Sinosauropteryx may not have been a feathery dinosaur after all.Proc. R. Soc. Lond. B

Feathery dinosaurs might not have been as common as experts thought, according to researchers who analysed a fossil of a creature previously thought to have feathers, and found instead that it was bald.

The discovery calls into question the theory that the first feathers evolved not for flight but for insulation, and that they made their first appearance in relatively early dinosaur lineages that later evolved into modern birds. If these dinosaurs didn't have feathers, or feather-like structures, then feathers may have evolved at a later time, and been used for flight right from the start.

The fossil represents a dinosaur called Sinosauropteryx, which lived in the Early Cretaceous period roughly 140 million years ago. The specimen, found in Liaoning Province, China, has distinctive patterns seen in its skin. Previous studies of other related dinosaurs with similar markings have led experts to conclude that these dinosaurs were covered with downy 'protofeathers'.

But Theagarten Lingham-Soliar from the University of KwaZulu-Natal in Durban, South Africa, and his colleagues now say otherwise. Instead, they argue that these structures are degraded remains of collagen fibres, the main connective tissue in animals.

Decomposing theory

Lingham-Soliar and his colleagues produced high-resolution microscopic images of the fossil that, he says, show that these structures represent degraded soft tissue. The regular pattern of the fibres suggests that Sinosauropteryx had a frill of skin along its neck, back and tail. And the random orientation of some of the fibres, previously attributed to protofeathers, are more likely to indicate breakages in the regular pattern of collagen fibres as the dinosaur decomposed.

The researchers also say that the absence of herringbone-shaped patterns — which would be seen if feathers were present — puts another nail in the coffin for the protofeather suggestion. They publish their findings in Proceedings of the Royal Society B¹.

If Sinosauropteryx was indeed featherless, then it may be that feathers arrived on the evolutionary scene later than palaeontologists had thought.

But the discovery does not mean that Sinosauropteryx and its kin were not the forefathers of birds. "There's no need to panic," says David Unwin, a dinosaur expert at the University of Leicester, UK. "This doesn't in any way challenge the idea that dinosaurs had feathers and that dinosaurs gave rise to birds." The real argument now is when in evolutionary history feathers started to emerge. "Things may be more complex than we thought," he says.

And even if Sinosauropteryx was featherless, it is still possible to argue that feathers evolved for something other than flight. "There are many other dinosaurs between the microraptor [a bird-like dinosaur thought to glide, but not fly] and Sinosauropteryx with feathers that are not ostensibly flight feathers," says Unwin. This result simply throws into doubt the first step in feather evolution.

Unwin is happy to believe that this single specimen is probably featherless and has a fibrous structure, although he suggests this doesn't have to be collagen but could also be muscle fibre or elastin. Unwin wants to see an analysis of a much greater number of specimens, using a wider range of analytical techniques, before he is convinced either way about Sinosauropteryx and its feathers, or lack of them. "It's almost dangerous to only look at one or two specimens," he says.

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• References

  1. Lingham-Soliar T., et al. Proc. R. Soc. Lond. B, doi:10.1098/rspb.2007.0352 (2007).