Mammals are the most encephalized vertebrates, with the largest brains relative to body size. Placental mammals have particularly enlarged brains, with expanded neocortices for sensory integration, the origins of which are unclear. We used computed tomography scans of newly discovered Paleocene fossils to show that contrary to the convention that mammal brains have steadily enlarged over time, early placentals initially decreased their relative brain sizes because body mass increased at a faster rate. Later in the Eocene, multiple crown lineages independently acquired highly encephalized brains through marked growth in sensory regions. We argue that the placental radiation initially emphasized increases in body size as extinction survivors filled vacant niches. Brains eventually became larger as ecosystems saturated and competition intensified.

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Four different Hunter−Schreger Band (HSB) configurations were observed in the teeth of fossil and extant Perissodactyla. This variability exceeds that observed in Artiodactyla or

The end‐Cretaceous mass extinction allowed placental mammals to diversify ecologically and taxonomically as they filled ecological niches once occupied by non‐avian dinosaurs and more basal mammals. Little is known, however, about how the neurosensory systems of mammals changed after the extinction, and what role these systems played in mammalian diversification. We here use high‐resolution computed tomography (CT) scanning to describe the endocranial and inner ear endocasts of two species, Chriacus pelvidens and Chriacus baldwini, which belong to a cluster of ‘archaic’ placental mammals called ‘arctocyonid condylarths’ that thrived during the ca. 10 million years after the extinction (the Paleocene Epoch), but whose relationships to extant placentals are poorly understood. The endocasts provide new insight into the paleobiology of the long‐mysterious ‘arctocyonids’, and suggest that Chriacus was an animal with an encephalization quotient (EQ) range of 0.12–0.41, which probably relied more on its sense of smell than vision, because the olfactory bulbs are proportionally large but the neocortex and petrosal lobules are less developed. Agility scores, estimated from the dimensions of the semicircular canals of the inner ear, indicate that Chriacus was slow to moderately agile, and its hearing capabilities, estimated from cochlear dimensions, suggest similarities with the extant aardvark. Chriacus shares many brain features with other Paleocene mammals, such as a small lissencephalic brain, large olfactory bulbs and small petrosal lobules, which are likely plesiomorphic for Placentalia. The inner ear of Chriacus also shares derived characteristics of the elliptical and spherical recesses with extinct species that belong to Euungulata, the extant placental group that includes artiodactyls and perissodactyls. This lends key evidence to the hypothesized close relationship between Chriacus and the extant ungulate groups, and demonstrates that neurosensory features can provide important insight into both the paleobiology and relationships of early placental mammals.

A skeleton of a primitive rhinocerotoid from the Uintan (middle Eocene) of Utah provides the basis for a new genu s and species, Uintaceras radinskyi; Hyrachyus grande Peterson, 1919 is a nomen dubium. The anterior dentition of thi s specime n is unlike that of any other known rhinocerotoid, although it most resembles the anterior teeth of so me primitive rhinocerotids. The cheek teeth are primitive in morphology and similar to tho se of specimens referred to " Forstercooperia" grandis. North American specimens referred previously to Forstercooperia are here referred to this new genus. This taxonomic change implies that the evolution of indricotheriine hyracodontids was restricted to Eura sia. The postcranial skeleton of Uintaceras is robu st and shows many primitive characters. including a tetradactyl manu s. Uintaceras is the sister taxon of the Rhinocerotidae and demonstrates that rhinocerotids, at their origin, retained the primitive condition for many characters, includ ing some that have been used to unite rhinocerotids with other rhinocerotoids, such as indricotheriine hyracodontids.