Published in The Hindu on July 28, 2011
The discovery of Archaeopteryx, considered the basal bird, became central to our understanding of avialan (a group that includes all birds and some dinosaurian relatives) origin. In fact, it became central to our understanding of the evolutionary process — how the dinosaurs slowly evolved to become today’s birds.
Found 150 years ago
More importantly, the discovery was made in 1861 from Bavaria, Germany, just two years after Darwin had come out with his famous work — Origin of Species.
Such has been the importance given to Archaeopteryx that a great body of scientific work on bird evolution has always centred on this basal bird.
“Indeed, virtually all our notions about early avian evolution have been viewed through the lens of Archaeopteryx,” notes a news piece published on July 28 in Nature journal.
Not a basal bird
But a paper published in the same issue of Nature today (July 28) has not just dethroned Archaeopteryx from its high pedestal but also gone to the extent of clubbing the iconic basal bird along with other non-avian dinosaurs. In other words, Archaeopteryx does not belong to the avian group, leave alone being a basal bird.
Indeed, the last ten years have seen the discovery of many dinosaur fossils that share some unique characteristics of basal birds that are not seen in Archaeopteryx.
The latest find
The Xiaotingia zhengi specimen discovered and described in the Nature paper only strengthens the argument against the basal bird status conferred on Archaeopteryx, and in turn its position in the avialan group.
Earlier studies found some unexpected and striking similarities between Archaeopteryx and other deinonychosaurs (bird-like dinosaurs but not belonging to avialan group of dinosaurs), and proposed a “close relationship” between the two. But the authors of the paper have gone a step further. They have presented a numerical morphological (phylogenetic) analysis “supporting deinonychosaurian affinities for the Archaeopterygidae [to which Archaeopteryx belongs].”
Two main conclusions
The authors have drawn two main conclusions. First, there are striking morphological similarities between Archaeopteryx, the latest find (Xiaotingia) and other deinonychosaurs (bird-like dinosaurs). Second, these similarities in turn highlight the real differences between the ‘basal bird’ discovered 150 years ago and “other widely accepted basal avialans” discovered in recent times.
“There are few derived features shared by Archaeopteryx and basal avialans… thus documented morphological support for the avialan affinities of Archaeopteryx is fairly weak,” they write.
There is a word of caution by the authors. “Our phylogenetic hypothesis is only weakly supported by the available data,” they warn.
The latest discovery also sets the record straight regarding the many morphological features that were once considered as distinctly avialan — that they are in fact characteristic of the Paraves group (to which the avialans and the deinonychosaurs belong).
They cite the instance of lengthening and strengthening of the forelimbs. Such forelimbs indicate a dramatic shift in their function, and maybe “related to the appearance of a degree of aerodynamic capability.” The forelimb example is akin to the presence of flight features seen in both basal avialans and basal deinonychosaurs.