What features define a bird?
It seems like a fairly simple question, especially since birds are very different from other living groups of vertebrates like reptiles, but over the past decade a flood of new fossils has shown that many of the features we think of as being unique to birds first evolved in dinosaurs: hollowed-out bones, an endothermic metabolism, brooding on top of nests, and even feathers.
It is hard to draw the line between the most bird-like dinosaurs and most dinosaur-like birds. Most of the dinosaurs with avian features were not ancestors of birds, however, and a new fossil from China reveals that feathered dinosaurs were more diverse than previously thought.
The pigeon-sized dinosaur Epidexipteryx, announced this week in the journal Nature, looked something like a weird Mesozoic pheasant. Its body was covered with fuzz-like feathers, and it had two pairs of long, ribbon-like feathers on its short tail. The skull was even stranger: short and tall with the nose further back than would be expected. The dinosaur was buck-toothed to boot: its sharp teeth jutted forwards at the front of its mouth, forming a sort of scoop similar to what is seen in mammals that feed on insects.
Although it is quite unusual in many aspects, Epidexipteryx is recognizable as being one of the maniraptors, the same group that contains dinosaurs like Albertonykus and Velociraptor. Within this larger group, it most closely resembles a small feathered dinosaur announced in 2002 named Scansoriopteryx. (The authors call Scansoriopteryx by a different name, Epidendrosaurus, but they are now recognized to be the same dinosaur. Because the name Scansoriopteryx appeared first, it is the name used for the dinosaur now.)
This is significant because Epidexipteryx and Scansoriopteryx together are the closest relatives to early birds like Archaeopteryx and Jeholornis. The new analysis in the Nature paper places Epidexipteryx and Scansoriopteryx within a group called Avialae, which includes birds (Aves) and their closest dinosaur relatives. To explain this another way, Epidexipteryx and Scansoriopteryx were the dinosaurs most similar to birds without being birds themselves.
This does not mean that Epidexipteryx was the ancestor of the first “true” birds, however. It is presently not possible to place the known fossils in a straight line of descent from feathered dinosaurs to birds, but by determining the evolutionary relationships paleontologists can tease apart from which group of dinosaurs the first true birds evolved.
In the case of Epidexipteryx, it was certainly bird-like, but it lacked feathers on its arms that allow birds to fly and are seen in other feathered dinosaurs like Microraptor. In other parts of its anatomy, like its skull, it shares features with dinosaurs less closely related to birds. There was a branching tree of diversity among feathered dinosaurs, as evolutionary theory predicts,but it can be difficult to untangle the branches.
Of particular interest will be determining the age of Epidexipteryx and other feathered dinosaurs from the same place, like Pedopenna. The fossil was found in a fossil bed in Daohugou, part of Inner Mongolia, which was of Middle to Late Jurassic in age, or between 152 and 168 million years old. It is possible that Epidexipteryx and the other feathered dinosaurs there were older than Archaeopteryx, the oldest known bird. Again, this would not make Epidexipteryx a direct ancestor of members of Aves, but would rather illustrate that feathers and a diversity of feathered dinosaurs existed much further back in time than had been previously known.