PARANASAL SINUS SYSTEM EVOLUTION IN METRIORHYNCHOID CROCODYLOMORPHS

COWGILL T.; YOUNG M.; SCHWAB J.; WALSH S.; WITMER L.; 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 Mesozoic, metriorhynchoid crocodylomorphs adapted to life in marine ecosystems, transitioning from semi-aquatic predators into fully pelagic forms (Metriorhynchidae). Until recently this transition has largely focused on the osteological changes, with the endocranial changes being poorly studied. This is especially true for the paranasal sinus system. In extant crocodylians the rostrum has numerous pneumatic diverticula originating from both the nasal cavity and nasopharyngeal ducts, that become more extensive (in terms of size and number of diverticula) during ontogeny. To investigate the evolution of this sinus system we digitally segmented µCT scans of skulls of two basal metriorhynchoids (Pelagosaurus typus and Eoneustes gaudryi), four derived pelagic metriorhynchids (Metriorhynchus superciliosus, Cricosaurus araucanensis, Cricosaurus schroederi, and Torvoneustes coryphaeus), and two adult and juvenile longirostrine crocodylians (Gavialis gangeticus and Tomistoma schlegelii) for comparison. We found metriorhynchoids to have exceptionally reduced paranasal sinus systems, solely comprising the antorbital sinus. In basal metriorhynchoids the paranasal sinus extent and morphology is most similar to juvenile longirostrine crocodylians, suggesting evidence of paedomorphosis in Metriorhynchoidea. The antorbital sinus is largely indistinguishable from the dorsal alveolarcanal in basal metriorhynchoids, and we propose that they shared the same cavity. In Metriorhynchidae, the antorbital sinus has a conical morphology and extends posteriorly through the postnasal fenestra into the orbit, creating an accessory suborbital sinus. The function of the suborbital sinus is unknown, but its association with the dorsal pterygoideous muscle possibly allowed active ventilation of the paranasal sinus system indicating a respiratory function. Alternatively, expansion and contraction of the sinus through the postnasal fenestra could have enabled metriorhynchids to cope with water pressure changes when diving. The nasopharyngeal ducts in basal metriorhynchoids are similar to extant crocodylians, but in metriorhynchids they are dorsoventrally enlarged and bordered ventrally by thickened palatines. The larger transverse area of the ducts could have enabled stronger ventilation, especially if metriorhynchids had increasedlung capacity. The nasal cavity posterior in most metriorhynchoids is dorsolaterally expanded, likely for housing the enlarged salt glands.