A REVISION OF THE CROCODILIAN FAUNA OF THE UPPER PALEOCENE AQUIA FORMATION OF VIRGINIA AND MARYLAND

In the past, Eosuchus minor was the only taxon of crocodilian identified to species from the upper Paleocene (Thanetian stage) near-shore marine Aquia Formation, although a second and undescribed larger crocodilian had also been reported. Recent collecting indicates there are more crocodilian taxa present in the Aquia Formation than previously recognized. The aforementioned larger crocodilian species is now tentatively identified as cf. Thoracosaurus clavirostris, the holotype having been originally described from the upper Paleocene (Thanetian) Vincentown Formation of New Jersey. Both E. minor and T. clavirostris are found throughout the Aquia Formation. Additionally, a mandible of a dyrosaur (cf. Hyposaurus sp.) has been found in the lower Aquia (Piscataway Member) as well as teeth of an alligatorid that cannot be assigned as yet to any lower taxonomic level. In the upper Aquia (Paspotansa Member), heavily worn but seemingly ziphodont (laterally compressed and serrated) crocodilian teeth have been identified tentatively as a planocraniid (formerly Pristichampsidae). Thus, the number of putative crocodilian taxa known from the Aquia is increased from 2 to 5.

Eosuchus, Thoracosaurus, and in particular Hyposaurus were almost certainly marine-going crocodilians, but the alligatorid probably occupied a fresh or brackish water habitat. The planocraniids are thought to have been terrestrial carnivores with a similar habitus to the sebecosuchians. Both the alligatorid and planocraniid remains were likely transported into the nearshore marine depositional environment from an inland location by coastal rivers.

The species Hyposaurus rogersii has long been know from the late Cretaceous (Maastrichtian) through early Paleocene (Danian) of New Jersey, South Carolina and Alabama, However, if the Aquia dyrosaur proves to be a species of Hyposaurus it is significant in that it would be the latest occurrence of the taxon known in the fossil record. The discovery of ziphodont crocodilian teeth in the Aquia Formation also suggests that a planocraniid may have been present in the fauna, however more material than isolated teeth must be found to establish its identity with any certainty.