Tuesday, 11 November 2014

Pterosaur bone mass

The last post here was on pterosaur skeletal pneumaticity, and while I said I was going to continue this discussion in the next post, I'm going to take a side-road for a bit and talk about my first research paper, which has just come out! It's still related though, and ties in with these questions nicely.

Estimating pterosaur bone mass using CT scans

Summary of Martin and Palmer 2014 [1]

At the beginning of my MSc, my supervisor (Colin Palmer) and I wanted to look at estimating pterosaur bone mass using CT scans. Total mass of pterosaurs is a controversial topic, with different methods and authors coming up with very different results, which you can read about in a previous post (and this one) if you are interested. It is essential to accurately estimate mass in pterosaurs as they were the largest animals to ever fly, an mass is extremely important in flight. The key thing to know here is that one method for estimating pterosaur body mass relied on the relationship between skeletal mass and total mass in birds [2], and applied this relationship to pterosaurs by estimating skeletal mass geometrically (i.e. a long bone is a hollow cylinder) [3]. Colin was interested in using computed tomography (CT) scans to estimate bone mass, to see how different (if any) the mass would be using this method. I agreed that it would be an interesting project, and started on my MSc at the University of Bristol.

The basic principle was that by calculating the cross-sectional area of bone in several slices throughout the bone (approximately every 5-10 mm), bone volume (as in the actual volume of bony material) could be calculated through integration. Then, mass can be estimated by applying a density and multiplying by the volume. This was done for a number of bones, but we only published on 3 first wing phalanges (the first big finger bone in the wing, herein referred to as WP1). We were then able to compare the results directly to the previous method used by Witton [3] thanks to him kindly sharing his dataset with us (thanks again Mark!).
CT scans through a pterosaur wing bone showing the shaft (A,B) and proximal head (C,D) cross-sections. Top images show unmodified CT scans, bottom images show reconstructed cortical bone and removed matrix used for area calculations. Image from Martin and Palmer [1]
What we found was quite different from what we had expected. We generally assumed that the mass would be somewhat similar to what Witton found. However, we found that all three bones were about twice as heavy using this method as previous estimates, which made us wonder what that means for the rest of the skeleton.
Table indicating measurements from 3 WP1s including mass estimates. Note the differences between mass estimates in our method and in the previous method. From Martin and Palmer [1]
While there are differences between our method and Witton's original study, he could only do what was available to him, which for many reasons, did not include CT scans. However, he did suggest in his original paper that using CT scans would be another way to do this study and likely would be more accurate, so credit to Mark for that! It was a pragmatic method at the time, and well done using the materials available to him at that point.

So why is the mass so much more using the CT method? There are several possible explanations for this. First of all, the original method did not account for trabeculae, which did add 10-15% of mass in our study. Another explanation is that the cortical thickness used by Witton (which was calculated using a regression model derived by someone else) was consistently lower than what we found in the CT scans (see the table above), which also would affect the mass. And finally, one point that is related to the last one is that the original method did not account for the variation within the cortical thickness throughout the bone.

And what does this all mean? While this information, as well as some additional new data suggests that the wings of pterosaurs were heavier than previously estimated. This isn't really a big surprise when noted that some previous estimates suggest that the pectoral muscles (the muscles around the shoulder) in pterosaurs account for 30-40% of the total body mass [4]. While these muscles are mainly used for flight, they would also be the main muscles for take off if pterosaurs did take-off using their forelimbs to launch as has been suggested [5].

This study made us wonder what the rest of the skeleton would look like if we calculated it using CT scans, which has lead to my PhD project at the University of Southampton. The amount of bone tissue in the wing bones is related to both mass and pneumaticity, which are both subjects I am interested in, as they all related to the biomechanics and flight capabilities of pterosaurs. If anyone would like to see the paper and does not have access, let me know!

Next up, I'll talk about quantifying and comparing the amount of air (pneumaticity) found within the skeletons of pterosaurs, looking at different bones, and different pterosaurs, another paper that Colin and I have published on the topic.

Just wanted to say thanks to everyone that helped me with CT scans and along with this project that I am so happy is finally out! This includes: first and foremost thanks to Colin Palmer for putting up with me the last 2 years, and to Mark Witton for sharing lots of things along the way, and also Davide Foffa, Lorna Steel, Lauren Howard, Dave Martill, the staff at Muvis, Mike Habib, Emily Rayfield, and many more! I'm so happy to finally have this paper out :)
EDIT: Also, this should seem obvious, but I'm going to add it anyways. Many thanks goes out to my wonderful partner in crime Josh Silverstone for helping through the last 7 years (and especially the last 3), and for helping me with figures of course!

[1] Martin, EG and Palmer, C (2014) A novel method of estimating pterosaur skeletal mass using computed tomography scans. Journal of Vertebrate Paleontology 34: 1466-1469.
[2] Prange, HD et al. (1979) Scaling of skeletal mass to body mass in birds and mammals. American Naturalist 113:103–122.
[3] Witton, MP (2008) A new approach to determining pterosaur body mass and its implications for pterosaur flight. Zitteliana, Reihe B 28:143–158.
[4] Strang, KA (2009) Efficient flapping flight of pterosaurs. Ph.D. disserta- tion, Stanford University, Stanford, California, 295 pp.
[5] Habib, MB (2008) Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana, Reihe B 28:159–166.