Monday, 17 September 2012

Palaeontology is Real Science Part 1: Muscle reconstruction

This week's Mesozoic Mondays post is going to be a bit different from usual. Since working at Jurassic Forest, I've been amazed by how many people don't consider palaeontology to be a real science, and how surprised they are to see a speaker talk about their research. In particular, I remember one set of parents who were engineers, that came to watch a particularly technical talk on theropod tail muscles. After the talk, the father said "I had no idea so much science was involved in palaeontology"! In response to this, I'm going to do a few posts based on this, showing some of the different scientific techniques palaeontologists use in their research. It's not all digging and assembling skeletons you know! 

To continue with the topic mentioned above, I'm going to talk a bit about muscular reconstruction first. It may sound trivial, but reconstructing fossil muscles is extremely important to understand how an extinct animal was able to move, and it's also very difficult to do correctly. There are two important pieces of information needed to reconstruct fossil muscles. The first, is something called the extant phylogenetic bracket (Witmer 1995). To first understand this, you must understand phylogenies. A phylogeny is something like an evolutionary family tree that shows how different groups of animals are related to each other, based on different features that the animals share, or new derived features. By using living animals (extant) that 'bracket' the extinct form in question on either side, an idea of how the muscle may have appeared in the bracketed animal may be possible. This is by assuming the theory of parsimony, that it is more likely that if something is present before and after, it is most likely also present in the evolutionary middle, rather than losing the feature and re-evolving it. I know, it's a bit complicated! For a good explanation of phylogenetic bracketing, check out the Wikipedia page, which is quite good and full of examples. One example is that theropod dinosaurs are bracketed between crocodiles, their closest living relatives that evolved before dinosaurs, and birds, which of course evolved from dinosaurs. Anything present in both crocodiles and birds was likely present in theropods, whereas something that is not present in either probably wasn't in theropods.
Example of a cladogram showing the phylogenetic relationships of modern reptiles (including birds).  An example of extant phylogenetic bracketing is also seen. The four-chambered heart is present in both crocodilians and birds, and therefore most likely present in dinosaurs. Image from

Now, once you know what the animals look like that evolved before and after the animal in question, the next key is to look at muscle scars. For this, I'm going to use a paper by Persons and Currie (2011a) from the University of Alberta as an example, as it is open access, so anyone can download it if they would like. By looking at the caudal (tail) vertebrae of the theropod dinosaurs Carnotaurus and Aucosaurus, muscle attachment sites can be viewed. By combining muscle attachment sites with the phylogenetic bracket, it's possible to determine what muscles attach where, and possibly how big they were. Persons and Currie (2011b) used a combination of both methods by dissecting modern caimans to understand their musculature in order to reconstruct the tail musculature of Tyrannosaurus. Although still controversial, this new muscle reconstruction suggested that Tyrannosaurus was a pretty fast guy, using large muscles that ran from its tail to its leg to run. Pretty cool stuff coming out of the University of Alberta! You may have guessed by now that it was actually a talk by Scott Persons that spurned the "paleontology is a real science" discussion. 
Caudal vertebra of Aucosaurus showing a sequence of muscle scars from the ischiocaudalis and caudofemoralis muscles. (Persons and Currie 2011a)
That was an introduction into how palaeontologists can reconstruct muscles from bones, and hopefully it taught you something you didn't know! Muscle reconstruction is really quite important in order to understand some aspects of behaviour and locomotion, as seen in the study of Tyrannosaurus. If it was really as fast as suggested, he was definitely an active hunter rather than a scavenger like some people suggest. Of course, this isn't an easy thing to do since most fossils are incomplete, and it can be difficult to see things like muscle scars. It can also be difficult to determine the phylogenetic position of an extinct animal, and changing the position can greatly change the reconstruction. That being said, this is a very common method used in palaeontology, and it's very important to understand the anatomy and muscular systems of living animals in order to better understand extinct animals!

a Persons, W.S., and Currie, P.J. 2011. Dinosaur speed demon: the caudal musculature of Carnotaurus sastrei and implications for the evolution of South American abelisaurids. PlosOne 6: e25763. --> Freely available online, so take a look if you're interested!
b Persons, W.S., and Currie, P.J. 2011. The tail of Tyrannosaurus: reassessing the size and locomotive importance of the M. caudofemoralis in non-avian theropods. The Anatomical Record 294: 1442-1461. --> unfortunately not open access.
Witmer, L.M. 1995. The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils. In: Thomason, J (ed.) Functional Morphology in Vertebrate Paleontology. pp. 19-33. Cambridge University Press.

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