Pterosaur jaw shape – what does it mean?

I’ve been a little bit busy and haven’t had a chance to blog about the most recent paper I was involved with, on pterosaur jaw disparity! This paper has been a long time coming, and was my lead by my first ever Master’s student, Charlie Navarro. This project came out of his MSc thesis at the University of Bristol, which I helped supervise a few years ago. I’m so happy, and so proud that we finally managed to get it published, with a lot of help from our co-author and one of Charlie’s other supervisors, Tom Stubbs!

The idea for this paper was to look at the disparity and variation of pterosaur jaws, to see if we could learn anything about their evolution or any trends related to niche partitioning or ecomorphology. In order to do this, Charlie collected as many images of pterosaur jaws in lateral view, where they were undistorted (we think) and preserving roughly the original shape. You can then put landmarks on specific points that can be found on every specimen, which a computer program can then quantitatively analyse and tell the differences or similarities between the different jaws. These kinds of studies have been done on a large number of animals to understand feeding or jaw evolution, including herbivorous dinosaurs, placoderms, and crurotarsans.  Unfortunately, thanks to all things pterosaurs, we didn’t have as many samples as we would have liked, or as good of a spread through time, so we weren’t able to do any real analyses of diversity through time or disparity through time.

Morphological variation in pterosaur lower jaws. The illustrated taxa are (a) Raeticodactylus, (b) Rhamphorhynchus, (c) Pterodaustro, (d) Cycnorhamphus, (e) Tapejara, (f ) Anhanguera, (g) Dsungaripterus and (h) Quetzalcoatlus. The selected jaws are illustrated to highlight shape disparity in the lateral pro le and are not plotted to scale. Jaws (b), (c) and (e–h) are based on illustrations by Jaime Headden. Image from Navarro et al. 2018

We divided the different jaws based on taxonomic groups based on the Andres phylogeny, and dental guilds (e.g. toothless, heterodont, multi-cusped, etc), and tested them using two different analyses. Interestingly, we found a lot of overlap, regardless of which test we used, and whether or not we looked at it from a taxonomic point of view or a dental point of view. Despite the variation seen in the jaws as seen in the image above, pterosaur jaws just aren’t all that different. They tend to be pretty long and skinny, with a tooth row at the same level as the jaw joint. The one feature that does seem to make a difference, is the presence of a large ventral mandibular crest seen in some pterosaurs (like Tapejara above).

Pterosaur lower jaw morphospace based on elliptical Fourier outline analysis. Two-dimensional morphospace plots are based on the rst two principal components axes. (a) Distribution of broad taxonomic groups (convex hulls) with numerous mandible shapes illustrated and labelled. (b) Phylomorphospace illustrating branching patterns. (c) Dental guild distributions (convex hulls). Taxonomic groups in (a) and (b) are ‘basal’ non-monofenestratan, Archaeopterodactyloidea, Pteranodontia and Azhdarchoidea. Shape variation along PC1 and PC2 is plotted. From Navarro et al. 2018.

This is a kind of interesting find. We know that pterosaurs were ecologically diverse – some lived near water and were dip-feeding piscivores, while some were shell-crushing waders, and others were even terrestrial foragers and insectivores. However, despite this variation in ecology, there doesn’t seem to be a connection between pterosaur jaw shape and ecological features like dental guild or jaw shape.

Though we didn’t test specifically for functional characters, a limitation of our dataset and the preservation of the jaws, there is some functional signal in some of the axes. Some pterosaurs have a lower articular joint, based lower than the tooth row, which is related to the occlusion of upper and lower tooth rows. Additionally, both Pterodaustro and Dsungaripterus, pterosaurs with very unique lower jaws, showed extreme positionings in the PC2 axis, related to their individual feeding strategies.

So what does this mean? Well first, it looks like a lot of jaw variation with respect to the mandibular crests is more related to socio-sexual selection rather than anything biomechanical or functional. It also looks like there isn’t much of a trend in size, so in general, jaw shape and size can’t be used as a determinant for diet or ecology. Fortunately, a PhD student at Leicester, Jordan Bestwick, is doing some cool work on micro-wear in pterosaurs that can hopefully help us out a bit more. Unfortunately, that doesn’t work so well on pterosaurs that don’t have teeth… so we’re not really any closer to solving that mystery!

Reference:

Navarro, Martin-Silverstone, and Stubbs. 2018. Morphometric assessment of pterosaur jaw disparity. Royal Society Open Science 5: 172130.