MICROANATOMICAL ANALYSES (PALEOHISTOLOGY AND COMPUTED TOMOGRAPHY) REVEAL TWO HELOCHELYDRID TAXA IN THE WOODBINE FORMATION, CENOMANIAN, TEXAS

Currently, nine distinct turtle genera (eight cryptodires and one pleurodire) are recognized from the Arlington Archosaur Site (Cenomanian Woodbine Fm, Tarrant County, Texas). Among these were the Helochelydridae, an endemic Laurasian family typically known from freshwater continental deposits later in the Cretaceous of Europe and North America. Helochelydrid species may be distinguished by their shell morphologies, primarily, the arrangement of small tubercles along the growing edges of bony plates in the shell. Tubercles can coalesce, or merge, with adjacent tubercles to form ridges that are arranged across the shell surface in known diagnostic patterns. Additionally, Helochelydrid taxa are distinguishable using microanatomical characters — an effective method even when gross morphological evidence is distorted by wear or other factors. We performed standard paleohistological techniques and computed tomography (CT) scanning on a sample of helochelydrid material from the AAS. Our results confirm the presence of Naomichelys at the AAS based on its diagnostic symmetrical raised pustules. Additionally, microanatomical analyses reveal a second smaller helochelydrid taxon, differentiated from the more common Naomichelys by its distinctly smaller body size and dorsal tubercles of the shell that coalesce into distinct serpentine ridges, very similar to Solemys vermiculata. Until now, Naomichelys is the only described helochelydrid in North America. Our results show that at least two helochelydrid genera were present in North America by the Cenomanian. Furthermore, our results suggest that the Solemys lineage was found in North America earlier than in Europe.