Was the first snake a sea serpent?

Fossil snake
The fossil snake Tetrapodophis, as preserved and as reconstructed in life. Credit: Alessandro Palci, Michael Caldwell and Michael Lee.

The evolution of snakes is a slippery debate but new research is pointing to an aquatic rather than underground primordial origin.

One of the enduring controversies in evolution is why snakes evolved their long, limbless body.

Many scientists now think snakes evolved their serpentine form for burrowing.

However, a new study of a primordial fossil snake reveals that it was probably aquatic – which means that snakes evolved their long bodies for eel-like swimming in their transition from the dinosaur age.

An Australian-Canadian team led by Professor Michael Lee and Dr Alessandro Palci (Flinders University and the South Australian Museum) re-studied one of the most important and controversial fossils of modern times, Tetrapodophis.

This 120 million-year-old creature lived during the age of dinosaurs in what is now Brazil, and was a tiny snake-like creature that retained four short legs.

It has been described as the ‘Archaeopteryx (after the famous dinosaur-bird ‘missing link’) of snake evolution.

It was described by fossil experts last year as a primitive snake and widely interpreted as a worm-like burrower, thus supporting the idea that snakes evolved underground.

However, the new study reveals that Tetrapodophis had the wrong body shape for burrowing, and instead possessed a suite of adaptations that are typical of aquatic animals.

“The shape of the animal also doesn’t quite fit with a burrower,” says SA Museum Earth Sciences researcher Professor Lee, from Flinders University School of Biological Sciences Professor of Evolutionary Biology.

“For instance, it has a long slender tail and four slender legs, something you don’t see very often in burrowing snakes and lizards today,” Professor Lee.

Research associate Dr Palci says the limb bones are “weak, poorly-ossified, and rather flipper-like. We see these traits only ancient marine lizards called mosasaurs”.

New evolutionary trees place the marine mosasaurs as the nearest relatives to snakes, and other primitive snakes.

“Furthermore other primitive snakes, such as the two-legged Pachyrhachis from the Middle East, are definitely aquatic,” Dr Palci says.

“We need to seriously contemplate the idea that snakes had aquatic origins,” says Professor Lee.

The radical new ideas about the aquatic habits of Tetrapodophis add to the debate surrounding this exquisite fossil, he says.

“When it was first described, there were questions about the legality of its provenance, whether this privately-owned fossil might be perpetually available for study, and even its identity as a primitive snake.

“These issues are still brewing, and the latest study helps cement this tiny reptile as one of the most important and controversial fossils of our times.”

‘Aquatic adaptations in the four limbs of the snake-like reptile Tetrapodophis from the Lower Cretaceous of Brazil’, by Michael S.Y. Lee, Alessandro Palci, Marc E.H. Jones, Michael W. Caldwell, James D. Holmes and Robert R. Reisz, has been published online by Cretaceous Research (Elsevier).

The co-authors are Professor Robert Reisz (University of Toronto), Professor Michael Caldwell (University of Alberta) and Dr Marc Jones and Mr James Holmes (both from the University of Adelaide).


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