HT: PZ Myers

Very cool find. As PZ said:

It’s a busy time for transitional fossil news—first they find a fishapod, and now we’ve got a Cretaceous snake with legs and a pelvis. One’s in the process of gaining legs, the other is in the early stages of losing them [snip] It must be rough being a creationist right now—the data against their mythology just never stop coming.

Before we discuss the new fossil itself, a little background. Pythons, boas, and the like (constrictors) are believed to be considerably more primitive than other living radiations. This is supported by genetic data as well as the fact that they retain vestigial hindlimbs–or spurs–that in some cases protrude outside the skin as little bony prominences. Edward T. Babinski has pictures here. He also points us in the direction of a paper/thesis/thingy that discusses a possible way in which limblessness might’ve evolved from a general lizard stock. The paper itself is pretty interesting, though technical. However, what really sets it apart is the fact that instead of pulling transitional forms out of thin air (like I had to do in my own thesis), the paper points us toward a modern lizard genus (Tetradactylus) which provides us with living breathing examples of not one but multiple transitional forms including one with tiny but funtional forelimbs, but no hindlimbs at all.

As a teaser, here’s a pic from the thesis:

This is really an excellent paper, so I’m actually going to spend more time on it than I do discussing the new fossil.

The main conclusion to be drawn was that transition is primarily functional and only secondarily morphological. This is proven by the most “primitive” species of the three, Tetradactylus seps, which hardly shows any difference in body shape compared to a “normal” lizard but is able to fold down both pairs of limbs in dense grass and move like a snake

Anyone who’s owned or watched a skink has seen them do that as well when moving through dense crap. First time I saw it I was stunned…seeing the obvious question of transition and evolution begging to be asked, that’s when I really became interested in the subject. Which was more or less the same motivation for the guys that actually got off their asses to research it like Berger-Dell’Mour (the study’s author).

He continues with:

The selective advantage of this ability was demonstrated to me, as the actual “predator” on the spot, in a little patch of grassland on the Cape Peninsula, where Tetradactylus seps occurs together with a skink of about the same size, Mabuya homalocephala. After a short early summer’s rainstorm the lizards were warming up in the top layer of the grass patch. On being approached, both kinds of lizard would dive into the dense grass. The skink, however, could be heard rustling on the ground, and occasionally, one would even see blades of grass moving at the top.
On the other hand, all one would hear or see of Tetradactylus seps was a quick movement when it took fright. It was clear that it moved on in the dense grass for quite some distance, but for the human observer it was impossible to determine in which direction and for how far. A bird of prey or a mongoose would definitely have more chance to catch a Mabuya than a Tetradactylus in that area. The possibility of approaching prey unapprehendedly should also be of advantage to the lizard.

So now we’ve got a demonstrated selective advantage to slithering, both in predation avoidance and predation success. Fair ’nuff. But if the guy in Figure 1 can slither AND walk, why would it make any sense for those legs to keep shrinking? And so we read on:

The general body shape of a “Schleiche” has already been attained in T. tetradactylus[Figure 2). That is, the mean index of body length over head length (as a standard) of T. africanus is only slightly greater than that of T. tetradactylus: The swiftness for moving through the grass like a snake should be much improved if the overall length of body with a constant diameter (see Gans 1975) that is used in locomotion increases. Similarly, miniaturized limbs facilitate gliding past stems of grass.

In other words, the speed of slithering increases the longer you are, at a given body diameter. And obviously the smaller your limbs, the less likely they are to snag on things; even if they’re held against the body, they do protrude a bit. For whatever reason, slithering becomes more important than walking in some of the lizards that have taken this path. This would probably have something to do with the effects on predation and prey mentioned earlier. The fact that ‘transitional’ species exist to this very day implies that there is a balance that needs to be struck, with the size of limbs and the body diameter/length ratio depending on how important the two locomotor methods are to the animal.

He also went on to explore the issue of whether the process is teleologic in nature, i.e. whether it’s an inevitability. ‘Built-in tendencies’ to evolve in a certain direction are referred to as orthogenetic processes. Like Cope’s Rule of animals of a given family getting bigger over time. Or the popular conception that Man is the great and unavboidable consequence of primate evolution (which is held even by many who think they believe in Darwinian evolution). This author beats the hell out of that notion, showing, once again, that natural selection is as Darwin himself said directionless and purposeless.

All in all, we now not only have a convincing impetus behind the evolution of slithering and limb reduction/loss, we also have reasonably decent living models of the transitional stage.

Ok, now on to the new fossil find.

As I said, pythons and boas still retain hindlimb spurs, but their pelvi are incomplete to nonexistent. Some older fossils, generally thought to be of the same radiation (macrostomata) often have slightly more complete hindlimbs and hip bones, but none of them have a sacrum. The new find, Najash rionegrina–which is at the moment the basal macrostomatan, the oldest snake yet found, andthe most primitive snake known to man–does have a sacrum.

I could show you a picture of a snake’s sacrum (and I will later), but just to put it into functional and comparative context, I’m going to ask you to grab your butts (so make sure you’re alone). Ok, I’m not really going to ask you to do that, but right where the lower back ends, and your pelvic girdle (hip bones) starts, you can feel/see that that piece right there that articulates on your spinal column feels like a bunch of vertebrae that have been smooshed and fused together. And embryologically that’s exactly what the sacrum is.

In other words, unless I’m misinterpreting things, Najash rionegrina is the only snake, fossil or otherwise, in which we see vertebral fusion along the body.

The following pic was shamelessly stolen from PZ’s blog:

Click for larger. You can sorta see the vertebral fusion, especially in the ventral picture (psv and sav). Other things to notice is that the other two bones that make up the pelvic girdle–the Ischium(isc) and Ilium (ili)–are present. The retention of features in the limb bones themselves are also notable. you can see a more or less full-featured femur (tro & fem) including what appears to be well-formed medial and lateral condyles (the femoral part of the knee).

Very cool find.

For a more general look at snake evolution, this link ain’t bad.