Argument About Snake Evolution Rekindled by Fossil

March 16, 2000

By Patrick Rizzo

NEW YORK (Reuters) - Did snakes first slither onto land from the sea, or did their ancestors first evolve on land?

That's one of the questions a fossil snake with legs has raised for scientists who thought they had already sorted out the origin of serpents.

The snake fossil, found more than 20 years ago in a limestone quarry near Jerusalem, represents a new species, according to researchers writing in the current issue of the journal Science.

"Haasiophis terrasanctus" -- named after Hebrew University paleontologist George Haas who bought the fossil from a West Bank quarry -- was about three feet long (0.9 meters) and lived in the shallow waters of a Cretaceous sea that covered part of the Middle East during the days of the dinosaurs.

It is the second limbed snake to come from Ein Yabrud, a 95 million-year-old bed of sedimentary rock that also yielded "Pachyrachis problematicus," another important fossil with clues to the origins of snakes.

Scientists believe that modern snakes are descended from lizards and that they lost their limbs over time.

Remnants of these limbs can still be found in the anatomy of boa constrictors and pythons, just as the remnants of tails can be found in human anatomy.

But a description of Haasiophis by Olivier Rieppel of the Field Museum in Chicago, Eitan Tchernov of the Hebrew University of Jerusalem and other colleagues writing in the journal Science has researchers once again puzzling whether snakes evolved from sea-going lizards, or from lizards which lived in seashore burrows.

"That question has been around for a long time," Rieppel said in a telephone interview.

In the 1970s, when Haas first described Pachyrachis, he thought that the well-developed hind limbs and advanced skull characteristics meant that the fossils weren't from snakes at all. Instead he thought they were reptiles related to a species of huge ocean-going Cretaceous lizards called mosasaurs.

Then in 1997, certain features in Pachyrachis' skull led scientists to put it at the root of the snake's family tree as a sort of "missing link" between mosasaurs and true snakes that at some point took to the land.

Rieppel and his colleagues now argue that traits found in Haasiophis and Pachyrachis are more akin to those of modern snakes, like the ability to unhinge their jaws to eat things larger than their heads. Such traits, the scientists say, mean that they are more closely related to modern snakes.

So they "cannot be related to primitive mosasaurs," Rieppel said.

Rieppel believes the ancestors of modern snakes were burrowing lizards that lived on land, but he acknowledges that the West Bank fossils do not provide clear answers to the question.

Worldwide there are about 2,700 different species of snakes, very few of which are marine animals.

"And so we are back to not knowing what kind of an origin snakes had," Rieppel said.

Dr. Harry Greene, professor of ecology and evolutionary biology at Cornell University, said the most intriguing aspect about the West Bank fossils is they may show that certain "atavistic" traits can re-evolve if the right genes are triggered.

The West Bank fossils may be snakes whose limbs re-evolved, making them "real snakes, just extinct real snakes" with legs, Greene said.

Greene postulates that if animals like the West Bank fossils could re-evolve limbs, then other animals that have certain genes they never lost but whose "triggers" are dormant could re-evolve those traits.

Maybe humans will end up with tails again.