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Age of Mammals

First mammals

Gorgonops, a therapsid reptileGorgonops, a therapsid reptileMammals are descended from mammal-like reptiles, called pelycosaurs, that lived on Earth about 300 million years ago. The first warm-blooded animals—being warm-blooded is a basic feature of all mammals—were probably the cynodonts, which first evolved about 260 million years ago. Small, fast-moving carnivorous reptiles from the therapsid group, the cynodonts probably had fur instead of scales. The first true mammals made their appearance about 210 million years ago. These tiny, shrew-like animals lived in a world already dominated by the dinosaurs, so they generally adopted a nocturnal lifestyle.

Mammal-like reptiles

ThrinaxodonThrinaxodonThrinaxodon was a cynodont. A small carnivore with a very un-lizard-like, upright posture, it lived in southern Africa about 240 million years ago. Its ability to eat and digest its food quickly—using its efficient teeth—provided it with internal heat, meaning it was probably a warm-blooded animal. It is most likely Thrinaxodon had a fur coat as this would have helped prevent its internal heat from escaping and so keep its body at a stable temperature. In its skull there was a hard palate separating its mouth from the nasal passage, meaning it could eat and breathe at the same time.

All mammals today—giraffes, tigers, whales, kangaroos and humans included—are descended from creatures like tiny Morganucodon.

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