Re-assessment of late Campanian (Kirtlandian) turtles from the Upper Cretaceous Fruitland and Kirtland Formations, San Juan Basin, New Mexico, USA

Abstract

The fossil turtles from the Upper Cretaceous Fruitland and Kirtland formations (late Campanian; Kirtlandian) have been known for more than 100 years. We re-assess and revise these Late Cretaceous testudine taxa from the San Juan Basin, New Mexico, USA, and discuss their biostratigraphic distribution. We recognize the following valid taxa as present: the bothremydid Chedighaii hutchisoni; the pleurosternid Compsemys sp.; the baenodds Denazinemys nodosa and Scabremys gen. nov., established for the distinct species S. ornata, previously included in Denazinemys; and Boremys grandis, though it is a rare taxon. The non-baenodd baenid Neurankylus baueri is recognized as a valid species. Two additional non-baenodd baenid taxa, Thescelus hemispherica and T. rapiens, are retained as distinct species and are not considered synonymous with T. insiliens. We also recognize a small indeterminate kinosternoid similar to that reported from the Campanian of Mexico. The two adocids Adocus bossi and A. kirtlandius are retained as distinct species. The nanhsiungchelyid Basilemys gaffneyi sp. nov. is established, whereas Basilemys nobilis is considered a nomen dubium because it lacks the diagnostic features that would allow referral to any known valid species. We recognize three trionychids: Aspideretoides austerus and A. robustus (new combination), and an unnamed plastominine. We synomize Aspideretoides fontanus and A. vorax with A. austerus. Aspideretoides ovatus is considered a subadult of A. robustus. The unnamed plastominine may represent a new genus and species. Turtles of the Fruitland-Kirtland formations resemble other late Campanian turtle assemblages from western North America, and are part of the characteristic vertebrate fauna of the Kirtlandian land-vertebrate age. The upper Fruitland and lower Kirtland formations (Hunter Wash local fauna) have greater turtle taxonomic diversity than the upper Kirtland Formation (Willow Wash local fauna). This apparent decrease in taxonomic diversity is interpreted as being real and reflects a shift in depositional (channel) environments to a more terrestrial one, a pattern which is seen in other North American Late Cretaceous settings.