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Reptiles

The serpent’s paws

Fossil found in Argentina strengthens the hypothesis on the terrestrial origin of snakes

JORGE GONZALEZThe Najash rionegrina: rekindling a debate that began in the 19th centuryJORGE GONZALEZ

“My intuition says that the oldest is a terrestrial species, those that we call fossorial snakes, since they spend the greater part of their time hidden beneath stones or crawling through tunnels” It was with this suspicion that Hussam Zaher, a researcher at the Zoology Museum of the University of São Paulo, ended an interview granted to this magazine in July of 2002. Zaher was speaking about the origin of serpents. At that time, the debate concerning the theme was heated and intense. The dispute, locked in by way of published scientific articles, placed two teams and interpretations at opposite poles.

The Canadian Michael Caldwell and the Australian Michael Lee guaranteed: snakes had come from the marine environment. Zaher disagreed and contested: even in remote times, they were animals that lived on firm ground. During the next four years few new pieces of information surfaced and the discussion went cold, although the two sides didn’t step back from their arguments. Now, in an article published last month in Nature, Zaher describes a fossil found in 2002 in the province of Rio Negro, in the south of Argentina, which relights the polemic question and strengthens the suspicion of the terrestrial origin of snakes. It is   an animal with legs, one meter in length, which lived some 90 million years ago. “It’s the most primitive serpent that we know of”, says Zaher. “It has the characteristics of a primitive and fossorial species and was removed from an area of continental sedimentary rock. These are elements that confirm terrestrial origin and discard the marine environment.”

The fossil, in an excellent state of conservation and almost complete, was discovered by a paleontology team led by the Argentinean Sebastián Apesteguía, from the Bernandino Rivadavia Argentinean Museum of Natural Sciences, who immediately invited Zaher to participate in the description of the animal. The work began in Argentina and finished in Brazil. Attentive to the minimum details and analyzing each millimeter of dozens of minute bones, the researchers did not take long to find the secret hidden by the new specie. The serpent presented, in an evident and definite manner, two sacral vertebrae – located in the animal’s pelvis region, which are responsible for the fixation and support of hind legs, which were around 20 millimeters in length. “This is an unprecedented characteristic, which doesn’t exist in any other of the current species not even in the serpents with feet described up until now”, he says. “This is the most primitive of the serpents ever discovered.” There is evidence that the musculature of the legs would have been well developed, which indicates that these organs would have been used in an intense and constant manner, for example, when moving.

An analysis of the region behind the cranium revealed a short mandible, which would limit the movements and indicates the incapacity of swallowing large prey. “To this extent, it bears resemblance to the Aniloideas and the snake fossil Dinilysia, two groups that are close to the foot of the evolutionary tree of serpents”, compares Zaher. With brownish  scales and rings almost black toned rings in an artistic reconstruction, the mother of all serpents was named Najash rionegrina. The first name makes reference to the biblical animal that inhabited the Garden of Eden and seduced Adam and eve; the second pays tribute to the region where the fossil was found.

The snake of Israel
The controversy over the origin of serpents goes back to the 19th century when the American paleontologist Edward Drinker Cope presented, for the first time, the idea that these animals had sprung from a marine environment and were the successors of the mosasaurs, large lizards now extinct, that had also inhabited the seas. This thesis was again taken up with major repercussions in 1997, when Messrs. Caldwell and Lee published in Nature an article in which they described the Pachyrhachis problematicus, a serpent with hind legs and around 95 million years old, found in the archeological site of Ein Yabrud, in Israel – an area formed by sedimentary rocks. The two researchers guaranteed: this was the missing link between the mosasaurs and the serpents of today. On reading the article,  Zaher was not convinced. “I noted a series of inaccuracies”, he recalls.

A more consistent reply could only be given two years later when the Brazilian,  in partnership with Olivier Rieppel, the fossil curator at the Field Museum in Chicago, the United States, had access to a true copy of another fossil of a snake with feet – which later would be called Haasiophis terrasanctus -, found in the same region of Israel. The animal’s description. Published by Science during 2000, had shown, in the cranium, specialized dentition in the roof of the mouth and mobilized via the mandible. For professor Zaher, these were the characteristics of a group of current snakes, the macrostomates, which include the boa constrictor and the rattlesnake (Crotalus durissus). “We showed that both the Pachyrhachis and the Haasiophis could not be considered the most primitive snakes known, at the base of the evolutionary tree of serpents, since they had too much in common with the macrostomates, which form the lineage closest to modern serpents”, he says. “In the two species, as well as the feet being accessories and non-functional, the sacral vertebrae were already incorporated into the thorax, further evidence of more recent evolution.”

The discovery and description of Najash represents, at least momentarily, the solution to the scientific impasse. The mother of serpents joins up with a series of other ancestors recently announced: at the beginning of April, Tiktaalik roseae, a fish with feet discovered by American paleontologists on an island in Canada, has become known to the public at large. In 2003, Chinese researchers had already found fossils of dinosaurs with wings, which could correspond to the oldest ancestors of current birds. In the opinion of Zaher, the efforts of the paleontologists and the more detailed knowledge of the Argentinean, Chinese and Canadian sedimentary basins are some of the main items responsible for feeding the debate over the origin of major groups of animals, helping to complete the gaps and to build the history of the lineages over time in a more precise manner. Zaher knows that the description of the most primitive serpent known until this moment is rekindling the ancient debate. “We are waiting for the reactions.”

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