The fossil record of the “primordial serpent” is a marine iguana –

Scientists assume that the ancestors of today’s snakes still had four legs. They were then gradually reduced in adaptation to the buried lifestyle. In 2015, paleontologists thought they had found fossil evidence for this scenario – the ancient four-legged serpent. Now scientists have reanalyzed the supposed relationship between snakes and lizards and concluded that the fossil in question is not a snake but a dolichosaurus, a long-legged sea lizard.

For snakes, limbless locomotion seems to offer evolutionary benefits in their respective habitats. They wander through deserts and rainforests, sometimes inhabit underground tunnels, and many species can even swim and dive. But when and how did their limbs retract? In 2015, scientists thought they could answer this question. In Brazil, 110 million years ago, in a layer of early Cretaceous rock, they found a fossil they called “Tetrapodophis” – a four-legged serpent. The fossil, nearly 20 centimeters long, was as long and narrow as a snake, but had four tiny limbs. It therefore seemed to bridge the evolutionary gap between lizards and snakes.

But there is no snake

A team led by Michael Caldwell of the University of Alberta in Canada re-examined Tetrapodophis and came to a different conclusion: “The most important finding of our team is that Tetrapodophis is not actually a snake and has been misclassified,” says Caldwell. . “Rather, all aspects of its anatomy are consistent with the anatomy of a group of extinct Cretaceous marine iguanas known as dolichosaurs.”

The first to describe Tetrapodophis assumed that the animal does not live in water, but in underground passages, where the advantage is a narrow, elongated body structure. They also suspected that the animal might flex its body to move around and detach its jaws while feeding, much like modern snakes. This is contradicted by the findings of Caldwell and his colleagues: “We found that Tetrapodophis exhibits aquatic adaptation and there is no evidence of contractile behavior or the ability to detach the jaw for feeding,” the authors say. Numerous other anatomical features, such as the shape of the teeth, skull, vertebrae and ribs, also show that it was not a snake but a Dolichosaurus.

Tetrapodophis fossil. (Photo: Michael Caldwell)

Imprints on both halves of the rock

One reason for the discrepant results is that Caldwell and his colleagues analyzed the fossils in more detail than the discoverers did: “When the rock containing the specimen was split, the skeleton and skull were on opposite sides of the plate, each missing piece left on the opposite side.” Caldwell explains. “The original study only described the skull and ignored the shape of the castings, which retained several features that clearly show that the Tetrapodophis did not have a snake’s skull – not even a primitive snake.”

The missing evolutionary link between snakes and lizards remains a mystery. “It has long been known that snakes belong to a group of four-legged vertebrates that have lost limbs during evolution. Somewhere in the fossilized history of snakes there must be an ancestor form that still had four legs, ”Caldwell said. “There are many evolutionary questions that the discovery of a four-legged serpent fossil can answer, but only if it is a real specimen.”

Legal contested copy

One of the challenges in studying Tetrapodophis was to have access to the specimen at all. “There was no proper authorization for the original shipment of the specimen from Brazil, and since its original publication, it has been held in a private collection to which researchers had limited access. This situation caused a strong reaction in the scientific community, ‘says co-author Tiago Simões of Harvard University, Cambridge.

The authors of this study were able to see the specimen when the owner lent it to a German private museum. “In our re-description of Tetrapodophis, we highlight the important legal status of the specimen and emphasize the need for it to be returned to Brazil, not only in line with Brazilian legislation but also with international treaties and increasing international efforts to understand the impact of colonial practices on science,” says Simões.

Source: Michael Caldwell (Alberta University, Canada) et al., Journal of Systematic Palaeontology, doi: 10.1080 / 14772019.2021.1983044

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