CRANIAL NEUROVASCULATURE AND PNEUMATICITY IN THALATTOSUCHIAN CROCODYLOMORPHS

YOUNG M.; SCHWAB J.; DUFEAU D.; DOLLMAN K.; WITMER L.; HERRERA Y.; WALSH S.; ZANNO L.; BRUSATTE S.; XING X.

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

During the Mesozoic, thalattosuchian crocodylomorphs underwent a major macroevolutionary transition, evolving from semi-aquatic taxa into pelagic forms (Metriorhynchidae). In order to investigate the land-to-sea transition seen in Thalattosuchia, and the evolution of the pelagic metriorhynchids, our team CT-scanned and digitally segmented the cranial endocasts of 16 extinct and extant crocodylomorphs. Our results document that early diverging crocodylomorphs (?sphenosuchians?) had distinct brain, vasculature, and sinus morphologies, noticeably different from extant species. However, protosuchian-grade taxa had the entire suite of pneumatic structures seen in extant crocodylians, suggesting crocodylian sinus patterns originated over 200 million years ago. Interestingly, ?protosuchians? had highly pneumatic crania (far more so than extant species) with pneumatization of the laterosphenoids and frontals. Thalattosuchians had a unique array of endocranial vasculature and pneumatic anatomies; with hypertrophy of venous sinuses and vasculature canal endocasts. Compared to the circulatory patterns of extant species, thalattosuchians would have had far greater blood flow entering and exiting the orbital and nasal regions. This corresponds to their proportionally large orbits, and hints that the salt glands observed in Metriorhynchidae evolved at the base of Thalattosuchia. All thalattosuchians had less extensive cranial pneumatic sinus systems when compared to ?sphenosuchians?, ?protosuchians? and extant species. Rather than having discrete diverticula, the sinuses were confluent with the tympanic cavity and hard to individualize. The lack of the intertympanic diverticula suggests thalattosuchians had poor acoustic coupling of the middle ears, with limited directional hearing when compared to extant species. Our results suggest that some of the major soft tissue adaptations that underpinned the metriorhynchid radiation into the pelagic realm occurred much earlier in thalattosuchian evolution, prior to the reorganisation of the post-cranial skeleton. This corresponds to our already published work on the inner ears: the major endocranial changes occurred at the base of Thalattosuchia, with incremental changes occurring towards and within Metriorhynchidae.