In 2015, a team of paleontologists described a strange fossil that was extracted from the Crato Formation, an area in Brazil abundant in the remains of prehistoric animals. But this one was very unusual: it was long like a snake and had four legs. So, he earned the title of the missing link of said species.
According the investigation, Tetrapodophis it would have been a primeval ground snake measuring 15 cm. long including its long tail. About their limbs – with five digits each – the authors suggested that they were used to burrow, hunt large prey, and hold their mates during mating.
Why did the find of this strange fossil seem to end a long debate in science?
Image of Tetrapodophis amplectus, a unique fossil so far. Photo: Dave Martill / University of Portsmouth
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An unclear evolution
Scientists have long searched the fossil record for a lost species that connects lizards and the first snakes. According to their theories, the find of such a fossil would culminate in the debate about how snakes evolved with elongated bodies and without limbs 120 million years ago.
In this regard, there are two theories. The first of these suggests that the lizards lost their limbs in the ground after starting to dig and adjusting to underground life. Thus, it is believed that their front legs disappeared first and then the back ones.
The second hypothesis, on the other hand, holds that they arose from aquatic lizards and their physiognomy was extended to be able to swim like eels. In both environments, this animal was able to develop a long, smooth body.
Scientists debate whether snakes lost their legs in the sea or on land. Photo: Composition / Flinders University / South Australian Museum / Julius T. Cstonyi
Since its discovery, Tetrapodophis amplectus held for a time the podium of the candidate that filled the evolutionary void of snakes; however, this theory has recently been refuted.
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Lizard, not snake
New evidence for Tetrapodophis amplectus, published in a study of 2021, reveals that it was never an earth snake, but a tiny sea lizard that lived in the Cretaceous period.
According to authors Michael Caldwell and Robert Reisz, the specimen is a dolichosaurus, a marine lizard related to snakes and mosasaurs, for that reason the confusion when classifying it.
The key clues confirming its identity were hidden in the two rock slabs where the fossil was found. The 2015 study only looked at a stone slab and not its counterpart.
In one slab, for example, most of the skull bones were “flattened like an eggshell,” while the natural mold was intact in their counterpart. “It is in the natural mold that we see some other characteristics that are lizard, not snake,” Caldwell told LiveScience.
According to a new study, the first description of Tetrapodophis was incomplete as the slab counterpart where the fossil was found concealed essential clues. Photo: Max Caldwell
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On the other hand, the body of the specimen had long, straight ribs, characteristics that suggested it was a swimmer and not a burrower. This structure, moreover, did not have two stabilizing systems that could have allowed it to slide back and forth.
On its four legs, the authors point out that the front legs were vestigial compared to the back ones, which were more developed. It should be noted that both were not ossified, so they could not walk or even hold their prey.
Although it is not a snake, scientists emphasize that Tetrapodophis continues to be a lizard as amazing as it is strange.
“There are many evolutionary questions that could be answered by finding a four-legged snake fossil, but only if it is real. Our team’s main conclusion is that Tetrapodophis amplectus is not in fact a snake and was misclassified, ”Caldwell said.
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