Some, if not all, snakes used to have legs, and now new research suggests snakes lost their limbs by growing them more slowly or for a shorter period of time.
The research, outlined in the latest issue of the Journal of Vertebrate Paleontology, strengthens the belief that snakes evolved from a lizard that either burrowed on land or swam in the ocean.
In either case, its legs must have become less useful as the animal evolved over time.
"If something is not useful it can regress without any impact on the (animal's) survival, or regression can even be positive, as for here if the leg was disturbing a kind of locomotion, like for burrowing snakes or swimming snakes," lead author Alexandra Houssaye told Discovery News.
For the study, Houssaye, from the National Museum of Natural History in Paris, and her colleagues analyzed a fossil snake named Eupodophis descouensi. The prehistoric snake lived during the Cretaceous Period in what is now Lebanon.
To better examine the snake, the scientists used a new imagining technique called synchrotron-radiation computed laminography (SRCL). With an intense, high-energy beam of X-rays, the SRCL deeply penetrated the fossil as the researchers rotated it. The result was thousands of two-dimensional images that were later compiled into a three-dimensional model of the ancient snake's hips and ultra tiny 0.8-inch legs.
"Synchrotrons are enormous machines and allow us to see microscopic details in fossils invisible to any other techniques without damage to these invaluable specimens," said co-author Paul Tafforeau of the European Synchrotron Radiation Facility.
The new 3-D model determined that Eupodophis, in its lifetime, had two small regressed hind limbs and no front limbs. The leg visible to the researchers was bent at the knee, possessing four anklebones but no foot or toe bones. The high tech imaging further showed that the internal architecture of the leg bones strongly resembled that of modern terrestrial lizard legs.
Houssaye said "the legs were very regressed" in this snake, which she added is not the world's oldest snake, but it comes pretty close.
"The oldest snake remains are dated to 112 to 94 million years ago, and this snake is dated to around 90 million years ago," she explained.
Yet another snake from the same time period, Najash rionegrina, is also thought to have had two small rear legs, strengthening the theory that snakes once had legs and evolved from a lizard with limbs. Hussam Zaher of the University of Sao Paolo in Brazil, and a colleague found Najash (which means "snake" in Hebrew) at the Rio Negro province of Argentina.
Najash retained a sacrum, a bony feature that supported the pelvis. Zaher believes this feature was lost as snakes evolved from lizards. The fossil snake from Argentina is also thought to have lived on land, since it was found in what was once a terrestrial environment. Zaher said its anatomy suggests this snake lived in burrows.
Houssaye, however, does not think the case is yet closed as to whether or not snakes evolved from a marine or land-based lizard.
"The question of snake origin should not be resolved in the next 10 years," Houssaye said.
She is, however, hopeful that all of the separate teams working on this puzzle can one day pinpoint what species was the common ancestor of all snakes.