Paleobiology of a new "macrobaenid" turtle (Testudinata: pan-cryptodira) from the Blackleaf Formation, Montana

Abstract

Across the Mesozoic fossil record, the stratigraphy between the Lower and Upper Cretaceous of North America has historically been understudied. Some fossils from this interval suggest diverse assemblages with both basal and derived turtles in many western North American formations. However, the fragmentary nature of these fossils has prevented more thorough reconstructions of their morphology, interrelationships, and ecology. A small fossil turtle, MOR 14891, was recently discovered in the Cenomanian-age Vaughn Member of the Blackleaf Formation in southwest Montana. This specimen includes a carapace and plastron surrounding other previously obscured elements, most of which are from the cranium and mandible. The specimen was micro-CT-scanned and digitally rendered using the software package 3DSlicer. Reconstructions of digital models were made using Blender. Additional synchrotron scans were acquired for the largest block. This permitted a subsequent description of the preserved elements. This specimen represents one of the most complete mid-Cretaceous North American turtle fossils found. Recent stratigraphic studies suggest the Blackleaf Formation dates to 100-101 Mya. As a result, MOR 14891 predates the oldest North American macrobaenid by about 8-9 myr. Morphological features related to the internal carotid artery canals and parabasisphenoid suggest this specimen represents a new and oldest North American member of the problematic "Macrobaenidae." To test this identification, a global Bayesian phylogenetic analysis was conducted, placing MOR 14891 within a clade of "macrobaenids/sinemydids" branching just stemward of Testudines. MOR 14891 falls in a polytomy with other "macrobaenids"/ "sinemydids", with moderate node support. More specifically, MOR 14891 plots as a sister taxon to Ordosemys with similarly moderate node support. The presence of such an unusual, early representative of the group provides valuable insight into the diversity of the Blackleaf Formation and the evolution of North American ecosystems through an underrepresented time range during the Cretaceous.