Jul 05 2012

Feathers More Common Among Dinosaurs

The story of feathered dinosaurs is one of the strongest success stories for evolutionary theory, and continues to be a thorn in the side of evolution deniers. They cannot help but expose their ignorance and deception when they clumsily try to deny the implications of feathered dinosaurs. So I tend to revel, just a bit, in each new significant discovery of a feathered dinosaur. The latest discover, recently published in PNAS, provides evidence that feathers were even more common among dinosaurs than previous evidence demonstrated. There is some debate among experts about how prevalent protofeathers and feathers were, and so a new piece of solid fossil evidence helps clarify the debate further.

After Darwin published his theory of evolution one of the early challenges to the idea of evolution, which includes the claim that all life on earth is related through common ancestors, was that there were significant gaps between major groups of living creatures. Birds, for example, seem to be their own group without a close connection to any other group. They are, of course, related to vertebrates. But if evolution were true then there must be fossil evidence connecting birds to another group, such as reptiles.

Right around that time the first specimens of Archaeopteryx were discovered (the London specimen in 1861 and the Berlin specimen in 1874). These are beautiful transitional fossils showing features of both theropod dinosaurs and birds – exactly as Darwin predicted. For decades Archaeopteryx stood as the sole species bridging birds and dinosaurs, and creationists did everything they could to deny the implications of this fossil. They tried calling it a hoax, then just a regular dinosaur, then just a regular bird. They also argued that, whatever it is, there is no proof that it is actually transitional between dinosaurs and birds – a non sequitur that betrays their lack of understanding of evidence in science.

The fact remains that if evolution is true, and birds did evolve from dinosaurs, then we would expect there to be many fossil species occupying the morphological space between birds and dinosaurs (the vast majority of which will not be on the exact line from dinos to birds, but on another phylogenetic path within that evolutionary space). Incidentally, many paleontologists now include birds in the clade of dinosaurs and call what we traditionally think of as dinos “non-avian dinosaurs.” But I will just use the term “dinosaur” in this common sense.

Further, starting around the 1980s paleontologists started finding the motherload of feathered dinosaurs in China, with numerous species fleshing out the timeline between dinos and birds. There were specimens both closer to birds than Archaeopteryx and others closer to non-avian theropods. There are specimens with protofeathers not yet adapted to flight. In other words, there is a branching bush of feathered dinosaurs in the right time to represent a group of animals from which birds evolved. That particular gap in the evolutionary tree of life was nicely filled in.

Further still, paleontologists started to discover specimens of known dinosaurs, like velociraptor and T-rex, that show signs of feathers. It now seems that this whole group of dinosaurs, known as the coelurosaurs (a subgroup within the theropod dinosaurs) has some kind of feather-like structures.

The new discovery is significant because it is a well-preserved specimen, showing clear protofeather filaments, in a theropod dinosaur that is closer to the trunk (base) of that group than any previous specimen. This means that feathers evolved earlier in the theropod clade than previous evidence demonstrated, and that, therefore, feathers were even more common within theropods. The new species is called Sciurumimus albersdoerferi and is a juvenile.  From the abstract:

Sciurumimus albersdoerferi represents the phylogenetically most basal theropod that preserves direct evidence for feathers and helps close the gap between feathers reported in coelurosaurian theropods and filaments in ornithischian dinosaurs, further supporting the homology of these structures.

The following cladogram demonstrates the relevant relationships:

Previous evidence showed feathers in theropods from Dilong to Confuciusornis. Sciurumimus takes feathers back four branching points on the above cladogram, meaning that it evolved earlier in the group and that many more dinosaurs likely had some kind of feathers. Of course, we may discover other specimens that bring it back further still. This is likely not the final word on the earliest origin of feathers within theropods.

I also notice that this specimen is a juvenile. Perhaps in some species feathers existed only in the young dinosaurs, as an adaptation for insulation, for example, and that adult specimens lost their downy feathers. Therefore we may need to find well-preserved juveniles (like this specimen) to really find out how prevalent protofeathers were.

Creationists are stuck arguing that it is nothing but a coincidence that the group hypothesized to the related to modern birds is turning out to have feathers and protofeathers as a common feature. Nothing changes the fact, however, that this is a verification of a prediction made by evolution. The ability to successfully make predictions about what will be discovered in the future is powerful vindication for any scientific theory.

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