Crocodiles look ancient. Maybe it’s something to do with the eyes, the armor, and the teeth that remind us of the Age of Reptiles. Or maybe it’s simply because crocs are often used as window dressing to set Mesozoic scenes that gives us the impression that they’ve always been watching from just beneath the surface of the water. Whatever the reason for these alligator impressions, though, paleontology has undeniably shown that these archosaurs are far from the “living fossils” we love to portray them as.
Paleontologist Julia Molnar and her coauthors set the record straight in the very first line of their latest paper. “The lineage leading to modern Crocodylia has undergone dramatic evolutionary changes in morphology, ecology, and locomotion over the past 200+ Myr.” While it’s true that crocs in the flavor of “semi-aquatic ambush predator” of one lineage or another have been around since the Jurassic, focusing on these amphibious carnivores blinds us to the wider variety of crocodylomorphs that have come and gone over the past 245 million years. There were terrestrial pipsqueaks that ran on their tippy-toes, crocs that spent almost their entire lives at sea, and, of course, the 40-foot monsters that snatched dinosaurs from the water’s edge, among others. And as Molnar and colleagues demonstrate, one way to see this diversity is in the spine.
Today’s alligators, crocodiles, and gharials get around in a surprising variety of ways. They’re accomplished swimmers, they can drag their bellies along the ground or push up into a “high walk”, and little crocodiles can even gallop. But are these recent specializations, or are some of their capabilities ancient holdovers from the long, long history of their greater family? To investigate this question Molnar and colleagues created virtual models of five extinct crocs to see how their trunk flexibility matched up with their mode of life, checked against a model of a Nile crocodile spine verified by trunk-bending experiments on a carcass from the living species.
The spread of crocs in the new study bridged land and water. Two of the earliest, Terrestrisuchus and Protosuchus, were little terrestrial predators, while Pelagosuchus, Steneosaurus, and Metriorhychus document the change from semi-aquatic crocs to ones that propelled themselves around the seas with paddle-shaped limbs and fluke-tipped tails. From these reconstructed lifestyles Molnar and colleagues predicted that the land-dwelling crocs that moved more like mammals would have had spines that were more flexible up-and-down than from side-to-side and that the marine species would show increasing stiffness of the trunk to cope with moving through the water at speed, but their results yielded some surprises.
Millions and millions of years before the first whales took the plunge, the thalattosuchian crocs transitioned from nearshore life to one out in the open ocean. And, much like the whales, the prehistoric crocodiles went through a similar process of increasing flexibility in the spine in amphibious forms followed by greater trunk stiffness among the species that were full-time swimmers. Compared to Pelagosaurus, Molnar and coauthors found, the increasingly aquatic Steneosaurus and Metriorhychus had spines that were stiffer from side-to-side as their tails took one more of the propulsive work. These crocs swam in a variation of what dolphins do today, keeping the body rigid to plow through the water while all that power comes from swishes of the tail.
Based on the similar biomechanical lines of logic, Molnar and coauthors predicted that the early, land-dwelling crocs Terrestrisuchus and Protosuchus would have trouble bending side-to-side but would be flexible in the up-and-down plane. This would fit with the way they moved, with vertical movements of the spine as they pumped their legs forward-and-back beneath their bodies. But this isn’t what the researchers found. Terrestrisuchus, which would have more of a mammal-like walk than any of its relatives in the study, had a spine that was more flexible from side-to-side than in the vertical plane, and, in fact, would have been even stiffer along that axis because of a set of osteoderms – bony armor – that ran down the vertebral column. Trackways have confirmed that crocs like Terrestrisuchus really did walk with more upright limbs, but, Molnar and colleagues point out, the way the spine and legs worked together must have been different than we see in mammals.
Paleontologists have found plenty of other prehistoric crocs that could be thrown into the mix. But even from these five, it’s clear that crocs have not been in stasis since they first trotted out onto the evolutionary scene in the Triassic. The species we see around us today are really just a sliver of what once existed, and are specialized creatures in their own right rather than being stagnant holdovers from the depths of the Mesozoic. And given how much they’ve changed since their origin, I can’t help but wonder what might happen in the future. Should today’s crocodylians survive us, might any of them reprise the roles their predecessors took on land and in the seas?
Molnar, J., Pierce, S., Bhullar, B., Turner, A., Hutchinson, J. 2015. Morphological and functional changes in the vertebral column with increasing aquatic adaptation in crocodylomorphs. Royal Society Open Science. doi: 10.1098/rsos.150439