Tuatara skull joints

Now that I got my vanity post, let’s continue the blow-by-blow details of this issue’s other papers with a real monster of a publication, one that normally would have to be printed as a monograph, leading to the death of a whole forest. To give you a quick idea of how big it is: there are no less than 86 figures, but it is not a coffee-table book. The final version of the manuscript text file has 75 pages in Word alone, not counting those for references and figure captions (another 45 pages)! And the authors are not only prolific in their science writing, the lead author Marc E.H. Jones was so kind to also provide the most detailed (and thus interesting) set of answers to our blog post questions.

Marc and his co-authors Neil Curtis, Michael J. Fagan, Paul O’Higgins and Susan E. Evans report on the joints in the skull of the Tuatara (Sphenodon), a relative of snakes and lizards. This is not the first time that this team of authors has sent a monster paper our way, and it is no coincidence that both their previous works in PE dealt with Sphenodon as well: one on the head and neck muscles associated with feeding (Jones et al. 2009), and a technical article on how to visualize muscles using three-dimensional computer models, using the previous work’s topic as an example. In the latter, the authors manage to pack 23 beautiful colour figures into meagre 18 pages (slackers!), while the former has 51 figures on 56 pages. Now you probably think that this covers the Tuatara rather well? No, sorry to say, but Marc and colleagues were unfaithful to PE, and published even more stuff elsewhere. It is this kind of thoroughness we need, and if PE’s free-of-charge policies, especially for colour figures, helps getting us all this data, then we’re on the right track!

Figure 20 from Jones et al. 2011. Just to water your mouth......

I’ll shut up now and save you the bother of reading another 30 or more lines of me fawning over the high detail figures, the thoroughness of the research, the attention to detail, detail, and again detail, and the importance of this paper for a long-time research project of mine, and will just let Marc tell you about it all below the break:

Can you summarize your research in two to five sentences understandable for lay people?

We survey how skull bones of the New Zealand tuatara reptile (Sphenodon) meet one another and evaluate the implications of this for skull mechanics. The joints (or sutures) between the bones take a variety of forms. They can be simple abutting contacts, overlaps or interlocking joints that involve several overlaps. Such overlaps provide a lot of surface area for fibrous tissues that can resist and buffer the stresses that arise from feeding in order to maintain rigidity of the skull. The degree of overlap between bones is larger in adult skulls (compared to juveniles) which may be associated with their ability to bite more forcefully and a tendency to feed on harder prey.

What is the most important thing about your research presented now in Palaeontologia Electronica? The one-sentence essence?

The skull bones of the tuatara overlap one another in a complex fashion that appears suited to resist the stresses that arise during feeding.

Why did you pursue this topic?

The functional role of cranial joints remains both poorly understood and contentious (e.g. Herring 2000). Some scientists argue they are not relevant to skull mechanics and exist only to permit growth. In humans and other mammals several of the joints between skull bones can fuse in adults, an observation that suggests once growth is completed they are no longer required and they play little role in skull mechanics. However, in many reptiles such as the tuatara the cranial joints remain open after adult size is reached. This suggests they are involved in how the skull responds to mechanical loading and that their morphology is to some extent related to this (either through growth or selection). As they have different material properties to the surrounding bone their presence alone must have functional consequences. The skull of the tuatara has been described repeatedly but the exact three-dimensional relationship between the skull bones and their appearance in isolation has been generally ignored. A survey of this aspect of anatomy would provide a solid basis for making functional comparisons and analyses using Finite Element computer models (e.g. Moazen et al. 2009).

Were your findings unexpected? Do they overturn a paradigm or otherwise surprise experts / the public?

The vast majority of skull descriptions are concerned only with the shapes of bones according to their external appearances. The joints between the bones are often neglected from discussion or, are at best, described as 2 dimensional lines rather than the three-dimensional ribbon-like volumes they actually are. The underlapping processes of bones are also often left out of descriptions.

The near absence of fine or complex interdigitated joints like those found in fish, mammals, turtles and crocodiles was surprising. Work on experimental animals such as pigs and sheep have led to suggestions that overlapping sutures are found in areas of tension whereas interdigitations are found in locations of compression. Therefore, we might expect both sorts of joints to be present in all skulls.

In general there does not appear to be much capacity for movements between the individual bones which supported previous suggestions that the tuatara has an “akinetic” skull. However, the joints around the premaxilla look like they might allow the base of this bone to pivot very slightly when under loading. Examining the histology of the soft tissue involved would help confirm this.

In background reading, I was struck at how many times cranial joints were described as interdigitated but the plane in which the interdigitation occurs is not specified. If the interdigitation is restricted to one plane this may have important implications for skull mechanics. Therefore, when describing those few joints with interdigitations we made certain that their orientation was clearly reported and for future description suggest a means of categorising the three main types.

What are the implications of your results on future research?

In the paper we evaluate the distribution of skull bone overlaps and the network of thickened bony ridges in light of expected loading from the teeth, muscles attachment sites, and jaw joints. We provide hypotheses that we intend to test with future computer modelling work and comparative studies. The descriptions and images of disarticulated tuatara bones will also assist comparisons with other lepidosaurs. In particular, the fossil tuatara known from cave deposits in southwest England and Wales (e.g. Gephyrosaurus, Clevosaurus, Diphydontosaurus and Planocephalosaurus, Evans 1980; Whiteside 1986; Fraser 1988) are typically known from isolated skull bones rather than articulated skulls. Furthermore, examination of the cranial joint histology is another obvious and potentially extensive avenue of future work.

Are there implications of your research outside science, i.e., in technology, business, everyday life?

The study of cranial joints and their functions has implications for medicine, where early closure (e.g. as in craniosynostosis) can have serious consequences. During my PhD I had dealings with a senior craniofacial surgeon based in Swansea and he was very interested to hear about the reptile suture anatomy I had started researching; even requesting a copy of my thesis once completed.

Did anything funny, weird or otherwise noteworthy happen during the research that lead to your paper in Palaeontologia Electronica?

Lots of things happened during the research because parts of it began in 2003 (the drawing of certain figures etc.) but nothing in particular stands out. I think we are all just glad it’s finally finished and out there for others to discuss.

Why did you choose to publish your results in Palaeontologia Electronica?

As we focus entirely on an extant taxon, our paper may not seem obviously suited to a journal with “Palaeontologia” in the title. However, a precedent for anatomical descriptions of extant taxa in Palaeontologia Electronica was set over five years ago when Bever et al. (2005) published a detailed description of the braincase and osteoderms of the living crocodile lizard (Shinisaurus). This paper included only cursory reference to the fossil record but has subsequently proven to be a highly useful resource for descriptions of fossil squamates (e.g. Conrad and Norell, 2006; Evans et al. 2006; Mead et al. in press). Similarly, we expect that our paper will also be relevant to those working on the cranial anatomy of both fossil and extant reptiles. We also had a good experience with our previous two papers on Sphenodon muscles in this journal (Curtis et al. 2009; Jones et al. 2009). There are currently very few journals able to accommodate long anatomical descriptions and multiple colour figures.

Will you use electronic publishing in the future, and why? What are the drawbacks and advantages from your perspective?

Yes but with some caution. Based on informal discussions with colleagues within life sciences there does still appear to be a certain level of snobbery against using electronic publishing but (for better or worse) it is likely that the role of electronic publication is going to grow and grow. For anatomical work, the advantages include fewer restrictions on figures and descriptive text.

What could be done to make Palaeontologia Electronica a better publishing venue?

Our experience with Palaeontologia Electronica has generally been very good with the editorial team being extremely supportive and helpful at all stages of the publication process. However, there are two things about the journal which could be improved.

Firstly, more recognition from search engines and citation counters. This is our third paper in Palaeontologia Electronica and we’ve found our previous papers are sometimes difficult to locate using search engines and citations are often ignored by research community websites such as Research ID.com.

Secondly, during the submission process it would be useful to have the option to check files after they’ve been uploaded but before final submission. At the moment you select all the files to upload in one go and they are submitted straight away. When you have lots of figures and the file names are limited to 8 characters it can be easy to make a silly mistake that can cause confusion later.

References

Bever, Gabe S., Bell, Christopher J., and Maisano, Jessica A. 2005. The ossified braincase and cephalic osteoderms of Shinisaurus crocodilurus (Squamata, Shinisauridae), Palaeontologia Electronica 8,4A:36p, 2.1MB; http://palaeo-electronica.org/2005_1/bever4/issue1_05.htm

Conrad, Jack L. and Norell, Mark A.2006. High-resolution X-ray computed tomography of an Early Cretaceous gekkonomorph (Squamata) from Öösh (Övörkhangai; Mongolia), Historical Biology, 18:405-431 http://www.informaworld.com/smpp/content~db=all~content=a769171150

Curtis, Neil, Jones, Marc E.H., Evans, Susan E., O’Higgins, Paul, and Fagan, Michael J. 2009. Visualising muscle anatomy using three-dimensional computer models – an example using the head and neck muscles of Sphenodon. Palaeontologia Electronica, 12,7T:18p; http://palaeo-electronica.org/2009_3/194/index.html

Evans SE. 1980. The skull of a new eosuchian reptile from the Lower Jurassic of South Wales. Zoological Journal of the Linnean Society 70: 203–264. DOI: 10.1111/j.1096-3642.1980.tb00852.x

Evans SE, Raia P, Barbera C. 2006. The Lower Cretaceous lizard genus Chometokadmon from Italy. Cretaceous Research, 27:673-683 doi:10.1016/j.cretres.2006.03.004

Fraser NC. 1988. The osteology and relationships of Clevosaurus (Reptilia: Sphenodontida). Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 321: 125-178.  http://www.jstor.org/stable/2396805

Herring, S.W. 2000. Sutures and craniosynostosis: a comparative functional, and evolutionary perspective, chapter 1, p. 3-10. In Cohen, M.M. and Mclean, R.E. (eds.), Craniosynostosis. University Press, Oxford, UK.

Jones, Marc E.H., Curtis, Neil, O’Higgins, Paul, Fagan, Michael, and Evans, Susan E., 2009. The head and neck muscles associated with feeding in Sphenodon (Reptilia: Lepidosauria: Rhynchocephalia). Palaeontologia Electronica 12,7A: 56p; http://palaeo-electronica.org/2009_2/179/index.html

Mead J, Schubert BW, Wallance SC, Swift SL. In press. Helodermatid lizard from the Mio-Pliocene oak-hickory forest of Tennessee, eastern USA, and a review of Monstersauria osteoderms. Acta Palaeontologica Polonica http://www.app.pan.pl/article/item/app20100083.html

Moazen M, Curtis N, O’Higgins P, Jones MEH, Evans SE, Fagan MJ, 2009. Assessment of the role of sutures in a lizard skull: a computer modelling study. Proceedings of the Royal Society B 276:39–46. DOI: 10.1098/rspb.2008.0863

Whiteside DI. 1986. The head skeleton of the Rhaetian sphenodontid Diphydontosaurus avonis gen. et sp. nov., and the modernising of a living fossil. Philosophical Transactions of the Royal Society London B312:379–430 http://www.jstor.org/stable/2396214

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