146-65 million years ago

sheep brain

The modifications of the placental nervous system included an enlarged cerebrum with many shared regions.

The ability to move the hand with fine, skilled movements evolved before primates and is present in a number of mammalian lineages, such as mice. In most placentals, the lateral cortico-spinal tract is no longer located in the dorsal column (as in monotremes, marsupials, ungulates and rodents). In placental mammals, there is an increase in the number of small and medium sized cells in and around trigeminal nucleus and a reduction in forward extension of efferent facial nucleus. Placental mammals possess multiple nuclei of the anterior, medial, and intralaminar groups and a lateral posterior-pulvinar complex, unlike monotremes.
In placental mammals, the cerebrum increased in size. Its expansion over the midbrain ends the "midbrain exposure" typical of other groups of vertebrates. Most placental mammals further increase the surface area of the cerebrum by developing gyri and sulci. This wrinkling of brain tissue to increase surface area is not unique to the cerebrum: a number of brain areas have increased their size through folding such as substantia gelatinosa, nucleus laminaris, dentate nucleus , inferior and superior olivary nuclei, lateral geniculate nuclei and cerebellar cortex. Folding of the cerebrum exists in some marsupials. While the majority of placental mammals possess gyri and sulci on the surface of the cerebrum, smooth cerebrums are known in some rodents, insectivores, and prosimians. The presence of the lipid lysophosphatidic acid (LPA) to the developing brain of a mouse converts its smooth cerebral cortex to a gyrated one, resembling the brain surface of higher mammals.
A corpus callosum developed in placental mammals to connect the two cerebral hemispheres (although some marsupials have a dorsal corpus callosum). In placental mammals, the hippocampus is positioned more laterally. Eutherian mammals share M2 primary and secondary motor fields. Placental mammals possess the following sulci: intercalates, rostralis, occipito-temporalis, suprasylvian, coronal, ansate (which contributes to the central sulcus in primates), and an interparietal (Simian) sulcus. Placental mammals lack oil droplets in retinal cones.