Neuroanatomy of Kayentachelys aprix and Eileanchelys waldmani provide insights into the early evolution of the turtle brain.

Even though many early stem turtles are known from relatively well-preserved skulls, their neuroanatomy remains poorly understood, limiting insights into key cranial and ecological transitions. Here we reconstruct the brain, nerves, inner ears, olfactory endocasts and arteries of two early stem turtles-the Early Jurassic Kayentachelys aprix and the Middle Jurassic Eileanchelys waldmani-based on high-resolution imaging. These species document key phases of turtle cranial evolution. Our analysis documents intermediate conditions of Jurassic mesochelydians between earlier Triassic stem turtles such as Proganochelys quenstedtii and crown Testudines. We show that changes in the canalis cavernosus, geniculate ganglion positioning, and braincase architecture are related to cranial stiffening in turtles. Whereas Kayentachelys aprix retains plesiomorphic features of Triassic testudinatans (e.g., separation of recessus scalae tympani and cavum acustico-jugulare; flat processus interfenestralis morphology) or intermediate features (e.g., cranio-quadrate space modified to short canalis cavernosus; clearly tympanic stapes but with robust morphology; reduced prootic foramen but absence of secondary braincase wall of parietal-pterygoid contact), Eileanchelys waldmani shows essentially 'modern' braincase architecture, including a ventrally inclined processus interfenestralis and a fully developed cavum tympani. Additionally, anatomical traits associated with olfaction and hearing provide insights into the paleoecology of these taxa, supporting a terrestrial lifestyle for Kayentachelys aprix and aquatic adaptations in Eileanchelys waldmani. Our study highlights the utility of neuroanatomical data in refining hypotheses of turtle cranial evolution and ecology, and underscores the importance of Jurassic stem turtles for understanding the origins of crown-group traits.