The fossil suggests that in both kinds of ancient animals, the front limbs shrunk before the hind legs.
"[The new fossil] shows that in Cretaceous lizards, and by analogy, snakes, the forelimb was lost in stages … before any obvious reductions of the rear limb assembly," Caldwell said.
Matthew Brandley, of the University of California, Berkeley, noted that the ancient fossil appears to have much in common with some modern lizard species.
"Limb reduction has evolved over 25 times in lizard groups we see today," Brandley said.
"We almost always see the forelimb reducing before the hindlimb [in lizards today]," he added.
"This fossil is interesting because it tells me that the same trend has been going on for a very long time."
Ocean Origins?
Despite its unusual features, the Adriosaurus fossil had long been overlooked by scientists. First collected from a limestone quarry in Slovenia in the 19th century, it sat on a shelf in a city museum in Trieste, Italy, until its recent rediscovery.
By demonstrating the antiquity of evolved limb loss in lizards, the fossil is now likely to play a role in a longstanding scientific debate over the evolutionary origin of snakes.
It was once widely believed that snakes first appeared on land, and some recent fossil discoveries still strongly support this view.
Other fossils, however, suggest the first snakes may have been sea creatures like Adriosaurus.
Forelimb reduction and body elongation in early marine lizards and snakes may have been adaptations to a watery environment, Caldwell said, but the connection is far from proven.
"What we can say is that limb loss has occurred many times [in different animal groups], and that in the majority this has occurred alongside other aquatic adaptations."
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