The discovery of skulls of animals about the size of turkeys which lived 70 million years ago in what is now the Gobi Desert provided one key; the other was in Madagascar, where archaeologists found a 65-million-year old fossil of a raven-sized bird with dinosaur-like features.
Although the public has generally thought that fossil skeletons such as the famous Archaeopteryx, a 140-million-year old "dinosaur with wings", provided incontrovertible proof that birds evolved from dinosaurs, palaeontologists have been less sure.
In recent years, they have suggested that birds and dinosaurs evolved separately, and that the cold-blooded Archaeopteryx, might have been an accident of evolution that later disappeared. Meanwhile, the warm-blooded ancestors of birds would have evolved separately, surviving the "dinosaur killer" asteroid which hit the Earth 65 million years ago to become the feathered animals we know today.
But the findings in the Gobi Desert of Mongolia were of turkey-sized animals which walked on two legs, and had a number of bird-like features. Yet they also had dinosaur features: long necks and tails, and - unlike any bird living today - stubby forearms that ended in a single, blunt claw.
Reporting today in the science journal Nature, Luis Chiappe and researchers from the American Museum of Natural History, New York, and Gorge Washington University, Washington DC, say the fossils revealed an important physical characteristic normally seen only in birds. The animals were capable of "prokinesis" - the movement of the snout up and down independently from the rest of the skull. This allowed the creature to open its mouth wide to eat large food items, and is considered an advanced characteristic of birds. "These strange creatures were actually early birds," the team said.
That in itself suggests that such an ability arose early in bird evolution - and that it happened in the time of the dinosaurs.
The second plank of evidence is reported tomorrow in the journal Science. A team led by Catherine Forster of the State University of New York found fossils of a type never before seen, including wing bones of a bird, but also a long tail and "a huge, sickle-shaped killing claw" that resemble such features in meat-eating theropod dinosaurs.Reuse content