Archaeopteryx Abandoned

A DEITY couldn't have planned it better. Just two years after On The Origin of Species was published, a fossil found in Germany gave Charles Darwin's controversial ideas an almighty boost. Archaeopteryx sported a mouthful of teeth and armfuls of feathers - facts that Darwin's supporters immediately leapt on as evidence that birds descended from dinosaurs.

This week, Darwin's "strange bird" has finally lost its perch on the lowermost branch of the bird evolutionary tree (see "Archaeopteryx knocked off its perch as first bird"). ¹

The main players here are an assemblage of bird-like dinosaurs — oviraptorosaurs and deinonychosaurs (including troodontids and dromaeosaurids) — and dinosaur-like birds that belong to the avialans. Deinonychosaurs and avialans together comprise a group known as Paraves, with oviraptorosaurs being a bit more distantly related. As more fossils of basal members of each of these groups have been collected, distinctions between the groups have predictably blurred, and some species have bounced around from group to group. For example, Anchiornis, which recently made headlines for its colourful plumage, was originally regarded as a basal avialan, then a basal troodontid, and is now considered an archaeopterygid. ²

But why is this such a big deal? Archaeopteryx has always been something of a celebrity and has monumental 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

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

In truth, this chapter of the scientific story is just beginning. Just as Xiaotingia moved Archaeopteryx out of the birds, the next find could move it back in — or to somewhere else within this fuzzy tangled knot that makes up the origins of birds and bird-like dinosaurs. That said, during this sesquicentennial anniversary of Archaeopteryx, which is being honoured with exhibits and commemorative coins, the bitter irony may be that it may not have been the bird we've always thought it was. But Archaeopteryx will remain an icon of evolution, perhaps even more so now, providing compelling evidence that, as we should expect, evolutionary origins are rather messy affairs. ⁶