Scientists at the University of Portsmouth have discovered a four-legged fossil believed to be an ancestor to modern day snakes.
Marked in stone
Deep in the forests of Brazil, David Martill and colleagues made a discovery that would shake the foundations in the evolution of snakes. Buried deep in limestone, a fossil snake-like in shape was uncovered. However the fossil contained pelvic, forelimb, and hindlimb bones, inconsistent with any modern day snake.
They named their newly discovered fossil Tetrapodophis amplectus, which quite literally translates to the words “four-footed snake” and “embracing”. After making their initial finding, Martill and colleagues headed back to lab to study the structure of the fossil in greater detail. Upon close examination, Martill noted how the skull and midsection of the creature shared traits found commonly in modern day snakes.
Martill then compiled all the structural traits together with some DNA data in order to construct what is known as a phylogenetic tree. This tree uses data from closely related animals and constructs an evolutionary map of how each animal evolved in relation to the others. For T. amplectus, Martill included data from many modern day snakes, as well as data from lizards, iguanas, and other snake-lizard fossils previously discovered.
The phylogenetic tree showed that indeed T. amplectus branched away from modern day snakes roughly 125 million years ago, during a time known as the Cretaceous Period (think T-rex, Triceratops, and Velociraptor). One group lost their legs and became the snakes of today, while the other group was comprised of the snake-like animals with feet.
So why did the snakes lose their legs anyways?
As an animal's midsection gets longer, its limbs usually shrink in size (such is the case with weasels or wiener dogs). One possible explanation is that it becomes harder and harder to coordinate your extremities the farther apart they are on your body. Snakes took this fact to the extreme in both senses, trading usable legs for a lankier body and choosing to navigate by slithering.
There have been two major schools of thought on why snakes have developed such a unique body structure not found in any other land vertebrate. One theory suggests that snakes adapted to living in a watery environment where legs would be somewhat of a nuisance. As time when on, legged snakes developed a morphology closely related to eels. This type of lifestyle can be observed today, as many water and sea snakes exist across the globe.
Another theory suggests that snakes adapted their bodies to live as burrowers. A narrower waist would allow a hungry snake to easily follow its unlucky prey down a hole. This theory is backed up by the fact that some of the closest relatives to snakes, such as the Komodo dragon and iguana, live as burrowers. The finding presented by Martill and company also seem to point towards this theory, as the narrow skull and flatter back of T. amplectus points towards an adaptation to living underground in burrows.
While T. amplectus did have legs, they were most likely not used for walking. T. amplectus’ species name, translating to “embracing”, was given because of how the limbs were structured; small and segmented with grasping claws. Martill suggested that the early snake used its extremities for grasping onto prey, or maybe even for clutching other snakes for mating. Whatever the reason was, as snakes evolved to move and hunt with an elongated body, the usefulness of their legs went the way of the dinosaurs.