A proposal has been made to remove beloved Archaeopteryx from the bird family tree and push it over to some non-avian dinosaur subtree. This is not the first time that the ancient species has had its position on the tree of bird life threatened, but this time it may be for real. The proposal is reasonable.
I’ve talked about this issue before, but I’m bringing it up now because there is a new paper, just out. Lets get right down to business and start with the abstract:
Archaeopteryx is widely accepted as being the most basal bird, and accordingly it is regarded as central to understanding avialan origins; however, recent discoveries of derived maniraptorans have weakened the avialan status of Archaeopteryx. Here we report a new Archaeopteryx-like theropod from China. This find further demonstrates that many features formerly regarded as being diagnostic of Avialae, including long and robust forelimbs, actually characterize the more inclusive group Paraves (composed of the avialans and the deinonychosaurs). Notably, adding the new taxon into a comprehensive phylogenetic analysis shifts Archaeopteryx to the Deinonychosauria. Despite only tentative statistical support, this result challenges the centrality of Archaeopteryx in the transition to birds. If this new phylogenetic hypothesis can be confirmed by further investigation, current assumptions regarding the avialan ancestral condition will need to be re-evaluated.
maniraptorans is a group of dinosaurs that includes birds. It consists of Avialae (which includes birds) and Deinonychosauria (which does not include birds).
Here’s what happened. They found new fossils in China. When they looked at these fossils they realized that some of the traits thought to be unique to the Avialae group of maniraptorans are actually not, but rather, are found in a larger group that includes both the bird-inclusive Avialae and non-birds. (This larger group is the Paraves).
“Archaeopteryx was [thought to be] a bird because it had feathers and nothing else had them. But then other animals started being found that had wishbones, three-fingered hands and feathers. Heck, even T. rex had a wishbone. So one by one we’ve learned Archaeopteryx ‘s uniquely avian traits weren’t so unique. The writing was really on the wall,” says Lawrence Witmer, a palaeontologist at Ohio University in Athens.
Here’s the thing. A valid phylogeny (tree of relatedness) requires a perfect understanding of both the timing (in relation to speciation, not chronology) and polarity of traits. You need to know which trait gave rise to which trait; The direction of the change has to be unambiguous. This means you also have to know which species did not have the trait. If you see a trait in what you think is a new species but not in its ancestors, you may have a case for a particular evolutionary tree. But if it turns out that the trait in question is also found in the ancestor, your tree is invalid.
If in fact Archaeopteryx was misplaced before, it was because a trait thought to be derived in that group was actually primitive to it and found in a larger group, and is thus disqualified as a distinguishing character. After this, other traits became more important in positioning Archaeopteryx, and thus the shift.
Among the key traits responsible for this shift are features of the head of the animals studied. Archaeopteryx has the head of a Deinonychosauran, not an Avialan.
Huxley is very happy to see this new development.
Xu, X., You, H., Du, K., & Han, F. (2011). An Archaeopteryx-like theropod from China and the origin of Avialae Nature, 475 (7357), 465-470 DOI: 10.1038/nature10288