Here at Monsters Real and Fantastic, we are avid snake stans. We have covered before how cute they are, but we also admire them for just how successful they are. There are 2,900 species of snakes worldwide, which is pretty solid! All of mammalia is a little over 5,000 species. And yet, as you might notice, not a one of them has legs. It’s kind of a defining feature of the group, but it really seems like a drawback from the point of view of us bipeds. How exactly did such a strange feature come to be so firmly entrenched?
Some snakes, such as pythons, still have what appear to be the remnants of leg bones in their hind limbs. Their pelvis remnants no longer attach to the spine, and the femur ends not in a foot but a simple stud with a keratin sheathe to make a strange little claw. These obviously aren’t used for walking anymore of course, but they still have an important role. Found mostly in males, these spurs are used for combat (which is hilarious, Imma stab you with my little leg bits) and “tickling females as part of a courtship ritual (which is also pretty hilarious). And that’s all that’s left of snake’s legs; tickle knives.
As we might expect from having hind leg remnants but none for front legs, there is evidence that snakes retained their hind limbs longer than their front. Several fossil species of snake are known with hind limbs. The first species known were marine snakes from the late Cretaceous, genera like Pachyrhachis, Haasiophis, and Eupodophis. For a while, these were the most primitive known snakes, leading many to suggest that snakes were close relatives of the Mosasaurs, the massive aquatic lizards of the Mesozoic, and they had an aquatic origin. By this logic, losing their limbs was an adaptation to streamline the body, letting them wiggle through the waves more efficiently. Basically, they were becoming like reptilian eels.
However, more recently, a legged snake more primitive than they has surfaced, dating back to roughly the same time. Najash rionegrina, unlike all other known fossil snakes, retains a pelvis attached the ribcage, like it is in most tetrapods, and a fused sacrum. Najash was also entirely terrestrial, suggesting that either snakes were already losing their limbs on land or Najash had reduced its limbs in the ocean, then returned to land for some reason.
An even earlier legged snake could change the story. From about 10 million years prior to our known two-legged snakes, this is more primitive and more representative of the ancestry of our serpentine friends than they are. Much like all living snakes, it was a predator, with bones in its stomach and hooked teeth. More importantly, they were identified as burrowers, with features like reduced spines on their vertebrae that would allow them to more easily move underground.
Now, it may seem odd to lose your limbs as an adaptation for burrowing. After all, moles and other burrowing animals have massive claws for digging, so a lack of limbs seems like a disadvantage. However, there are two ways around this. One is by being small and digging in loose soil. Some snakes and most modern earthworms dig this way, using their heads to push through the earth. Of course, even without adaptations for digging on your own, you can specialize in entering other people’s burrows to hide from predators or look for prey. In this case, a slender body with reduced limbs is great. This is why wiener dogs have such short limbs; they were bred to crawl into the holes of badgers (it’s also why they’re the most aggressive breed of dogs).
However, there is a lot of argument over the actual taxonomy of Tetrapodophis. Some researchers, such as Michael Caldwell of the University of Alberta, don’t believe that Tetrapodopis is a snake relative, citing a lack of certain defining features like hooked teeth (the skull does show backwards-facing teeth, but this is claimed to be a result of decay rather than the living condition) and zygosphenes, specialized joints between the vertebrae of modern snakes. Instead, he identifies this supposed snake as a dolichosaurid, a branch of aquatic varanid lizards with ties to the mosasaurs. This would still make it quite closely related to snakes, but would also make it another aquatic animal, backed up further by what appear to be fish bones in its stomach.
The authors of the initial study, however, are still backing up Tetrapodophis‘ snake status. They claim that there is no evidence that its teeth changed their position in death, and it has a single row of belly scales just like all modern snakes. The supposed fish bones in the stomach are also complete with limbs, which makes them more likely belonging to a land animal. Unfortunately, it is difficult to settle this dispute, as the original fossil belonged to a private museum and has since been bought and removed. Nobody is sure where it is, so nobody can study it further and confirm its placement.
With the jury out, it is difficult to say what the origin of snake limblessness actually is. If Tetrapodophis is truly a snake or even just an ancestor of them, they likely lost their limbs for easier movement underground. If it isn’t, they may have reduced them to streamline their movement through water. It may have even been for a third, unknown reason that we just don’t have the fossil evidence to guess at. Whatever the case, after evolving this feature, they have continued to succeed with that body plan for millions of years, and likely will continue to do so for millions more.
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