Section 1

Origin and Evolution of Mammals

• Six key characteristics of mammals are endothermy, a fully divided heart, hair, milk production by females, a single jawbone, and a diversity of complex teeth.

• Mammals belong to an ancient group of animals called synapsids. Synapsids have a skull with one opening in a bone behind the eye socket.

Vocabulary endothermy (p. 861) mammary gland (p. 861) synapsid (p. 862)

therapsid (p. 862) monotreme (p. 863) oviparous (p. 863)

• Mammals are probably descended from a subgroup of early synapsids called therapsids. Therapsids had legs that were positioned beneath their body and had complex teeth.

• Small mammals first appeared in the Triassic period. Mammals underwent adaptive radiation after the Cretaceous mass extinction of dinosaurs.

marsupial (p. 863) viviparous (p. 863) placental mammal (p. 863)

placenta (p. 863)

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