ental historical, sociological and even political importance. It was discovered, with perfect timing, in mid-1861, less than two years after Darwin's Origin of Species hit the bookstalls. With its blend of avian and reptilian characteristics (not to mention the charismatic beauty of the fossils themselves), Archaeopteryx was seemingly the ideal evolutionary intermediate, instantly entering the debates over evolution in Victorian England and elsewhere, and gaining prominence in textbooks.
Given this iconic role, Archaeopteryx has also been in the cross-hairs of creationists, and remains a lightning rod for political debates and legal proceedings about teaching evolution in schools. Of course, Xu and co-workers' finding only deepens the impact of Archaeopteryx by highlighting the rich evolutionary nexus of which it is a part, but how the ever-clever creationist community will 'spin' it remains to be seen.
Politics aside, the historical importance of Archaeopteryx stands, even if we need to add the footnote that current evidence no longer regards it as the oldest bird. The impact of losing Archaeopteryx from the avian clan is, nevertheless, likely to rock the palaeontological community for years to come simply because, for the past century and a half, these familiar fossils have guided almost all scientific thought about the beginnings of birds. The late John Ostrom, the most influential modern worker on avian origins, began his seminal 1976 article, itself entitled 'Archaeopteryx and the origin of birds', with the statement, “The question of the origin of birds can be equated with the origin of Archaeopteryx, the oldest known bird.” Indeed, virtually all our notions about early avian evolution have been viewed through the lens of Archaeopteryx. Hundreds of publications (including several of my own) draw on the structure of Archaeopteryx to formulate and evaluate hypotheses about birds. Some published phylogenetic analyses have even used Archaeopteryx as the sole representative of birds. To what species do we now turn to ground our understanding of early birds? 3
These observations necessitate a re-evaluation of widely accepted hypotheses of paravian phylogeny; such an exercise will have significant implications for our understanding of avialan origins and related issues such as the origin of flight. 4
The primary source is an article about the discovery of a fossil called Xiaotingia. Most of the article deals with a description of the fossil, including the number of teeth, the size and shape of its forelimbs, and various other skeletal elements. Based on all these measurements of Xiaotingia, they conclude that Archaeopteryx wasn’t a bird at all. Here is perhaps the most important paragraph summarizing the analysis.
When Xu et al. ran a phylogenetic analysis combining the attributes of Xiaotingia with those of Archaeopteryx, other basal avialans, deinonychosaurs, and oviraptorosaurs, not only did Xiaotingia and Anchiornis cluster with Archaeopteryx, but these archaeopterygids now were yanked out of Avialae and placed in Deinonychosauria. In other words, Archaeopteryx was no longer a bird. Surprised by this outcome, the authors re-ran the analysis with identical parameters, but this time omitting Xiaotingia. The result was that Archaeopteryx was restored to Avialae as the most basal bird. This experiment affirmed how crucial Xiaotingia is to understanding the evolution of advanced theropods. 5
Here’s what that means. They wrote a computer program that compares selected physical characteristics to decide how closely related various species are and assigns them to various groups. The comparisons that the computer makes are all based on what the programmer tells it to compare, and how heavily to weight each feature. The validity of the conclusion depends entirely upon the subjective decisions made by the programmer.
When they ran the program without including data about Xiaotingia, the computer program said that Archaeopteryx was a bird. When they added Xiaotingia, the computer changed its mind and said that Archaeopteryx wasn’