Legged Snake
Legged Snake Fossil May Uproot Family Tree

By Michael Milstein,
Discovery.com News

March 17, 2000 -- A newly described legged fossil snake that languished in a museum drawer in Israel since the 1970s may uproot the snake family tree. The fossil suggests that the slithering reptiles evolved from secretive, burrowing lizards and not fearsome, crocodile-like mosasaurs that ruled the seas while dinosaurs ruled the land.

The snake, described in this week’s issue of the journal Science, casts doubt on an earlier suggestion that another legged snake from the same marine deposits near Jerusalem represents a link between modern snakes and the Cretaceous marine reptiles known as mosasaurs.

"These specimens are both such well-developed snakes that their ancestors had to be fairly advanced terrestrial snakes, not marine reptiles," says paleontologist Olivier Rieppel of the Field Museum of Natural History in Chicago, Ill., one of the scientists who studied the newfound snake.

The newest specimen provides the latest evidence in a debate over the origin of snakes that began in the 1860s when famed paleontologist Edward Drinker Cope suggested that snakes had inherited detachable jaws from the gaping, flexible jaws of mosasaurs. Two researchers further argued in 1997 that an Israeli fossil snake called Pachyrhachis problematicus was an ancestor of all snakes and that its rudimentary legs were remnants of the flippers mosasaurs used to speed through water after prey.

But Israeli paleontologist G. Haas, who had originally described Pachyrhachis in 1979, died before revealing a second and more complete legged snake that lay unnoticed in a museum drawer until Michael Polcyn of Southern Methodist University spotted it a few years ago. Polcyn, Rieppel and three other researchers describe that second specimen in this week’s Science as Haasiophis terrasanctus.

They conclude that both it and Pachyrhachis resemble modern, advanced snakes such as pythons and boas and are not more primitive, ancestral snakes that belong at the root of the snake family tree. Their conclusion undermines any evolutionary connection between snakes and mosasaurs, leaving the alternative possibility that snakes descended from burrowing lizards that lost their legs. They surmise the two fossil snakes later regrew tiny legs, which were too small to be of much practical use.

Modern pythons have a rudimentary hind limb, usually little more than a claw of cartilage tipped with bone that they use during mating and occasional fighting, and it is possible that Haasiophis' leg served a similar purpose, Rieppel said.

Analysis of the "spectacular" new specimen "really tells us that snakes were much more likely to have a terrestrial origin," says Harry Greene of Cornell University. "We know there are instances in the fossil record of terrestrial creatures taking to the sea -- whales, for instance, are descended from land animals -- but not the other way around."
 

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