for National Geographic News
Since the 19th century naturalists have debated whether the ancestors to modern snakes lost their limbs at sea or on land. Recently the discovery of early marine fossil snakes with tiny hind limbs reignited the controversy.
In addition, new genetic evidence may prove that snakes evolved on land.
"The big question has been, why would a four-legged animal lose its limbs and develop an elongated body?" said evolutionary biologist S. Blair Hedges of Pennsylvania State University. Hedges is co-author of the first reptile DNA study to include all living families of lizards and many snakes. That new analysis adds to mounting evidence that snakes are not descended from giant extinct marine reptiles.
"The jury is definitely still out on whether living snakes had a terrestrial or marine origin," commented Rick Shine, a snake expert at the University of Sydney in Australia. "But this new study is exactly the kind of research we need to clarify the evidence."
Sink or Swim?
One long-held theory is that snakes are closely related to some group of terrestrial lizards and lost their limbs on land. Many burrowing animals, from weasels to worm lizards, have smaller limbs today. "For animals wiggling around in small holes and crevices, it makes sense that limbs would get in the way," Hedges said.
Today many primitive snakes live in soil or leaf litter. Furthermore, many burrowing animals, such as the blind mole rat, have lost their eyes in the course of evolution. Snakes' eyes are so unusual that many experts believe they re-evolved from those of burrow-dwelling ancestors with much-reduced vision.
Another idea, first formally suggested by Victorian fossil hunter and evolutionary biologist Edward Drinker Cope (1849-1897), argues that snakes lost their limbs at sea and are closely related to the extinct marine lizards called mosasaurs.
Despite pronounced differences (body size for example), snakes share some skeletal features with mosasaurs, such as a loose jaw that allows the swallowing of large prey. Contemporaries of the last dinosaurs, paddle-limbed mosasaurs inhabited seas of the Cretaceous period, which ended about 66 million years ago. Mosasaurs grew up to 60 feet (18 meters) in length. Under this scenario, snakes are also related to mosasaurs' undisputed living ancestor, the fearsome monitor lizard.
For many decades the burrowing-ancestor theory had held sway. But in 1997, researchers described several marine snake fossils with tiny hind limbs. The discovery re-invigorated the marine theory. Paleontologists behind some of these fossils believed they were the oldest snakes yet discovered and a missing link with mosasaurs.
To arbitrate in the debate, Hedges and a colleague at Pennsylvania State University, evolutionary biologist Nicholas Vidal, compared the genetic similarity of 64 species, including all 19 families of living lizards, and 17 of the 25 families of living snakes.
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