New Mexico Geological Society Annual Spring Meeting — Abstracts

Sea turtles, specifically members of the family Protostegidae, reached gigantic size (up to 4 m long) during the Late Cretaceous and have been documented extensively from the deposits of the Western Interior Seaway in the mid-continent of North America. Here, we describe the first record of a giant sea turtle from New Mexico. NMMNH (New Mexico Museum of Natural History and Science) P16079 and P-16104 consist of three carapace fragments of a large sea turtle that were recovered from the Pierre Shale of the Raton basin, Turkey Creek Canyon area, Colfax County, New Mexico. The fragments are all very thick, with a maximum thickness of 4.2 cm, and possess dermal patterning that consists of a series of thin, radial grooves. Portions of this material have previously been identified as mosasaur cranial elements, but the thickness and dermal patterning of the specimens are not consistent with a mosasaur.

The New Mexico carapace fragments compare well with material of Archelon ischyros, a giant protostegid sea turtle recovered from the Pierre Shale of Kansas and South Dakota. However, the lack of more extensive material precludes a genus- or species-level identification. The index ammonoids Didymoceras and Baculites collected from nearby localities indicate a late Campanian age, either the Didymoceras cheyennense or Baculites compressus zones, for the turtle fossils. This first recognition of a giant sea turtle from New Mexico adds to our knowledge of the marine fauna along the western margin of the Late Cretaceous Western Interior Seaway.