Many different kinds of animals coexisted during the Jurassic period, from primitive birds to two-legged, large-jawed dinosaurs. As different as they sound, paleontologists have found extensive evidence that they share a common ancestry. Two new studies of fossils from China show that bird-like dinosaurs predate the earliest official birds, and shed light on the origin of feathers.

Scientists have generally accepted that birds and theropods, bipedal dinosaurs à la Tyrannosaurus rex, share a common history somewhere back down the genetic line, but they've had trouble locating ancestors of the two groups. The early appearance of the first known bird-like animal, the archaeopteryx, relative to theropods has been particularly confounding, and has even been taken as evidence by some paleontologists that the two groups are unrelated.

A new fossil of a theropod has been uncovered in China that may help settle the matter. The remains are of an alvarezsaur, a small bipedal dinosaur that paleontologists have previously suspected was related to birds, and date to the start of the Late Jurassic period. That extends the records of alvarezsaurs by 63 million years, and places their origin at least a few million years earlier than the archaeopteryx.

The fossils of the alvarezsaur had two shrunken digits on its hands, a possible indicator that that the species' arms were well on their way to becoming some sort of specialized appendage. However, even the existence of a dinosaur with disappearing fingers still leaves a gap of millions of years to fill in the history of birdlike species.

The precise order of other adaptations was also the subject of a recent find. Some time ago, a few theropod specimens discovered in China were said to show evidence of having had feathers, although the claim was controversial. A number of scientists said the evidence was just degraded collagen, but others insisted that dinosaur's skin had some kind of filament covering, such as hair or feathers, possibly even the robust vaned feathers found in modern bird wings, the kind used for quills.

Paleontologists have now used scanning electron microscopy to study some of the theropod fossils and found melanosomes, the organelles that create color in feathers, which were previously thought to be unable to survive fossilization. The researchers noted that the fossilized remains of the melanosomes were packed and layered in a configuration found in modern feathers.

Two prominent varieties of melanosomes can create brown or black shades, and the paper's authors were able to find both in their samples. One dinosaur, the Sinornithosaurus, had filaments that were one color or the other in different locations, suggesting it may have been striped or had another prominent pattern, while the Confuciusornis had both colors in a single feather.

While the findings definitely appear to be feather precursors, the selective pressure that drove their evolution remains elusive. Scientists speculate there could be a number of reasons for the feathers and coloration, including camouflage or communication; then again, maybe the theropods were just cold. Either way, the existence of these feathers will help in deriving the details of the dinosaurs' appearance, as well as their relation to modern birds.

Nature, 2010. DOI: 10.1038/nature08740

Science, 2010. DOI: 10.1126/science.1182143

Listing image by NASA