several others were left without explanation. A new silicone endocast prepared from Continued.

FMNH PF 3090 provided the basis for a comprehensive revision in which many previously unidentified structures and several bilaterally symmetrical features are recognized.

Once the cranial endocast is oriented correctly, its morphology is more readily established (fig. 38). In many respects, it is remarkably similar to the digitally generated endocast of ‘‘ Cobelodus ’’ and to the Virgilian braincase OUZC 5204 described above (figs. 24–30, 36). Crucially, the specimen has not split through the sagittal plane as Zangerl and Case (1976) suggested; instead, the fracture surface is close to the midline only ventrally, but then passes obliquely through the left side of the braincase farther dorsally. Consequently, considerably more than half of the endocast is present (including the telencephalic, mesencephalic, and diencephalic chambers) and almost two-thirds of its dorsal surface is exposed. The right side is essentially complete, although some features have unfortunately been obliterated during earlier preparation and are seen only on the left side (e.g., the glossopharyngeal canal). Several bilaterally symmetrical structures are evident, including both sides of the large hypophyseal chamber (although its distal extremity is still filled with matrix and cannot be seen), the main exit of the facial nerve, the abducent foramen, and parts of both saccular chambers.

Many features in the endocast are readily identified by comparison with ‘‘ Cobelodus ’’ and OUZC 5204. One obvious landmark is the deep notch formed by the apex of the dorsum sellae (fig. 38A), adjacent to the oculomotor foramen (misidentified as the optic foramen by Zangerl and Case, 1976; in fact, that feature is not preserved). Only the proximal part of the hypophyseal chamber is preserved, so its depth is uncertain (although it must have been as long and narrow as in ‘‘ Cobelodus ’’). Zangerl and Case (1976: 121) remarked that ‘‘the specimen does not exhibit a fossa hypophyseos’’. They attributed this to an artefact of preservation and did not understand the true nature of the specimen, but their observation is nevertheless important because the preserved part of the hypophyseal chamber certainly lacks any communication with the basicranium, as in ‘‘ Cobelodus ’’.

The roof of the hypophyseal chamber merges with the telencephalic region anterodorsally. This is very narrow from side to side (its broken cross section is indicated by a dashed line in fig. 38B). The olfactory tracts presumably diverged farther anteriorly, since there is no trace of them in the endocast but the olfactory capsules are spaced widely apart in X-rays of compression fossils (see below). The roof of the cerebellar chamber is fairly featureless, apart from a small mass of calcified cartilage along the dorsal midline that may represent the position of a pineal foramen. The position of the trochlear foramen (correctly identified by Zangerl and Case, 1976) is evident on the lateral wall of this region, more or less dorsal to the oculomotor foramen. The feature identified by them as the oculomotor foramen is identified here as the combined exit for the trigeminal and anterodorsal lateral line nerves. Farther ventrally, the foramen for the facial hyomandibular trunk lies just in front of the anterior ampulla. The abducent foramen (correctly identified by Zangerl and Case, 1976) is located even farther ventrally, near the base of the dorsum sellae.

Very little of the labyrinth endocast is preserved, apart from low mounds representing the anterior and external ampullae and the utricular recess. However, the position of the anterior semicircular canal is indicated by a deep diagonal groove in the roof of the vestibulolateral chamber. This shows that the canal was only thinly sheathed by cartilage, as in OUZC 5204 and unlike in ‘‘ Cobelodus ’’. The vestibulolateral chamber was not recognized as such by Zangerl and Case (1976: 121), although they correctly determined that it is a paired structure; they simply described it as ‘‘a very pronounced, posteriorly facing pit in the roof of the neurocranial cavity’’, but had ‘‘no idea as to its significance’’. The posterior end of this chamber seems unnatural and may have been slightly overexcavated during preparation.

The dorsal surface of the medullary region is poorly preserved and the area surrounding the fontanelle has been overexcavated during its original preparation. Consequently, some of its features are difficult to interpret. At the dorsal midline, medial to the vestibulolateral chamber there is a rounded projection, corresponding to the ‘‘chimney’’ for the posterior dorsal fontanelle in ‘‘ Cobelodus ’’ (pdf: fig. 38). It would have been flanked by the anterior semicircular canal after they passed over the roof of the vestibulolateral chamber (there is a shallow groove for the right canal in the roof of the vestibulolateral chamber). At the left posteriormost extremity of the endocast, there is a small protuberance that probably represents part of the (largely unexcavated) left posterior semicircular canal (?psc: fig. 38). Unfortunately, the posterior part of the medullary chamber has not been excavated and none of its features are evident. It cannot be determined from the endocast whether a persistent otico-occipital fissure was present, and passages for the vagal and occipital nerves cannot be seen. Most of the left glossopharyngeal canal has been excavated, but the right one is still filled with matrix. The specimen has split longitudinally through the occipital cotylus, which can be seen in sagittal section ( Zangerl and Case, 1976: fig. 8A). Below the cotylus is a continuous unexcavated line of tesselate cartilage, which probably represents the basicranium. The space between this cartilage and the occipital cotylus probably contained an aortic canal, although this cannot be observed. Inspection of the counterpart suggests that parts of the basicranium and interorbital septum are preserved intact, but are still unprepared.

X-RAYS: Zangerl (1965) advocated the use of X-ray stereo images as a means of obtaining anatomical data in strongly com- pressed fossils, and much of his subsequent investigation of Cobelodus aculeatus was based on analysis of X-ray stereopairs ( Zangerl and Case, 1976). Indeed, many of the best compression fossils of C. aculeatus are unprepared and can be investigated only from X-ray plates (including FMNH PF 7347, on which the original reconstruction was mainly based; fig. 39). Some specimens have been prepared mechanically to remove calcified cartilage leaving natural impressions, and silicone casts of these specimens are informative (see following section). The fossils have clearly suffered considerable diagenetic compression, which can be mimicked by reducing the y-axis dimension of the digital ‘‘ Cobelodus ’’ contour-based surface rendering by 90–95%, to create an orthographic ‘‘biscuit’’ that closely resembles the compression fossils (fig. 40). Assuming that the braincase in C. aculeatus was originally as deep as in ‘‘ Cobelodus ’’, only 5–10% of the original depth is available for stereographic analysis of the X-rays. Despite this limitation, Zangerl and Case (1976) probably established the original depth of the neurocranium fairly accurately.

Stereo X-rays of the neurocranium in Cobelodus aculeatus have been reinterpreted here using the three-dimensional ‘‘ Cobelodus ’’ braincase as a guide. The braincase in Cobelodus aculeatus is wide and blunt anteriorly, with a short ethmoidal region that includes a subtriangular precerebral fontanelle (widest anteriorly), flanked by the olfactory capsules. Most of the ethmoidal region is lightly calcified and transparent to X-rays. More radio-opaque regions (which are strewn with minute pyrite crystals) include the margin of the precerebral fontanelle and parts of the olfactory capsule walls (especially laterally). Mesial and slightly posterior to the capsules are paired pyrite-lined channels. These are bilaterally symmetrical and appear to be real anatomical structures, possibly marking the course of the orbitonasal canal through the postnasal wall.

Immediately behind the ethmoidal region in Cobelodus aculeatus X-rays is an extensive radio-opaque central area extending back behind the orbits. This area consists of two distinct regions, but one is located directly above the other and they can be distinguished only in stereo images. The larger dorsal area extends from the postnasal wall, narrowing and then broadening again within the orbit and eventually merging with the upper part of the postorbital arcade. This is interpreted as the main cranial endocast, since it is apparently restricted to the upper part of the neurocranium as in ‘‘ Cobelodus ’’. The more ventral area runs posteriorly from the anterior part of the orbit and widens slightly toward the ventral part of the postorbital arcade, and probably represents the hypophyseal fenestra. This region is flanked by the suborbital shelf, which is narrower than in ‘‘ Cobelodus ’’ although its anteroposterior extent is about the same.

X-rays of Cobelodus aculeatus reveal a row of pyrite crystals along the margins of the supraorbital shelves, following a symmetrical course above both orbits. The row passes almost straight back from the ethmoid region, then makes a distinct midorbital inward flexure around a notch in the supraorbital shelf (possibly for the dorsal part of the eyeball) before continuing posterolaterally toward the lateral part of the postorbital arcade. A similar notch is present in the supraorbital shelf of ‘‘Stethacanthulus’’ meccaensis ( Williams, 1985: plate 2, fig. 1; see below). There is no evidence of a pineal foramen in C. aculeatus .

As in ‘‘ Cobelodus ’’, the postorbital process in Cobelodus aculeatus is delicate, narrow, and directed posterolaterally. The lateral commissure was chondrified, forming a continuous postorbital arcade around a voluminous jugular canal and bearing an articular surface for the palatoquadrate otic process ventrolaterally.

In X-rays, some parts of the otic and occipital regions are much clearer than others (fig. 39). Faint traces of a median aortic canal are evident in the floor of the braincase beneath the occipital cotylus, and can be traced anteriorly between the otic capsules. Behind the postorbital arcade, this canal apparently branches into short, paired canals that open into elongated areas corresponding to the basicranial foramina observed in peels of other compression fossils (see below). However, it would be extremely difficult to interpret the aortic canal in these compression fossils without prior knowledge of the arrangement in ‘‘ Cobelodus ’’. The semicircular canals are difficult to follow in X-rays, as only those parts that are lined by pyrite crystals are seen clearly. Comparison with ‘‘ Cobelodus ’’ suggests that the central region of the medullary chamber has collapsed onto underlying structures, so that very little is discernible apart from a small triangular area in the expected position of the posterior dorsal fontanelle. A periotic process is faintly visible on the widest part of the otic capsule, but is partly obscured by the postorbital arcade, which has collapsed over it.

The occipital region is generally well preserved and has resisted compaction better than the otic capsules. Paired dorsal and ventral paroccipital processes are evident surrounding the occipital cotylus. The latter is as narrow from side to side as in ‘‘ Cobelodus ’’, not expanded laterally as in Cladodoides , Tamiobatis , and Orthacanthus . The occipital arch is short, and is separated at least posterolaterally by a narrow clear band just behind the posterior semicircular canal, probably representing a narrow oticooccipital fissure. A broad opening is present on each side of the occipital region, probably representing the glossopharyngeal canal. No canals for occipital nerves are discernible.

COMPRESSION SPECIMENS: The braincase in all the compression fossils of Cobelodus aculeatus examined was invariably crushed dorsoventrally. Its appearance in lateral view is therefore conjectural, although the original depth of the orbit can be estimated from associated elements (e.g., depth of the palatoquadrate otic process).

The ‘‘ Cobelodus biscuit’’ (fig. 40) does not allow for differential diagenetic compaction and consequently lacks certain features that commonly show up in the compression fossils. Taphonomic artefacts formed by diagenetic compression are so common in compression fossils of Cobelodus aculeatus that some have been misinterpreted as anatomical features. For example, curved ridges extending posterolaterally from the posteriormost paired basicranial foramina were thought to mark the position of paired internal carotids ( Schaeffer, 1981: fig. 13B). These ridges are interpreted here as the borders of paired collapse structures, where thinner parts of the basicranium associated with the glossopharyngeal canals, orbital arteries, and palatine rami became crushed against resistant internal structures such as the occipital cartilage and dorsum sellae, forming shallow craters on each side of the ventral midline (fig. 41). The same pattern of internal features is revealed when a clipping plane is introduced into the ‘‘ Cobelodus ’’ surface rendering to remove part of its ventral surface (fig. 42), as well as in orthographic three-slice mode views of the CT scan (fig. 43). Taphonomic artefacts may also have affected the interpretation of the braincase in Ornithoprion presented by Zangerl (1966).

Comparison of Cobelodus aculeatus compression fossils and the ‘‘ Cobelodus ’’ surface rendering suggests that the shape of the basicranium behind the postorbital arcade is very similar, and some of the compression specimens show a distinct ‘‘waist’’ where the arcade merges with the margins of basicranium (e.g., FMNH PF 8011; fig. 44). The area lateral to this ‘‘waist’’ probably represents a flattened lateral otic fossa in the outer capsular wall.

Most of the compression fossils examined are broken at the level of the postorbital arcade, so canals and foramina farther anteriorly are usually missing. A narrow suborbital shelf is discernible in the peel of FMNH PF 7345 (fig. 41) as well as in the Xray of FMNH PF 7347 (fig. 39), although in both cases it is narrower than in ‘‘ Cobelodus ’’ (cf. fig. 8). In the peel of FMNH PF 7345 there is an uneven area of cartilage in the expected position of the basicranial fenestra, but no features could be identified with certainty. Although the presence of an interorbital septum cannot be confirmed from the compression specimens, their interorbital region is clearly too narrow to accommodate the endocast in the manner suggested by Zangerl and Case (1976: fig. 7). As in ‘‘ Cobelodus ’’, the ventral ramus of the postorbital arcade is short anteroposteriorly but projects a considerable distance posterolaterally (fig. 37D). At the distal end of this arcade is a transverse articular surface for the palatoquadrate, corresponding to the ridge and groove in ‘‘ Cobelodus ’’.

The paired foramina within the anterior paired collapse structures are considered to be real morphological features rather than taphonomic artefacts because they match foramina in the ‘‘ Cobelodus ’’ braincase. Zangerl and Case (1976: 117) described a single pair of foramina, more or less posteriorly directed with ‘‘ very short canals behind them which converge and appear to meet a short distance behind the foramina.’’ They suggested that the larger paired collapse structures represented passages for the internal carotids and concluded that the pair of foramina farther anteriorly housed the orbital arteries. That interpretation is clearly at odds with the arrangement of foramina and canals in ‘‘ Cobelodus ’’ and is probably erroneous (though understandable, since their comparison was limited to platybasic Paleozoic elasmobranchs in which paired aortic canals are present; e.g., Cladodoides wildungensis , Tamiobatis vetustus ; Gross, 1937; Romer, 1964). Schaeffer (1981: fig. 13) presented an alternative interpretation of the basicranial circuit in Cobelodus aculeatus in which the posterior depressions supposedly marked the entrance of the internal carotids rather than their exit, and the (paired) aortae were not enclosed by cartilage. The problem with both hypotheses is that the posterior collapse structures show no evidence of real openings in the cartilage. Furthermore, in specimens in which these collapse structures are weak or absent, there are no foramina behind the pair already noted (e.g., FMNH PF 7324; fig. 45). However, a second pair of foramina is present farther anteriorly within the smaller anterior collapse structure, anterolateral to the main pair. Comparison with ‘‘ Cobelodus ’’ suggests that the paired foramina described by Zangerl and Case (1976) probably housed the aortae shortly after they diverged, and the second pair farther anterolaterally probably housed the orbital arteries as they re-entered the basicranium (lda, ora; figs. 41, 44–46). As in ‘‘ Cobelodus ’’, there are no basicranial grooves for the internal carotids, suggesting that the basicranial circuit in C. aculeatus was similarly specialized (fig. 14).

Many of the compression fossils show a posterior median opening for the dorsal aorta, flanked by paired ventral paroccipital processes. Some specimens also show part of the basioccipital fovea (e.g., FMNH PF 7472; fig. 46). The lateral walls of the otic capsule are usually crushed in compression specimens, splitting along the external semicircu- lar canal, but in the counterpart to FMNH PF 7345 (which provides a rare opportunity to examine the dorsal surface of the braincase) there is evidence of a periotic process like that in ‘‘ Cobelodus ’’ (fig. 47). The supraorbital shelf in this specimen is wider and extends farther anteriorly than in ‘‘ Cobelodus ’’, and there is a notch in the midorbital part of the supraorbital shelf (confirming the earlier observation of X-rays). In most of the compression specimens, prismatic calcification of the otic capsules is usually broken and the individual tesserae can be scattered, possibly explaining why the periotic process is rarely observed.

It is concluded that the general morphology of the braincase in Cobelodus aculeatus and ‘‘ Cobelodus ’’ was probably similar, but diagenetic compaction of compression fossils has generated a suite of taphonomic artefacts (especially in the basicranium), obscuring or even obliterating many features. Crucially, the concretion specimen of C. aculeatus shows many similarities to ‘‘ Cobelodus ’’ in endocast morphology (once its orientation is properly established), showing that there was a deep dorsum sellae and an interorbital septum. X-rays and compression fossils also confirm the presence of a median dorsal aorta and a periotic process. Such unusual features indicate a close phylogenetic relationship and probably represent symmoriiform synapomorphies. Cobelodus aculeatus has a wider supraorbital shelf and a narrower suborbital shelf than ‘‘ Cobelodus ’’, suggesting that they are not conspecific and may not even be congeneric.

A new reconstruction of the braincase in C. aculeatus is presented here in which it is shown as tropibasic (fig. 48), somewhat different from previous interpretations, although Zangerl and Case (1976: 115) deserve credit for noting the presence of a narrow interorbital septum in their preliminary (un- published) reconstruction founded on X-rays and compression fossils. In hindsight, it was only their misinterpretation of the threedimensional endocast discovered later that misled them into thinking C. aculeatus had a much wider interorbital region.

Since the cranial morphology in ‘‘ Cobelodus ’’ and Cobelodus aculeatus was probably very similar, it is likely that much of the cranial cartilage in ‘‘ Cobelodus ’’ is missing anteriorly, which would account for its somewhat squat appearance, the incomplete precerebral fontanelle, and lack of olfactory capsules (figs. 6–8). The supraorbital shelf is wider and better developed in C. aculeatus , and merges gradually with the postorbital arcade. By contrast, the arcade in ‘‘ Cobelodus ’’ is angled sharply outward from the supraorbital shelf, and forms a distinct lateral projection in dorsal view (cf. figs. 7, 48A). In dorsal view, the otic and occipital regions in C. aculeatus and ‘‘ Cobelodus ’’are remarkably similar apart from the slightly more anteriorly positioned periotic process and the slightly longer occipital arch in C. aculeatus . Ventral views of the ‘‘ Cobelodus ’’ and C. aculeatus braincases are also similar (figs. 8, 48B), except that the orbital arteries were more completely enclosed by cartilage in C. aculeatus . Since the lateral view is conjectural in C aculeatus there is little point in comparing it with ‘‘ Cobelodus ’’.

Figures

Fig. 36. Virgilian symmoriiform braincase from Texas, OUZC 5204. A, lateral view of right side; B, anterior view; C, posterior view; D, dorsal view; E, ventral view. Scale bar 5 10 mm.