Limusaurus – an herbivorous ceratosaur?

Drawing of the Limusaurus skeleton

Well, what can I say about Limusaurus? Quite a lot actually and I hope to do so over the next few posts. Obviously this animal is generating quite a bit of excitement and / or controversy so it’s well worth commenting on at some length for a variety of reasons. The focus of the paper and by extension most of the media and blogging interest is in the hand and the implications it has for the identity of the fingers of derived theropods and of birds. I will deal with that in due course but this is also incredibly interesting as another probably herbivorous theropod in a lineage full of animals that are anything but vegetarians.

It is no surprise that a number of theropods changed to herbivory from carnivory at some point, or at least were probable omnivores. Theirizinosaurs are considered herbivorous, and the ornithomimids and oviraptorosaurs were probably omnivorous in the main with some species possibly being exclusive herbivores. What is surprising though is that Limusaurus is a ceratosaur and thus part of a clade which would normally not be associated with this kind of lifestyle at all – at least the others are consistent.

Photo of the Limusaurus holotype. THe yellow arrows indicate a crocodilian in association.

Ceratosaurs such as the eponymous Ceratosaurus, bizarre Carnotaurus and Abelisaurus while obviously having their differences do have one thing in common, namely being tanking great animal with shredding carnivorous teeth. Because, well, they killed and ate their dinner and it had a tendency towards a) being alive, and thus unlike plants, b) could run away. It is then very odd to find an herbivorous genus floating around among this bunch of carnivores, but why do we think it has gone down this route?

Preserved gastroliths with a 1cm division scale bar - these are really small.

Well, for a start Limusaurus doesn’t have carnivorous teeth – it has none at all. Instead there is a place for a bird-like beak and spaces for blood vessels and nerves in the bones of the jaws that suggest one was present. Just having a beak of course does not exactly make it an herbivore (as any modern eagle or vulture) but it certainly shares this feature and its general morphology with those dinosaurs we do know are herbivores like ceratopsians and ankylosaurs. Secondly, although derived ceratosaurs often reduced the hand to quite amazing degrees, the more basal forms kept ‘normal’ theropod hands with big predatory claws and thus it is therefore probably significant that Limusaurus as the most basal ceratosaur has a reduced had with no big unguals. Again, hardly absolute but it points in the same direction. Finally inside both specimens of Limusaurus (yes there are two specimens for those not paying attention at the back) there is a patch of gastroliths (literally ‘stomach stones’) such as are seen in animals like Caudipteryx. You don’t need anything to mash up soft and easily digested meat in your guts but if you have no teeth or mechanism to chew up touch plants this would be pretty handy. The presence of gastroliths therefore also points towards a herbivorous lifestyle. Limusaurus was then, if not a true herbivore, it had a strong bias towards herbivory as part of an omnivorous diet. It is certainly hard to argue that it was a predator in the manner of its close relatives.

The implications of this are themselves interesting. First of all it means that a basal member of a basal clade of theropods adopted herbivory far earlier than we thought. Limusaurus may itself be Middle Jurassic in age, but one must assume that there were herbivorous and / or omnivorous ancestors going back potentially much further in their evolutionary history. The other herbivorous / omnivorous theropods appear much later both in temporal and phylogenetic positions. This suggests (to me at least) that becoming herbivorous might be ‘easier’ then we imagined even for theropods (since ancestral dinosaurs were carnivorous, the sauropodomorphs and ornithischians at least have evolved herbivory). We now have several lineages of theropods that independently acquired herbivory and it happened much earlier than we thought, and in a clade we would not have expected to see it in – who knows, there might even be an herbivorous allosaur or tyrannosaur out there.

This also makes the evolution of the ceratosaurs more interesting with an herbivore in their ranks. First off, Limusaurus is recovered in our analysis as the sister-taxon to the little known Elaphrosaurus from the Late Jurassic of eastern Africa. This implies that this animal itself may also have been herbivorous if it is such a close relative. Enticingly, Elaphrosaurus, while known from a pretty complete skeleton has no skull preserved so this is certainly a possibility. Oli Rauhut is working on a big new description of this taxon and there are potentially indicators in the skeleton that can reveal a tendency for herbivory or carnivory so there is mo re to come on this issue at least. Secondly, Limusaurus and Elaphrosaurus as a pair are the basal most ceratosaurs, and with the taxa basal to them (other theropods) and more derived than them (other ceratorsaurs) all being predatory, what does this mean for the evolution of herbivory? It’s tempting to reason that perhaps early ceratosaurs shifted from carnivory to herbivory and later ones revered to carnivory. However, it is of course more parsimonious to think that this is simply a single change to herbivroy in one lineage than two independent reversals (and of course such complex issues as the reacquisition of teeth, the predatory manus and so on). Still, when we see a general conservatism of diets in other theropod lineages (no matter what they are eating) Limusaurus stands out next to its cousins as a dramatic example of a reversal of lifestyle.

Elaphrosaurus mount in Berlin (with various reconstructed bits, most notably the skull). Image courtesy Mike Taylor

Another point here is that Limusaurus looks rather like ornithomimosaurs, oviraptorosaurs and modern ostriches in terms of the general body plan. For that matter it looks rather like Effigia, the crocodilian ‘ornithomimid’-mimic that was described a few years back. It’s not a great piece of evidence admittedly, but it does seem to be a nother case of convergent evolution towards a constrained bauplan for archosaurian herbivores. In fact it’s worth pointing out that Elaphrosaurus at some point was mistaken for an ornithomimosaur, adding fuel to the fire that this may be an herbivore too and adding weight to the idea that it’s a close relative of Limusaurus.

So there we are, an herbivorous Middle Jurassic ceratosaur. We never expected to find it, and there it is. What other surprises yet lie in wait? I’m really not sure I might want to guess. There is still lots more to talk about with Limusaurus however so come back for the (possibly) exciting, and certainly complex and confusing issues of finger homology and the rise of birds (via ceratosaurs).

Xu, X., Clark, J.M., Mo, J., Choiniere, J., Forster, C.A., Erickson, G.M., Hone, D.W.E., Sullivan, C., Eberth, D.A., 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(18):940-944.

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