Aspideretoides virginianus ( Clark 1895 ) Robert & Weems, 2014

Robert & Weems, E., 2014, Paleogene chelonians from Maryland and Virginia, PaleoBios 31 (1), pp. 1-32 : 12-14

Aspideretoides virginianus ( Clark 1895) n. comb.

( Figs. 9A–F, 10)

Synonymy — Trionyx virginianus Clark 1895 , Amyda

virginiana ( Lynn 1929), Aspideretes virginianus ( Weems 1988) .

Type species — Aspideretoides virginianus ( Clark 1895) .

Diagnosis –Costal plates up to 26 mm thick; sculpture consists of ridges and grooves oriented at right angles to the sutural borders; a few to many ridges cross the grooves, breaking them up into circular and elongate pits up to 5 mm in diameter (after Hay 1908).

Expanded diagnosis —Carapace elliptical in shape, longer than wide, adult specimens over 60 cm in length; neurals and costals very thick with a ridge and groove sculpture pattern oriented generally at right angles to sutural boundaries and generally broken up by ridges crossing the grooves into circular to elongate pits; preneural present. Plastron strongly reduced; xiphiplastron similar in shape to that of Aspideretoides foveatus but less elongated anteroposteriorly, posterior border concave rather than convex as in A. foveatus , and much larger.

Holotype — USNM-9354 , fragments of two costal elements.

Hypodigm — USNM- 11944, a largely complete carapace illustrated in Weems (1988); CMM-V- 4756, a partial carapace found by Peter Kranz; CMM-V- 4768, largely complete right xiphiplastron found by Gary Grimsley; CMM-V- 4766, proximal portion of scapula found by Thomas G. Gibson.

Locality, horizon, and age —The type material came from the high bluffs between Potomac Creek and Aquia Creek along the southern shore of the Potomac River in Stafford County, Virginia, as did the nearly complete carapace described in Lynn (1929) and Weems (1988). The scapula fragment was found by Thomas G. Gibson in “Zone 4” of the Aquia Formation in these same bluffs. The right xiphiplastron was found in the Blue Banks south of Liverpool Point on the east bank of the Potomac River in “Zone 2” of the Aquia Formation, Charles County, Maryland. All of these specimens came from the lower part of the Piscataway Member of the Aquia Formation; late Paleocene (early Thanetian).

Remarks —Clark’s “ Trionyx ” virginianus was referred by Weems (1988) to the genus Aspideretes because the carapace bears a large preneural element, which is present in that genus and not in the genus Trionyx . Since then, however, it has become apparent that the presence of a preneural is a primitive feature among the Trionychidae that has persisted in several quite different lineages, and that the genus name Aspideretes is applicable only to some Asian trionychines ( Meylan 1987, Vitek 2012). Gardner et al. (1995) have erected the genus Aspideretoides to include Late Cretaceous to Early Cenozoic western American trionychine species that retain a preneural and also have similar and distinctive plastra. As discussed below, the discovery of a “ T.” virginianus xiphiplastron that is very similar now makes it appropriate to assign this species to Aspideretoides .

The referred xiphiplastron clearly pertains to Aspideretoides virginianus because (1) it comes from an animal too large to be the enigmatic basal Aquia or pre-Aquia trionychid “ Trionyx ” halophilus, (2) its ridge and trough sculpture pattern is like that of A. virginianus and unlike the pitted pattern of “ T.” halophilus, and (3) this xiphiplastron was found at a locality that has produced numerous carapace examples of “ A.” virginianus and no examples of “ T.” halophilus. For all of these reasons, this xiphiplastron can be confidently assigned to A. virginianus . Its discovery for the first time allows comparison with the plastra of other trionychids known from the Late Cretaceous and Early Cenozoic of North America.

North American Late Cretaceous and Cenozoic trionychids include representatives of both the subfamilies Plastomeninae Hay 1902 and Trionychinae. Most plastomenines have a preneural, but they also have a plastron that is much less reduced than in this species and most other trionychines; therefore referral to Plastomeninae is not indicated. Among Paleocene and Eocene trionychines, Apalone and Oliveremys differ from A. virginianus in that they both lack a preneural element. Axestemys is similar to A. virginianus in that it retains a preneural and attains a large size (greater than 60 cm), but it shows no close similarity in its xiphiplastra. The right xiphiplastron discussed here bears considerable resemblance to the xiphiplastra of Aspideretoides ( Fig. 10), so reference of the species virginianus to the genus Aspideretoides is indicated. The detailed morphology of this xiphiplastron, however, is not identical to that of any known species of Aspideretoides , so the species name remains valid.

Although the holotype of A. virginianus is very fragmentary and not directly diagnosable, the supplementary material discussed here and in Weems (1988) does provide adequate information to allow a unique diagnosis. The large size of the supplementary xiphiplastron and the large size and exceptional thickness of the supplementary carapace specimens indicate that all of this material pertains to a very large trionychine turtle comparable in size to the turtle from which the holotype specimens came. The only other trionychid turtle that possibly has ever been found in the Aquia Formation is “ Trionyx ” halophilus which, as discussed above, more probably is only present in the basalmost Aquia Formation as material reworked from the immediately underlying Brightseat Formation. Even if “ T.” halophilus did survive into the base of the Aquia Formation, however, it is a much smaller species than A. virginianus and has a distinctly different surface sculpture. Considering that all of the material assigned here to A. virginianus was found within a very restricted stratigraphic interval (lower Piscataway Member of the Aquia Formation) in outcrops less than ten miles apart, there is no reason to doubt that all of the large, thick-shelled trionychid specimens from the Piscataway Member of the Aquia Formation represent a single species.

The xiphiplastron of A. virginianus has a healed fracture which is discernable near its external and internal margins (shown in Fig. 10) but untraceable across its central region. This indicates that the xiphiplastron was fractured when the turtle was young, but the animal survived and its fracture healed almost entirely before the animal died.