Bird's—eye view: because modern dinosaurs are flying all around us, examining them closely can offer new insights into the lives of their fossilized ancestors

Natural History, May, 2005 by Matthew T. Carrano, Patrick M. O'Connor

The array of the dinosaurs that flourished during the Mesozoic era was as dazzling as any bestiary ever imagined; not even medieval fantasies of griffins and unicorns could compete with the fabulous record of fossils in rock. Yet with a single exception, the entire dinosaur lineage was obliterated 65 million years ago. The sole dinosaurian representatives to survive the cataclysm were the birds, a group that has since radiated into virtually every environment on the planet.

The suggestion of an evolutionary link between dinosaurs and birds originated with several late-nineteenth-century biologists, most notably Darwin's friend Thomas Henry Huxley. At first welcomed, the hypothesis was later disregarded by most biologists and treated with skepticism through much of the twentieth century. But in the past three decades, the hypothesis has roared back to life, with almost overwhelming support. The latest evidence for the link has come from the spectacular recent discoveries of a number of feathered dinosaurs in China.

To many a casual eye, the case is made by the presence of feathers on the fossils. But feathers only highlight one of the most visible similarities between the two groups. Biologists classify birds among the dinosaurs not only because both groups have (or had) feathers, but also because they share a suite of other, characteristic anatomical traits. One of those important traits is the "pneumaticity" of the skeleton: certain dinosaurs possessed bones riddled with air pockets, which during life were linked with the pulmonary, or breathing, system of the animal. Much the same is the case with many birds today.

The classification of birds as dinosaurs also implies that many other so-called avian features are better thought of as dinosaurian. And similar anatomies could imply that the bodies of birds and dinosaurs functioned similarly. Moreover, one may also learn a great deal about dinosaur biology by contrasting their features with the anatomical and biomechanical characteristics of other, more distantly related vertebrates. It is the birds, though, that have carried the torch of dinosaurian biological heritage from the Mesozoic through global calamity to the present day. Modern paleontologists, in large part by the light of that torch, are elucidating the paleobiological characteristics of those long-dead, long-buried, long-obscured animals.

To understand what one can learn about dinosaurs from the study of birds, it is useful to sketch how the two groups are related. A discipline of biology known as cladistics, or phylogenetic systematics, investigates the evolutionary relationships among organisms by charting their anatomical similarities. Cladistic hypotheses about such interrelations often take the form of a branching diagram called a cladogram. Each junction on the cladogram indicates an evolutionary event that split one lineage into two. Each of the two descendant lineages shares one or more features inherited from the ancestor at the most recent junction, and those shared features define different groups. To examine the relations within and between groups of organisms is also to chronicle the sequence by which those groups' features evolved.

According to the leading cladistic hypotheses, birds are descended from within the group of theropod dinosaurs. Theropods are quite familiar to most people, if not necessarily by that name: members include giant Tyrannosaurus, sickle-clawed Velociraptor, and birdlike Ornithomimus. Theropods such as Herrerasaurus, from the Middle Triassic are among the earliest known dinosaurs. [For a summary chart of geologic periods, see "Up Front," page 6.]

Theropods, like birds, were bipedal animals. All of them share several key features: thin-walled bones, a foot with three main toes, and a joint in the lower jaw. Early theropods sprit into two groups, the herrerasaur-like primitive theropods, and a group called the neotheropods, which included most of the familiar predatory dinosaurs [see the branch of the illustration on pages 40 and 41 outlined in blue]. Early neotheropods, known as the coelophysoids, were common in the Late Triassic and Early Jurassic.

As the neotheropods emerged as a separate group, they shared an important "birdlike" trait--the furcula, often (in birds) called the wishbone. The furcula is formed by the fused left and right clavicles, and in modern birds it acts as a spring between the powerful flapping wings. Clearly, though, the furcula did not function in that capacity in the earliest neotheropods. Although its original role remains unclear, it may have helped neotheropods control their forelimbs.

By the end of the Early Jurassic the theropods sprit again, giving rise to the ceratosaurs (a group that includes Ceratosaurus) and tetanurans (a diverse group that includes Allosaurus, Spinosaurus, Tyrannosaurus, Velociraptor, and a number of others). The tetanurans are named for their tails, which were less flexible than those of their forebears. Like the hand of a modern bird, the tetanuran hand had only three fingers; the tetanurans' wrist was more specialized, and their entire forelimb more birdlike, than the corresponding anatomy of any of the earlier theropods.