Snakes
When I was a kid, I was fascinated by snakes. I loved their slinky, slithery movements and bone-defying flexibility -- and I was also endlessly frustrated by my inability to tell whether they had very long legless bodies or just very long tails attached to a skull, or what.
(For the record, they’re mostly body, with short necks and tails. You can tell by looking at where the ribs are in a skeleton.)
As major reptile groups go, snakes are a very recent development -- they’re younger by far than lizards, crocodiles or turtles, having only emerged late in the Cretaceous. They've been around for less than birds or mammals have, too -- and there’s a reason for that, as it’s thought that snakes arose in part in response to arrival of mammals.
The precise origin and development of snakes isn’t extremely well-documented, because snake skeletons are fragile and easily destroyed and so don’t turn up too often in the fossil record. What we do know is that the first snakes were relatives of the early monitor lizards, as monitors are the snakes’ closest living relatives, and probably resembled them.
Reasonably well-preserved snake fossils start appearing in the tail end of the Cretaceous, only a very short time before the famous extinction. These creatures still had vestigial limbs, often reduced to a single hind pair, and were burrowers (this in part counteracts the fragility of their skeletons, as burrowing animals are typically in an excellent spot to become buried and preserved and tend to be well-represented in the fossil record -- there’s nothing quite so nice in paleontology as a self-burying fossil). It’s not that clear what their ancestors were like, but they were probably something not too different from the modern-day earless monitor, another burrowing species.
That burrowing habit, specifically, is quite likely what caused the snakes’ leglessness. Burrows, as a rule, are cramped spaces -- digging is tiring work, so most animals make tunnels just wide enough for their bodies. This in turn means it’s a good thing to have a compact body, because it cuts down on how much dirt you have to move. Burrowing mammals, like gophers or ferrets, achieve this by having short, powerful limbs and low profiles, keeping their legs tucked against their chests when moving and essentially turning themselves into furry little tubes. They can do this because their high metabolisms allow them to support themselves on legs kept erect beneath their bodies, a stance that requires a lot of energy to maintain.
Lizards, with lower metabolisms, cannot do this, and keep their legs in a sprawled position. This is a problem for burrowing lizards, because it drastically widens their profiles -- and in turn forces them to either waste a lot of energy in digging wider tunnels or deal with cramped, tight conditions and toes that smack against tunnel walls with every step. The early snakes couldn’t pull their legs underneath themselves, so they did the next best thing -- they got rid of them altogether.
This of course begs another question -- if lizards has so much going against them in a tunneling lifestyle, why did they adopt it begin with? There are a number of possible answers, and it’s likely that they all played a factor. Burrows offer shelter from predators, weather extremes and temperature fluctuations, to begin with, and these would all have benefited a lizard capable of heading underground. However, the late Jurassic and the Cretaceous also saw a new clade of animals come along that was characterized by, among other things, extensive burrowing habits and a tendency to feature on the menus of modern-day snakes.
“I don’t like where this is going...”
Where the prey goes, predators follow -- and a sudden boom in tunneling prey followed by a boom in tunneling predators is probably not coincidental. Even if the early mammals weren’t the reason those early lizards headed underground to begin with, they certainly encouraged them to stay there.
Later snake species left their tunneling origins fairly early and adapted to a variety of different environments, but their burrow-dwelling origins likely did a lot to save their hides during the end-Cretaceous extinction. Burrowing animals tend to do comparatively well during such events, as the ability to retreat underground lets them ride out the brunt of climate and temperature extremes.
Having a diet that mostly consisted of another widely-surviving group probably didn’t hurt much, either.
After the metaphorical dust settled, snakes did fairly well for themselves. They colonized most terrestrial habitats in fairly short order, and some reached gigantic sizes -- Titanoboa, a genus that arose five million years after the dinosaurs died out, is thought to have reached or exceeded thirteen meters (nearly forty-three feet) in length. The origin of snake venom isn’t really all too clear, but probably happened in the early Cenozoic as well. This was a major development for snakes, as it made them much more efficient predators -- earlier species had relied on physically subduing their prey, which was both less likely to successfully kill their prey and more likely to get them injured.
Snakes also developed very specialized organ arrangements to suit their lifestyles, although to rarity of preserved soft tissues makes these difficult to trace and time. Their organs are thin and elongated, and snakes have only one working lung, the right one -- in many cases, the left lung is entirely absent.
A slow-worm, which is neither a snake nor a worm.
The basic snake body plan is evidently a very successful ones for their lifestyle, because multiple families of lizards developed into it independently of either true snakes or each other. Most, like the ancestral proto-snakes, are either burrow-dwellers or live in thick undergrowth. Very few are as specialized as true snakes, however -- many have vestigial limbs, whether stubby legs or simply claws, and almost all retain two functional lungs.
Amateurs.










