Ecomorphological and allometric signatures in endocranial shape in crocodylomorphs

TURNER, A.H.; BRUSATTE, STEPHEN L.; POL, D

Congreso; 80° Annual Meeting of the Society of Vertebrate Paleontology; 2020

Society of Vertebrate Paleontology

Many ecological transitions are associated with morphological transformations as organisms adapt to new environments. Extinct crocodylomorphs evolved an impressive range of ecologies (e.g., rivaling those of mammals) that contrast the semiaquatic lifestyle of modern crocodylians. The earliest diverging crocodylomorphs were predominantly small terrestrial predators. However, by the Jurassic marine thalattosuchians had evolved towards their eventual conquest of pelagic domains. Notosuchians, on the other hand, included terrestrial forms ranging from small herbivores and omnivores to large predators?the latter group surviving well into the Cenozoic. The extent to which the neuroanatomy of these taxa tracked their ecological specialization and associated skeletal adaptations is not well established. Preliminary data, based largely on extant sampling, have suggested that brain morphology in extant crocodylians retains ecomorphological signal.To explore how ecological preferences among fossil crocodylomorphs is reflected in brain morphology, we used a high-density 3D morphometric approach on cranial endocasts, broadly sampling extant and extinct crocodylomorphs. We analyzed whole endocasts as well as focused on the cerebrum, optic lobe, cerebellum, and medulla, individually. We found that overall endocranial shape exhibits phylogenetic clustering with major clades occupying different, but in some cases overlapping, regions of morphospace. Especially distinct are the basalmost crocodylomorphs and the notosuchians. To interrogate these patterns further, we used a time-calibrated phylogeny of Crocodylomorpha to perform a suite of comparative phylogenetic analyses. Results indicate that allometry accounts for nearly 10% of total endocranial shape variation after correcting for phylogenetic structure. Surprisingly, habitat preference and overall shape variation for each brain region shows little association. Our current data suggest that neuroanatomical evolution in crocodylomorphs is more complex than might typically be expected for saurians, with convergence of brain morphology among distantly related and ecologically distinct taxa. Significant brain regions such as the olfactory tract and bulb as well as the pituitary/infundibulum, which remain to be explored, may add needed clarity. The findings of the present study contrast strongly with other aspects of neurosensory anatomy in crocodylomorphs, namely that inner ear morphology tracks aquatic habitat evolution.