Meeting Abstract
Dyrosaurids, specialized marine relatives of crocodylians, are one of few archosaur lineages to survive the K-Pg extinction and present a unique combination of morphology and ecology absent in living crocodylians. Little is known about their endocranial anatomy, leaving questions about their neurosensory adaptations unaddressed. We CT scanned a well-preserved skull of Rhabdognathus, a Paleocene dyrosaurid from Mali. We focused on three specific internal structures: the cranial endocast; inner ear; and paratympanic sinuses. The Rhabdognathus endocast showed novel features including a dorsal venous system that appears to communicate with the external skull table, enlarged tympanic bullae that meet at the endocranial midline, and elongate olfactory tracts forming half the total endocast length. The tracts end in paired olfactory bulbs with complex bony projections—a unique morphology perhaps serving to increase surface area for olfaction. Rhabdognathus has a novel conformation of its paratympanic system. The anterior and posterior divisions of the lateral Eustachian canal meet outside the skull and a unique duct was discovered connecting the pharynx to the adductor chamber. These findings require a reinterpretation of the associated external foramina in dyrosaurids and potentially their close relatives pholidosaurids. The inner ear exhibits aspects of both Pelagosaurus and Gavialis. The common crus is spherical, as in Gavialis, but significantly expanded. The cochlear duct is shifted anteriorly as in Gavialis. The semicircular canals appear pyramidal as in Pelagosaurus unlike the spherical shape of crocodylians. This is the first time dyrosaurid braincase and neurosensory features have been examined using CT scans. Our work reveals novel morphologies in the ear, paratympanic sinuses, and olfactory bulb that may relate to dyrosaurid adaptation to a marine habitat.
