Mammal Beginnings
The first mammals were small, shrewlike animals that were about 5 inches (12 cm) long. Related to today's monotremes, they first appeared during the Triassic Period, about 220 million years ago. They were descended from reptiles called synapsids , which appeared about 300 million years ago. These primitive mammals evolved into different groups during the Jurassic and Cretaceous periods (208-65 million years ago). Most of these early mammals were carnivores (meat eaters), but some, such as the tree-living multituberculates, which ranged from animals the size of mice to some as big as beavers, ate plants. The ancestors of today's marsupials, insectivores and primates first appeared in the Cretaceous Period (145-65 million years ago). When the dinosaurs died out at the end of the Cretaceous Period, these modern mammals spread to every continent and evolved into thousands of new species. The First Mammal - Megazostrodon - Megazostrodon, which lived in Africa abou...