91av

A FOSSIL snake with well-developed rear limbs has fired the debate over
whether snakes’ ancestors lived on land or in the sea.

The 95-million-year-old Haasiophis terrasanctus was found in marine
deposits north of Jerusalem. In 1996, researchers who studied a similar legged
snake called Pachyrhachis decided that it was an ancestral snake that
had never lost its rear legs. As Pachyrhachis was a sea predator, they
concluded that it—and all snakes—had evolved from marine lizards
called mosasaurs
(91av, 30 November 1996, p 17).

But Olivier Rieppel of the Field Museum in Chicago thinks they got it wrong.
He says Haasiophis could dislocate its jaw to swallow large
prey—a feature that first appeared in advanced “big-mouth” snakes such as
pythons. This suggests that far from being ancestral, both Haasiophis
and Pachyrhachis are advanced snakes that had re-evolved legs. Snakes
such as pythons retain limb buds, says Rieppel, so re-evolving limbs isn’t a
problem. He thinks the first snakes evolved from lizards that burrowed in soil
or leaf debris.

However, Mike Caldwell of the Canadian Museum of Nature in Ottawa says that
Rieppel’s analysis fails to compare the legged snakes with mosasaurs, which he
considers their closest relatives. That could mistakenly put primitive legged
snakes among more modern ones.