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MESOZOIC ERA

TRIASSIC PERIOD

251-201 million years ago

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MAMMAL

The first mammals evolved by the Mid-Late Triassic Period. Although many people associate fossil mammals with the Cenozoic Era (the "Age of Mammals") the first two thirds of mammalian history occurred during the Mesozoic Era (the "Age of Reptiles"). For about 160 million years, primitive mammals lived in a world dominated by reptiles. The majority of these mammals were small (the largest being about the size of a groundhog) and probably were more active at night.