Nocturnal giants and brainy birds : ecological evolution across early avian divergences

Torres, Christopher Robert
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The identification of fossilizable correlates to ecology is essential for estimating the biology of extinct organisms. Among birds, reconstructions of brain shape and wing shape can elucidate macroevolutionary patterns in sensory and locomotory ecology. Here, I use these data to estimate traits ancestral to the crown avian divergence that may have contributed to their exceptional survival of the end-Cretaceous extinction event, as well as to the basal divergences within Palaeognathae, a clade marked by repeated losses of flight and gigantism. First, I use digital brain reconstructions to investigate the evolution of palaeognath sensory ecology. I find relatively large olfactory bulbs in taxa occupying forested environments where visual cues used in foraging are likely to be limited. Elephant birds exhibited extremely reduced or obsolete optic lobes, a condition only observed among extant birds in nocturnal taxa. I interpret these results as evidence for nocturnality among elephant birds. Next, I reconstruct the wing of an exceptionally preserved specimen of the Early Eocene lithornithid Calciavis grandei. Lithornithids are stem flying palaeognaths, and investigations of properties of their wings informs estimation of ancestral traits in Palaeognathae. My comparison of flight parameters estimated for Calciavis with representatives from across Aves show that it was likely capable of sustained flight and migratory behavior, consistent with previous hypotheses the ancestors of extant palaeognath lineages may also have been capable of sustained flight. Finally, I use data from a new, nearly complete skull of Late Cretaceous (~70 Ma) near-crown bird Ichthyornis to interrogate hypotheses proposed to explain patterns of dinosaur survivorship of the end-Cretaceous mass extinction and reassess bird body size evolution. These hypotheses demand identification of traits unique to survivors in Aves. However, identification of such traits is complicated by data available from stem birds. Ichthyornis exhibited a wulst and segmented palate, features with controversial evolutionary timing. The origin of Aves is marked by larger, highly flexed brains indicating selection for relatively large telencephela and eyes but were not uniquely small bodied. Sensory system differences, potentially linked to these shifts, may help explain avian survivorship relative to other dinosaurs.