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Snakes Evolved on Land, New Fossil Find Suggests

Nicholas Bakalar
for National Geographic News
April 19, 2006
 
An ancient snake with hips connected to its spine might be proof that slithery serpents originated on land, not in the water, a new fossil find reveals.

The fossil snake—which has a primitive pelvis and robust, functional legs outside the ribcage—dates from about 90 million years ago.

Sebastian Apesteguía, a researcher with the Argentine Museum of Natural Science, says the new fossil is not the oldest snake fossil ever found. Older marine snakes have been unearthed in North Africa and Eastern Europe.

But the species, named Najash rionegrina, is the earliest limbed snake ever found in a fully terrestrial deposit, he says.

N. rionegrina was discovered in Argentina's Rio Negro province, about 700 miles (1,130 kilometers) southwest of Buenos Aires (see map).

Apesteguía and colleague Hussam Zaher, of the Zoological Museum of the University of São Paulo in Brazil, describe the find in tomorrow's issue of the journal Nature.

Land or Water?

Many living snakes, such as pythons, have the vestiges of legs that are not attached to the backbone and simply hang from the body.

(See a related photo of a python that burst after trying to eat an alligator in the Florida Everglades.)

By contrast, Apesteguía said, "In Najash the hip was connected to the vertebrae, so it has a sacrum. No other known fossil or extant snake is so primitive as to retain this feature."

The sacrum is the bony structure that connects the spine to the hips in vertebrates, including humans.

The animal's sacral region would have made its legs well suited to digging or crawling, the researchers say, giving weight to a land-based origin for snakes.

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 Jurassic—150 million years ago," Apesteguía said, "but there are no fossils.

"During the early Cretaceous—120 million years ago—they 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-Cretaceous—about 95 million years ago—leaving 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 prey—a 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 hypothesis—and I am sure they will still advocate it—to convince others in the community," he said.

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