The origin of snakes is relatively well researched and has some surprising, at least to me, members of the snake family. I think we should start by defining what makes a snake a snake and then look at the fossil record.
All snakes belong to a clade known as Ophidia which includes snakes and those extinct animals more closely related to snakes than other non-snake squamates (what most people would call lizards are non-snake squamates). Interestingly, more and more evidence is mounting to include members of the Mosasaur family within snakes as they share many characteristics with pythons, more on this later. The appearance of the Ophidia clade is suggested to lie in the early to mid Cretaceous period (145-65 million years ago). Characteristics of modern snakes, but not all snakes, includes scaled skin, limblessness (not entirely true for pythons, boas and extinct species), moderate to extreme extension of the thorax, and decrease in the number of cervical, lumbar and pelvic vertebrae.
Fossil evidence:
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| Tetrapodophis amplectus from http://www.sci-news.com |
Snakes have a somewhat spotty fossil record. Delicate bones and the large number of ribs with a lack of more robust leg bones, or robust skulls leads to the rarity of snake fossils. However, many do exist and allow for significant, if not total, understanding of the origin of snakes. In fact, the earlier mentioned 145 million years ago is in the process of being disproven with a recent discovery in Alberta that may be as old as 167 million years. The skulls of these new species place them firmly in the snake classification. Another fossil, which is younger, Tetrapodophis amplectus, is a four legged animal that has all the body features of a snake in addition to having four limbs. The overlap between limbless fossils and those with two or four legs that all bear the derived anatomy of snakes makes a very tangled picture. However, it is not uncommon to have this kind of variability in the evolutionary process. Though two theories have been hotly debated, burrowing vs marine swimming, the burrowing theory is finding support from CT scans. These suggest snakes lost their limbs to burrow.
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| Mosasaur from the Oceans of Kansas collection: http://oceansofkansas.com/mus-mosa.html |
When snakes have legs...:
Not all snakes, fossil or modern, are limbless. Modern members of the python and boa family have a structure called pelvic spurs. These spurs are remnants of their leg bones, referred to as vestigial structures. I discussed snake legs in a previous post about limbs. Cornell University has a short video about snake toes. Then we have the ancient marine reptiles of the Mosasaur family. Research is building to show that they at least share a more recent common ancestor with snakes than other reptiles while other research suggests that they might in fact be snakes.
So where does that leave us...
Research suggests that snakes evolved from burrowing lizards. Based in the fossil record there seem to be multiple species with legs that possess many of the derived characters that make a snake a snake. This information gives weight to the idea that snakes are evolved from a much earlier lizard which would have had four limbs. These limbs were later lost, mostly, but retained in full, perhaps, by the mosasours and partially until the modern time in the pythons and boids. In reality, scientist are still combing the fossil record and DNA, in the form of comparative genomes, to produce a complete family tree for snakes and their ancestors. Even venom origins are helping paint a better picture of snake evolution.
