Stephanie E. Pierce, Megan Williams & Roger B.J. Benson (2016)
Virtual reconstruction of the brain and sinuses of the early Jurassic marine crocodylomorph Pelagosaurus typus (Thalattosuchia).
PeerJ Preprints 4:e2539v1
Thalattosuchians were highly specialised marine archosaurs of the Jurassic and Early Cretaceous, and represent a peak of aquatic adaptation among crocodylomorphs. Relatively little is known of their endocranial anatomy or its relevance for the evolution of sensory systems, physiology, and other aspects of biology. Nevertheless, such data have significance for two reasons: (1) thalattosuchians represent an important data point regarding adaptation to marine life in tetrapods; and (2) as early-diverging members of the crocodylian stem-lineage, thalattosuchians provide information on the evolutionary assembly of the brain and other endocranial structures in crocodylomorphs. Here we use µCT data to virtually reconstruct the brain and sinuses of Pelagosaurus typus, an early thalattosuchian with plesiomorphic traits of relevance to the split between the two major subgroups: Teleosauroidea and Metriorhynchoidea. Interpreted in a phylogenetic context, these data indicate several endocranial features as likely synapomorphies of Thalattosuchia, including: a pyramidal morphology of the semicircular canals, the presence of an elongate cochlear duct that may indicate enhanced hearing ability, the presence of large, paired canals extending anteriorly from an enlarged pituitary fossa, a relatively straight brain (possibly due to the presence of large, laterally placed orbits) and an enlarged dorsal longitudinal sinus that is confluent with the paratympanic sinus system. Notably, we document a large expansion of the nasal cavity anterior to the orbits in Pelagosaurus as an osteological correlate of an enlarged salt gland previously only documented in Late Jurassic metriorhynchoids. This is the first direct evidence of this structure in early thalattosuchians. Pelagosaurus also shares the presence of paired olfactory tracts with metriorhynchoids, and shows an enlarged cerebrum, which may also be present in teleosauroids. Taken together, our findings indicate that physiological and sensory adaptations to marine life occurred early in thalattosuchians evolution, predating the origins of flippers, tail flukes, and hydrodynamic body forms seen later in metriorhynchoids.