The advent of the description of Limusaurus and the associated hypothesis of digit homologies in the paper is likely to generate quite a lot of interest in dinosaurian and avian circles. However, it will, I suspect, also generate a fair bit of confusion in the short and perhaps even long term, even if the hypothesis is rapidly refuted or is not adopted by the palaeontological community. It’s a problem that comes up occasionally in palaeontology (and I imagine other fields too) and is worth commenting on at least a little.
The issue is that there are (for now) two working hypotheses for the homologies of digits in most theropods, either as I-II-III, or as II-III-IV. Until one or the other wins out (or even if one does not) we have to be very careful with how we refer to theropod digits. Obviously Limusaurus has generated much discussion in the office and by e-mail during the writing of the paper, and we were forced to resort to referring to ‘traditional’ and ‘new’ interpretations of patterns (or even single digits) to avoid confusion and remove ambiguity. Others are going to have to follow this pattern in their descriptions and analyses or an author may be referring to a digit II in the new sense and a reader take it as a digit II in the traditional sense and the two are very different.
In the short term this issue will probably also fuel a number of new investigations, revisions of old ones, or duplicates to see how the two different hypotheses affect the results of their work. Some initial work in the paper suggests that the II-III-IV hypothesis is more parsimonious in cladistic analyses by reducing the number of transitions in theropods, and while this is only tantalising evidence that the new hypothesis may be correct, it does illustrate the importance of investigating both hypotheses in parallel.
In the long term the presence of two hypotheses or the rejection of the traditional one (if that happens) may become really confusing. We have more than 150 years of research papers describing theropod digits as I-II-III and this may change. It does not make them obsolete, and of course it is relatively easy to mentally flick the switch when reading in order to change I-III as II-IV but it is complex. More importantly, when a new student or non-specialist gets to the literature (let’s say in 20 years or so), unless he meets the debate of the hypotheses early on, he will really struggle to work out how and why the transition occurred from one to the other and which papers are right and how the digits should be interpreted. I was stuck in a realm of confusion over a very similar issue during my PhD studies where I missed the fact that there were (and indeed still *are*) two different definitions of the archosaurs. I was reading papers that would each be using a different definition but were not always explicit about the differences or which definition they were using and it was quite a time before I found out and was able to put it into context and sort it out in my own mind. I can imagine something similar happening here in the future (depending on how it all works out, obviously).
Right, that leaves me pretty Limusaurus-ed out for now. No doubt the debate will rage, perhaps on here, but also elsewhere over exactly what this is and what it means. It will take time, but regardless of the eventual result there is some fascinating research to come.
Xu, X., Clark, J., Mo, J., Choiniere, J., Forster, C., Erickson, G., Hone, D., Sullivan, C., Eberth, D., Nesbitt, S., Zhao, Q., Hernandez, R., Jia, C., Han, F., & Guo, Y. (2009). A Jurassic ceratosaur from China helps clarify avian digital homologies Nature, 459 (7249), 940-944 DOI: 10.1038/nature08124