Dietary Diversity and Evolution of Cranial Adaptations in Turtles

Abstract

The evolution of the turtle skull has involved a series of unique modifications resulting in increased bite force stemming from cranial elongation, rearrangement of jaw adductor musculature, and reduction of cranial kinesis. Loss of teeth, development of the keratinous rhamphotheca, and evolution of a highly flexible neck have also driven turtle cranial evolution. Dietary diversity across the clade has resulted in a diverse range of morphological variations of the turtle cranial Bauplan, including those associated with suction feeding and tongue‐dependent food uptake.The Cenomanian Arlington Archosaur Site (AAS) from the Woodbine Formation of Texas was home to numerous mid‐Cretaceous turtle taxa. This diverse faunal assemblage allows an evaluation of how dietary adaptations can facilitate partitioning of the available niche space among co‐occurring turtle species. Baenids, such as Trinitichelys maini at AAS, are typically reconstructed as being primarily carnivorous, although there is evidence that some taxa were molluscivorous. The AAS helochelydrid Naomichelys was likely a generalized omnivore, being more terrestrial than the other AAS turtle taxa. Bothremydids, such as the AAS species Pleurochayah appalachius, have a wide range of dietary specializations including durophagy and suction‐feeding. These adaptations are evidenced by the broad range of bothremydid cranial morphologies including expanded triturating surfaces and an inflated face. Highly aquatic trionychids are predominantly piscivorous, although they will also opportunistically hunt other small prey such crustaceans and amphibians. Many trionychid species must be fully submerged under water in order to properly swallow their food in contrast to some semi‐aquatic turtles. A newly identified protostegid turtle reveals a marine component to the AAS turtle fauna, suggesting the presence of an open ocean piscivore. Finally, an intriguing new lower jaw with well‐developed denticles indicates the presence of an obligate herbivore among the AAS turtle fauna. This diverse array of dietary proclivities and adaptations reveals how AAS turtle communities divided the available ecospace in order to avoid interspecific competition for available resources among turtle taxa and with the other AAS herpetofauna.