Mammals Members of the traditional class Mammalia include about 5000 species that range in size from mice to whales. Characteristics unique to mammals include mammary glands in females, which produce milk to nourish young, and hair. Mammals also breathe air, have four-chambered hearts, and regulate their internal body temperature. DOL•60–DOL•64

▸ The First Mammals True mammals first appeared during the late Triassic Period, about 220 million years ago. They were very small and resembled modern tree shrews, like the one in Figure 26–13. While dinosaurs ruled, mammals remained generally small and were probably active mostly at night. New fossils and DNA analyses suggest, however, that the first members of modern mammalian groups, including primates, rodents, and hoofed mammals, evolved during this period. After the great dinosaur extinction at the end of the Cretaceous Period, about 65 million years ago, mammals underwent a long adaptive radiation. Over millions of years, mammals diversified, increased in size, and occupied many niches. The Cenozoic Era, which began at the end of the Cretaceous Period, is usually called the Age of Mammals.

▸ Modern Mammals By the beginning of the Cenozoic Era, three major groups of mammals had evolved—monotremes (MAHN oh treemz), marsupials (mahr SOO pee ulz), and placentals. These three groups differ in their means of reproduction and development.

Only five species of the egg-laying monotremes, including the duckbill platypus, exist today, all in Australia and New Guinea. Marsupials, which include kangaroos, koalas, and wombats, bear live young that usually complete their development in an external pouch. Placental mammals—which include most of the mammals you are familiar with—have embryos that develop further while still inside the mother. After birth, most placental mammals care for their young and nurse them to provide nourishment.

FIGURE 26–13 Early Mammal Look-Alike The first mammals appeared on Earth about 220 million years ago. They may have resembled this modern tree shrew and probably, like this tree shrew, ate insects.

26.2 Assessment

Review Key Concepts

1. 1. Review Name the group of animals whose ancestors were related to the earliest chordates.

   2. Compare and Contrast Why did scientists classify Pikaia as a chordate instead of as a worm?

2. 1. Review What two aspects of evolutionary history does the cladogram of chordates show?

   2. Explain How do nonvertebrate chordates differ from other chordates?

   3. Interpret Visuals According to Figure 26–5, which chordate feature evolved earlier—endothermy or lungs?

Apply the Big idea

Evolution

3. Recall what you learned about plant evolution in Chapter 22. Based on the evolutionary changes shown in the cladograms on pages 636 and 758, identify the first major adaptations that allowed plants and chordates to live on land. In what ways are chordate adaptations to life on land similar to plant adaptations to life on land?

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