Weird Lizard Fossil Reveals Clues to Snake Evolution, Experts Say
for National Geographic News
|March 26, 2007|
A 95-million-year-old marine lizard with minuscule front legs may shed new light on the evolution of modern reptiles, particularly snakes, scientists have reported.
The fossilized remains of the reptile represent the earliest known example of a lizard evolving toward a limbless state, according to experts who described the new species.
The creature's vestigial, or no longer functional, forelimbs barely protrude from its long, snakelike body.
Although its rear legs were of normal size, researchers said the lizard was probably an eel-like swimmer that spent little time on land.
Michael Caldwell, of the University of Alberta, in Edmonton, led the team that made the discovery. He said the lizard, dubbed Adriosaurus microbrachis or "small-armed Adriosaurus," belongs to the lizard group most closely related to snakes.
Intriguingly, Caldwell noted, the new fossil dates to the same period as fossils of primitive snakes that also retained their hind legs.
"This animal appears to have been aquatic, like the rear-limbed snakes from the Middle East," Caldwell said.
He and Italian paleontologist Allessandro Palci reported the discovery in the March issue of the Journal of Vertebrate Paleontology.
Look, No Hands
Experts believe all lizards and snakes descended from a common reptilian ancestor that walked on four legs and lived on land.
Adriosaurus and early snakes probably evolved their elongated bodies and shortened front legs independently of one another, Caldwell noted.
But the similarities between the two may help explain the evolutionary origin of the snakelike body form.
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."
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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