ECOMORPHOLOGICAL VARIATION IN ENDOCRANIAL SHAPE IN THALATTOSUCHIAN CROCODYLOMORPHS

SCHWAB J.; YOUNG M.; HERRERA Y.; WITMER L.; WALSH S.; WATANABE A.; TURNER A.H.; WILBERG E.; GIGNAC P. M.; BRUSATTE S.

Congreso; 9th International Meeting on the Secondary Adaptation of Tetrapods to Life in Water; 2021

Crocodylomorphs evolved a rich diversity of ecomorphotypes during their long evolutionary history. Appearing during the Late Triassic, approximately 230 million years ago, crocodylomorphs were initially terrestrial and possibly facultatively bipedal. However, they quickly diversified especially after the Triassic-Jurassic boundary, into a wide range of semiaquatic, freshwater and marine forms. During the Early Jurassic, Thalattosuchia was the first crocodylomorph group to make the land-to-water transition. They evolved from their terrestrial ancestors and gradually moved back into the ocean. Thalattosuchians comprise two subgroups, the teleosauroids which occupied predominantly brackish and coastal environments during the Jurassic and Early Cretaceous, and the metriorhynchoids which transitioned from coastal ecosystems to open ocean environments. Within the latter, the crown Metriorhynchidae evolved paddle-like limbs, a hypocercal tail and a streamlined body. Metriorhynchids are the only archosaurs known to have adapted to an obligately pelagic lifestyle. Such evolutionary transitions involve several skeletal transformations related to new physiological requirements. Besides osteological changes, endocranial sensory systems are key to understanding such transitions, and can reveal how ecology changed over long time-scales. To explore these ecomorphological adaptations we used CT scans of extant and extinct crocodylomorphs and a high density 3D morphometric approach to extract brain endocast shape data. We analysed the whole endocast as well as individual sections: cerebrum and olfactory tracts, optic lobes, cerebellum, medulla oblongata and pituitary. We performed statistical analysis (e.g., principal component analysis) to test whether and which features characterise the land-to-water transition in thalattosuchians. Our results reveal that the overall endocranial shape shows a phylogenetic signal with major clades clustering together in morphospace. Thalattosuchians generally have a more elongated and tubular brain endocast compared to their modern relatives, which might be related to their elongated skull-shape. However, there are additional ecomorphological differences between pelagic metriorhynchids compared to the semiaquatic teleosauroids and the basal metriorhynchoid Pelagosaurus typus, with them occupying distinct morphospace region. One main feature is that pelagic taxa seem to have a significantly larger pituitary fossa compared to semiaquatic forms. Our results suggests that brain endocast morphology shows phylogenetic and ecomorphological signal, similar to other neurosensory systems such as the bony labyrinth, that became more compact in pelagic metriorhynchids. Those neurosensory changes combined likely helped metriorhynchids adapt to their new open ocean realm.