BRAINS, GIANT DINOSAURS, AND SWIMMING CROCODYLOMORPHS: NEUROSENSORY CHANGES DURING MAJOR EVOLUTIONARY TRANSITIONS IN MESOZOIC ARCHOSAUR

BRUSATTE S.; YOUNG M.; SCHWAB J.; WITMER L.; HERRERA Y.; WALSH S.; MCKEOWN M.; MUIR A.; WILLIAMSON T.E.; CARR T.D.

evento virtual

Congreso; The Society of Vertebrate Paleontology 80th Annual Meeting · Virtual 2020; 2020

The Society of Vertebrate Paleontology

Archosaurs originated ca. 250 million years ago and diverged into two lineages: one including birds and dinosaurs, the other leading to crocodylians. Both lines diversified in the Mesozoic, experienced spates of dominance, and underwent evolutionary changes as they faced changing environments and two mass extinctions, ultimately surviving as 10,000+ species today. Little is known, however, about how brains and sensory systems related to, and perhaps underpinned, major evolutionary transitions in archosaurs. We here focus on neurosensory changes during two of the most remarkable transformations in Mesozoic archosaurs: the development of enormous body size in tyrannosauroid dinosaurs and the transition from terrestrial ancestors to open-ocean swimmers in thalattosuchian crocodylomorphs. We used high-resolution computed tomography scanning to construct digital endocasts of the endocranium (approximating the brain), nerves and vasculature, and associated pneumatic sinuses of over two dozen fossil and extant archosaurs, focusing particularly on metriorhynchids (the thalattosuchian subgroup, which were the only archosaurs to ever develop fully pelagic lifestyles) and tyrannosauroids from the mid-to-Late Cretaceous that span the acquisition of gigantic size (>ca. 1 ton). For thalattosuchians, we find there was little change in overall brain shape or relative size as metriorhynchids evolved from nearshore semiaquatic species into pelagic forms. The brains of close thalattosuchian outgroups (e.g., Eopneumatosuchus) and teleosauroids share with metriorhynchids an elongate, tubular shape and a long, horizontal pituitary fossa with enlarged orbital arteries. Metriorhynchids, however, have an expanded dural venous sinus system, particularly pronounced in taxa like Metriorhynchus brachyrhynchus, which may be related to diving or thermoregulation. For tyrannosauroids, we find that the large, tubular brain with an expanded dorsal peak but reduced flocculus, characteristic of the largest Late Cretaceous species, first developed in smaller, more basal taxa like Timurlengia. Voluminous air sinuses surrounding the brain evolved later, in concert with a great increase in body size in taxa such as Bistahieversor. In both thalattosuchians and tyrannosauroids, there is no marked change in brain anatomy associated with their major ecological and evolutionary transitions, although there are associated sinus changes. Brains, thus, were not evidently drivers of these transitions.