Scientists believe that the ancestors of today’s snakes still had four legs. These were then gradually reduced to adapt to a digging way of life. In 2015, paleontologists believed they had found fossil evidence for this scenario – a primitive four-legged snake. Now researchers have re-analyzed the alleged link between snakes and lizards and have come to the conclusion: The fossil in question is not a snake, but a dolichosaur, a long-limbed marine lizard.
For snakes, limbless locomotion appears to offer evolutionary advantages in their respective habitats. They meander through deserts and rainforests, sometimes inhabit subterranean passages, and many species can even swim and dive. But when and how did their limbs recede? In 2015, the researchers thought they could answer this question. In Brazil they found a fossil in a rock from the early Cretaceous 110 million years ago, which they called “Tetrapodophis” – four-legged serpent. The fossil, about eight inches long, was long and narrow as a snake, but had four tiny limbs. It seemed to close the evolutionary gap between lizards and snakes.
But no snake
A team led by Michael Caldwell from the University of Alberta in Canada has now re-analyzed Tetrapodophis and comes to a different conclusion: “The most important conclusion of our team is that Tetrapodophis is not really a snake and has been misclassified,” said Caldwell. “Rather, all aspects of its anatomy are consistent with the anatomy of a group of extinct Cretaceous marine iguanas known as dolichosaurs.” The new study reveals that numerous features that researchers concluded in 2015 that it was a snake act, have been incorrectly described and interpreted.
The first description of Tetrapodophis assumed that the animal did not live in water, but in tunnels underground, where a narrow, elongated body was an advantage. They had also suspected that the animal could contract its body to move and that it could hook its jaw when ingesting food, just like today’s snakes. This is contradicted by the findings of Caldwell and his colleagues: “We find that Tetrapodophis exhibits aquatic adaptations, and there is no evidence of contracting behavior or the ability to unhook the jaw for ingestion,” according to the authors. Numerous other anatomical features such as the shape of the teeth, skull, vertebrae and ribs show that it was not a snake but a dolichosaur.
Imprints on both halves of the rock
One reason for the differing results is that Caldwell and his colleagues analyzed the fossil more thoroughly than the discoverers: “When the rock in which the specimen was located was split, the skeleton and skull ended up on opposite sides of the plate, with the missing pieces left their marks on the opposite side, ”explains Caldwell. “The original study only described the skull, ignoring the shape of the prints, which retained several features that make it clear that Tetrapodophis did not have the skull of a snake – not even that of a primitive snake.”
The missing evolutionary link between snakes and lizards remains a mystery. “It has long been known that snakes belong to a tribe of four-legged vertebrates that have lost their limbs in the course of evolution. Somewhere in snake fossil history there must be a precursor that still had four legs, ”said Caldwell. “There are many evolutionary questions that could be answered by finding a four-legged snake fossil, but only if it is a real specimen.”
Legally disputed copy
One of the challenges in the investigation of Tetrapodophis was to get access to the specimen at all. “There were no proper permits to originally ship the specimen from Brazil, and since it was originally published it has been kept in a private collection to which researchers have had limited access. This situation has generated violent reactions in the scientific community, ”says co-author Tiago Simões from Harvard University in Cambridge.
The authors of the current study were able to inspect the specimen when the owner loaned it to a privately run German museum. “In our re-description of Tetrapodophis, we draw attention to the important legal status of the specimen and emphasize the need for its return to Brazil, not only in accordance with Brazilian law, but also with international treaties and increasing international efforts to counter the effects of colonialist practices in science, ”said Simões.
Source: Michael Caldwell (Alberta University, Canada) et al., Journal of Systematic Palaeontology, doi: 10.1080 / 14772019.2021.1983044