A flying leap?
来源：未知 作者：车箨懋 时间：2019-03-08 06:17:01
By Matt Walker EARLY birds didn’t learn to fly by jumping out of trees, or by running along the ground chasing flying insects, as most palaeontologists would argue. According to a study that’s taken another look at the fossil record, they developed wings to jump onto prey from ledges and rocks. Most experts are split into two camps over the origins of bird flight (This Week, 8 May, p 10). Some believe that small reptiles took to climbing trees and developed prototype wings and feathers that helped them glide down back to the ground. Others insist that fast-running dinosaurs developed feathers for insulation, then used them to take ever greater leaps into the air to snatch flying insects. But now Adrian Thomas of Oxford University and his colleagues have re-examined the fossil record of early birds, including the most famous, Archaeopteryx. They now believe flight evolved in a pouncing protobird or “proavis” that used its feathers to control and balance its body during a feet-first, predatory leap. This pounce turned into a swoop, which finally led to true flapping flight (Proceedings of the Royal Society B, vol 266, p 1259). Thomas points out that fossils of Caudipteryx, a primitive feathered cousin of Archaeopteryx, have symmetrical feathers, which could provide enough drag to stabilise a short fall. But the ground-up and tree-down theories require the first birds to have evolved asymmetrical feathers, which can generate lift. Both popular theories also predict that early birds became lighter as their wings evolved. But fossils show that birds already had functional wings before they began to shed excess weight from their tails and skulls. A pouncing proavis could remain heavy well into the swooping phase of its development, say the researchers. The pouncing proavis model also explains why Archaeopteryx has fully feathered wings attached to an unremarkable reptile skeleton. The body did not seem to have adapted for flight. “It’s very odd, and you have to work hard to see how the other theories could fit this,” says Thomas. But in the pouncing theory, an advanced wing could easily appear alongside a simple body. “Birds evolved from predators that specialised in ambush from elevated sites, using their raptorial hindlimbs in a leaping attack,” Thomas concludes. Jeremy Rayner, a bird aerodynamics expert from the University of Bristol, says it’s an interesting idea, but he’s not convinced. For instance, Caudipteryx’s feathers are soft and floppy like down, and it’s difficult to see how they could have created enough drag,