Endocranial Anatomy of a Paleocene stem Waterfowl (Aves, Anseriformes)

DEGRANGE, FEDERICO JAVIER; TAMBUSSI, CLAUDIA PATRICIA; DEMMEL FERREIRA, MARÍA MANUELA; WITMER, LAWRENCE; SANTILLANA, SERGIO NÉSTOR

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina, 2018; 2018

Morphology of the avian brain is of critical importance for studies on the evolution of modern birds, and especially important is the information provided by stem avian fossils. Here we present a reconstruction of the endocranial morphology based on computed tomography of a stem representative of Anseriformes of early Paleocene age (Danian). The fossil (MLP 07-III-1-1) comes from the López de Bertodano Formation (Maastrichtian–Danian) of Seymour Island, West Antarctica. The brain cast shows a wide telencephalon, small optic lobes, large olfactory bulbs, a short and wide cerebellum, and a wide medulla oblongata. Viewed laterally, it displays a slight flexure of the main brain axis, and the telencephalon completely covers the mesencephalon in dorsal view. No telencephalic Wulst is present, suggesting that this innovative avian neurological peculiarity was not recognizably developed 65.5 Myr ago. Morphology was closer to that of the Eocene anseriform Presbyornis due to the presence of large olfactory bulbs, stout flocculi that are caudolaterally directed, and also the absence of the Wulst. Absence of the Wulst and small optic lobes seems to indicate that stem Anseriformes were not strongly visually oriented birds whereas its large olfactory bulbs would denote significant olfactory capabilities. In crown Anseriformes (as in most modern birds), the condition is the opposite. Consequently, the brain morphology of the new Antarctic fossil is consistent with morphological trends seen in living birds: replacement of olfactory sensory input by enhanced vision.