Bumpy Bones: Fossil hints that dinosaur had feathery forearms

September 19, 2007 at 1:55 pm

Several knobs on a forearm bone from a 1.5-meter-long predatory dinosaur provide the first direct evidence of substantial feathers on a dinosaur that large.

FOSSIL PHRENOLOGY. Regularly spaced bumps on the 80-million-year-old fossil of a velociraptor (arrows, top), like those on the comparable bone of a modern turkey vulture (middle and bottom), probably supported substantial feathers. Science

Scientists first described fossils of Velociraptor mongoliensis in 1924, but the creature gained pop-culture notoriety in the 1993 film Jurassic Park. Researchers have unearthed remains of only about 20 velociraptors, and most discoveries have taken place in the past 15 years or so, says Alan H. Turner, a paleontologist at the American Museum of Natural History in New York City.

Now, Turner and his colleagues have scrutinized the 80-million-year-old fossils of a velociraptor unearthed in the Gobi Desert in 1998. Sixty percent of the specimen’s bones have been recovered, and the scientists found a feature not seen on other velociraptors: a series of six regularly spaced bumps on the ulna, one of the bones in its forearm. Each protuberance is about 0.8 millimeter across and is spaced about 4 mm from its neighbor.

On modern birds, such bumps, called quill knobs, are where ligaments connect the major flight feathers of the wing to the bone. Not all birds have quill knobs, and the number of such bumps can vary even among birds of the same species, says Turner. However, the presence of quill knobs on a bone is a sure sign that feathers were attached there, he adds.

The bumps on the velociraptor fossil have a rough texture, as the quill knobs of modern birds do, which further indicates that ligaments were attached there, says Turner. Although he and his colleagues found only six such protuberances, their spacing on the bone suggests that up to 14 large feathers—some not attached to quill knobs—could have adorned each forelimb. The researchers report their findings in the Sept. 21 Science.

Some fossils of several small dinosaurs that are closely related to velociraptors, such as those of the genus Microraptor, have included feathers (SN: 1/25/03, p. 51). Members of those species typically measure no more than 1 m long, and they presumably glided from tree to tree (SN: 1/27/07, p. 53). Velociraptors, however, were not only larger than microraptors but also too hefty to get airborne. The 1.5-m-long specimen that Turner and his colleagues analyzed probably stood about 1 m tall at the hip and weighed around 15 kilograms.

Nor would just a few large wing feathers have helped velociraptors regulate their body temperatures, says Turner. But the animals could have used such feathers to shield a nest from sunlight or from the ever-prying eyes of predators. The feathers might also have increased maneuverability, enabling the animals to make faster turns or higher leaps, Turner notes.

The presence of quill knobs suggests that the feathers had some sort of aerodynamic, load-bearing function, says Thomas R. Holtz Jr., a vertebrate paleontologist at the University of Maryland at College Park. “This goes to show that even a well-known dinosaur like Velociraptor [mongoliensis] can still provide surprises,” he adds.