Apr 05 2012

A Feathered Tyrannosauroid

Yet another feathered dinosaur has been discovered in China – this one is a relative of T. rex and is the largest creature with feathers, extant or extinct, to have been discovered. Yutyrannus lived about 125 million years ago and is an early cousin of the T. rex.

This is one of my favorite paleontological stories – in fact, it’s one of my favorite science stories because it demonstrates one of the ways in which evolution is testable. When Darwin first proposed his theory the fossil record was very scant. Further, the number of species currently alive is a very small portion of the number of species to have ever lived (scientists are fond of saying that most species to have ever lived are extinct).

This means that when we look at living things in order to infer the relationships among them, different plants and animals might appear to cluster in separate groups. Actually a thorough survey of living things shows a pattern of nestled categories that are blurry at the edges (I’m looking at you, platypus) and nicely reflects a branching pattern of common descent. But still, there do appear to be distinct groups. This appearance, however, is an artifact of the incompleteness of the sample of all living things represented by those species that are still alive.

Birds are a classic example. Birds appear to be a very distinct group with features (i.e. feathers) shared by all birds but absent in all non-birds. “Aha,” said creationists at the time The Origin of Species was published, “there are no connections between major groups and if evolution were true then we would expect to find connections all over the place.” In fact some creationists still say this today, but now they have a growing body of evidence to deny.

This challenge to evolution created an excellent opportunity to test the theory that all life on earth is related through common descent with modification. If evolution were true, and we are just missing a lot of pieces to the puzzle of life on earth, then we would expect to find fossil evidence of extinct species that fill in the gaps between living groups. If evolution were not true, and the different “kinds” of life were all separate creations, then we would not expect to find fossils which fill in the gaps (beyond some chance correlations).

In 1861, just two years after On the Origin of Species was published, the Berlin Specimen of Archaeopteryx lithographica was announced. This is a specimen of an early bird, complete with modern-looking feathers. The specimen also displays many primitive features that are reptilian, in fact that are very similar to theropod dinosaurs. Archaeopteryx, in short, is a beautiful transitional fossil between two major groups – birds and dinosaurs. It is half bird, half dinosaur. Really, it doesn’t get better than this (although there are many other examples of stunning connections between major groups). Archaeopteryx was taken, rightly, as confirmation of Darwin’s new theory – at least by the scientific community.

Creationists, however, stubbornly ignored the implications of this fossil and variously dismissed it as a hoax (it’s not) or as just a weird bird with teeth and a bony tail. Other specimens of Archaeopteryx have been found over the years, many beautifully preserved with feather impressions. But for a long time it was isolated as the only fossil connection between birds and dinosaurs. Over that same time, the century following Darwin, evolutionary theory became much more fleshed out, as did the fossil record. We began to appreciate that evolution is a complex branching bush (rather than a ladder or a linear progression), and that successful species tend to experience adaptive radiation, filling the environment with variations on basic morphological themes.

Then in the 1980s paleontologists hit pay dirt in China, discovering fossil beds with other feathered dinosaurs. Over the last three decades they have continued to find more and more feathered dinosaurs, and evidence that known dinosaurs (like velociraptor) had primitive plumage. They found primitive feathers that were not used for flight, dinosaurs with all four limbs adapted for flight, and everything in between. Now we have a whole host of feathered dinosaur specimens at the correct time in geological history, clustered in one region of the world. This is exactly the kind of thing that evolutionary theory predicts should be the case – wide distribution of an adaptation, one branch of which led to flight and early birds. The fossils also show that feathers themselves had ancestors that were clearly not adapted for flight, and so served other functions.

I can’t help feeling that every new feathered dinosaur discovery is like a dagger straight into the heart of creationist denialism of evolution. I have to wonder if it makes them squirm, even just a little. Probably not, the power of rationalization is just too great, but it’s fun to think about.

With this latest discovery we have more evidence that feathers were a common adaptation in this branch of the dinosaur order, even existing on some of the big boys – the tyrannosauroids. This specimen also adds to the debate over whether or not T. rex had feathers. Given how common feathers were in this group, the answer is that T. rex probably did sport some sort of plumage. It’s possible that just the young ones had downy feathers to keep warm, but adult T. rex did not have any feathers. It is also possible adults had feathers for display or some other purpose. The discovery of yutyrannus makes it more plausible and likely that even adult T. rex had feathers. We cannot know for sure until we find a T. rex specimen preserved in such a way that feather impressions would be visible. That would be an exciting find.

Meanwhile, I can enjoy the latest feathered dinosaur discovery, and welcome yutyrannus as the largest member of this group (pending definitive data on whether or not T. rex had feathers).


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