EVOLUTION OF THE NEUROVASCULAR SYSTEM WITHIN THE SNOUTS OF METRIORHYNCHOID CROCODYLOMORPHS

BOWMAN C.I.; YOUNG M.; SCHWAB J.; WITMER L.; WALSH S.; HERRERA Y.; BRUSATTE S.

evento virtual

Congreso; The Society of Vertebrate Paleontology 80th Annual Meeting · Virtual 2020; 2020

The Society of Vertebrate Paleontology

During the Jurassic, metriorhynchoid crocodylomorphs underwent a major evolutionary transition, adapting to life in open marine environments. The osteological changes that occurred during this transition are well known (tail fin, flippers, loss of osteoderms) but the endocranial ones are not. In other secondarily marine tetrapods, such as cetaceans, the sensory and physiological demands of the underwater life necessitated a shift in internal rostral anatomy. In order to investigate if these changes are a common response to life in the sea, we digitally segmented the rostral neurovascular from CT scans of eight extant and extinct crocodylomorphs. Our sample includes metriorhynchids (Cricosaurus araucanensis, C. schroederi, Metriorhynchus superciliosus, and Torvoneustes coryphaeus), basal metriorhynchoids (Pelagosaurus typus and Eoneustes gaudryi), and a juvenile and adult example of two extant longirostrine species (Tomistoma schlegelii and Gavialis gangeticus). We found that metriorhynchoids have reduced antorbital sinuses positioned ventrally to the dorsal alveolar canals at the posterior of the snout, which is a condition shared with extant juvenile crocodiles but not with extant adults, which have large antorbital sinuses positioned dorsally, laterally and ventrally around the dorsal alveolar canals. This arrangement of smaller sinuses may have been beneficial in the high pressure environments the metriorhynchoids may have encountered when diving in the ocean. Trigeminal innervation is markedly different between the metriorhynchoids and extant taxa. Extant crocodylians have a complex network of nerve channels that are spaced to fully innervate all sides of the rostrum, whereas basal metriorhynchoids had fewer, larger channels, with a linear arrangement of openings on the skull. Furthermore, metriorhynchids had a greatly reduced trigeminal system, implying that facial somatosensation was no longer a principal sense; an intriguing possibility as during metriorhynchoid evolution, orbit size rapidly increased. Metriorhynchoids possess a pair of canals which connect the oral cavity to the nasal cavity through the palatine bones, and we hypothesise that these had a function in thermoregulation as these animals had large brains and large eyes and therefore may have used highly vascularised tissues on the roof of the mouth as a ?heat sink? to shed excess heat.