Contrasting brains and bones: neuroanatomical evolution of turtles (Testudinata)

Turtles are relatively species-poor, but anatomically very distinct from other reptiles, which yields longstanding debates about their origin among amniotes. On the other hand, an abundant and diverse fossil record, documenting repeated habitat transitions, makes turtles one of the best models for investigating morphological changes associated with different functions and ecologies. Here, we review the literature on the neuroanatomy and sensory evolution—including cranial innervation and circulation—with considerations on available physiological and functional studies. We provide comparative neuroanatomical descriptions based on digital dissections of soft and hard tissues, particularly scrutinizing the relation between brain tissue and the endocranial cavity, including new quantitative data. Some level of correspondence between these is integral for paleoneurological inferences from osteology, but this is rarely addressed for non-avian reptiles. We submit that turtle endocasts are poor predictors of soft tissue anatomy, yet they can still provide insights, for example on brain size evolution. We show that the turtle brain and braincase exhibit allometric changes through ontogeny, making adult braincases a poorer fit to brain tissue form than is the case in juveniles. Finally, we summarize open questions and caveats in the areas, aiming to provide a framework for future research on turtle neuroanatomy.