By Will Dunham

WASHINGTON (Reuters) - Snakes have been slithering on Earth far longer than anyone ever realized.

Scientists on Tuesday described the four oldest-known snake fossils, the most ancient of which was a roughly 10-inch-long (25 cm) reptile called Eophis underwoodi unearthed in a quarry near Oxford, England, that lived about 167 million years ago.

The remarkable fossils from Britain, Portugal and the United States rewrite the history of snake evolution, pushing back snake origins by tens of millions of years.

Until now, the oldest snake fossil dated from about 102 million years ago, said University of Alberta paleontologist Michael Caldwell, who led the study published in the journal Nature Communications.

Scientists say snakes evolved from lizards, and a number of previously discovered fossils of primitive snakes featured small back legs.

Those described on Tuesday did not include entire skeletons, but the researchers say all four may have had some form of reduced forelimbs and hind limbs.

That does not mean they walked. "It seems probable that they were slithering, so to speak, though the limbs might still have been used for grasping," Caldwell said.

"Snakes have generated fear and fascination since ancient times," said paleontologist Sebastián Apesteguía of Argentina's National Scientific and Technical Research Council (CONICET) and Universidad Maimonides, another of the researchers.

"However, we know very little about their very origins," added Apesteguía, who speculated the first snakes may have appeared about 190 million years ago.

The four snakes lived during the age of dinosaurs. Eophis, the oldest, was a swamp dweller that probably ate small minnows, insects and tadpoles.

The largest, named Portugalophis lignites and discovered in a coal mine in central Portugal, measured about 4 feet (1.2 meters) and was 155 million years old. It may have eaten small mammals, young dinosaurs, lizards, birds and frogs.

Diablophis gilmorei, a snake from about 155 million years ago, was found in western Colorado. It was a bit larger than Eophis and probably ate similar prey.

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Parviraptor estesi, found in sea cliffs near Swanage, England, was about 2 feet (60 cm) long and 144 million years old.

Caldwell said the four snakes' skull anatomy was similar to modern snakes and other fossil snakes. Caldwell said the characteristic snake skull design likely emerged before these reptiles acquired their elongated and legless body plan.

None of the four was venomous, Caldwell said. The oldest venomous snake fossils are about 20 million years old.

(Reporting by Will Dunham; Editing by Eric Beech)