SKULL of a previously unstudied fossil shows that both it and another key fossil are more like modern snakes than scientists thought.

Maybe you don't care where snakes come from--or just wish that they'd go back. But, in fact, a hot debate has erupted over the origins of these slithering beasts. A new analysis of an old fossil stands to overturn the current theory that snakes evolved from the mosasaurs, huge lizards that swam the Cretaceous oceans some 144 to 165 million years ago.

Scientists first placed serpents' ancestors in the sea only a few years back when they found the fossil Pachyrhachis problematicus at Ein Yabrud, a collection of 95-million-year-old marine deposits in Israel.

They examined this limbed creature's skull and determined that it had neither the fixed jaws of lizards nor the highly flexible ones of snakes but rather a modified, mosasaur-like gape that must represent some intermediate evolutionary step.

It now turns out, however, that that interpretation was incorrect. Researchers led by Olivier Rieppel of the Field Museum in Chicago and Eitan Tchernov of the Hebrew University in Jerusalem have recently reached new conclusions after comparing P. problematicus to another, previously unstudied fossil snake from Ein Yabrud. They report their findings in the March 17 issue of Science.

The new specimen, named Haasiophis terrasanctus, had been stashed away in a museum drawer. But it was extremely well preserved and so enabled the group to better scrutinize its evolutionary relationships. Using H. terrasanctus as an anatomical guide, they found that it was most likely a close relative of P. problematicus--and that both were far more like modern boas and pythons than any mosasaur.

LIMB of a 95-million-year-old fossil snake is probably too small to have been used for locomotion.

Indeed, the fossils bear skulls similar to those found in modern snakes that can almost completely unhinge their lower jaw. This feat makes it possible for the animals to eat mice, rats and other prey that are larger than the diameter of their own heads. And given these flapping jaws, H. terrasanctus and P. problematicus cannot be related to primitive snake ancestors.

At the same time, it is unclear just how modern these fossils can be because they both have stubby legs--presumably a leftover feature from more ancient snakes. One guess is that during evolution, such hind limbs vanished and reappeared. Says Rieppel: "Since our fossil record of snakes is very poor, we can't exclude the possibility that limbs in snakes were lost not just once in the beginning but several times throughout their history." Let's just hope that walking snakes never make a comeback.