New Mexico Geological Society Annual Spring Meeting — Abstracts

Hoplochelys crassa was originally described from the Paleocene Nacimiento Formation of New Mexico in the San Juan Basin, New Mexico. Following the original description, numerous species of the genus were subsequently named based on shells of varying degrees of completeness. Among these was H. bicarinata, which has two parallel ridges on each side of the midline rather than one, as in other Hoplochelys. In the last 10 years, all Hoplochelys species have been synonymized with H. crassa with little formal explanation. We suggest that this may have been premature because an examination of the shells in the collections of the New Mexico Museum of Natural History and Science reveals that two distinct morphological groups appear to be present. One has a single ridge and a broad first neural, here termed Hoplochelys n. sp., from the Puercan interval of the Nacimiento Formation. This does not correspond to the morphology of any previously published Hoplochelys. The other has two distinct double ridges and an elongate first neural that we tentatively refer to H. crassa. This has been found in both the Puercan and Torrejonian intervals of the Nacimiento Formation. We thus conclude that the retention of two taxa is justified based on the differences in the proportions of the neural bones. Regarding paleoecology, this may suggest the competitive exclusion principle in these turtles during the Puercan, with only H. crassa surviving into the Torrejonian, although non-shell material must be analyzed to confirm or refute this possibility.

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