Paleontologists in Lebanon have recently uncovered the 95 million years-old fossil remains of a snake. No ordinary specimen, however, this snake was equipped with vestigial legs, giving scientists an excellent look at the regressive evolutionary processes that resulted in the anatomical structure of the modern snake. Using a process known as computed laminography, whereby hundreds of 2D X-ray images of the fossil where compiled into a single 3D representation, scientists at the Museum National d'Histoire Naturelle in Paris, France, were able to acquire a detailed and high-resolution 3D image of the leg bones. These images showed that, while the legs did articulate with the pelvis, they lacked bones for both ankles and feet, indicating that their function as mobile appendages had been long before this creature's existence.
While fossil and molecular evidence shows quite conclusively that snakes began to transition towards ‘leglessness’ around 150 million years ago, there are two competing hypothesis regarding the selection pressures that drove the leg regression. The first is known as the burrowing lizard hypothesis, which proposes that a shift towards burrowing lifestyles in ancestors of varanid lizards, such as monitor lizards, caused their legs to become obsolete. The fused eyelids that now make up the thick, transparent covering of the snakes’ eyes, the loss of external ears and the ability to hear by sensing terrestrial vibrations are characteristics of snakes that are also suggestive of transitions towards a fossorial, or burrowing, lifestyle. The second hypothesis is known as the Aquatic Mosasaur Hypothesis, which suggests that the aforementioned characteristics points to adaptation to an aquatic environment. Increased corneal thickness to hold off osmotic pressure, loss of external ears, and the ability to hear by sensing changes in pressure, much like the lateral line in fish, are characteristics that can also be construed as consistent with a transition towards an aquatic lifestyle.
The Aquatic Mosasaur hypothesis has long been touted as the most likely, but modern molecular studies have shown that snakes may not be as closely related to mosasaurs, a family of extinct marine lizards, as was once thought. Snakes are still show greater genetic relatedness to mosasaurs than to varanid lizards, though, giving the upper hand in the debate. Unfortunately, it does not appear as of yet that the legged fossil provides any substantial evidence for either hypothesis. Still, it is quite fascinating to see a fossil such as this, one of three legged snake fossils in the world, as it gives us an excellent look into one of the numerous dilemmas that makes the debate over the origin of species so interesting.
Posted by Connor Finnerty (2)