BBC Science Reporter James Morgan provided a fascinating story this week about the latest in a string of feathered dinosaur fossils from Liaoning Province in China. This one, named Epidexipteryx by lead researchers Fucheng Zhang and Xing Xu of the Chinese Academy of Sciences, is unusual for a couple of reasons.
First, it had a typical maniraptor theropod body plan: a pigeon-sized, agile runner with three claws on its grasping forearms and lots of needle-sharp teeth. It was also covered with “protofeathers” – hairlike feathers without the central shaft and barb structure seen in modern contour feathers. But its tail was shortened to a stub and adorned with four elongated, shafted “proper” tail feathers. Paleontologists say these tail feathers – which seem quite obviously to have had some function in display – have never been described in a fossil animal that didn’t also have feathers somewhat modified for flight on the forearms. This means that the ability to grow feathers with a shaft and vane made of zipped-together barbs arose independently from the selective pressure that shaped such feather structures for flight. Which came first, the flight or the feather? The surprising answer now seems to be “feather”, although it’s still possible that Epidexipteryx could have been secondarily flightless, i.e., it evolved from a flying ancestor with proper, vaned flight feathers on its arms.
The question of “first” is also relevant and unusual regarding this creature. If you’re familiar with some of my views on avian evolution, you may know that I’ve never jumped on the “birds from dinosaurs” bandwagon. As new fossils were described over the last 10 years or so that indicated the existence of multiple feathered dinosaurs, the door on the “birds from earlier, non-dinosaur reptiles” hypothesis seemed to have slammed shut. There were clearly multiple maniraptors back in the day with skeletons that were structurally almost identical to that of the “first bird” Archaeopteryx, and several of these were feathered animals – some may even had had some rudimentary flying ability.
But I’ve reserved final judgment because every one of these feathered dinosaur fossils has been dated to Cretaceous, or at least late Jurassic age: they appear later in the fossil record than Archaeopteryx. When you consider that Archaeopteryx was pretty darn close to a modern bird in structure (its teeth and long, bony tail notable exceptions), and that its fossils are dated to about 150 million years ago (MYA), then there is no way that something less birdlike from, say, 100 MYA could have been ancestral. For all the amazing similarities in those feathered dinosaurs that provide support for the “birds from dinosaurs” hypothesis, the common ancestor of Archaeopteryx and the maniraptors must be something much older still than Archaeopteryx. This has led some to conclude that perhaps the maniraptors evolved from birds, rather than the other way around!
Epidexipteryx provides a new and interesting wrinkle. It is clearly less birdlike than Archaeopteryx, though also feathered, albeit with the odd mix of ancient and “modern” feather structure. It might just be considered one more feathered dinosaur from Liaoning but for the fact that it is dated earlier than Archaeopteryx: mid Jurassic, 158-162 MYA.
So Epidexipteryx has focused our attention again to where it should be if we want to describe the evolutionary origins of birds: the early Jurassic. We may never have access to fossils that will solve the puzzle once and for all, but as we keep digging deeper for the solution to that puzzle, we approach the time of the origins of dinosaurs themselves. This potentially makes the “dinosaurs from birds” hypothesis not so crazy after all.