OK by now the ‘secret’ is probably out and the ‘new basal avialian’ that my colleagues and I described earlier this year, Anchiornis, turns out not to be an avialian (or avian if you prefer) at all, but in fact is a very basal troodontid. Those new specimens I mentioned at the time brought in a ton of extra detail and information that allowed researchers to firm up the diagnosis of this animal and show conclusively that it is indeed a troodontid. There are some important and interesting lessons to take from this, both in terms of theropod and bird relationships and how new information changes perspectives. To try and avoid me rambling on and keeping the issues clear, I have listed them:
1. Our initial assessment was not ‘wrong’ as such, we merely did not have all the information that would have revealed the troodontid identity of this animal. The original diagnosis was well supported by the information we had and the accompanying systematic analysis. We rarely have complete animals to work from and often have to work out an organisms ancestry based on a few key characteristics, if those are missing it can be very tricky.
2. In this case the missing piece was the skull which, for archosaurs, is incredibly important in learning about an animal. For cladistic analyses at least half the characters often reside in the skull, jaws and teeth, so with that missing you immediately have your information cut in half. Add to that any other missing pieces, distortion or compression of the fossil, pieces hidden under each other, or split and broken, or of course only having the thing in 2D because it’s preserved in a slab of rock and not as a 3D set of bones and you can see why we can’t always make as accurate designations as we would like.
3. By extension of the above points, this shows just how similar taxa are at the point where dromaeosaurs, troodontids and avialians (which together make up the Paraves) diverge. There are still longstanding discussions and conflicting analyses over exactly which groups is related to which and in what way. Even with detailed analyses using complete who skeletons some of these relationships are still subject to debate. If anything Anchiornis clouds the issue a little and certainly illustrates how intimately linked these groups are.
4. While we already knew that troodontids had feathers, including on their legs, this is a particularly good example and does extend our knowledge of how these evolved and were distributed around the theropod tree. Since we now have basal avialians (Archaeopteryx), basal troodontids (Anchiornis) and dromaeosaurs (Microraptor) with feathered legs then it seems clear that this is likely to be a primitive characteristic for paravians and that many, and possibly all, of them had these kinds of plumes.
5. However, it does not mean that they were all parachuting, gliding, or even flying – merely that they had long leg feathers. The feathers on Anchiornis are symmetrical and thus provided no (or at least low / minimal) lift and the animal shows none of the specialisations of Microraptor to gliding or of Archaeopteryx to powered flight. It does raise the possibility that other paravians developed something similar however given the fact that they had the apparent exaptations (also called pre-adaptations) of elongate and broad leg feathers.
The paper itself focuses more on the age of the fossil and the implications there of early troodontids and I’ll be returning to this shortly in another post, but right now I’m at SVP and thus rather busy.
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