How Birds Evolved From Dinosaurs: Fossil and Molecular Evidence

Introduction

Evolutionary studies, particularly the evolution of birds, have always evoked tremendous curiosity and fascination among the scientific community as well as the general public. The idea that birds actually evolved from dinosaurs dates to the second half of the nineteenth century, when two of the greatest scientists of that period — Richard Owen and T. H. Huxley — proposed this hypothesis backed by documented anatomical evidence. The discovery of Archaeopteryx, an archetypal primitive bird, provided the basis for this hypothesis. Subsequent discoveries of abundant and well-preserved fossils of both dinosaurs and birds have offered extraordinary proof of the dinosaurian origin of birds, the beginnings of flight, and the origin of feathers before the beginning of flapping flight (Zhou, 458; Norell, Gaffney, and Dingus, 11).

Brief History of Dinosaurs

It is believed that dinosaurs first evolved during the Late Triassic and included both herbivores and predators, among them four-toed and bipedal — that is, bird-like — predatory dinosaurs. This period was marked by the appearance of dinosaurs such as Coelophysis, which were bird-footed, flesh-eating, and gracile. The dinosaurs survived the end of the Triassic well into the Jurassic period, when many other kinds of reptiles also flourished. This was a period when birds became common in the air, in the water, and on land. However, very little evidence of birds has been found from most of the Jurassic period; fossils such as Archaeopteryx appear only from the end of the Jurassic, approximately 150 million years ago (Paul, 5).

The bipedal predators, or theropods, were obligatory bipeds. Prior to the end of the Triassic period, the neotheropods had split into two lines: the ceratosaurs and the tetanurans. These tetanurans proliferated on all continents by the mid-Jurassic and gradually divided into two main groups — the allosauroids and the coelurosaurs. It is the coelurosaurs that eventually gave rise to birds by the end of the Jurassic period (Sereno, 2141). The coelurosaurs comprised both large- and small-bodied theropods with characteristic features in the limb bones, vertebrae, and skull. They included ornithomimids, tyrannosaurids, therizinosaurids, and maniraptors — the group that included birds (Sereno, 451).

The Dinosaurian Origin of Birds: Theory and Evidence

It was the discovery of an amazingly bird-like 150-million-year-old fossil in the swamps of Germany in 1869 that started the debate about the dinosaurian origins of birds. Archaeopteryx had long, strong legs with three clawed toes quite like those of birds. Its head, however, bore reptilian jaws, and its spine extended into a reptile-like bony tail. The most remarkable feature was the presence of feathers — a unique characteristic of birds — on both sides of its tail bones (Davies, Evolution). Later findings of bird-like dinosaurs in the early 1990s from Mongolia and Argentina gave further credence to this theory. The fossilized remains of Mononykus from Mongolia and Alvarezsaurus from Argentina blurred the boundary between flightless birds and running bipedal dinosaurs.

The most recent significant discovery was a feathered dinosaur fossil found in China. This species, Anchiornis huxleyi — a small, approximately 13-inch theropod whose forelimbs and body were feathered — is believed to be even more primitive than Archaeopteryx. Anchiornis is probably closer to the point at which divergence from dinosaurs took place and a new line of "dinosaurs with wings" first evolved. These fossils have provided invaluable information about the evolution of feathers, wings, and flight among theropods, ultimately paving the way for the appearance of modern birds (Long and Schouten, 25; Platt, New Feathered Dinosaur Found).