"Somewhere, something incredible is waiting to be known" Carl Sagan
sábado, 29 de octubre de 2011
Palaeontology - Archaeopteryx restored as 'first bird'
A new evolutionary tree has once again defined archaeopteryx as the most primitive of birds. The new research appearing today in Biology Letters is by evolutionary biologist, Dr Michael Lee of the University of Adelaide and the South Australian Museum and Dr Trevor Worthy of the University of New South Wales.
A rethink of the evolution of dinosaurs and birds has placed Archaeopteryx back at the top of the bird list [Credit: Michael Lee]
The magpie-sized archaeopteryx, which lived 150 million years ago in Bavaria, had feathers and flew like a bird, but had a long tail and serrated teeth like a reptile.
"It was immediately recognised as half bird-half reptile when it was first discovered," says Lee.
Archaeopteryx has generally been seen as the most bird-like of dinosaurs, and the most primitive ancestor of birds.
But earlier this year archaeopteryx was knocked off its perch by the discovery of a dinosaur that appeared to be a very close relative from China - the ground-dwelling Xiaotingia zhengi.
This small agile meat-eating dinosaur had feathers and hollow bones, and ran around on two feet.
The discovery of X. zhengi led to a new evolutionary tree that placed archaeopteryx in a group with bird-like dinosaurs like velociraptor of Jurassic Park fame, rather than in a separate branch that evolved into birds.
"Archaeopteryx lost its exalted place in bird evolution," says Lee.
But, this new evolutionary tree presented a problem because it placed archaeopteryx in a group of dinosaurs that either didn't fly at all or glided in a way that was not bird-like.
Lee says, it meant that bird flight most probably evolved more than once and archaeopteryx possibly evolved flight independently of birds in a case of what's called "convergent evolution".
As far as evolutionary theory goes, such scenarios are not particularly elegant. So Lee carried out a new analysis of the data to see what he could find.
He found that roughly the same number of traits put archaeopteryx with dinosaurs as put it with birds.
"If you just count the number of traits it's basically a 50:50 call," says Lee.
Lee says the traditional method of building evolutionary trees weighs each trait equally and tries to find a tree that fits the majority of them.
But, he says this is not necessarily the best approach because some traits are more reliable than others, because they evolve slowly and are more likely to be conserved through time.
For example, a back bone is likely to evolve just once, but a particular fur colour could evolve multiple times, and is thus a less reliable trait for building evolutionary trees.
Lee borrowed a technique called 'maximum-likelihood', which is used to build evolutionary trees based on genetic data and applied it to fossil data.
The technique can determine the rate of evolution of different traits and gives extra weight to more reliable slow-evolving traits.
Lee found the bird traits in archaeopteryx were more reliable than the dinosaur traits and when this is taken into consideration you get an evolutionary tree that restores archaeopteryx to its original position as 'first bird'.
"It puts it back where people have always thought it belonged," he says.
Lee says despite the findings, the exact position of archaeopteryx in the evolutionary tree is hard to pin down because there's a whole swarm of fossils that lie between dinosaurs and birds.
But the findings do suggest that the maximum-likelihood method is a useful tool for analysing the fossil record, he says.
Lee says he would like to see the method used to examine evolution rates in the fossil record of other groups such as mammals.
"The standard view of mammal evolution is that it was an evenly spaced progression," he says.
"I think the really hot question is did they evolve towards mammalness in a gradual stately fashion or did it happen in fits and starts. These methods can actually shed light on that."