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Flying Reptile Fills an Evolutionary Slot
14 October 2009 (All day)
Charles Darwin just got a 200th birthday present: a newly discovered fossil of a crow-sized flying reptile. Named Darwinopterus ("Darwin's wing") in his honor, the find closes a gap in the fossil record and might provide an important new clue about how natural selection works.
Darwinopterus is a pterosaur, a member of the group of reptiles that soared above the landscape for much of the Mesozoic era. They appeared about 220 million years ago and vanished 65 million years ago, along with the dinosaurs.
For years, scientists have puzzled over the fact that the fossil record features two types of pterosaurs but no intermediates. Looking vaguely like primitive pheasants, the oldest members of the group had long tails and short skulls, along with short hands. The more recent reptiles featured short tails, long hands, and the familiar elongated skulls. "Up until now, the transition between the primitive and derived species has been a great mystery," says paleontologist Stephen Brusatte of the American Museum of Natural History in New York City.
Darwinopterus, which is about 160 million years old, constitutes "a perfect intermediate form between the two groups," says Brusatte, who was not involved in the find. The fossil's 20-centimeter-long skull and neck resemble those of the later group of pterosaurs, and the rest of the 50-centimeter-long body looks more primitive. The animal's appearance is "contrary to what we expected," says paleontologist and co-author David Unwin of the University of Leicester in the U.K.
The fossil of the predatory airborne creature, which Unwin and Chinese researchers describe online today in the Proceedings of the Royal Society B: Biological Sciences, supports a concept called modular evolution. It states that different parts of an animal can evolve at different rates--with some structures evolving particularly rapidly. Modular evolution could explain the hybrid nature of Darwinopterus. The skull might have been much more advanced than the rest of the body because its evolution accelerated, whereas the rest of the body idled. Rather than small changes accumulating over a long period of time in various parts of the body, Unwin says, natural selection acted on the entire module.
Brusatte agrees. "Darwinopterus is an intriguing snapshot of modular evolution in action," he says.