A fossil found in Germany has clouded debate about the advent of feathers, the key event in the theorized evolution of birds from small dinosaurs 176-146 million years ago.
The find shows a juvenile dinosaur that was a small meat-eating predator around 75 centimeters (30 inches) long.
By all expectations, this creature, dubbed Juravenator starki, should have had feathery filaments on its body.
It was a small group called the Compsognathidae, which includes the first-known feathered dinosaur, Sinosauropteryx.
And it was a cousin and a near-contemporary of the earliest-known bird, Archaeopteryx, which lived around 150 million years ago in the Late Jurassic period.
But the exquisitely preserved fossil shows no feathers or even the structures for holding them, just a scaly skin, according to a paper published on Thursday in Nature, the British weekly science journal.
Authors Ursula Goehlich of the University of Munich and Luis Chiappe of the Natural History Museum in Los Angeles believe the find shows that the adoption of feathers was a far more complex process than previously thought.
They argue that some lineages of dinosaurs clearly took to feathers later than others.
In a commentary, Chinese palaeontologist Xu Xing, one of the most renowned dinosaur specialists in the world, suggests that J. starki, while a fascinating find, may distort the picture because it was a juvenile, and had yet to develop feather-bearing structures.
He also wrote that whatever the explanation, the discovery of Juravenator has enriched knowledge of early feather evolution. It could also indicate where future research could be concentrated.
"Juravenator may complicate the picture, but it makes it more complete and realistic," he said in the commentary.
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