Παρασκευή, 28 Οκτωβρίου 2011

Και πάλι ως πτηνό, χαρακτηρίζεται ο αρχαιοπτέρυγας!

The Archaeopteryx is back in the bird category, with Australian researchers deeming this ancient creature to be more like a primitive bird than a bipedal theropod, as was suggested earlier this year.


A study published in July 2011 by palaeontologists from the Institute of Vertebrate Palaeontology and Palaeoanthropology of the Chinese Academy of Sciences in Beijing, challenged the idea that Archaeopteryx, was a primitive bird and knocked it off the avialae branch, reshuffling it into the Deinonychosauria tree.


Now, thanks to a more sophisticated anatomical analysis, this claim has been challenged, as researchers from the University of Adelaide and the University of New South Wales found results conflicting with the Chinese researchers' findings. Their study was published this week in the journal Biology Letters.

"The [Chinese] author's paper was fine and certainly did not make any unjustified claims," said Mike Lee, co-author of the paper and a senior researcher in palaeontology from the South Australian Museum. "The authors were careful to emphasise that their results were suggestive rather than definitive and that further research was required."

Icon of evolution and natural selection

The first complete specimen of Archaeopteryx was discovered in 1861 in Germany. It lived around 150 million years ago and had a combination of bird- and dinosaur-like features, including sharp teeth, three fingers with claws, a long bony tail, broad feathered wings, and the ability to fly.

"Archaeopteryx is arguably the most famous fossil and an icon of evolution," Lee said, "so a robust consensus of its position in the reptile family tree is important. Ironically, the discovery of more and more "missing links" between birds and dinosaurs makes this job harder."

Chinese palaeontologists had uncovered the fossil of an unknown species which had the hallmarks of being a bird-like dinosaur. Named Xiaotingia zhengi, its position put the place of the Archaeopteryx as the earliest and most primitive bird in jeopardy, eventually being replaced by the newly uncovered fossilised species.

A raptor relative or an early bird?

The Chinese researchers argued that "the Archaeopteryx is not a bird, but is ancestral to the Velociraptor", said Xing Xu, lead author of the July paper published in Nature.

By using a Bayesian maximum-likelihood method, the Australian researchers found a majority of the dinosaur's evolutionary tree to be the same as if an Unweighted Parsimony based model was used, save for the Archaeopteryx, which was re-united with the other birds.

"While parsimony is simpler, faster and easier to understand, likelihood uses slower and more complicated mathematical models," Lee said, "but it recognises that certain traits change more slowly than other traits and accounts for this when trying to find evolutionary trees."

Unweighted parsimony remains the most common method of analysing morphological data and treats changes across all branches of the evolutionary tree equally. However, it is prone to giving misleading results, especially with long genetic branches.

Birds and dinosaurs blended together

However in contrast, the Bayesian likelihood methods used in the study places a certain amount of emphasis on how fast an evolution occurred. Although such analytical methods are more commonly used in the field of molecular biology, its foray into palaeontology and morphology can be very useful in determining evolutionary trees.

"Birds and dinosaurs are now blended together by a series of transitional forms which differ from each other in very subtle features," Lee said, "making it increasingly difficult to precisely place any fossil - including the Archaeopteryx - in a single exact spot within this evolutionary column."

Phylogenetic relationships between the species and genus are often focused on searching for their ancestral root. Organisms each have distinct features, called character states, on which the absence or presence of, can be enough to determine an evolutionary mutation.

A trait is homologous if the evolution occurs once and is unreversed. However, if the cause of a trait is not similar ancestry but independent evolution, termed homoplasy, the analysis can provide misleading evidence of phylogenetic relationships.

"While there will always be more challenges to the claim [of Archaeopteryx being a bird or not]," commented Thomas Rich, senior curator and palaeontologist from Victoria Museum, "We should use different statistical methods to analyse the same problem, and get more statistical data from existing specimens and fossil evidence to further our understanding of the phylogenetic position of Archaeopteryx."

cosmosmagazine.com


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