Seeking adaptations for running and swimming in the vertebral columns of ancient crocs
A guest post by Dr. Julia Molnar, Howard University, USA (this comes from Julia’s PhD research at RVC with John & colleagues)
Recently, John and I with colleagues Stephanie Pierce, Bhart-Anjan Bhullar, and Alan Turner described morphological and functional changes in the vertebral column with increasing aquatic adaptation in crocodylomorphs (Royal Society Open Science, doi 10.1098/rsos.150439). Our results shed light upon key aspects of the evolutionary history of these under-appreciated archosaurs.
Stomach-Churning Rating: 5/10; a juicy croc torso in one small photo but that’s all.
As fascinating as modern crocodiles might be, in many ways they are overshadowed by their extinct, Mesozoic cousins and ancestors. The Triassic, Jurassic, and early Cretaceous periods saw the small, fast, hyper-carnivorous “sphenosuchians,” the giant, flippered marine thalattosuchians, and various oddballs like the duck-billed Anatosuchus and the aptly named Armadillosuchus. As palaeontologists/biomechanists, we looked at this wide variety of ecological specializations in those species, the Crocodylomorpha, and wanted to know, how did they do it?
Of course, we weren’t the first scientists to wonder about the locomotion of crocodylomorphs, but we did have some new tools in our toolbox; specifically, a couple of micro-CT scanners and some sophisticated imaging software. We took CT and micro-CT scans of five fossil crocodylomorphs: two presumably terrestrial early crocodylomorphs (Terrestrisuchus and Protosuchus), three aquatic thalattosuchians (Pelagosaurus, Steneosaurus, and Metriorhynchus) and a semi-aquatic modern crocodile (Crocodylus niloticus). Since we’re still stuck on vertebrae (see, e.g., here; and also here), we digitally separated out the vertebrae to make 3D models of individual joints and took measurements from each vertebra. Finally, we manipulated the virtual joint models to find out how far they could move before the bones bumped into each other or the joints came apart (osteological range of motion, or RoM).
Above: Video of a single virtual inter-vertebral joint from the trunk of Pelagosaurus typus (NHMUK) showing maximum osteological range of motion in the lateral direction (video). Note the very un-modern-croc-like flat surfaces of the vertebral bodies! (modern crocs have a ball-and-socket spinal joint with the socket on the front end)
While this was a lot of fun, what we really wanted to find out was whether, as crocodylomorphs became specialized for different types of locomotion, the shapes of their vertebrae changed similarly to those of mammalian lineages. For example, many terrestrial mammals have a lumbar region that is very flexible dorsoventrally to allow up-and-down movements during bounding and galloping. Did fast-running crocodylomorphs have similar dorsoventral flexibility? And did fast-swimming aquatic crocodylomorphs evolve a stiffer vertebral column like that of whales and dolphins?
Above: Video of how we modelled and took measurements from the early crocodylomorph Terrestrisuchus gracilis (NHMUK).
Our first results were puzzling. The Nile croc had greater RoM in side-to-side motions, which makes sense because crocodiles mostly use more sprawling postures and are semi-aquatic, using quite a bit of side-to-side motions in life. The part that didn’t make sense was that we found pretty much the same thing in all of the fossil crocodylomorphs, including the presumably very terrestrial Terrestrisuchus and Protosuchus. With their long limbs and hinge-like joints, these two are unlikely to have been sprawlers or swimmers!
So we started looking for other parts of the croc that might affect RoM. The obvious candidate was osteoderms, the bony scales that cover the back. We went back to John’s Freezer and got out a nice frozen crocodile to measure the stiffness of its trunk and found that, sure enough, it was a lot stiffer and less mobile without the osteoderms. If the fairly flexible arrangement of osteoderms in crocodiles had this effect on stiffness, it seemed likely that (as previous authors have suggested; Eberhard Frey and Steve Salisbury being foremost amongst them) the rigid, interlocking osteoderms running from head to tail in early crocodylomorphs would really have put the brakes on their ability to move their trunk in certain ways.
Another cool thing we found was new evidence of convergent evolution to aquatic lifestyles in the spines of thalattosuchians. The more basal thalattosuchians, thought to have been near-shore predators, had stiffness and RoM patterns similar to Crocodylus. But Metriorhynchus, which probably was very good at chasing down fast fish in the open ocean, seems to have had greater stiffness. (The stiffness estimates come from morphometrics and are based on modern crocodiles; see here again, or just read the paper already!) A stiff vertebral column can be useful for a swimmer because it increases the body’s natural frequency of oscillation, and faster oscillation means faster swimming (think tuna, not eel). The same thing seems to have happened in other secondarily aquatic vertebrate lineages such as whales, ichthyosaurs, and mosasaurs.
So, our results were a mixed bag of adaptations particular to crocs and ones that seem like general vertebrate swimming specializations. Crocodylomorphs are important because they are the only group of large vertebrates other than mammals that has secondarily aquatic members and has living members with a reasonably similar body plan, allowing us to test hypotheses in ways that would arguably be impossible for, say, non-avian dinosaurs and birds. The take-home message: crocodylomorphs A) are awesome, and B) can teach us a lot about how vertebrates adapt to different modes of life.
Another take on this story is on our lab website here.