At the beginning of the Tertiary, no large land animals were present and the surviving amphibians, reptiles, mammals, and birds found the land surface covered with lots of plants and an abundance of insects. Figure 7.21 shows the evolution of placental mammals. It can be seen that various mammalian
orders had already radiated from the oldest order, Insectivora in the upper Cretaceous, while the remainder departed in the Paleocene. The important innovation of the mammals was to let their embryos grow in an uterus inside the body, in contact with the mother's blood supply over a placenta. Together with milk-feeding of the infants and intensive child care (which is also done by reptiles and birds), this invention led to a high survival rate of the offspring.
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