Other orders

Abstract

Abstract Midway through the Mesozoic, ~196–190 Ma, mammalian brains reached a size typical of Paleocene eutherians, which lived at least 120 million years later. Across all mammals, body mass increased during the Paleocene without much, if any, relative increase in brain or cortex size. Later, the cortex expanded independently in several mammalian lineages. Cortical enlargement in cetaceans most closely resembled what happened in primates: a cortical expansion in toothed whales by the late Eocene (in parallel with Eocene primates); another in oceanic dolphins during the middle-to-late Miocene (in parallel with monkeys). In contrast, cortical expansion in other large-cortex mammals, such as ungulates and carnivores, had a different, mostly later time course. Like primates, several carnivore lineages developed a relatively large cortex independently. In mammals, a larger brain almost always resulted from cortical expansion, but there were exceptions.