Laura Erickson's For the Birds

Dinosaurs

Where did birds come from? Dinosaurs? Some other reptiles? Ornithologists and paleontologists have debated the question for over a century. Nowadays just about all of them subscribe to one of just two main theories about the origin of birds. One is called the “pseudosuchian thecodont hypothesis.” This theory suggests that birds evolved about 230 million years ago from thecodont reptiles. The other, the “dinosaur theory,” suggests that birds evolved about 150 million years ago from theropod dinosaurs.

There is no dispute whatsoever that birds evolved from diapsid reptiles–the same group that led to all modern reptiles except turtles. There are two clearly distinguished groups of diapsids. No one believes that birds evolved from the line that led to modern snakes and lizards. Birds are placed in the Archosauromorpha, which includes the ancient thecodonts and their descendants–crocodiles, dinosaurs, pterosaurs, and birds. Of animals living today, ornithologists consider birds most closely related to crocodiles.

The question isn’t whether birds evolved from thecodonts, but WHEN. Thomas Huxley hypothesized that birds evolved directly from dinosaurs. He found the similarities between one dinosaur, Compsognathus, and birds very telling. But Huxley was mainly comparing modern birds like chickens to Compsognathus–he didn’t make his comparisons using the oldest known bird fossil, Archaeopteryx. The dinosaurs most related to birds according to this theory were species like Deinonychus and Velociraptor. At the end of the movie Jurassic Park, as the velociraptor changes before our eyes to a flying bird, we see a vivid graphic representation of Huxley’s hypothesis.

Robert Broom, another Paleontologist, thought Huxley was wrong and set forth the pseudosuchian thecodont hypothesis. Broom saw great similarities between fossils of Euparkeria, a small, 230-million-year-old thecodont, and birds, and believed that birds evolved from the thecodonts millions of years before dinosaurs came about. Broom’s theory remained the most popular one until the 1970s.

In 1973, another paleontologist, John Ostrom, renewed interest in the dinosaur theory when he proposed that Deinonychus and Archaeopteryx were so similar that they were most certainly related. People disputing his theory said that the similarities could be due to convergent evolution.

Archaeopteryx certainly looks like a feathered dinosaur. But it has some features that are found in NO dinosaurs, such as a fully reversed hallux and highly curved claws (both important for arboreal species, and awkward for land species). And Archaeopteryx has birdlike skull bones–the occiput and quadrate bones–and conical teeth devoid of serrations found in theropods. And its feathers have all the structures found in modern bird feathers. So paleontologists opposed to the dinosaur believe that Archaeopteryx and the theropods came from a common ancestor, but diverged before the Cretaceous. In support of them, the entire group of theropods, the dinosaurs thought to have given rise to birds, is limited to the Cretaceous period, while Archaeopteryx was found in the Jurassic (BEFORE the theropods were around).

So–I don’t know what the answer is, but it’s certainly intriguing to think about it. The pressures that led to the mass extinction of dinosaurs also led to the rise of Resplendent Quetzals and hummingbirds. Now, our own species’ proclivity to squander more and more natural resources is leading to the enormous success of rats, mice, crows, gulls, and sparrows at the expense of lovely specialists. With luck we’ll be able to work out the origin of various species before we cause the extinction of our own.