Trionyx, Forskal, 1775

Robert & Weems, E., 2014, Paleogene chelonians from Maryland and Virginia, PaleoBios 31 (1), pp. 1-32 : 11

“ Trionyx ” halophilus Cope 1869a

( Figs. 8L–N)

Specimens —CMM-V-4761, neural element found by Peter Kranz; CMM-V-4760, seventh right costal found by Michael Smigaj.

Locality, horizon, and age — The neural was found at the foot of the bluffs west of Fort Washington on the shore of the Potomac River, Prince Georges County, Maryland. The specimen probably came from the base of the Piscataway Member of the Aquia Formation (late Paleocene, early Thanetian), but it seems likely it was reworked from the Brightseat Formation (early Paleocene, Danian). The costal was found at a site along Central Avenue just east of Washington, D.C., in Prince Georges County , Maryland. The specimen came either from near the base of the Piscataway Member of the Aquia Formation (late Paleocene, early Thanetian) or from the Brightseat Formation (early Paleocene, Danian) .

Description —Neural and costal both have a distinctive surface sculpture composed of rounded shallow pits separated by flat-topped interconnected ridges. Posterior border of costal abutted a relatively small eighth costal.

Remarks —There is little that can be gleaned from these specimens that would allow any detailed identification within the family Trionychidae . The relatively small size of the eighth costal, as indicated by its nested border with the seventh costal, is typical of trionychine turtles but also is found in some cyclanorbines. Therefore, no definitive subfamily placement can be made. The pitting pattern on the external surface of the bones is distinctly different from that of the described Aquia Formation trionychid species “ Trionyx” virginianus Clark 1895 referred below to the genus Aspideretoides Gardner, Russell & Brinkman 1995 so this material pertains to a different taxon of trionychid turtle. The pattern on the surface of these bones is quite similar to that of “ Trionyx ” halophilus ( Cope 1870, Hay 1908), which was described from Maastrichtian strata in Delaware ( Baird and Galton 1981). Aspideretoides species ( Gardner et al. 1995), Axestemys puercensis ( Hay 1908) , and Oliveremys Vitek 2011 have a somewhat similar pattern of pits. Oliveremys is not known to range below the middle Eocene and has not been reported from anywhere in eastern North America, so it is very unlikely that this material would pertain to that genus. Axestemys and Aspideretoides are known to range from the Upper Cretaceous into or through the Paleocene ( Williamson and Lucas 1993, Hutchison and Holroyd 2003, Jasinski et al. 2011), so “ T.” halophilus might pertain to one of these genera.

Unfortunately, the holotype material and the specimens described here are too fragmentary to assign to any genus with certainly, so the matter cannot be resolved. Therefore this material is referred to “ Trionyx ” halophilus for the sake of nomenclatural stability, while recognizing that the type material is not diagnosable and this species is a nomen dubium. It is possible that these specimens represent a turtle that survived into the very beginning of deposition of the Aquia Formation, but the total absence of any similar remains at any horizon above the very base of the Aquia Formation suggests that it is far more likely that these specimens originated in the Brightseat Formation and were reworked into the base of the immediately overlying Aquia Formation.