A REVISED VIEW OF THE TURTLE FAUNA OF THE UPPER CRETACEOUS FRUITLAND-KIRTLAND SUCCESSION IN NEW MEXICO

The turtle faunas of the two members of the Fruitland Formation have received little attention. A new collecting campaign in the Bisti/De-na-zin Wilderness has substantially altered understanding of these faunas. The fauna of the lower, Ne-nah-ne-zad Member has not been previously published on but is dominated by trionychoidean turtles, including Adocus, Basilemys, and indeterminate trionychids. The fauna of the upper, Fossil Forest Member is dominated by the turtle Adocus with more rare occurrences of Basilemys and Trionychidae. Baenid material, common in higher strata, is almost entirely absent from the surveyed strata, only occurring as small shell fragments. The turtle faunas of the Fruitland Formation and the lower, Hunter Wash Member of the Kirtland Formation have previously been reported to be baenid dominated. Recent field work in the Bisti/De-na-zin Wilderness area suggests that this is not entirely correct. While the fossil assemblages of the upper Hunter Wash and De-na-zin members of the Kirtland Formation appear to have a strong bias toward baenid turtles, the Fruitland Formation shows a strong bias toward trionychoidean turtles, particularly the genus Adocus. Also of note is the unusually common occurrence of Basilemys in the lower, Ne-nah-ne-zad Member of the Fruitland Formation, which was previously interpreted to represent coastal swamps overlain by strata deposited on more inland floodplains. This trend in the turtle faunas from trionychoidean dominated to baenid dominated fits inferred environments of deposition. More terrestrial animals like Basilemys should have been commonly brought into an aquatic setting and preserved in a swamp environment. The dominance of Adocus in the fauna of the Fruitland was unexpected based on previous studies. Extant dermatemydid turtles are common aquatic herbivores on the eastern slope of Central America, and the similar anatomy of these turtles and Adocus suggests a similar ecological niche, which coastal swamps would provide for in abundance. As such, their abundance in the coastal swamps of the Fruitland fits with their inferred ecology. The ecological diversity of turtles from the Fruitland-Kirtland succession is greater than previously thought, with a distinct coastal to more inland faunal change indicated moving up the stratigraphic section.