Thursday, March 16, 2006

Juravenator and the complex pattern of feather evolution

I normally like to post news here, but since this is a science and natural history blog (plus, apparently, random ramblings about other things) I'll post something that isn't very novel: a new theropod dinosaur published in Nature this week. The big news about Juravenator starki is what it doesn't have: for once, Nature is running an article about a dinosaur that doesn't have feathers.

Type specimen of Juravenator starki from the Late Jurassic of Germany. From Göhlich and Chiappe 2006

Ultraviolet and normal light photographs of the tail, showing patch of scales. From Göhlich and Chiappe 2006

The reason this is of interest to some palaeontologist is the fact that the authors' analysis suggested that it "should" have had feathers. That is, it was a type of theropod dinosaur that nested in the tree where feathers are predicted for the common ancestor.

Phylogeny showing the relationships of Juravenator with other theropod dinosaurs. From Göhlich and Chiappe 2006

Birds have scales and feathers needn't always cover the remainder of the body. The preservation of feathers in Juravenator's nearest relative known to have feathers, Sinosauropteryx is apparently limited to a mindline "mane" of filaments, an may not necessarily have covered the bodies. Whether or not this was the true condition in life could, potentially, be disputed. However, it follows from the enormous diversity of theropod dinosaurs that their feather distributions (both within and among taxa) were considerably more varied than previously though.

Tyrannosaurs are known to have patches of scaly skin, but recent discoveries show that their ancestors probably had feather-like structures. Thus, at some point, feathers must have been either incompletely covering the animal or lost and gained over varying degrees. Moreover, it appears that estimates of feather covering in dromaeosaurs were dramatically underestimated It would appear that our interpretation of feather evolution paints, perhaps too conservatively rather broad coating of feathers on just about anything descended from the common ancestor of all coelurosaurian dinosaurs.

Xing Xu, who has described many of the Chinese feathered dinosaurs, wrote a News and Views piece which raised some interesting cautions about these results: For one, we don't know that Juravenator did not have feathers. All we know is that parts of its body had scales. Fossilization is biased against feather preservation and those few records we have are remarkably rare. The specimen is apparently a juvenile and may, in fact, create a false signal pulling the animal to a particular part of the tree. Normally, this would be the other way around: juvenile characters tend to make you look more 'primitive'. However, Juravenator clumps with a group of small theropod dinosaurs which may share character similarities simply related to the fact that they're small, and not any real common ancestry. It's possible that this animal is more primitive, but unites with these other animals due to bias.

Göhlich, U.B. & Chiappe, L.M. 2006. A new carnivorous dinosaur from the Late Jurassic Solnhofen archipelago. Nature 440: 329–332.

Xu, X. 2006. Palaeontology: Scales, feathers and dinosaurs Nature 440: 287-288.

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