To understand a group of organisms, you have to start at their very beginning. The name “archosaur” translates as “ruling reptiles”, and the name is indeed appropriate. In the form of dinosaurs, pterosaurs, phytosaurs, and others, archosaurs dominated the earth during the Mesozoic. Today, archosaurs have more species than any other single group of vertebrates living on land–around 10,000 birds (the surviving lineage of dinosaurs) as well as around two dozen species of crocodilians.
No doubt, archosaurs are successful. But what were the seeds of this success? What paths has the group taken during its 245 million year history? Archosaurs started out as small, land-dwelling organisms, before flowering into gators, tyrannosaurs, and condors. One of the key fossils for following this evolution is Euparkeria (pronounced “yoo-par-KAIR-ee-uh”).
Euparkeria was a small carnivore, about a meter in maximum body length, that scurried around what is now South Africa approximately 245 million years ago (15 million years before the oldest definitive dinosaur fossils). Euparkeria wasn’t a true archosaur, but evolutionarily sat close to the group. Other, closer relatives to archosaurs were aquatic, and adaptations for life in the water overprint a layer of anatomical variation upon the ancestral condition. So, the anatomy and inferred lifestyle for the terrestrial Euparkeria are probably a better approximation for those in the first archosaur, even if the animal is a little more distantly related than others. And luckily, we have ten different specimens of Euparkeria with exquisite preservation; over the past 100 years, it has provided a treasury of data. But even for the best known of fossils, there is always more to learn.
A new study by Gabriela Sobral, Roland Sookias, and colleagues, published in Royal Society Open Science, details the anatomy of the braincase in Euparkeria. Although aspects had been previously explored, micro-CT scanning allowed a close look into areas of the skull largely inaccessible to previous researchers. The bones around the brain house important sensory structures such as the inner ear, providing details on hearing as well as sense of balance. Furthermore, the spatial relationships, size, and shape of the various bones that make up the braincase help greatly with reconstructing evolutionary relationships–the presence or absence of certain bones, as well as their form, are often quite similar within closely related animals, even if other aspects of their anatomy are vastly different. So, the new paper provides a wealth of information for paleontologists interested in understanding the life and times of early archosaurs.
To learn more about the study, I got in touch with Dr. Sobral, who is a paleontologist at Universidade Federal de Santa Catarina, in Brazil. She was kind enough to field a few questions about the new paper.
Euparkeria is (at least for paleontologists) a pretty well-known animal–was there anything you discovered in this study that was particularly surprising for you and your co-authors?
GS: Euparkeria is known from a few fairly complete specimens. Because it is the closest terrestrial relative of archosaurs, it has been used as a model for their early evolution. The braincase of Euparkeria was described in detail by Gower & Weber in 1998 when discussing on the phylogenetic relationships of birds – if they were closer to dinosaurs or to stem archosaurs like Euparkeria. However, very little was known about its inner ear.
I am the most experienced of the team in inner ear anatomy, and I had only worked with the ornithopod dinosaur Dysalotosaurus before. The inner ear of Dysalotosaurus looked completely different from the one of Euparkeria. The semicircular canals of Dysalotosaurus are smaller and thicker. The floccular fossa was much shallower and the cochlear duct of Dysalotosaurus was shorter, too. Being a more derived taxon than Euparkeria in relation to birds, I was expecting the inner ear of Euparkeria to be overall bulkier, but it turned out to be slenderer with regard to all these structures. These structures are related not only to the sense of hearing, but to balance as well. A slender anatomy indicates Euparkeria was a more agile animal than Dysalotosaurus. I was expecting a stronger phylogenetic signal in the anatomy of the inner ear of Euparkeria – the more “primitive”, the slower – but we found out exactly the opposite. It made us conclude that these aspects of the inner ear are anatomically more related to the ecology and biomechanics of Euparkeria than we had expected, perhaps related to the fact that Euparkeria was a carnivore, whereas Dysalotosaurus was a herbivore.
Quite a few people were involved in this project–did you have a particular focus as part of the research team, and if so, what was it?
GS: I was analyzing braincase materials in South Africa when I came across the beautiful, acid-prepared specimen of Euparkeria. I borrowed it from the Iziko Museum and brought it to Berlin to be scanned. At some point I was asked by Roland and Richard about my plans on the material because they were interested in it, too – Roland was researching euparkeriids for his PhD thesis. They told me they had the Cambridge specimen with them, and mentioned Anjan and Roger were scanning the other two braincases as well, including the holotype. A collaboration was thus the obvious result of this.
I had the inner ear part of the work ready already, and together we added a more complete anatomical description of the braincase and the discussion on the relationship anterior braincase wall of Euparkeria with that of living crocodylians and birds. As I had also worked with more basal groups, I had a slightly different focus than the others. Euparkeria is the classical model-taxon for archosaurs in a “tree up” view, but I was also interested in a “tree down” approach: not having Euparkeria as a starting point for archosaur evolution, but as one of the ending points in diapsid evolution. How does the braincase and inner ear morphology of Euparkeria compare to other, more basal groups? What are the changes we see and their implications for the biology of these animals? This was a differential in our work, and the others liked it.
It takes a lot of time to produce high-quality computer models from CT scan data–and it’s often tedious. What did you do to help pass the time while making the reconstructions?
GS: Hahaha, segmenting can be a long work indeed – but it can also be relaxing. It’s like mechanically preparing a fossil. I like writing papers, but it’s good to take breaks. When I segment I usually have some music on, which I normally find difficult to listen to if I have to concentrate and think. So I take the time to catch up with the latest albums of the bands and artists I like, or I listen to podcasts or audiobooks. During my PhD, I worked a lot with 3D modelling and segmentation, so you end up getting used to the tools and doing your job quicker, too. It took me only a week or so to segment, model and prepare the figures for all the braincases.
Euparkeria is from South Africa, the research was completed in Germany, and you are now based in Brazil. How does the Euparkeria study relate to your current research in Brazil? What’s next in the world of archosaurs?
GS: Because of its integrative nature, I think the braincase is in a unique position for offering insights on the paleobiology of vertebrates. Braincase formation in reptilians follows some patterns that I believe to be able to track in the fossil record. Also, the formation of the anterior braincase wall seems to be related to certain functional aspects of reptilians. During the course of my PhD in Germany, I became very interested in development. I tried to explore these points in this latest paper, but I reached a point where there was simply no information available. I just had a research grant approved, and now I want to explore braincase development in crocodylians by applying some techniques I acquired during my PhD – to answer some of the questions raised with the braincase of Euparkeria, and many others. And this is what I think is next in the world of archosaurs: there is still so much to learn in terms of gross anatomy and embryological development. There is surprisingly very little information available for crocodylians – they were classically seen as big lizards, and research has concentrated on the latter because they are easier to obtain. Thankfully, this scenario is slowly changing. I hope to be soon able to contribute to the advancement of this field.
For more information, check out the open access paper, available via Royal Society Open Science.
Sobral G, Sookias RB, Bhullar B-AS, Smith R, Butler RJ, Müller J. 2016. New information on the braincase and inner ear of Euparkeria capensis Broom: implications for diapsid and archosaur evolution. Royal Society Open Science 3: 160072. http://dx.doi.org/10.1098/rsos.160072