Appearing like the punchline to an evolutionary riddle, a new fossil snake with legs has emerged from 95 million year-old deposits near Jerusalem. Its sedimentary surroundings suggest a seafaring lifestyle for this ancient reptile, but its advanced anatomy could overturn a current theory about the marine origin of snakes.
This intriguing new species, dubbed Haasiophis terrasanctus in the 17 March issue of Science, is the second limbed snake to come from the site of Ein Yabrud, an ancient marine environment broadly similar to the still, coastal waters of today’s Bahamian reef.
The first such species, Pachyrhachis problematicus, plays a pivotal role in a scenario that places the ancestor of snakes in the sea. In support of Pachyrhachis’ position at the base of the serpent family tree, some paleontologists have noted features in its skull that they believe single it out as a transitional link between mosasaurs–gigantic swimming lizards of the Cretaceous (144-65 million years ago)–and true snakes. This view contrasts dramatically with the traditional view of small terrestrial or burrowing lizards as snake ancestors.
A group of scientists, led by Olivier Rieppel of the Field Museum in Chicago, Illinois and Eitan Tchernov of the Hebrew University of Jerusalem in Israel, brought Haasiophis into the midst of this origins controversy after the fossil had spent years in nameless limbo in a museum drawer. Their description of the extremely well-preserved fossil, along with an analysis of its evolutionary relationships, led the scientists to conclude that their new species was close kin to Pachyrhachis. Their analysis also indicates, however, that these two snakes were not primitive ancestors, but advanced snakes similar to modern boas and pythons. The new anatomical interpretation suggests that neither Pachyrhachis nor Haasiophis have anything to do with snake origins.
The finding does undermine the theory that Pachyrhachis represents an evolutionary link between marine reptiles and true snakes. Snakes like boas and pythons have a distinctively mobile skull structure that allows them to nearly unhinge their jaw in a formidable gape and "walk" their skull over their prey, dining on meals larger than the diameter of their own head. The two species of fossil snakes from Ein Yabrud appear to have skull architecture similar to these modern serpents. Previous studies of Pachyrhachis had concluded that the snake was incapable of such kinetic feats, instead adopting a modified gape similar to that of the mosasaurs as an intermediate step between the rigid skull of lizards and the mobile skull of higher snakes.
"We went back and looked very carefully at the skulls of Pachyrhachis, Haasiophis, and lizards like mosasaurs, especially features like the braincase, the dentition, and the joint in the middle of the lower jaw," says Rieppel. "The better preservation of Haasiophis allowed us to use its anatomy as a guide, and gave us the background to see just how much these fossils looked like advanced snakes."
But a riddle remains: why do these two snake species have hind limbs? If legs were the norm for snake ancestors, it would make sense to see the species’ advanced anatomy as only superficially similar to more modern snakes. On the other hand, the stubby limbs on the fossil snakes might represent an evolutionary reversal, where snakes with advanced skull design regain hindlimbs that were lost or perhaps greatly reduced in their ancestors. Rieppel and his colleagues counted the number of evolutionary steps involved in each possible scenario, and concluded that the redevelopment of limbs was a more likely story.
"We know of at least 62 lizard and snake lineages that have undergone some degree of limb reduction," Rieppel notes. "Since our fossil record of snakes is very poor, we can’t exclude the possibility that limbs in snakes were lost not just once in the beginning, but several times throughout their history."
Rieppel said that it is difficult to tell how the legs themselves might have been used, since they are too small in relation to the animal’s whole body to have any locomotor function. Modern pythons have a rudimentary hindlimb, usually little more than a "claw" of cartilage tipped with bone that they use during mating and occasional fighting, and it is possible that Haasiophis’ leg served a similar purpose.
American Association For The Advancement Of Science. March 2000.