Paleoneurology of the Proboscidea (Mammalia, Afrotheria): Insights from Their Brain Endocast and Labyrinth

Abstract

The elephant brain is famous for its higher than average encephalization quotient, memory capacities, large cerebellum, large facial and trigeminal nerves, and the extensive repertoire of complex behaviors and social interactions it produces, the last of which being supported by infrasonic communication. The evolutionary history of Proboscidea is amongst the bestdocumented among mammals but knowledge of the group's paleoneurological history remains comparatively fragmentary. Here, we summarize and build upon more than 150 years of research on the evolution of the proboscidean nervous system. We find that the morphology of the endocranial cast and bony labyrinth of the basal-most proboscideans is consistent with the generalized plesiomorphic conditions for placental mammals (e.g. linearly organized brain parts, low encephalization quotient, presence of a secondary common crus), whereas their conditions become essentially elephant-like in the Elephantimorpha around the Oligocene. This suggests that a higher encephalization quotient and adaptations to lowfrequency hearing (e.g. loss of the secondary bony lamina) evolved in parallel with the formation and evolution of a trunk, adaptation to a drier environment, and a higher body mass. We hypothesize that these structures co-evolved as a response to the changing climate in the Oligocene.