Tuesday, July 17, 2018

Taxonomy Tuesday: Snakes are lizards

I'm trying something new in an effort to post more often then once a month.

Once a week, on "Taxonomy Tuesday," I'll write a short post about some taxonomic weirdness that people might not think about. On this maiden voyage of "Taxonomy Tuesday," we'll talk about snakes...which are lizards.

Now, when most people think about reptiles, they think about turtles, crocodilians, lizards, and snakes as if all four groups are wholly separate from each other (nobody ever thinks about amphisbaenians). The tree diagram in their heads looks something like this, as if snakes and lizards split from a common ancestor:

Behold the magic of Microsoft Paint.
But in reality it's more like this:

I just realized I left off tuataras...which are not lizards.
"But snakes don't have legs!"

Indeed they do not, but that can't be their distinguishing feature, because lizards lose their legs every chance they get. Among the more well-known examples of non-snake legless lizards are slow worms and glass lizards. They can be distinguished from snakes most readily by having eyelids, an external ear (snakes lack both of these) and a non-forked tongue. Apart from those two groups, however, there are many more lizards from diverse families that have also lost their limbs:

Lialis, a pygopodid gecko. IT'S A GECKO.
  • Pygopodids lack forelimbs entirely but have vestigal flaps for hindlimbs. Astoundingly, these lizards are geckos.
  • Dibamids are limbless and include species that are blind or nearly blind and deaf. They are specialized burrowers and may be the most basal living squamate group. They look like worms.
  • Anniellids, or the American legless lizards, are found exclusively in California.
  • Gymnophthalmids (GYM-noff-THAL-mids), or speckled lizards, is a large family that contains some species that have all four limbs and others with reduced limbs. Different taxa have different limb/less proportions.
  • Skinks, which make up a very large radiation of rather shiny-bodied lizards, include many groups that have independently lost their limbs, some of which only have tiny vestigal flaps remaining and others that have lost their limbs entirely.
The legless skink Acontias
  • Amphisbaenians are extremely specialized burrowing lizards that, like dibamids, look more like worms than reptiles. Bipes may be the most publicly-known genus thanks to the Internet. Amphisbaenians were, for a long time, considered a unique clade of squamates until molecular evidence moved them into the Lacertoids (along with gymnophthalmids). Lacertoids are what you probably imagine when you think of a generic lizard that isn't a gecko, iguana, or chameleon.
An amphisbaenian (Amphisbaena, in fact)
So if snakes are just another group of legless lizards, and sets them apart?

Jurassic snake Diablophis by Julius Csotonyi
Snakes probably evolved in the Mid-to-Late Jurassic from an anguimorph ancestor. This large group includes varanids (monitor lizards) but also Gila monsters, beaded lizards, the extinct marine mosasaurs and a couple other extinct semi-marine groups. Although there is some debate about whether the ancestral snake was a burrower or a swimmer, it should be noted that all other legless lizards listed above are either burrowers or live in the leaf litter of a forest. While there are sea snakes (and sea kraits), they may not represent the original snake.

Within varanoids, there's still some debate about exactly who snakes are closest to. The most recent phylogenies that I've seen (and I haven't seen a ton) indicate that snakes are a sister group of mosasaurs. Interestingly, Cope proposed this relationship back in 1869: snakes and mosasaurs were united in a monophyletic Pythonomorpha but that concept fell out of favor until being revived in the last few years.

Snakes have a fused, transparent eyelid (a bit like geckos), a forked tongue (shared by other anguimorphs), and very reduced skull bones, some of which can dislocate and others that can slide against each-other. Thanks to this, snakes are able to fit through heads around prey items much wider than they are. Compared to other legless lizards, many features of a snake's internal organs are unique compared to non-snake lizards. For example, snakes don't have any lymph nodes. Many snakes are venomous, but venom is more widely distributed in anguimorphs and may actually be more widely distributed in lizards generally, so being venomous, by itself, isn't a feature unique to snakes.

Titanoboa, a 40-foot long Columbian snake from the Paleocene, by Jason Bourque
Snakes are also, obviously, the most successful group of legless lizards by a mile and have adapted to a huge variety of environments and ecological roles.

Thanks to Gabriel Ugueto for corrections! 


  1. Here’s a quick historical note on Pythonomorpha you never really hear: Cope actually coined it to refer to the mosasaurian line only. As far as I can ascertain this is the only way Cope himself ever used the name. At some point (I don’t know when) it was used to refer to the Mosasauria+Ophidiomorpha node and, well, here we are. Here’s a link to the paper where he named it: https://www.biodiversitylibrary.org/part/243473#/summary

  2. Neat idea!

    For next Tuesday, I suggest the counter-intuitive fact that jawed fish are jawless fish, phylogenetically speaking.

  3. I was surprised to learn that Sea Snakes, at least Hydrophiinae (the true/aquatic sea snakes), evolved recently only 10-15 million years ago. They probably crossed the Pacific Ocean after the Panama Isthmus closed because there aren't Sea Snakes in the Atlantic.
    Sea Kraits evolved seperately 15-20 million years ago.