In my previous post I offered you a quick look inside the world of paleontology and the new insights scientists are drawing from the fossil record of dinosaurs. Let’s take a closer look at how wing feathers may have evolved for the purpose of brooding, as opposed to the purpose of flight.
The image above was published in the journal, Nature, by the team of scientists who uncovered the incredible fossil of a brooding oviraptor that died while defending its nest from a sandstorm. It’s a “reconstruction,” or schematic drawing in which the various bones and eggs that were preserved in their specimen are complemented by the addition of some missing bones to complete the skeleton and an outline of the nest of sand, as well as an outline of the animal’s body shape.
That’s a nice diagrammatic way to look at how a dinosaur parent could nestle down on its eggs to keep them warm or shade them from hot sun or fend off a desert rain squall. What was left for Mark Orsen and me to do after this paper was published was to explain how feathers might have evolved to help the oviraptor shelter its eggs and the hatchlings that would come from them.
This is another of Mark’s images. Here he shows how the short feathers of a primitive bird-like dinosaur could have increased in size over time, driven by evolutionary survival pressure. Three stages of evolution are shown. In the first, a short feathered ancestor can cover only a relatively small area with its “wing” feathers. This is a survival disadvantage because only very few eggs or babies can be protected, compared to the nice big clutch of eggs shown in the diagram above. This ancestor would never have been able to shelter all the eggs that were found in the fossil nest.
Over millennia, pressure to raise more young, or to protect them as they grew bigger, would naturally lead to the lengthening of the feathers. One nice feature of this theory is that it is gradual, not sudden. Other theories of wing feather evolution suggest the animals were jumping out of trees and flapping their arms with stubby feathers or trying to glide with them. Still others suggest that the animals were trying to leap up off the ground and fly. All of these flying-came-first theories fail to solve the problem of the intermediate stages. That is, what happened during the times when the feathers were too short for flight or gliding? A lot of crash-landings, it would seem.
On the other hand, Mark and I have proposed a theory in which every length of feather, from the stubby ones to the long ones seen on the right, would be a useful length — it just depends on how many babies, or how large a brood, the adult was trying to shelter. Each little increment of feather lengthening meant another egg could be laid, or a larger baby could be protected. These are key survival issues for any species of nesting creature, from tyrannosaurs to chickens.
So, maybe it really did happen that way. Maybe the wing, which we naturally associate with flight, evolved first for the other purpose we sometimes forget, namely brooding. That’s what Mark and I think.