Snakes Probably Lost Their Legs Swimming, Not Burrowing

Our single precious Tetrapodophis specimen comes from Brazil and is 120 million years old. Professor Mike Lee of Flinders University, Australia, told IFLScience: “This is far more important than Archaeopteryx where we have 11 specimens.” Just as Archaeopteryx represents the point where dinosaurs were growing wings to become birds, Tetrapodophis is a snapshot of the point where the ancestors of snakes were losing their limbs, which had been reduced to small appendages of increasingly little value.

^{The one and only fossil of Tetrapodphis compared with an artist's impression of the living thing. Alessandro Palci and Michael Lee/Flinders University/South Australian Museum}

Feet and hands are so useful that no animal gets rid of them without evolutionary pressure. One possible reason is to glide gracefully through water with minimal resistance. Another is to burrow into small spaces without protuberances getting in the way. A previous study suggested that Tetrapodophis looked more like a burrower, but Lee disputes this.

Lee told IFLScience the burrowing conclusion was based on “a relatively short tail and the shape of the backbone.” However, Lee concluded, Tetrapodophis' tail has been maligned and was actually “intermediate in length.” The near-absence of a spinal crest, on the other hand, does make Tetrapodophis look like some modern burrowing species. As Lee explained: “If that was the only evidence, I would say it has something to it.”

^{Tetrapodophis had body proportions that don't match any modern reptiles, although they are closer to burrowing species than swimmers. Lee et al./Cretaceous Research}

However, in Cretaceous Research, Lee and co-authors present several pieces of evidence contradictory to the burrowing theory. Most potent of these, Lee told IFLScience, is that “the first finger is very robust,” compared to the rest of the digits on Tetrapodophis' atrophied limbs. This is standard for flippers, providing support for a hydrofoil shape that assists swimming, but serves no purpose underground.

^{Tetrapodophis had digits that looked far more like the structure of flippers than limbs designed to support weight. Lee et al. Cretaceous Research.}

In addition, Lee said in a statement: “[Tetrapodophis] has a long slender tail and four slender legs, something you don't see very often in burrowing snakes and lizards today."

Co-author Dr Alessandro Palci added that the limb components were “weak, poorly-ossified, and rather flipper-like. We see these traits only in ancient marine lizards called mosasaurs."

Lee also noted that Pachyrhachis, another primitive snake that post-dated Tetrapodophis by 20 million years and only had two remaining limbs, was definitely aquatic.

Nevertheless, Lee acknowledged to IFLScience, this may not be a simple case of aquatic versus burrowing. “The common ancestor of snakes may have had small legs, and these may have been lost several times in different environments,” Lee said. “Today there are also some snakes living in intertidal zones, and are good both at swimming and burrowing. Something similar would explain why snakes adapted so well to both environments.”