1232
Journal of Paleontology 91(6):1228–1243
is available at the Mace Brown Museum of Natural History at the College of Charleston.
Expanded description.—One of the two new specimens (CCNHM 893) consists of an associated skull and lower jaws (Figs. 2, 3). Skull fragments associated with the type specimen of P. charlestonensis are quite similar to comparable parts of the skull described here, which also is similar to the skull of the Miocene species P. grandaeva (Weems and Sanders, 2014). In dorsal view, P. charlestonensis has a broad snout but otherwise a fairly standard cheloniid arrangement of skull elements, with orbits facing laterally, parietals much longer than wide, pre- maxillae not fused together, and weak temporal emargination. Sulci marking the borders of the dermal scutes of the skull are not preserved in this specimen. In ventral view, P. charlestonensis has a well-developed secondary palate that completely covers the vomerine pillar but does not extend quite as far back as the anterior borders of the fossae temporalis inferior openings. The antero-lateral borders of the vomer are concave,which causes the anterior end of the vomer to have somewhat restricted contact with the premaxillae. Vomer and premaxillae are nearly equal in length on the secondary palate. The pterygoids have amid-ventral ridge and their processus pterygoideus externus are reduced and have a strongly rounded border. The skull of P. charlestonensis is distinctly different from the skulls of Carolinochelys wilsoni and Ashleychelys palmeri in a number of features (Fig. 4). The snout ofP. charlestonensis (width/length ratio=0.60) is relatively shorter than the snouts of A. palmeri (0.64) and C. wilsoni (0.72). The palate of P. charlestonensis (palate length/snout length ratio=0.86) is relatively longer than the palates of C. wilsoni (0.63) and A. palmeri (0.79). Procolpochelys charlestonensis is like A. palmeri, and unlike C. wilsoni, in that the vomerine ridge is not visible in ventral view, the supraoccipital ridge is thin, the processus pterygoideus externus are well developed, the palatine is a major contributor to the antero-lateral rim of the fossa temporalis inferior, the vomer is as long or longer than the premaxilla on the surface of the secondary palate, and the parietals are longer than they are wide. Procolpochelys charlestonensis is likeC. wilsoni, and unlikeA. palmeri, in that the prefrontal is located on the antero-dorsal rim of the orbit rather than on the dorsalmargin, the tip of the snout is angular rather than rounded, and the pterygoids are very narrow at their most constricted mid-length point. These similarities and differences demonstrate that these are three distinctly different genera of turtles. The well-preserved lower jaws (Fig. 3) allow useful
comparisons with the lower jaws of the other two pancheloniid species in this fauna (Fig. 5). In P. charlestonensis, the high coronoid ridge on the dentary, the expansion of the symphysial region of the lower jaws into a broad triturating surface, and the absence of a strongly upturned tip to the beak all indicate that this animal had a powerful, dominantly crushing bite. The Meckelian grooves on each jaw ramus also are notably short and rounded in dorsal appearance. This extends the crushing lateral edges of the lower jaws far rearward relative to most cheloniid turtles, which resulted in a greatly shortened posterior jaw region. This appears to be an adaptation for increasing the crushing force that could be generated by the jaw musculature. In sharp contrast (Fig. 5), the lower jaws of A. palmeri are
relatively much narrower, the distal tip of the beak is much sharper, and the jaw rami are much straighter and far less robust. These characteristics suggest that A. palmeri was adapted to shearing food rather than crushing it (Parham and Pyenson, 2010). The lower jaw of C. wilsoni is intermediate in its conformation, though somewhat closer to Procolpochelys in terms of its robustness. The second newspecimen ofProcolpochelys charlestonensis
(CCNHM 300.1) includes a nearly complete carapace and plastron (Fig. 6), of which the carapace is quite comparable in its size and overall morphology to the holotype carapace of P. charlestonensis (Fig. 7) except for the much narrower costoperipheral fontanelles in the new specimen. The narrowness of these fontanelles indicates that this animal was much older and moremature than the holotype at the time of its death. Even so, the costoperipheral fontanelles remained open throughout life (Fig. 6, lower right) and did not tend to close up with age, as in C. wilsoni (Fig. 7). The new specimen unfortunately does not preserve the shallow grooves that mark the borders of the dermal scutes that overlay the bones of the carapace, but these are present in the holotype. The rib-free peripheral element in the posterior shell lies between the seventh and eighth costal ribs and not between the sixth and seventh costal ribs. Comparative illustrations of the plastra of the three Oligocene
cheloniid species from South Carolina plus the Miocene species P. grandaeva are shown in Figure 8. Themost notable difference among these is that the central fontanelle of the plastron of C. wilsoni is much narrower than that of either A. palmeri or P. charlestonensis. Although the central fontanelle is about equallywideinboth A. palmeri and P. charlestonensis, the length of the central fontanelle in P. charlestonensis is relatively about twice as long as the central fontanelle in A.
palmeri.The mid-lateral fontanelles are much wider in A. palmeri than they are in the other two genera, so A. palmeri had amore reduced plastron than either C. wilsoni or P. charlestonensis.
Expanded diagnosis.—Large turtlewith a deep skull andwide but angular beak; dorsal and lateral skull surface faintly ornamented by ridges, grooves, or pits in its anterior region; orbits round with prefrontals forming their antero-dorsal borders; frontals form only a small portion of the dorsal orbit border but medially project strongly forward along the midline between the prefrontals; parietals longer than wide; supraoccipital process elongate; ventral surface of skull has an elongate secondary palate with the vomerine ridge covered and thus hidden from view, premaxillae and vomer of nearly equal length on the secondary palate surface; longitudinal ridges present on the palate surface of themaxillae; palatines form the antero-medial border of the fossa temporalis inferior; pterygoids very narrow in their mid-length region, with a promi- nently developed processus pterygoideus externus along the antero-lateral margin of each; planar joint between the sixth and seventh cervicals; carapace moderately convex, up to 110cm in length, markedly longer than wide with the central (neural-costal) portion of the carapace widest across the second costal region; tenth peripheral has no attachment socket to receive the rib of either the seventh or eighth costal; costoperipheral fontanelles wide and persistent throughout life; dorsal surface smooth or only very faintly sculptured; vertebral scutes hexagonal and about as wide as long; juvenile carapace with normal pancheloniid thickness, but
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