Scientists have unearthed a fossil of a primitive snake with stubby legs. The 95-million-year-old specimen, described in today's issue of Nature, may be the long-sought missing link between snakes and their lizard ancestors.
The fossil was actually discovered about 20 years ago by amateur collectors at a limestone quarry near Jerusalem. A professor at Hebrew University in Jerusalem concluded it was a Cretaceous-era reptile and named it Pachyrhachis problematicus, publishing a description in an obscure journal. The specimen languished on a shelf at Hebrew University until 1995, when a professor there invited geologist Michael W. Caldwell of the University of Alberta in Canada to take a second look at it.
It didn't take long for Caldwell and a colleague, zoologist Michael S. Y. Lee of the University of Sydney in Australia, to realize they may have the long-sought missing link between snakes and early reptiles. "I'd say it is the Archaeopteryx of snakes," says Lee, referring to the species that provided the first fossil evidence that modern birds may have evolved from reptiles. The Jerusalem fossil has a well-developed pelvis and hindlimbs like a lizard, but it also has a slender, elongated body and narrow skull characteristic of modern snakes. "It has some but not all of the features of snakes, showing that the highly distinctive snake body plan arose gradually," says Lee. Why are snakes legless? "In gradually using more and more of the trunk for locomotion, they used their legs less and less and gradually lost them," Lee says.
That the fossil was unearthed in limestone suggests P. problematicus lived in water and was closely related to the huge marine lizards called mososaurs. This finding runs counter to the prevailing view that snakes evolved from terrestrial lizards. "It's a very interesting idea," says Nicholas C. Fraser of the Virginia Museum of Natural History, "but it needs a lot more detailed work before everyone is going to fully accept what they say."