The tortuous noses of ankylosaurs and the long-haul of science
This week saw the release of my work on the crazy winding nasal passages in ankylosaurs. Our study has attracted much interest and has been picked up by multiple media outlets including The Daily Mail, Nova and the Washington Post. Yet as great as it is to see our work finally making it out for everyone to read (via Open Access in PLOS ONE), I find myself equally as happy to finally say goodbye to this project.
This feeling of pride and relief is not uncommon for scientific studies. Many of us pour so many hours / days / years into a project that by the time it is finally ready for release into the wild, we are sick and tired of seeing the damned thing.
My interest in the noses of ankylosaurs started about four years ago, but curiosity surrounding the nasal system of ankylosaurs persisted in paleontology since the seventies with paleontologist, Teresa Maryańska. Broken skulls of the small ankylosaur, Pinacosaurus grangeri, revealed what looked like little pockets and shelves along the nasal cavity. Maryańska interpreted these as respiratory turbinates (more on that later). A little later on paleontologist, Walter Coombs saw similar structures in a broken skull of Euoplocephalus tutus, which he interpreted as paranasal sinuses. That interpretation remained until 2008 when my advisor, Lawrence Witmer and WitmerLab lab technician, Ryan Ridgely, CT scanned the skulls of E. tutus and the large nodosaurid, Panoplosaurus mirus. The results of their scans came as a surprise. It turned out the various sinuses and turbinates that were previously interpreted for these nasal passages, were in fact all just part of one elongated nose (Witmer and Ridgely 2008).
An elaborate nasal passage such as this cried out for an explanation. At the time, Witmer and Ridgely offered up some suggestions based on the physical implications of such a nose. This included heat exchange and sound production along with potential increased olfaction. A few years later, paleontologist, Tetsuto Miyashita, re-examined the skull of E. tutus and found that the olfactory recess (where all of the sense of smell would be located) was a pretty standard-issue size for a dinosaurs, and that these elongated nasal passages were unlikely there to increase the sense of smell (Miyashita et al. 2011). That narrowed down the choice to air conditioning and sound production. Neither function was particularly easy to determine in an extinct animal.
Then I came along.
My PhD with Dr. Witmer involved the use of Computational Fluid Dynamics (CFD) to simulate air flow in the noses of modern animals, with the goal of reconstructing airflow and soft tissues in dinosaur noses. Ankylosaurs had come on my radar as I prepared my PhD proposal. The thinking being that if we can figure out a way to reconstruct soft tissues using airflow as a guide (reverse-engineering, if you will) then we might be able to go a step farther and look at some of the implications of these soft-tissue reconstructions. CFD allows for multiple analyses of airflow including heat transfer. With this methodology in hand, it was time to crank on the nasal passages of ankylosaurs.
To do things properly, I looked at the initial scans of the specimens and interpretations of the nasal passages. I also visited multiple museums to look at their ankylosaurs and get a better idea of what these structures looked like in life and up close (image to the right shows Ruger Porter and myself looking at the palate of AMNH 5405, Euoplocephalus tutus).
Simulation of a dinosaur breath required an extensive review of the literature on modern-day relatives of dinosaurs (birds, crocodylians, and lizards). In particular I wanted to see what had been previously been done with modern-animals in regards to nasal heat transfer. The results of my studies were initially shown at the 74th meeting of the Society of Vertebrate Paleontology. At the time, I only had preliminary results of the nasal passages largely as they were preserved in the skulls. The paper, it turns out, was still a long way off.
During the next four years, work on the project would continue on, with other projects occasionally sneaking in along with new priorities such as my PhD defense, post doc applications, and job applications consistently stealing the spotlight. As such, I became very familiar with this project, including weaknesses in the analyses that needed addressing. A large edition to the paper that appeared post-SVP presentation, was the inclusion of secondary analyses.
The appeal of using digital models is that we can "break" them without worrying about permanently damaging the fossils. In this case, we had the opportunity to really test the effectiveness of these noses as air conditioners, by running them up against nasal morphologies that lacked various aspects of the originals. For instance, we looked at how well a simplistic, "basic" nose would work in the skulls of these animals. This was a nasal passage that only ran from the nostril down to the throat. This would be impossible to do in the real animals, but with a digital model it became a relatively simple nip and tuck job.
The interesting thing about the secondary analyses is that they really gave us an idea of just how much real estate these noses took up in the animals. For instance, the nasal vestibule (i.e., the longest part of the nasal passage in both dinosaurs) was 440 mm long in P. mirus. However, the skull was only 419 mm long (measured from tip of rostrum to occipital condyle). Fitting a nose with a length that would "better fit" this skull required chopping off more than half the nasal passage. E. tutus was even crazier. The nasal vestibule in this critter was 809 mm. All that length was wrapped inside a skull that was only 457 mm long. Fitting a more constrained nasal vestibule in these guys required chopping off 80% of that airway. These noses were ridiculously long.
They were also effective. When we ran our heat-transfer analyses we found that P. mirus and E. tutus were able to reclaim 73% and 84% of the heat used to warm the air. Whereas sticking a more simplistic nose in their place caused a drop of over 50% in that heat transfer efficiency. These guys needed their roller coaster noses.
Our initial results proved interesting, but without ecological context, they were of limited use. When we looked at how these nasal passages would perform in the environment we currently have interpreted them in (subtropical marshlands), our results took on new meaning. A nose that is super-good at reclaiming heat was probably not all that useful in a world that is already pretty hot and wet. However, since that heat transfer comes at a cost of heat loss to the underlying blood, we may instead be looking at a very effective cooling device. My colleague, Ruger Porter reconstructed the blood supply to the head of these—and other—dinosaurs, and he found an ample blood supply heading to the brain from the nose, If ankylosaurs were heating arteries that came from the body core, the cooled blood from those arteries would head back towards the brain via the venous system that Ruger had uncovered. Thus, it appears that the nose in these dinosaurs was acting as a coolant reservoir that offset hot blood coming from the body core. This is similar to the anatomical setup we observe in artiodactyls today with their carotid retes.
Among the many neat things that we uncovered during this project, was the fact that these dinosaurs, and potentially many more, were able to use their noses as cooling devices in a manner that was decidedly different from the one used by today's mammals and birds. Whereas we partition our noses up into many smaller channels, ankylosaurs and other large dinosaurs kept their nose as a single tube. It was a crazy long tube, but a single tube nonetheless. There are interesting implications of this too. A single tube for air conditioning means that air has to all be conditioned in sequence. In contrast, mammals and birds partition the air field into multiple smaller channels. This increases resistance in the nose, but it allows a smaller nose to condition air a bit faster. As such, these long, winding noses in ankylosaurs would not have allowed air to move through too fast, meaning that it would be harder for these dinosaurs to hyperventilate. This may have imposed a limit on their activity levels.That said, given their overall body plan, these dinosaurs probably weren't doing much running anyway.
So here we are. After 4+ years of work, our paper is finally out. It is probably the chapter of my dissertation I'm most proud of as it not only shows that we can reconstruct (to a degree) what the soft-tissue shape of the nasal passage looks like in a dinosaur, but we can also attempt to answer some fundamental questions about their physiology.
Where do we go from here? Well, I'm definitely not done with ankylosaurs yet. They are my favourite dinosaur group, and they remain one of our best glimpses into what the dinosaur nasal system was like. However, I am also looking at other aspects of nasal physiology that may be addressed, such as acoustic communication in hadrosaurs. Then there's the all important extant realm. Birds and reptiles have a dearth of comparative physiological information about them, especially when compared to mammals. One thing I am looking to accomplish over the next couple of years is to fill in some of these gaps. Without a strong foundation in the modern-realm, we limit what we can say about extinct animals.
For now, though. I'm going to take a break and enjoy the holidays.
Witmer, L.M., Ridgely, R.C. 2008. The Paranasal Air Sinuses of Predatory and Armored Dinosaurs (Archosauria: Theropoda and Ankylosauria) and their Contribution to Cephalic Structure. Anat. Rec. 291:1362–1388. Miyashita, T., Arbour, V.M., Witmer, L.M., Currie, P.J. 2011. The Internal Cranial Morphology of an Armoured Dinosaur Euoplocephalus Corroborated by X-Ray Computed Tomographic Reconstruction. J. Anat. 219 (6):661–675.