Like insects, some vertebrates evolved an egg adapted to tolerate dry conditions that accompany life on land. The amniotic egg has a desiccation-resistant shell and four membranes that permit gas exchange and management of waste products produced by the embryo (Chapter 42). These eggs must be fertilized internally before the eggshell is produced by the female because sperm cannot penetrate the shell. Amniotic eggs permit long development times, with the embryo’s nutrition supported by a large yolk or a placenta. They keep wastes separated from the embryo so they do not poison it, giving the embryo time to build more complex bodies than otherwise would be possible.

The amniotic egg can exchange gases while retaining water, and so permits the group of vertebrates known as amniotes to live in dry terrestrial habitats that amphibian eggs cannot tolerate. Amniotes include lizards, snakes, turtles, and crocodilians (about 6000 species of scaly animals commonly referred to as reptiles), as well as birds and mammals (Fig. 44.37). Most mammals, many lizards, and some snakes have evolved live birth rather than laying eggs. Nonetheless, they retain the specialized membranes characteristic of all amniotes. Instead of being wrapped around the embryo inside a tough egg shell, these membranes surround and protect the embryo inside the womb.

As shown in Fig. 44.31, amniotes form a monophyletic group with two major branches. One are the mammals, and the other contains turtles, birds, and the amniotes traditionally grouped together as reptiles. Like fish, however, reptiles are not monophyletic —birds are descended from reptiles, although you have to look closely to see the similarities between the two groups. Birds lack teeth, most of their scales have been modified into feathers (except on the legs and toes), and they are generally adapted for flight. Adaptations for flight include hollow bones that are light but remain strong, and a method of breathing that extracts much oxygen from each breath, permitting high-performance flight (Chapter 39).

One consequence of this streamlined, lightweight body is a lack of the heavy jaws that many other tetrapods use to grind up plant leaves or other low-nutrition foods. Instead, birds have their grinding organ, the gizzard (also found in other reptiles), inside their bodies near their center of gravity, where it interferes less with flight. Only one kind of bird, the weak-flying Amazonian Hoatzin, can actually feed on leaves. As discussed in Chapter 23, fossils show that birds diverged from dinosaur ancestors, and molecular sequence comparisons indicate that crocodiles are the closest living relatives of birds.

Quick Check 5 Why do bird feet have scales like those found on snakes and lizards?

All mammals are covered with hair and feed their young milk from the mammary glands for which the class Mammalia is named. Like many other groups of animals and plants, the early-branching mammals show intermediate stages in the evolution of the body plans that dominate on Earth today. The first fossils that clearly show hair appeared about 210 million years ago, during the Triassic Period. Like feathers in birds, hair permits retention of body heat. The earliest-branching living mammals, the monotremes, lay eggs, like birds or lizards (Fig. 44.38), but their hatched young drink milk secreted from pores in the skin of the mother’s belly. Among these are the Australian platypus and the echidna, or spiny anteater.

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The first mammals that gave birth to live young appeared in the Jurassic Period, about 160 million years ago. These animals gave rise to the two major living groups, marsupial and placental mammals. Marsupials include kangaroos, koalas, and related groups native to Australia, as well as the opossums found in the Americas. Their young are born at an early stage of development, and the tiny babies must crawl to a pouch where mammary glands equipped with nipples provide them with milk. Only later do fully formed juveniles emerge from the pouch and begin life on the ground.

The placental mammals are named for a temporary organ called the placenta that develops in the uterus along with the embryo, providing nutrition that enables the offspring to be larger and more quickly independent when born (Chapter 42). Most living mammals fall into this group. Placental mammals include the carnivores, such as lions and weasels; the primates, including monkeys, apes, and humans; and the hooved mammals, which include cattle, pigs, deer, and—perhaps surprisingly—their marine relatives, the whales.

All together, the familiar mammals mentioned so far number about 1000 species. The most diverse mammals, however, belong to two groups of mostly small animals, the bats and the rodents. Bats, which evolved flight and a form of sonar called echolocation to find food at night, include more than 1000 species. The largely plant-feeding rodents are as diverse as the large mammals and bats combined, numbering more than 2000 species. Beavers, gophers, rats, mice, hamsters, and squirrels are all rodents. Their evolutionary success is often attributed to innovations in tooth development. Their teeth are unusual in that they grow continually throughout life. This constant growth permits constant wear, and so allows these animals to gnaw through hard protective coverings to obtain nutrition from a remarkably wide variety of otherwise hard-to-access sources like the contents of seeds.

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