Eosphargis gigas (Owen) Lydekker 1889a

Robert & Weems, E., 2014, Paleogene chelonians from Maryland and Virginia, PaleoBios 31 (1), pp. 1-32 : 15-17

cf. Eosphargis gigas (Owen) Lydekker 1889a

( Figs. 11A–G)

Specimens —CMM-V-4777, a fragmentary nuchal element; CMM-V-4778, two fragmentary neurals, largely complete right hyoplastron, and proximal end of coracoid; all found by Gary Grimsley.

Locality, horizon, and age — All specimens were found near the base of the bluff on the Potomac River immediately west of Loyola Retreat , 1.7 mile north of Popes Creek in southern Charles County, Maryland; lower part of the Woodstock Member (Woodstock A) of the Nanjemoy Formation; early Eocene (Ypresian, nannofossil zone NP12) .

Description —Nuchal deeply indented, with a pronounced knob on its ventral surface for attachment to the top of the neural spine of the first dorsal vertebra. Neural thin and wide, with a low but well developed midline ridge and a beveled lateral margin bearing only faint indications of suturing to the adjacent costals, peripherals about as long as wide and roughly square-shaped with no sutural contact with the costals or any plastral element.

Remarks —These four bones belonged to a very large turtle with a much reduced carapace. The deep emargination of the nuchal, the medially ridged neurals that are wider than long, and the faint indication of a sutural attachment between the neurals and costals all indicate that this was a dermochelyid turtle. At one time two species of Eosphargis ( Eosphargis gigas and E. breineri ) were reported from the early Eocene of the North Atlantic region ( Nielsen1959), but the stratigraphic horizon of E. breineri later was revised from early Eocene to latest Paleocene ( Bonde 1987). Therefore only E. gigas , described by Owen (1880) from the early Eocene London Clay of England and also reported from the early Eocene of Belgium, presently represents the Dermochelyidae in the North Atlantic region in the early Eocene. A frontal from a dermochelyid skull, found in the Potapaco Member (Potapaco B) of the Nanjemoy Formation, was assigned by Weems (1999) to cf. E. gigas because it was fully comparable with that species but not really diagnostic. The remains reported here also are fully comparable with E. gigas but similarly are inadequate to be diagnostic. In the absence of any other described early Eocene dermochelyid species in the North Atlantic region, all of this material is provisionally assigned to E. gigas until such time as any other early Eocene species is recognized in the North Atlantic region.

In the early Eocene of the North Atlantic region, no dermal ossicles have been found anywhere that could be assigned to one of the more advanced dermochelyid genera that had an epithecal shell mosaic (e.g., Arabemys Tong, Buffetaut, Thomas, Roger, Halawani, Memesh, and Lebret 1999 , Cosmochelys Andrews 1919 , Psephophorus von Meyer 1847 or Dermochelys de Blainville 1816 ). The oldest reported dermochelyid dermal plates in the southeastern United States are from the late Eocene Clinchfield Formation of Georgia ( Parmley et al. 2006), but still older dermal plates have been found in the middle Eocene Cross Member of the Tupelo Bay Formation of South Carolina (material observed by author in the Charleston Museum collections). The oldest dermochelyid dermal plates from England are from the middle Eocene of Sussex ( Lydekker 1889b), which is comparable in age to the oldest ossicles known from the southeastern United States. In the southern hemisphere, Albright et al. (2003) reported rather similar dermochelyid ossicles from Antarctica that might be as old as early Eocene but more probably are also middle Eocene in age. The only documented occurrence of dermochelyid ossicles older than middle Eocene are ossicles assigned to Arabemys crassicutata Tong et al. (1999) . These were reported as being late Paleocene to early Eocene in age, but this age range is the maximum permissible. Based on the ranges of some of the co-occurring mollusks and the earliest known first occurrence of the teleost fish Eotrigonodon Weiler 1929 ( Thomas et al. 1999), the most likely age is early Eocene. Even so, this still is the oldest known occurrence of dermochelyid ossicles, and these are distinctly more primitive in their morphology and osteology than dermal ossicles found in the middle Eocene. It therefore seems that the dermochelyid epithecal shell mosaic first evolved in the Arabian region of the Tethys Seaway but its bearers did not spread into the North Atlantic Ocean Basin and elsewhere until the middle Eocene.

Description —Hyoplastron and hypoplastron of small size (combined length about 4.5 cm) with a prominent low knob developed on each that lies away from the proximal end of each bone about one-fourth the distance toward the distal end. No sutural border developed medially or laterally; the mid-line fontanelle expands in its central region into a sub-rounded vacuity. Suture line on hypoplastron indicates xiphiplastra were narrow and located close to the midline. Hyoplastron and hypoplastron constricted antero-posteriorly at their mid-length to form a saddleshaped concavity both anteriorly and posteriorly.

Remarks —The prominent low knobs on the external surface of the hyoplastron and hypoplastron are a distinctive characteristic that readily characterize this material as belonging to a lophochelyine pancheloniid. Lophochelyines were an abundant and diverse group of marine turtles