Birds Are Dinosaurs

Are Birds Just Feathered Dinosaurs?

The last of the dinosaurs are still flying around

Birds and dinosaurs are similar in many ways, not least in their ability to capture the human imagination. The evolutionary links between birds and dinosaurs are well known, and so people often wonder if the American Crow in their backyard, and the Eastern Kingbird divebombing it, are both modern-day dinosaurs.

The answer is yes: all birds are indeed dinosaurs. In fact, birds are the sole remaining descendants of the so-called theropod dinosaurs, a group that included Tyrannosaurus rex and a diversity of other dinosaurs, large and small. The theropod ancestry of birds was hotly debated in scientific circles in the 1970s and ’80s, and that controversy garnered a lot of attention in the popular press, but the origin of birds from theropod ancestors has since been supported by many new lines of evidence and is now nearly universally accepted by scientists who study bird and dinosaur evolution.

Many shared attributes link birds and theropods, from their brooding of eggs in nests to specialized features of their bone structure. But the most spectacular recent evidence for the dinosaur origins of birds is a series of exceptionally well-preserved dinosaur fossils discovered in China over the past decade or so. These 150-million-year-old fossils clearly show that some theropod dinosaurs had feathers, previously thought to be unique in birds. Other features of these fossils have allowed researchers to pinpoint the ancestry of birds in a sub-group of theropod dinosaurs called the maniraptors.

From Tyrannosaurus to Tyrannus, an ever-marching parade of change: Tyrannosaurus rex, Deinonychus, Archaeopteryx, Anhinga anhinga, and Tyrannus tyrannus (Eastern Kingbird)

art courtesy: Katherine A. Smith

Despite science fiction notions in popular culture, such as in Jurassic Park, it has not been possible to use DNA to explore the link between extinct dinosaurs and birds. DNA is a delicate molecule that degrades under even the best preservation conditions. The oldest DNA ever discovered is less than 100,000 years old. This means that while it is possible to obtain DNA from an ice-age woolly mammoth preserved in the arctic permafrost, no DNA remains in fossil dinosaur bones, more than a thousand times older.

Even without DNA, it has been possible to study dinosaur genetics indirectly. In 2007, a group of researchers used scans of both dinosaur and bird bones to show that the bone cells of birds and the dinosaurs most closely related to them were similarly small in size. Previous work had revealed that the size of these particular cells is related to the size of an animal’s genome (the overall amount of DNA in each of its cells). Smaller cells mean smaller genomes. Birds have unusually small genomes compared with other animals, and the small cell size seen in their dinosaur relatives is another feature that birds share with these dinosaurs.

Dinosaurs are still, of course, considered reptiles, leading to a natural follow-up question: if birds are really dinosaurs, and dinosaurs are reptiles, should birds be considered reptiles? The answer again is yes: herpetologists, the scientists who specialize in the study of amphibians and reptiles, take great pleasure in reminding their ornithologist colleagues that birds might correctly be thought of as “avian reptiles.”

story by: Irby Lovette, director / Fuller Evolutionary Biology Program

reprinted with permission: Cornell University Lab of Ornithology / BirdScope