SHELL HISTOLOGY OF THE STEM TRIONYCHIANS (ADOCUSIA) ADOCUS CF. BOSSI AND NANHSIUNGCHELYIDAE FROM THE CENOMANIAN (LATE CRETACEOUS) ARLINGTON ARCHOSAUR SITE, WOODBINE FORMATION, TARRANT COUNTY, TEXAS, USA

The Adocusia unites the Late Cretaceous Adocidae and Nanhsiungchelyidae of North America and Asia. In North America, Adocusian families are represented only by Adocus (Adocidae) and Basilemys (Nanhsiungchelyidae). However, the adocid fossil record of North America requires substantial revision for meaningful comparisons among coeval taxa. North American adocusian turtles are best known from freshwater sediments deposited shortly after the completion of the massive Western Interior Seaway, beginning in the Turonian/Coniacian. Distinct micromorphologies can be distinguished between the cryptodiran turtle families Adocidae, Nanhsiungchelyidae, and Trionychidae using thin histological sections. Here, we describe and compare the shell histology of the adocids and nanhsiungchelyids from the Arlington Archosaur Site, a Cenomanian locality (93-99 Ma) from the Woodbine Formation of Texas. This fossil assemblage comprises the earliest currently known adocusians from North America. We made histological thin-sections and analyzed the slides following standard histological practices. AAS adocid specimens display characteristics consistent with those of Adocus, including abundant Sharpey’s fibers organized into plies and reticulated vascular channels. The AAS nanhsiungchelyid specimens exhibit paleohistological traits consistent with other members of the family, including absence of plywood-like layers and minimal Sharpey’s fibers. However, they lack the characteristic wavy growth marks of Basilemys, and instead more closely resemble Asian species such as Hanbogdemys orientalis. This finding when combined with gross morphological traits suggest that the AAS nanhsiungchelyid is a new genus and not Basilemys. Prior to this study the histology of North American adocusians before the Campanian was unknown. Microanatomical studies of the AAS Adocusia increase nanhsiungchelyid diversity in North America. They furthermore indicate greater complexity of the biogeographic relationships among Asian and North America nanhsiungchelyid turtles.