Cochilius

MacPhee, R. D. E., 2014, The Serrialis Bone, Interparietals, “ X ” Elements, Entotympanics, And The Composition Of The Notoungulate Caudal Cranium, Bulletin of the American Museum of Natural History 2014 (384), pp. 1-69 : 1-69

Cochilius

Of the three interatherioid skulls submitted for CT scanning ( Cochilius volvens AMNH-VP 29651, Progaleopithecus tournoueri AMNH-VP 29603, and Protypotherium sp. MLP 12-1280), only the first produced good results and therefore comments will be largely limited to this specimen.

Macroscopically, Cochilius volvens AMNH-VP 29651 (figs. 13–15) is solidly constructed and, like other interatherioids (fig. 16), lacks the large piriform fenestra and dorsal midcranial hiatus seen in pachyrukhine hegetotherioids. In volume the epitympanic sinuses are in fact quite large, but the outline of their thecae on the caudal cranium is subdued because of the extreme robustness of the temporal ridges (fig. 13C).

A number of cranial sutures in AMNH-VP 29651 are either open, or, if fused, still identifiable in section by indicia (fig. 14C). On the dorsal surface of the skull, the external suture between the squamous part of the squamosal and the parieto-interparietal is apparent and punctuated by large foramina for rami temporales (fig. 13C). Sutural relationships are as in notoungulates generally, in that there is no evidence of any atypical sutures or subdivisions of bone territories (but see Entotympanics and Septum Bullae). The squamosal and the rostral crus of the ectotympanic are fused, and the sutural line between the bulla and the petrosal has largely disappeared (although its trace is still identifiable; fig. 13A).

Virtual sectioning of the skull (figs. 14, 15) reveals that the thecal covering is separate from all other bone territories except the squamosal, which unequivocally forms most of the theca; and the petrotympanic, which is continuously fused with the thecal wall in a small zone around the aditus and along the medial wall of the notably inflated retrotympanic process (fig. 15B, C). Roth (1903) claimed that the “protuberancia

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promontorium, as in other notoungulates in which this feature has been detected. Note long tongue of tentorial process of supraoccipital (tent pr soc) meeting tentorial process of parietal (tent pr par). Exoccipital and supraoccipital are completely fused, but the division between parietal and interparietal can still be identified.

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in roughly the same place as Roth’s “sutura squamoso-serrialis” (see fig. 13C). Note that fracture intersects the

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very large transverse sinus, also longitudinally positioned. In C (section through caudal portion of epitympanic sinus and retrotympanic process), it appears that rearmost part of petrosal was pneumatized in concert with retrotympanic process; thus, sidewall of epitympanic sinus in this region is probably both squamosal and petrosal in origin. This should be distinguished from conditions in D: here caudal portion of pars canalicularis helps to frame part of posttemporal canal, but trabeculated space (asterisk) seen internally within “mastoid” is a marrow cavity, unconnected to middle ear. Caudalmost part of epitympanic theca appears suspended in space, separated

from more medial parts of skull by relatively enormous posttemporal canal and sulcus for transverse sinus.

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mastoidea” was pneumatized in notoungulates, but this is probably not correct judging from conditions in Cochilius . In this taxon (fig. 15D) the caudal end of pars canalicularis presents large, trabeculated spaces, possibly for hematopoeic tissue, but these lack a connection with either the tympanic cavity or the epitympanic sinus. Unlike Paedotherium (fig. 12), alae are not significant and the petrotympanic has only limited exposure on the caudal aspect of the skull, where it helps to frame the aperture of the posttemporal canal. Protypotherium sp. MLP 12-2780 is apparently similar in these respects (fig. 17B, C).

In summary, the caudal cranium of adult Cochilius bears no evidence of a serrialis or unusual sutural relationships suggestive of additional elements. The epitympanic theca is largely formed by the squamosal, with small contributions from the compound petrotympanic. Because of crushing it is not clear whether the rostral crus of the ectotympanic contributes to the lateral part of the meatal roof. The caudal crus definitely does not, as there is a marked fissure (retrotympanic fissure; fig. 13B) between the two. As far as may be ascertained this last statement applies to other interatherioids as well.