Some parts of skeletons are very easy to identify and can be diagnostic right down to individual genera in some cases. Others are, sadly, far less easy to sort out and even very different species can have very similar features, or the range of variation can be so great that all manner of things can overlap making identification difficult at best. Such is the lot of theropod teeth (as we have seen before) and while there are *some* good characters which seem pretty conservative within groups, in general it can be hard to tell with confidence exactly which groups a given tooth might (or might not) belong to.
However, this doesn’t mean we can’t try and work this out and there are cases where positive (or at least reasonable) identifications can be made. And this can shed real light on the diversity and distribution of certain clades.
And so we move to the Shishugou Formation of Middle Jurassic age in China. The name might not be familiar but the theropods will be – Limusaurus, Guanlong, Haplocheirus and Zuolong are all from here and a giant tooth suggests the presence of Sinraptor or something very similar too. That’s already quite a bit of variety (ceratosaurs, tyrannosaurs and alvarezsaurs) but there’s more to come. A number of isolated theropod teeth collected a few years ago were given to one of Xu Xing’s Masters students as part of his thesis work and a paper on this has just been published. I feature in the authorship list in the obviously most important place of ‘last’ though Tom Holtz suggests that I should in fact advertise this as “special guest star”.
Anyway, this is a real challenge. Not only are many theropod teeth not diagnostic or highly variable, but the Middle Jurassic represents perhaps the peak of theropod diversity at least in terms of the clades that were around at the time. The early coelophysoids have died off by then but ceratosaurs, allosaurs, tyrannosaurs, spinosaurs, and every maniraptoran was, or should be, around. Ah. Still, Han Fenglu stepped up the the challenge and the result of his work and the rest of the team has come to fruition.
Perhaps unsurprisingly given the conditions not all of these teeth could be identified down to any especially narrow set of possibilities but a couple were very good candidates for deinonychosaurs teeth and not much else. For those that don’t know this is the name of the group that consists of dromaeosaurs and troodontids and thus are the closest dinosaurian relatives of birds. While the discovery of Anchiornis has finally pushed back the record of this group to before Archaeopteryx, these teeth predate even that. This marks them out then as (possibly, or dare I say probably) one of the oldest records of the group. Hardly groundbreaking stuff, but quite interesting none the less. Filling in all those gaps in the fossil record is generally a piecemeal and occasional occupation in any case, but any chip away at it should be welcomed.
What’s also interesting is that while many of the teeth could not be assigned to any particular group, many of them could be fairly confidently kept apart from taxa we know are present. You might not know quite what a tooth came from, but you can compare it to Guanlong and the rest and see if it’s a match. If not, you might well have something new there. In this case there were several possible newbies which means that not only might we have early troodontids and dromaeosaurs but there are probably other new theropod taxa waiting to be discovered in the Shishugou as well. Time will tell if all or any of these interpretations are correct, but they are certainly intriguing.
Han, F., Clark, J.M., Xu, X., Sullivan, C., Choiniere, J., & Hone, D.W.E. 2011. Theropod teeth from the Middle-Upper Jurassic Shishugou Formation of northwest Xinjiang, China. Journal of Vertebrate Palaeontology, 31: 111-126.