Saturday, November 5, 2011

Archaeopteryx is back on the Avialian tree...


Fig 1.- Archaeopteryx back on its perch?

In the midst of the discovery of the 11th specimen of Archaeopteryx, almost complete (missing the head) with extensive feather impressions, a new paper published in Biology Letters, challenged the recent view expressed a few months earlier that it was not an Avialan (bird in the restricted sense) after all. Australian scientists Michael S. Y. Lee and Trevor H. Worthy, using the same sets of 374 characters than those defined by Xing Xu and co-workers in their Nature paper, employed a different and more sophisticated mathematical method in their phylogenetic analysis and arrived to a somewhat different conclusion.

Xu et al. used the maximum parsimony approach to obtain their evolutionary tree of birds and bird-like dinosaurs, while Lee used the more sophisticated maximum-likelihood and the related Bayesian inference methods. Under maximum parsimony, the preferred phylogenetic tree is the one that requires the least number of evolutionary changes to explain the observed sets of characters (or traits). Whereas generally valid, this assumption can be problematic in cases such as when some of the traits are evolving much faster than others or when some taxa have very long branches. The maximum-likelihood method is a seemingly more powerful (and computationally intensive) parametric statistical technique that uses an explicit model for character evolution and therefore is not subject to the same pitfalls. Maximum likelihood will pick the most probable tree that explains the observed data.

Fig 2.- Simplified tree according to Lee & Worthy, 2011.

The most important result of the Australian team new analysis of bird ancestry is that it puts solidly (in the sense that the measured level of accuracy given by the analysis is higher than with the parsimony approach) Archaeopteryx back on the Avialian tree as a basal bird. One of the consequences is that the typical forelimb-powered flight of birds would have only evolved once, while deinonychosaurian dinosaurs such as Microraptor would have discovered four-winged flight. Interestingly and on the side note, the odd scansopterygids, appear in the maximum likelihood analysis as deeply nested within the Avialians.

Is this new phylogenetic analysis establishing with certainty the evolutionary position of Archaeopteryx as the ancestral bird? Probably not…  Pitfalls of the maximum parsimony method are reduced when taken more characters into account. We note for instance than in D. Naish & et al.’s study (2011), almost 3 times more characters (1025) were taken into account in a parsimony approach and the conclusion is somewhat similar to Xu et al. in the sense that Archaeopteryx is out of the Avialian tree.

References:

Michael S. Y. Lee & Trevor H. Worthy. 2011. Likelihood reinstates Archaeopteryx as a primitve bird. Biology letters. Published online before print.

Darren Naish, Gareth Dyke, Andrea Cau, François Escuillié and Pascal Godefroit. 2011. A gigantic bird from the Upper Cretaceous of Central Asia. Biology Letters. Published online before print. Electronic supplementary info.

Xing Xu, Hailu You, Kai Du and Fenglu Han. 2011. An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475: 465–470.

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