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The origin of snakes remains one of the most contentious evolutionary transitions in vertebrate evolution. The discovery of snake fossils with well-formed hind limbs provided new insights into the phylogenetic and ecological origin of snakes. In 2015, Tetrapodophis amplectus, a unique creature that lived 110 million years ago (Early Cretaceous epoch) in what is now Brazil, was described as the first known snake with fore- and hind limbs. New research shows that Tetrapodophis amplectus is instead a long-bodied dolichosaurid lizard.

In the shallows near shore, Tetrapodophis amplectus glides through a tangle of branches from the conifer Duartenia araripensis that have fallen into the water, sharing this habitat with a water bug in the family Belostomatidae and small fish (Dastilbe sp.). Image credit: Julius Csotonyi.

“It has long been understood that snakes are members of a lineage of four-legged vertebrates that, as a result of evolutionary specializations, lost their limbs,” said Professor Michael Caldwell, a paleontologist in the Department of Biological Sciences and the Department of Earth and Atmospheric Sciences at the University of Alberta.

“Somewhere in the fossil record of ancient snakes is an ancestral form that still had four legs. It has thus long been predicted that a snake with four legs would be found as a fossil.”

In the new study, Professor Caldwell and colleagues revealed a number of mischaracterizations of the anatomy and morphology of Tetrapodophis amplectus — traits that initially seemed to be shared most closely with snakes, suggesting this might be the long-sought-after snake with four legs.

“There are many evolutionary questions that could be answered by finding a four-legged snake fossil, but only if it is the real deal,” Professor Caldwell said.

“The major conclusion of our team is that Tetrapodophis amplectus is not in fact a snake and was misclassified.”

“Rather, all aspects of its anatomy are consistent with the anatomy observed in a group of extinct marine lizards from the Cretaceous period known as dolichosaurs.”

Tetrapodophis amplectus. Image credit: Martill et al., doi: 10.1126/science.aaa9208.

The clues to this conclusion were hiding in the rock the fossil was extracted from.

“When the rock containing the specimen was split and it was discovered, the skeleton and skull ended up on opposite sides of the slab, with a natural mould preserving the shape of each on the opposite side,” Professor Caldwell said.

“The original study only described the skull and overlooked the natural mould, which preserved several features that make it clear that Tetrapodophis amplectus did not have the skull of a snake — not even of a primitive one.”

Although Tetrapodophis amplectus may not be the snake with four legs, it still has much to teach paleontologists.

“One of the greatest challenges of studying Tetrapodophis amplectus is that it is one of the smallest fossil squamates ever found,” said Tiago Simões, a postdoctoral researcher in the Department of Organismic and Evolutionary Biology & Museum of Comparative Zoology at Harvard University.

“It is comparable to the smallest squamates alive today that also have reduced limbs.”

The team’s paper was published this week in the Journal of Systematic Palaeontology.

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Michael W. Caldwell et al. Tetrapodophis amplectus is not a snake: re-assessment of the osteology, phylogeny and functional morphology of an Early Cretaceous dolichosaurid lizard. Journal of Systematic Palaeontology, published online November 17, 2021; doi: 10.1080/14772019.2021.1983044

 



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