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What are Mammals? Mammals are amniotes like their relatives and reptiles. They all share certain features in the skull, jaws, and teeth. All mammals are warm-blooded, have hair, and produce milk through mammary glands, which is how they got their name. Humans are mammals, as are many of the animals that share our lives as pets or farm animals. Many of the earliest mammals were tiny and are represented in the fossil record only by teeth, the most durable parts of the mammalian skeleton. The first mammals appear in the Late Triassic.
First known fossil occurrence: Triassic.
Last known fossil occurrence: Quaternary. This group has living relatives.
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