PARANASAL SINUS SYSTEM, UPPER RESPIRATORY TRACT, AND SALT GLAND EVOLUTION IN THALATTOSUCHIAN CROCODYLOMORPHS

COWGILL T.; YOUNG M.; SCHWAB J.; WALSH S.; WITMER L.; HERRERA Y.; DOLLMAN K.; CHOINIERE J.; BRUSATTE S.

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

Thalattosuchian crocodylomorphs were a diverse clade that predominately inhabited coastal ecosystems during the Mesozoic. Thalattosuchia is comprised of two subgroups, Teleosauroidea and Metriorhynchoidea. Teleosauroids and basal metriorhynchoids were semi-aquatic predators that lived in freshwater, brackish and coastal environments. However, within Metriorhynchoidea there was a major macroevolutionary transition, analogous to that seen during cetacean evolution. Derived metriorhynchoids within the subclade Metriorhynchidae evolved flippers, a tail-fin and became obligately pelagic. Until recently the study of this transition has largely focused on osteological changes, while internal craniofacial changes have been neglected. 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. Herein we investigate the evolution of the paranasal sinus system along with the anteriormost part of the respiratory system (nasal cavity and nasopharyngeal ducts) in Thalattosuchia, by reconstructing the internal anatomy in CT scans of seven thalattosuchian skulls including one teleosauroid (Plagiophthalmosuchus gracilirostris), two basal metriorhynchoids (Pelagosaurus typus and Eoneustes gaudryi), and four derived pelagic metriorhynchids (Metriorhynchus superciliosus, Cricosaurus araucanensis, Cricosaurus schroederi and Torvoneustes coryphaeus). Our outgroup taxa were three extant crocodylians (Gavialis gangeticus, Tomistoma schlegelii and Crocodylus rhombifer) and one basal crocodyliform (Protosuchus haughtoni). We found that metriorhynchoids had exceptionally reduced paranasal sinus systems, solely comprising the antorbital sinus. In both the teleosauroid Plagiophthalmosuchus gracilirostris and the basal metriorhynchoid Pelagosaurus typus the antorbital sinus is partially located medial to a reduced external antorbital fenestra, and broadly communicating with the dorsal alveolar canal. In pelagic metriorhynchids, the antorbital sinus is more extensive than in basal taxa and possibly performed an active function associated with a hypothetical accessory suborbital diverticulum. Although our reconstructions alone are insufficient to confirm the presence of a suborbital diverticulum they are consistent with specimens that preserve a contiguous soft-tissue structure exiting the antorbital cavity into the orbit. The nasopharyngeal ducts in metriorhynchids are dorsoventrally enlarged compared to crocodylians and basal thalattosuchians, and bordered ventrally by thickened palatines. The larger transverse area of the ducts possibly enabled stronger ventilation. The nasal cavity in most metriorhynchoids exhibits posterior dorsolateral expansions, which we identify as a possible osteological correlate for hypertrophied salt glands. The acquisition of internal craniofacial adaptations shows a mosaic pattern, predating the major skeletal adaptations. We hypothesise that these internal craniofacial adaptations occurred while metriorhynchoid were still semi-aquatic.