Evolution Watch: A Sickle in the Clouds

A 70-million-year-old fossil bird recently found in Madagascar may just clinch the argument that birds descended from two-legged dinosaurs.

By Carl Zimmer|Monday, June 01, 1998
RELATED TAGS: DINOSAURS
For 139 years, ever since its delicate bones and feathers were found splayed on a limestone slab in a German quarry, Archaeopteryx has had the distinction of being the most primitive bird known. Its clawed wings, toothy beak, and long bony tail, most paleontologists now believe, show the transformation of a lineage of bipedal dinosaurs into birds. Since its discovery, the 145-million-year-old Archaeopteryx has perched alone on a low branch of the avian evolutionary tree. Now the old bird finally has company. In March, paleontologists described a 65- to 70-million-year-old bird from Madagascar, which, though younger than Archaeopteryx, is almost as primitive. In life it would have resembled a little winged Velociraptor.

Paleontologists found the fossil in a quarry in northwestern Madagascar. Scott Sampson of New York College of Osteopathic Medicine in Old Westbury, New York, came across a wing bone embedded in rock; suspecting that more of the skeleton might be hidden in the rock, he dug around the bone and shipped a yard-long block to the State University of New York at Stony Brook, where the fossils could be carefully extracted. Judging from the wing bone, the researchers thought they had a relatively ordinary bird (it even had nodes on one edge where feathers were once lodged)—until they uncovered the tail. Modern birds have a stub to anchor a fan of feathers; the Madagascar bird had a bony tail.

It just looked really primitive, says Catherine Forster of Stony Brook. I remember a preparator was working on the foot, and I joked that wouldn’t it be funny if the foot had a big sickle claw? She was remembering that in the 1970s John Ostrom of Yale had suggested that birds were closely related to dromaeosaurs, predatory dinosaurs with giant sickle claws on their feet. This group included dinosaurs such as Deinonychus and Velociraptor, which probably disemboweled prey with their claws. By the time Archaeopteryx had evolved, most paleontologists assumed, this weapon had dwindled to an ordinary bird claw. After she made her joke, Forster left the lab to teach an anatomy class. Half an hour later a colleague called from the lab to tell her that the bird did indeed have an inch-long sickle claw.

The researchers recovered part of the bird’s tail and spine, the pelvis, much of its legs, parts of its arms, and a shoulder blade. To determine its closest relatives, Forster compared it with seven birds and eight dinosaurs, using a computer to calculate the simplest evolutionary pattern that could produce their anatomic similarities. Her analysis showed that the Madagascar bird was nearly as primitive as Archaeopteryx, possibly located on the same side branch of bird evolution.

Forster and her colleagues have named the bird Rahonavis ostromi. Rahona is a Malagasy word, says Forster, with a double meaning: ‘cloud’ and ‘menace.’ We thought it was a good name for a flying killer.

Forster also honored Ostrom with the name because the fossil further supports the bird-dinosaur connection. Although most paleontologists subscribe to Ostrom’s ideas today, there are critics. Some argue that dinosaurs lost their fourth and fifth fingers while birds are missing their first and fifth. Still others have looked at dinosaur fossils found recently in China and have argued that they had crocodile-like lungs radically unlike those of birds.

Yet to Forster and others, the many primitive bird and birdlike dinosaur fossils found recently in Spain, China, and Argentina bolster Ostrom’s theory. Rahonavis, says Forster, is a particularly strong link in the chain. It’s a primitive bird, it looks almost exactly like Archaeopteryx, and it has a dromaeosaur foot, which is a pretty clear link to its dinosaur origins.
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