by James Lamb, Paleontologist
There are to date at least 21 different types of dinosaurs that are known to have had feathers. Even Velociraptor, a well-established dinosaur, appears to have had a feather coating, as a recently discovered specimen shows that it had the bumps on one of its forearm bones called “quill nodes” that indicate where feathers attach in modern birds. In fact, it appears very likely that all carnivorous dinosaurs (the group called theropod dinosaurs) had feathers, at least as juveniles. It also seems that the dromaeosaurs, the specific group of theropods that Deinonychus belongs to, are the closest relatives to birds. Many of the feathered dinosaur skeletons have the switchblade claw on their hind feet. Recently, there have been discoveries such as the “bristles” on the back on the small Triceratops relative Psittacosaurus, and just two months ago of what appear to be feathers on the heterodontosaurid dinosaur Tianyulong, that indicate that perhaps all dinosaurs had feathers, at least originally (although they may have been lost later by such groups as the hadrosaurs (duck-bills) which we know from skin impressions did not have feathers). This is because Psittacosaurus and Tianyulong are plant-eating dinosaurs that are not close relatives to theropods, and in particular heterodontosaurids are near the base of the dinosaur family tree.
So what did a dinosaur need with feathers?
The key question about the origins of flight in birds if dinosaurs are indeed their ancestors, is how did ground-dwelling animals become flying animals? Looking at the range of feather types in modern birds, Dr. Alan Brush has worked out the probable sequence of the evolution of feathers from a reptilian type scale. The problem is that the intermediate stages in feather evolution, say, halfway between a scale and a feather, are not capable of sustaining flight. That means that feathers were originally evolved for a purpose other than flight, a purpose for which intermediate grade feathers provided some useful function. It appears that the answer to this riddle is that feathers originally were for insulation. We now know from a variety of evidence that dinosaurs almost certainly had some form of warm-blooded metabolism, and the earliest dinosaurs were all small animals. Small warm-blooded animals lose body heat more rapidly than a large animal with the same metabolism. To prevent the loss of body heat, small warm-blooded animals need insulation. Dinosaurs had feathers for the same reason that mammals have hair – to stay warm. Indeed, birds and mammals are the only warm-blooded animals. Mammals inherited fur from their ancestors, birds inherited feathers from their ancestors. The intermediate stage of a feather may not work for flight, but it does work as insulation, at least better than a scale. Only later were they co-opted for use in flight. For many years though there hasn’t been a satisfying explanation for how theropod dinosaurs made the transition into the air. The final piece came a few years ago with the work of Dr. Ken Dial, and is known as WAIR, or wing-assisted-incline-running. I’ll give an example from personal experience how this works.
Some years back, while searching for dinosaurs, umm, fossil ones, in central Alabama, I was walking through a stretch of woods to get to a site, when I scared up a wild turkey in the underbrush. I’m not sure which of us was more surprised; turkeys generally rely on camouflage, and will hide quietly in a bush and let you walk right past them. Evidently, this one decided at the last instant the camouflage wasn’t working because the turkey exploded out of a bush within arms reach in a cloud of leaves and feathers and with a noise equivalent to what I thought of at the time as a charging rhino crashing through the underbrush. The turkey sprinted across the clearing, heading towards a large cedar tree. And then it did the oddest thing – it waited until it was right at the tree and then flew vertically up it, it’s chest just inches away from the trunk. Heart pounding, still checking to see if my pants were still dry recovering from the initial surprise, the whole thing having taken less than 2 seconds, I began to wonder why it had waited until the last instant to fly up the tree instead of taking off and flying in a shallower trajectory into the tree. What WAIR proves is that in fact the turkey did not fly into the tree at all, it RAN into the tree – vertically up the trunk.
Dial has spent many hours filming birds running up the trunks of trees. He has shown that a bird can run up a tree quicker from a short distance than it can fly into the tree. In fact, this happens so quickly that it can only be seen in high-speed video footage, which is why the turkey appeared to me to have flown up the tree. Further, birds only a day or two old exhibit this behavior, and also sometimes run up inclined surfaces and use it as a launching point to glide away from danger long before their wings are developed enough for flight. He has documented that birds can run up completely vertical surfaces in this manner, flapping the wings both as a propulsive assist and to help keep the feet pressed against the tree. It also turns out that they move their arms/wings in exactly the same manner that they do during active flight. This is almost certainly the origin of flight in dinosaurs – small dinosaurs with feathers evolved for insulation modifying a predator-escape behavior for a new purpose.
Next week: Birds of a Feather Part 3
James Lamb is a native Birminghamster with a nearly life-long interest in fossils. He collected his first fossil when he was 5 years old. Friends and family members assure him he has not matured much since then. James is curator of paleontology at McWane Science Center, and would like to one day see the absolute treasure trove of Alabama's fossil heritage revealed to the public. When not at work he wishes he were in the field digging up fossils. At home he enjoys reading, jogging, woodworking, and carving. He has been informed that he is in the habit of telling atrocious puns, but this comes as a surprise to him. James describes himself as a "science nerd".