We know when mammals first evolved because they left fossil evidence in the form of

Name: We know when mammals first evolved because they left fossil evidence in the form of
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Heterodont teeth are unique to mammals and can be seen in the fossil record, making them the best evidence for when mammals first evolved. Four chambered hearts, warm bloodedness, and new reproductive strategies evolved later in mammalian evolution and cannot be seen in early fossils.

We Know When Mammals First Evolved Because They Left Fossil