11.1.5 11.17: Birds are reptiles in which feathers evolved.

Soon after amniotic vertebrates appeared, two different evolutionary lineages began to diverge (FIGURE 11-27). One of these lineages is the mammals, the group to which humans belong. The other lineage is the one referred to in this chapter as “reptiles (including birds).” That is an awkward term, but the evolutionary tree shows why it is necessary. Surprisingly, birds (about 9,700 species) are one branch of the reptile lineage that also includes snakes and lizards (about 8,000 species), turtles (about 300 species), crocodiles and alligators (23 species), the New Zealand tuatara (2 species)—and the dinosaurs.

The characteristics that hold the bird-crocodile-dinosaur group together are mostly similarities in bones (especially the bones of the skull and legs) and DNA sequences. Reptiles are amniotes and thus have amniotic eggs. You are familiar with the hard-shelled eggs of birds, and some lizard, snake, and turtle eggs also have hard shells. Other species in these groups have eggs with a paper-like shell.

Most people find it confusing that birds are grouped with turtles, lizards and snakes, and alligators and crocodiles, and that confusion is easy to understand (FIGURE 11-28). After all, birds are covered by feathers and can fly, whereas the other reptiles have bare skin and do not fly. And there is an important physiological difference as well: birds are endotherms, meaning they use the heat produced by cellular respiration to raise their body temperature above air temperature, whereas other reptiles are ectotherms—they bask in the sun to raise their body temperature and seek the shade when the air is too warm.

How can animals as different as crocodiles and hawks be closely related? During the Mesozoic era, from about 250 million to 65 million years ago, dinosaurs were the dominant terrestrial vertebrates. The fossil record shows a remarkably clear series of animal forms that bridge the transition between bare-skinned dinosaurs and feathered birds. Birds are, in fact, just one group of dinosaurs.

The most apparent difference between reptiles (including most dinosaurs) and birds is the presence of feathers in birds. The fossil record reveals that feathers evolved before birds. Fossil evidence also suggests that the initial evolution of feathers probably had nothing to do with flight.

Among reptiles, many different species had feathers. A small, agile dinosaur called Sinosauropteryx, for example, living about 120 million years ago, had feathers that were simple spiky filaments on its neck, back, and tail, along with shorter filaments covering its body (FIGURE 11-29). The 2010 discovery of fossilized pigment-packed organelles—called melanosomes—within these feathers indicates that the feathers and filaments were probably brightly colored. Researchers believe that they may have been used by male Sinosauropteryx for courtship displays to females, as well as for aggressive displays to other males. Sinosauropteryx and many of the other feathered dinosaurs were latecomers, however. The pigeon-sized Archaeopteryx, which was among the first bird species, was already, by 147 million years ago, using its feathers to fly; by 140 million years ago, the skies were filled with birds.

Another big difference between birds and reptiles is that, as noted earlier, birds use internally generated heat to maintain a high body temperature, whereas reptiles rely on the sun to heat their bodies. This evolutionary change may have been related to feathers. As feathers evolved for display and flight, they may also have provided some insulation. This insulation may then have enabled the evolution of high rates of cellular respiration and the maintenance of a high and constant body temperature.