Euclastes roundsi ( Weems 1988 )

Robert & Weems, E., 2014, Paleogene chelonians from Maryland and Virginia, PaleoBios 31 (1), pp. 1-32 : 21

Euclastes roundsi ( Weems 1988)

( Figs. 14 D-H, 15)

Synonymy – Osteopygis roundsi (partem) Weems 1988,

Osteopygis wielandi (partem) Lynch & Parham 2003,

Osteopygis wielandi (partem) Parham & Pyenson 2010.

Holotype – USNM 412108 , crushed and slightly exploded skull imbedded in a nodule.

Referred specimen –Cast of largely complete skull ( CMM-V- 4825); original in the collection of Mel Gulotta who found the specimen.

Locality, horizon, and age – Holotype found in a bluff along the Pamunkey River upriver from U.S. Highway 301, Hanover County, Virginia; basal Piscataway Member of the Aquia Formation ; late Paleocene (early Thanetian). Referred specimen found in the Blue Banks south of Liverpool Point , Charles County , Maryland; “Zone 2,” Piscataway Member , Aquia Formation ( Clark and Martin 1901); late Paleocene (early Thanetian).

Description –Cranium slightly longer than wide, snout narrow but rounded and not elongate. Dorsally, parietals cover nearly half of the skull roof; supraoccipital does not extend beyond the parietals as far rearward as the squamosal horns. Ventrally, a secondary palate is well developed, with both narrow premaxillaries and vomer elongated back to half way across the suborbital vacuities and with maxillary-palatine flanges extending even farther back to either side and terminating in a small contribution to the palate by the pterygoids.

Remarks –The species Osteopygis roundsi was described by Weems (1988) and assigned to the genus Osteopygis . Since then, Parham (2005) has established that the concept of O. emarginatus had become a chimera with a pancheloniid skull improperly associated with a “macrobaenid” shell and skeleton. He concluded that the postcranial material should remain O. emarginatus but that the skull should be assigned to Euclastes wielandi . Thus, the holotype skull of “ O. ” roundsi was referred to Euclastes ( Lynch and Parham 2003) . The paratype postcranial shell elements of E. roundsi now are referred to Judithemys kranzi n.sp. The excellent skull cast of E. roundsi illustrated here ( Figs. 14 D-H) was made by Billy Palmer for the Calvert Marine Museum. The suture boundaries of the skull elements can be readily seen on both the original and the cast ( Figs. 15A, B). This specimen, by far the best example of this species yet found, differs considerably from the holotype ( Fig. 15E) in that the nasals and vomer of the new specimen are much narrower than in the holotype, the contribution of the pterygoids to the secondary palate is smaller, and the vomer lacks a strong midline ridge. This perhaps could indicate that two durophagous pancheloniid species are present in the Aquia, but both skulls are almost identical in absolute size and ecologically it seems very unlikely for two closely related species to be present in the same area at essentially the same time. With only two skulls of E. roundsi available for study, it is impossible to determine the range of individual variability that existed in the secondary palates of these turtles. However, it is significant that Zangerl (1971) described considerable differences in skulls of the closely related turtle Erquelinnesia gosseleti ( Dollo 1886) and suggested that these differences may reflect sexual dimorphism similar to that seen in the modern alligator snapping turtle ( Macrochelys temmincki (Troost in Harlan 1835). Therefore it seems best to consider the two available skulls of E. roundsi from the Aquia Formation as examples of a single species with a markedly variable palate region, possibly due to sexual dimorphism but otherwise with the same skull morphology. The secondary palate of both specimens of E. roundsi are much longer than the secondary palate of E. wielandi ( Fig. 15), and the premaxillaries and vomer also are much longer and narrower than in E. wielandi . These consistent and marked differences between the length of the palate region and constituent bones of E. wielandi and E. roundsi refutes the suggestion by Lynch and Parham (2003) and Parham and Pyenson (2010) that these two species are synonymous, though in all likelihood E. roundsi descended from E. wielandi . The secondary palate of the slightly younger (late Thanetian) species E. platyops Cope 1867 is much wider than in E. roundsi , and E. platyops also seems to have attained a much larger size. The palate of E. gosseleti is more similar to E. roundsi in its size and proportions than the palates of the other described species, but a striking difference is that the palatines of E. gosseleti enclose the vomer posteriorly unlike in any of the other species ( Parham 2005, fig. 3). No carapace material referable to Euclastes has been identified so far from the Aquia Formation, but it would presumably be similar to the carapace of the closely related turtle E. gosseleti ( Fig. 16).