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This 95-million-year-old legged snake tests serpentine origins.Mark Widhalm/The Field Museum

A newly described fossil snake with legs may have climbed higher into the snake family tree than previously thought.

The 95-million-year-old fossil snake, dubbed Haasiophis terrasanctus, is a relative of another legged fossil snake—Pachyrhachis problematicus—that some scientists think may be the earliest link between snakes and extinct marine lizards, like mosasaurs (SN: 4/19/97, p. 238).

However, the new fossil suggests that the two-limbed snakes are advanced big-mouthed snakes, like pythons and boas, rather than primitive serpentine ancestors, says paleontologist Olivier Rieppel of the Field Museum of Natural History in Chicago.

Rieppel and his colleagues compared the fossils' skulls with skulls from other snakes, living and extinct. They ignored the fossils' legs, which earlier studies suggested were characteristic of primitive snakes. It's the skull that makes the snake, and the legs are "not informative for the analysis," Rieppel says.

In the new study, published in the March 17 Science, Rieppel speculates that the fossil snakes could have lost, then re-evolved, limbs by turning off and on a genetic program for legs. So, regrowing legs would only require one evolutionary step, he says, while developing an advanced skull would take many.

By using only data from the skulls, Rieppel's group made it impossible to see important relationships between the fossils and other lizard groups, cautions paleontologist Michael W. Caldwell of the Canadian Museum of Nature in Ottawa. "The whole animal evolves, not just the head," he says.

Caldwell and his colleagues say that snakes evolved from ocean-dwelling lizards. The new classification of the fossil snakes as more-advanced forms makes it impossible to determine origins from these specimens, says Rieppel. "These two fossil snakes don't make the link [to mosasaurs]," he says.

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