Researchers agree that birds are dinosaurs, but when did dinosaurs start becoming birds? New excavations in Siberia reveal that one sure sign of birdiness, the presence of feathers, has very deep roots in the dino evolutionary tree; indeed, dinosaurs may have been sporting feathers from the very beginning of their existence about 240 million years ago.
The fossil record makes clear that birds were the only dinosaurs to survive a mass extinction about 66 million years ago, probably caused by a massive asteroid hitting Earth. But the past decade or two of research, which is marked by the discovery of thousands of specimens of early birds and flying dinosaurs, also shows that feathers were an early evolutionary innovation—even if they probably arose for reasons unrelated to powered flight, such as insulation or sexual display.
Just how early has been a matter of debate. Although the best evidence for feathers has been found in a group of meat-eating dinosaurs dating back to about 150 million years ago, and from which birds apparently evolved at about the same time, there have been sightings of bristly, filamentous structures in very distantly related plant-eating dinosaurs as well. Leading examples have been Psittacosaurus, a cousin of the horned dino Triceratops found in Asia and dated to perhaps 120 million years ago; and the 160-million-year-old Tianyulong, found in China and reported in 2009.
If these bristly structures represented early feathers, as researchers have increasingly come to think, it would mean that feathers evolved in dinosaurs that preceded the evolutionary split between so-called saurischians (which include the meat-eating species) and ornithischians (which comprise plant-eating species) more than 200 million years ago. (Despite their confusing name, the ornithischians are not related to birds, which are saurischians.)
“There is a near-consensus now that the simple bristlelike structures in Tianyulong and Psittacosaurus should correspond to the earliest developmental stage” of what researchers often call “protofeathers,” says Pascal Godefroit, a paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels. But he and others point out that hard evidence for this hypothesis has been lacking, largely because the single filaments found on these plant eaters lack the complexity of the protofeathers found on early meat eaters.
In 2010, a team led by geologist Sofia Sinitsa of the Institute of Natural Resources, Ecology and Cryology in the Siberian city of Chita, Russia, discovered some dino fossils in the nearby Kulinda valley. At first, Russian paleontologists were not impressed, because the specimens were fragmentary and poorly preserved. Sinitsa found more fossils in 2011 and 2012, and contacted Godefroit, whose work on early birds is well known, and other researchers. “We were completely shocked by her discoveries,” Godefroit says, because the new specimens had the kind of complex, multifilamented structures typical of protofeathers.
This week in Science, Godefroit, Sinitsa, and their colleagues report the details of six partial skulls and hundreds of skeletons of what they dub Kulindadromeus zabaikalicus (from Kulinda, the specific locality where it was discovered; dromeus, Greek for “runner”; and zabaikalicus, from the Zabaikal krai [region] in which Kulinda valley resides).
K. zabaikalicus, which could date to as early as 175 million years ago, has filamentous structures on much of its body, including its head and thorax; but the more complex featherlike arrangements are found mostly on its arms and legs, an arrangement typical of many feathered meat-eating dinosaurs. “For the first time we found more complex, compound structures, together with simpler hairlike structures” in a plant-eating dinosaur, Godefroit says, “that really resemble the protofeathers in advanced” meat eaters.
Godefroit and others do not claim that these early dinos could fly, because researchers generally agree that feathers evolved first for other purposes. But the findings do suggest that feathers might have been present in some of the very first dinosaurs, and perhaps were a widespread feature of all dinosaurs.
“Kulindadromeus seals the deal that some plant-eating dinosaurs had feathers, and is the best proof yet that feathers weren’t something that evolved only in the meat-eating dinosaurs,” says Stephen Brusatte, a paleontologist at the University of Edinburgh in the United Kingdom. “It tells us that feathers must have arisen earlier in dinosaur evolution than most of us previously thought, and maybe even the common ancestor of all dinosaurs had feathers.”
The finds are “a fantastic discovery,” says paleontologist Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. But he cautions that the fossils are still too fragmentary to be certain that the more complex feathery structures actually correspond to those found later in birds. On the other hand, Xu says, the large number of skeletons—which seem to represent different ages, from juveniles to adults—promise important new insights into the stages of feather development as dinosaurs grew up.