Virtual endocasts of the Paleogene Ctenodactyloidea and brain evolution in ctenodactyloid rodents

Asian ctenodactyloids were highly diverse in the Paleogene. As early rodents, they provide important insights into the earliest evolution of the rodent brain. Here, we describe the first virtual endocasts of the early Eocene Exmus mini and the Oligocene Bounomys ulantatalensis and compare them with extant taxa ( Ctenodactylus sp. and Pectinator spekei ). From the Eocene to the present, the endocasts of ctenodactyloids reveal a reduction in olfactory bulb size, increased neocorticalization, expansion of the temporal lobes, a decrease in petrosal lobule size, and an overall increase in encephalization. Specifically, the cerebrum changes from an ovoid to a teardrop shape due to lateral expansion of the temporal lobes, and the petrosal lobule becomes much smaller in extant taxa. A reduction in the olfactory bulbs and petrosal lobules, and an increase in body mass not proportional to brain size, were observed from the early Eocene E. mini to the early Oligocene B. ulantatalensis , a change that may reflect ecological shifts during the Eocene–Oligocene transition.