Dinosaurs are distinguished from other reptiles by the way their legs are joined to their hips so that their feet strike the ground beneath the body, much like humans. Thus other reptiles, such as the extinct pterosaurs and ichthyosaurs, and the living crocodiles and lizards, are not dinosaurs because their legs are sprawled to the side.
Dinosaurs come in all shapes and sizes. But two distinct groups can be recognized according to how the pelvic (hip) bones are arranged: Saurischia if arranged like a lizard’s, and Ornithischia if somewhat more complex like that of birds.
Evolutionary Relationship Between Bird and Dinosaur
Despite its name and a bird-like pelvis, Ornithischia does not appear to be the ancestor of birds. Saurischians, particularly from a group called Theropoda, have much more in common with birds. Theropod characteristics include a bipedal gait, 3-toed feet, hollow and thin-walled bones, short grasping forearms, and claws on fingers and toes (the familiar Tyrannosaurus is a theropod).
These features were identified in a number of early fossil birds, including the famous Archaeopteryx, which resembled another theropod called Velociraptor except for the feathers. One Archaeopteryx fossil was misidentified as a pterodactyl, a flying reptile, because the poor-quality fossilized feather imprints were initially overlooked.
Archaeopteryx is generally undisputed as a bird because well-developed feathers are found over much of its body. It was probably already capable of flight, albeit limited because of its relatively heavy bones (e.g. jaws). But critics claim that, if birds evolved from reptiles in a single family tree, we should find fossils with feathers intermediate between scales and the advanced feathers on Archaeopteryx.
Discoveries of Feathered Theropods
The first breakthrough came with the discovery in the 1990s of a theropod called Sinosauropteryx, which has simple feathers but no wings. Then more feathered theropod fossils were discovered at the same site in China, including ones with feathers more primitive than those of Archaeopteryx. The undeveloped state of feathers suggests these dinosaurs were incapable of flight.
However, these fossils were dated younger than 145 million years old, which is the approximate age of Archaeopteryx. Skeptics have argued that these feathered theropods could not have been ancestral to the birds because they were younger than Archaeopteryx.
Then came the announcement in 2009 that several new species of feathered theropods unearthed were more than 150 million years old, thereby supporting the idea that birds evolved from dinosaurs and Archaeopteryx evolved from one of the earlier birds. The suggestion that scale led to an unbranched hollow tube and eventually to flight feather is no longer a fanciful idea.
Terminological Relationship Between Bird and Dinosaur
Today, more than a dozen genera of feathered dinosaurs have been identified. With so many feathered dinosaurs discovered, features used to distinguish them from birds no longer apply. Not so long ago it was only birds that have toothless beaks. We now know prehistoric birds also had teeth, and some dinosaurs (e.g. Erlikosaurus) have a toothless beak as well. Even the furcula (wishbone) is no longer unique to birds as it is found in some feathered theropods (e.g. Microraptor).
We should remember that the definition and classification of a bird (or any organism for that matter) are a human concept. With more data, which no doubt will emerge, new ways of defining a bird will also emerge. For now, definition of a (especially early) bird is applicable also to a feathered dinosaur.
References
D. Hu, L. Hou, L. Zhang & X. Xu (2009) A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus. Nature 461, 640-643.
J.A. Long & P. Schouten (2009) Feathered Dinosaurs: the Origin of Birds. Oxford University Press.
X. Xu (2006) Scales, feathers and dinosaurs. Nature 440: 287-288.
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