Scientists who believe snakes evolved on land say they began as subterranean creatures.
Early snakes, the theory's supporters say, are closely related to scolecophidians, a living group of primitive land snakes that still have vestigial pelvic regions.
But proponents of a watery origin believe that snakes most likely evolved from extinct marine reptiles called mosasaurs, powerful swimmers that spent their entire lives in the ocean. (Explore an interactive world of ancient sea monsters.)
"Snakes probably evolved during the Jurassic150 million years ago," Apesteguía said, "but there are no fossils.
"During the early Cretaceous120 million years agothey exploded [into] several forms, including some terrestrial like Najash " and some aquatic.
The fossil record shows that terrestrial and aquatic snakes both existed by the mid-Cretaceousabout 95 million years agoleaving researchers unsure about which type evolved first.
The question, Apesteguía said, is, "Which is more primitive, the terrestrial Najash or the most primitive water snakes, a group called pachyophids?"
He points to evidence that marine snakes are less primitive: Their skull bones suggest that they could expand their mouths to ingest larger preya characteristic of modern snakes.
The marine snakes, Apesteguía concludes, "are ancient versions of modern snakes, not really primitive."
Making a Case
Olivier Rieppel is chair of the geology department at the Field Museum in Chicago, Illinois, and was not involved in the study.
He sees the find as convincing evidence that snakes originated on land.
"It's a most interesting fossil," he said. "It shows a character combination as would be expected in a snake that is [in the same classification as] all other known snakes.
"In that sense, it's a very important find from a critical period of the evolutionary history of snakes and in my mind settles the issue of a terrestrial origin of snakes."
S. Blair Hedges, an evolutionary biologist at Pennsylvania State University in University Park who was not involved in the study, says more fossils are needed to fully resolve the debate.
"But the discovery of this fossil snake with limbs from terrestrial sediments now swings the pendulum so far to the terrestrial [origins] camp that it will be hard for the advocates of the aquatic hypothesisand I am sure they will still advocate itto convince others in the community," he said.
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