Unit Five. Evolution of Animal Life
20. History of the Vertebrates
Birds evolved from small bipedal dinosaurs about 150 million years ago, but they were not common until the flying reptiles called pterosaurs became extinct along with the dinosaurs. Unlike pterosaurs, birds are insulated with feathers. Birds are so structurally similar to dinosaurs in all other respects that there is little doubt that birds are their direct descendants. All but a few taxonomists, however, still continue to place birds in their own class, Aves, rather than lumping them in with reptiles, because birds are such a distinct and important group.
Modern birds lack teeth and have only vestigial tails, but they still retain many reptilian characteristics. For instance, birds lay amniotic eggs, although the shells of bird eggs are hard rather than leathery. Also, reptilian scales are present on the feet and lower legs of birds. What makes birds unique? What distinguishes them from living reptiles?
1. Feathers. Derived from reptilian scales, feathers serve two functions: providing lift for flight and conserving heat. Feathers consist of a center shaft with barbs extending out (figure 20.20). The barbs are held together with secondary branches called barbules that hook over each other. This reinforces the structure of the feather without adding much weight to it. Like scales, feathers can be replaced. Among living animals, feathers are unique to birds. Several types of dinosaurs also possessed feathers, some with bands of color.
Figure 20.20. Feathers.
The barbs off the main shaft of a feather have secondary branches called barbules. The barbules of adjacent barbs are attached to one another by microscopic hooks.
2. Flight skeleton. The bones of birds are thin and hollow. Many of the bones are fused, making the bird skeleton more rigid than a reptilian skeleton and forming a sturdy frame that anchors muscles during flight. The power for active flight comes from large breast muscles that can make up 30% of a bird’s total body weight. They stretch down from the wing and attach to the breastbone, which is greatly enlarged and bears a prominent keel for muscle attachment. They also attach to the fused collarbones that form the so-called wishbone. No other living vertebrates have a fused collarbone or a keeled breastbone.
Birds, like mammals, are endothermic. They generate enough heat through metabolism to maintain a high body temperature. Birds maintain body temperatures significantly higher than those of most mammals. The high body temperature permits a faster metabolism, necessary to satisfy the large energy requirements of flight.
History of Birds
The oldest bird of which there is a clear fossil is Archaeopteryx (meaning “ancient wing” and shown in figure 20.21), which was about the size of a crow and shared many features with small, bipedal, carnivorous dinosaurs. For example, it had teeth and a long reptilian tail. And unlike the hollow bones of today’s birds, its bones were solid. Fossils recently discovered in China show that several species of dinosaurs possessed feathers or structures similar to feathers. In these, however, the feathers were most likely used for insulation or display. By the early Cretaceous, only a few million years after Archaeopteryx, a diverse array of birds had evolved, with many of the features of modern birds. The diverse birds of the Cretaceous shared the skies with pterosaurs for 70 million years.
Figure 20.21. Archaeopteryx.
Archaeopteryx lived about 150 million years ago and is the oldest fossil bird.
Today about 8,600 species of birds (class Aves) occupy a variety of habitats all over the world. The major orders of birds are reviewed in table 20.5. You can tell a great deal about their lifestyles by examining their beaks. For example, carnivorous birds such as hawks have a sharp beak for tearing apart meat, the beaks of ducks are flat for shoveling through mud, and the beaks of finches are short and thick for crushing seeds.
TABLE 20.5. MAJOR ORDERS OF BIRDS
Key Learning Outcome 20.7. Birds are descendants of dinosaurs. Feathers and a strong, light skeleton make flight possible.
A Closer Look
Are Birds Dinosaurs?
Archaeopteryx (pronounced "ahr-kee-OP-tuh-riks”) is the first bird for which we have clear fossil evidence. About the size of a crow, the first fossil was found in a 150-million-year-old limestone quarry in Bavaria in 1861. It had the clawed fingers and long bony tail of a dinosaur, with the wishbone and feathered wings of a bird.
For more than a century people have argued about Archaeopteryx. Did Archaeopteryx evolve from a dinosaur, or from some other reptile? The preponderance of evidence favors a dinosaur ancestor. Archaeopteryx is remarkably like a theropod dinosaur called Velociraptor. You may remember velociraptors as the scary creatures that stalked the kids in the kitchen in the film Jurassic Park. Like Velociraptor, Archaeopteryx has an unusual swivel-jointed wrist; a long, very deep shoulder blade; a fused collar bone (familiar as the "wishbone” of Thanksgiving turkeys); and many other shared features.
The convincing proof of a dinosaur-bird direct link was the discovery in 1996 in China of dinosaurs with feathers. The first of these, called Sinosauropteryx, had no wings, but was covered with a light featherlike fuzz. A dinosaur with a feather coat, or just stubble?
In 2010, researchers succeeded in answering that question. Paleontologist Mike Benton and colleagues at the University of Bristol, England reported that the simple blunt feathers of Sinosauropteryx contain tiny baglike organelles called melanosomes, each stuffed with either the black pigment melanin (the same pigment that makes a crow black), or the reddish-brown pigment pheomelanin (which gives a chestnut color similar to that seen in many racehorses). The presence of melanosomes is a common occurrence in the animal kingdom. They are found within the exterior cells of all the major groups of vertebrates. The discovery of melanosomes by the Bristol group proves beyond dispute that the bristles studding Sinosauropteryx are indeed feathers and not collagen fiber fuzz.
In birds today, the shape and arrangement of melanosomes helps produce the color of bird feathers. Using scanning electron microscopes to examine the surfaces of the dinosaur feathers, the University of Bristol researchers found that the Sinosauropteryx melanosomes were not randomly located along the tail feathers, but rather occurred in broad bands—the dinosaur had alternating orange and white rings down its tail!
In the years since Sinosauropteryx was discovered, another very exciting fossil has come to light from the same Chinese fossil fields. Called Caudipteryx (that's "caw-DlP-ter-iks”), Greek for "tail feathers,” the fossil dinosaur has large feathers on its tail and arms. Big feathers, not bristles, with much of the detailed architecture of modern bird feathers. Two of these feathered dinosaurs were discovered in 1998, and a third beautifully preserved specimen was reported a few years later.
While Caudipteryx has a handful of birdlike features, including feathers, it has many features in common with dinosaurs like Velociraptor, including short arms, serrated teeth, a velociraptor-like pelvis, and a bony bar behind the eye. Paleontologists who have studied the new fossils describe Caudipteryx as sitting on a branch of the dinosaur family tree between Velociraptor and Archaeopteryx.
It seems feathers are not a distinguishing trait of birds. They first evolved among the dinosaurs. Because the arms of Caudipteryx were too short to use as wings, feathers probably didn't evolve for flight. Instead, they could have served as a colorful display to attract mates (as they do in peacocks today), or as insulation (as they do in penguins today). Flight is something that certain kinds of dinosaurs achieved as they evolved longer arms. We call these dinosaurs birds.
Why then continue to assign birds to a separate class, Aves? If birds are dinosaurs, why not just toss them in with the reptiles, as dinosaurs are? Because classification must do more than just reflect the order in which organisms evolved. It also serves a very practical function. The feathers and flight of today's birds set them apart in such a fundamental way that most biologists, well aware of their dinosaur roots, still choose to assign them their own unique class, Aves. Others have abandoned Aves, and call birds another kind of reptile. Its a judgement call, and the jury is still out.