INSIGHTS INTO THE LAND-TO-SEA TRANSITION: MARINE CROCODYLOMORPH (CROCODYLOMORPHA: THALATTOSUCHIA) VASCULATURE AND SINUS SYSTEM ADAPTATIONS

YOUNG M.; SCHWAB J.; WALSH S.; WITMER L.; HERRERA Y.; ZANNO L.; CLARK J.; RUEBENSTAHL A.; XING X.; CHOINIERE J.; DOLLMAN K.; BRUSATTE S.

Bruselas

Conferencia; 17th Conference of the EAVP; 2019

While the osteological adaptations of secondarily aquatic vertebrates are well understood, the soft tissue ones are not. Here we use the Mesozoic crocodylomorph clade Thalattosuchia as an exemplar of the land-to-sea transition. During the Jurassic, thalattosuchians transitioned from shallow marine to open ocean habitats, with the subclade Metriorhynchidae convergently evolving osteological adaptations seen in other Mesozoic marine reptile groups (e.g. paddle-like forelimbs, hypocercal tail). However, the soft tissue adaptations underpinning the success of Thalattosuchia remain obscure.Our team CT scanned and digitally segmented the cranial endocasts of 15 extinct and extant crocodylomorphs. The thalattosuchians shared a unique array of intracranial vasculature and pneumaticity anatomies; with hypertrophy of the carotid-orbital vessels, temporoorbital vessels, transverse sinuses and possibly the cavernous sinus. Based on the blood flow patterns of extant species, thalattosuchians would have had far greater blood flow entering and exiting the orbital and nasal regions. This increase corresponds with their proportionally large eyes, and suggests that the salt glands of Metriorhynchidae evolved at the base of Thalattosuchia.All thalattosuchians had a less extensive paratympanic sinus system when compared with sphenosuchians, protosuchians and extant species. The recessus epitubaricum, otoccipital diverticula, and vestigial infundibular diverticula were confluent with the tympanic cavity (rather than discrete diverticula). While the intertympanic and quadrate diverticula were absent.Our results suggest that at least some of the major soft tissue adaptations that underpinned the metriorhynchid radiation into the pelagic realm occurred much earlier in thalattosuchian evolution, and occurred prior to their osteological and locomotory adaptations.