The vacant niches left by the non-avian dinosaurs and other vertebrates after the end-Cretaceous mass extinction, provided a crucial opportunity for placental mammal diversification. The general neurosensory organization exhibited by extant mammals has been maintained since the Late Triassic-Early Jurassic. Much later during the Eocene, fossils of early members of extant placentals display neurosensory innovations, such as a proportionally larger neocortex and a higher encephalization quotient (EQ), compared to their Mesozoic ancestors. However, few studies have focused on the brain of the oldest placentals that lived during the Paleocene. We analysed the neurosensory system of two species of arctocyonid ‘condylarths’, a likely nonmonophyletic group, including some species potentially implicated in the origins of extant ungulates. ‘Arctocyonids’ were of small-to-medium size, omnivorous and mainly terrestrial. We obtained cranial and bony labyrinth endocasts for Arctocyon and two species of Chriacus using high-resolution computed tomography. Both species exhibit plesiomorphic features shared with other early Paleocene mammals such as a relatively small lissencephalic brain with an EQ range of 0.07-0.31 using Eisenberg’s equation. The olfactory bulbs and the petrosal lobules represent 7% and less than 1% of the total endocranial volume, respectively. The neocortical height represents ~25% of the total endocranial height. Based on the cochlea, these species had hearing capabilities similar to extant wild boars. Agility scores between 2 and 3, show that these taxa were similar to the American badger and the crab-eating raccoon, suggesting that Arctocyon and Chriacus were moderately agile. These results support the growing evidence that early placentals had low EQs and less expanded neocortices compared to Eocene and later taxa, potentially indicating that complex neurosensory organization was not key to the placental radiation after the end-Cretaceous mass extinction. This research has been funded by Marie Sklodowska-Curie Actions: Individual Fellowship, European Research Council Starting Grant, National Science Foundation, and Belgian Science Policy Office.
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RBINS Staff Publications 2019
