Dinosaurs dominated most terrestrial habitats while populations of small mammals continued to evolve. By the middle of the Cretaceous period, about 100 million years ago, three different kinds of mammals had appeared. Modern mammals belong to one of these three groups. The first group is made up of monotremes. They are oviparous, meaning that they lay eggs. The second group is made up of marsupials. They are viviparous, which means that they give birth to live young. In marsupials, the young develop within a pouch on the mother's body for some time after birth. The third group is made up of placental mammals. They are also viviparous, but in this group, the fetus typically develops within the mother's reproductive system for a longer time than it does in marsupials. Also, in this group, the developing fetus receives nourishment through a blood-rich structure called the placenta.
Many scientists think that some sort of natural disaster changed Earth's climate and forced the dinosaurs into extinction about 65 million years ago, at the end of the Cretaceous period. This change opened up many new habitats and resources to mammals. So, mammals took over many of the ecological roles that dinosaurs previously had. Today, nearly all large terrestrial animals are mammals.
The earliest mammals, such as this species of the genus Eozostrodon, had to compete with the larger, dominant dinosaurs. These early mammals were active at night, when dinosaurs were probably less active.
www.scilinks.org Topic: Mammals Keyword: HM60906
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