New York Times, April 25, 2006Link
Charles Darwin was fascinated by snakes -- in particular, by the tiny hip and leg bones nestled inside boa constrictors and other species. They were some of the most striking cases of evolution's imprint. Snakes descended from walking ancestors, and as they adapted to slithering, their legs dwindled to a few vestiges.
It took more than a century after Darwin's death for paleontologists to find fossils of snakes with legs. In the last decade, they have found four species. The fourth, known as Najash rionegrini, was unveiled in the April 20 issue of the journal Nature, and it has reignited a debate about how snakes lost their legs.
The first report of a snake with legs came in 1997 from Michael Caldwell of the University of Alberta and Michael Lee of the South Australia Museum. Pachyrhachis problematicus, dating back 100 million years, sported a small pair of hind legs, each about an inch long. Comparing Pachyrhachis to other snakes and lizards, Dr. Caldwell and Dr. Lee concluded it was the most primitive snake yet found.
Dr. Caldwell and Dr. Lee also proposed that snakes evolved in the water. Among the evidence they cited was the that the Pachyrhachis fossil was found in marine rocks. It had adaptations for swimming like a flattened tail. The scientists also found that the closest relatives to snakes were extinct swimming lizards called mosasauroids.
Other experts had doubts. Hussam Zaher, now at the University of São Paolo in Brazil, said Pachyrhachis was not a primitive snake, pointing out advanced features like teeth in the roof of its mouth. Blind burrowing snakes, considered by many researchers to be the most primitive snakes, do not have those teeth.
Dr. Zaher has added fresh fuel to this debate with his report on Najash rionegrini, of which he was a co-author with Sebastián Apesteguia of the Argentine Museum of Natural Sciences. Dr. Apesteguia discovered the 90-million-year-old fossil in 2003 in the Río Negro region of Argentina. The name Najash is the Hebrew word for the snake of the Garden of Eden. After finding the fossil, Dr. Apesteguia invited Dr. Zaher to help him study its anatomy.
''From the first moment, it was obvious to me that it was important snake,'' Dr. Zaher said.
Najash had well-developed hind leg bones. Unlike Pachyrhachis or any other snake, it also had a sacrum, a special section of the backbone that attaches to the pelvis. ''I think it used its legs to help while it crawled,'' said Dr. Zaher.
Comparing the fossil to other snakes and lizards, Dr. Apesteguia and Dr. Zaher concluded that Najash was the most primitive snake. Their study suggests that Pachyrhachis evolved later, as Dr. Zaher had argued.
Dr. Zaher notes that Najash lived on land, a fact he says casts doubt on the ocean origin of snakes. Losing legs may instead have been an adaptation in early snakes for burrowing after prey.
Dr. Caldwell rejects these arguments. ''Najash is an important fossil, but it's not what it's claimed to be,'' he said.
Dr. Caldwell argues that Dr. Apesteguia and Dr. Zaher did not consider enough snakes and lizards in their comparison. If they had, they would have found that Pachyrhachis is the most primitive snake. Najash evolved millions of years later, Dr. Caldwell argues, as snakes moved onto land.
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