Wednesday, February 9, 2011

Scientists Uncover 95 Year-Old Snake Fossil with Vestigial Legs

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)


  1. I can't help but to read the competing hypotheses here without thinking about tiktaalik, the famous intermediate organism that signified the fish to reptile transition. Like snakes, tiktaalik had internal ears, a streamlined body shape and smooth skin, all of which were traits indicative of a predominantly aquatic environment. Perhaps, then, it can be assumed that snakes too have some aquatic homology--as indicated by snakes that already have aquatic or amphibious lifestyles.

    POSTED BY: Alexander Simolaris

  2. The evidence of a 95 million year old fossil is substantial. However, it is interesting that two hypotheses can be provided for the reasoning behind the supposed ancient legs of snakes. I feel that the first hypothesis is more on the right track because it gives reason as to why there snakes have adapted to no longer needing legs while connecting similarities with modern snakes.

  3. Alexander,

    Yes, I also thought of tiktaalik when writing this story, not only in terms of how these creatures represent a process of adaptation to a distinctly different environment, but also in how their evolution started from, and proceeded in, exactly opposite directions. Examples such as these always make me think of Alan Grant's line in Jurassic Park: 'Life finds a way.'

  4. Liz,

    I agree that this fossil will probably end up being significant in some way. The scientists featured in the article downplayed the fossil's importance, but paleontologists tend to be pretty resourceful considering what little they're usually given to work with.

    I also agree that it is quite interesting that both of these hypotheses are capable of explaining the same set of facts despite proposing entirely different explanations. Ambiguity is often the price life scientists have to pay for not starting from more fundamental premises, unlike physicists. Beats having to take Differential Equations, though.

  5. Evolutionary biology is such an interesting field, and combined with herpetology, we have ourselves a really cool article. I heard about this find in my herpetology class, and I am really interested in what these vestigial legs were used for, if anything. I am curious as to whether snakes had been simply phasing their legs out, or whether legs without ankles and feet were actually useful to them. It is very possible that these legs helped them to fill a certain niche, in fact, I am sure they did. If I had to generate my own hypothesis, I would say that certain snakes were secondarily evolving to be very like gymnophiona, or caecilians, as shown by the evolution of these snakes' eyes in that they closed over to aid in their fossoriality. And with fossorials, legs only get in the way. I cannot wait to see what becomes of this study!

    Posted by Derek Melzar