The entire skeleton of an exceptional four-legged snake was discovered in Early Cretaceous sediments in Brazil when it was still a part of Gondwana. The findings, published in Science this week, suggest that today’s sleek slithery serpents evolved from a burrowing ancestor and not a swimming one.
There are more than 3,000 species of snake today, and despite having no arms or legs, they’ve managed to climb, crawl, glide, and swim their way into a vast range of habitats, from deserts and rainforests to oceans and my apartment. Yet we’re still unclear about the origins of their unique body plan. Recently unearthed fossils have helped our understanding of the lizard-to-snake transition. Earlier this year, for example, the fossils of four new species who lived over a hundred million years ago were uncovered in museum collections. And while some fossil snakes have sported hindlimbs, no snake with four limbs has ever been reported until now.
This new articulated skeleton of a previously unknown species was discovered in the Crato Formation of Ceará, Brazil, and it came complete with soft tissues and even stomach contents. University of Portsmouth’s David Martill and colleagues analyzed the remarkable fossilized remains and conducted four phylogenetic analyses to see where it fits on the snake evolutionary family tree. They found that the new species is, in fact, an ancestor of our snakes today. They named it Tetrapodophis amplectus, and it lived 146 to 100 million years ago.
Close up of Tetrapodophis amplectus hands (left) and feet (right). Dave Martill, University of Portsmouth.
Tetrapodophis has many classic serpentine features: scales, hooked teeth, a long braincase, an elongated body (with over 150 vertebrae), and a short snout with a flexible jaw for swallowing large prey whole. It even boasts the spinal column structure that offers the extreme flexibility required to constrict prey (interestingly, a new study that came out just this week states that constrictors don’t suffocate their prey; they die of cardiac arrest).
Of course, the biggest differences are the four reduced limbs, which don’t appear to be used for locomotion or digging. The shorter exterior digits and lengthened second digit resemble the prehensile feet of some birds, sloths, and bats – suggesting that those limbs were adapted for grasping. Tetrapodophis may have used its forelimbs and hindlimbs to seize vertebrate prey or clasp another during mating. Or maybe it climbed trees. In any case, after the initial evolution of the familiar serpentine undulating locomotion, these sorts of limbs were repurposed for some other function. And its elongated body showed up before the loss of its forelimbs.
Finally, its skull and body proportions, as well as its reduced neural spines, are fossorial (or burrowing) adaptations. Since Tetrapodophis has a long trunk and a short tail – and not the long, laterally compressed tail that’s typical of aquatic animals – the researchers think that vipers and pythons alike evolved from creatures that burrowed, and not from marine ancestors like mosasaurs.
Tetrapodophis amplectus whole skeleton. The skull is on the counterpart slab. Also, notice the stomach contents. Dave Martill, University of Portsmouth.