There are many similarities between birds and some dinosaurs, such as Caudipteryx show in Figure 42-1. Three of these similarities include a flexible S-shaped neck, a unique ankle joint, and hollow bones. Birds are thought to have evolved from small, fast-running carnivorous dinosaurs during the Jurassic period (200-146 million years ago). Figure 42-1 shows the likely relationships between birds and other terrestrial vertebrates.
The oldest known bird fossils are classified in the genus Archaeopteryx and date from the late Jurassic period, about 150 million years ago. In the fossil in Figure 42-2a, the shapes of feathers are clearly visible. Feathers covered Archaeopteryx's forelimbs, forming wings, and covered its body and tail as well. Like modern birds, Archaeopteryx had hollow bones and a furcula (FUHR-kyoo-luh), the fused pair of collarbones commonly called a wishbone. The furcula plays an important role in flight by helping to stabilize the shoulder joint. Based on such similarities with modern birds, scientists think that Archaeopteryx could fly. However, Archaeopteryx also had several characteristics of its dinosaur ancestors, including teeth, claws on its forelimbs, and a long, bony tail. Figure 42-2b shows an artist's conception of what an Archaeopteryx might have looked like.
The evolution of a flying animal from nonflying ancestors entails many changes in anatomy, physiology, and behavior. According to one hypothesis, the ancestors of birds were tree dwellers that ran along branches and occasionally jumped between branches and trees. Wings that allowed these animals to glide evolved. Once gliding was possible, the ability to fly by flapping the wings evolved.
Another hypothesis draws on the fact that the dinosaurs most closely related to birds were terrestrial and states that the evolution of birds must have occurred on the ground, not in the trees. Wings may have originally served to stabilize the animals as they leapt after prey. Or they may have been used for trapping or knocking down insect prey. Over generations, the wings became large enough to allow the animal to become airborne.
A number of recent discoveries show that by the early Cretaceous period (146-66 million years ago), birds had begun diversifying. Sinornis, a 140-million-year-old specimen discovered in China in 1987, had some key features of modern birds, including a shortened, fused tail and a wrist joint that allowed the wings to be folded against the body. The diversification of birds continued throughout the Cretaceous period. Figures 42-2c and 42-2d show two birds from the late Cretaceous period.
Only two of the modern orders of birds had appeared by the end of the Cretaceous period. Birds survived the global catastrophe that is thought to have wiped out the dinosaurs, and then underwent a dramatic and rapid evolutionary radiation. By about 40 million years ago, most of the modern orders of birds had originated.
(d) Hesperornis figure 42-2
In this fossil of Archaeopteryx. (a), one can find characteristics of both birds and dinosaurs. These artist's renderings of three extinct birds are based on fossil evidence. Archaeopteryx (b) is the oldest bird; it still had claws on its wings. Ichthyornis (c) had strongly developed wings and was about 21-26 cm (8-10 in.) in length. Hesperornis (d) was considered flightless, but its well-developed legs made it a strong swimmer.
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