What good are hindlegs for snakes?

The fossil marine snakes Pachyrhachis and Haasiophis have well developed hindlegs but no front limbs. If the front limbs are lost because they are useless, then of what use are hindlegs in these aquatic snakes? After all, the manatee, the dugong, and the whales get by quite nicely without any hindlegs. Sea turtles, too, mostly use their hypertrophied front flippers to paddle themselves around. When newts swim, they fold their front and hindlegs against their bodies to minimize turbulence. So, if not for locomotion, the hindlimbs of these earliest known snakes must have been functional for some other purpose. Modern pythons use their vestigial hindlimbs during mating. Undoubtedly, the hindlimbs in Haasiophis and Pachyrhachis could have served a similar purpose. Is there, however, possibly some other function for these hindlimb? Since most snakes are oviparous, it appears that the last common ancestor of all living snakes is probably also oviparous. That means Haasiophis and Pachyrhachis were most likely oviparous as well, since no reptile or amphibian is known to have reacquired oviparity from an ovoviviparous ancestor. If these marine snakes laid eggs, they must have come ashore to do so, because otherwise the embryos would simply drown. If amniotic eggs must be laid on land, they must also be hidden from predators and from the Sun. Sea turtles accompanish both goals by digging holes with their hindlegs, laying the eggs inside the holes and then covering the holes with sand. Pachyrhachis and Haasiophis, since they have well-developed hindlegs, could have done the same.

That's a nice story. How are you going to test it?

It can be tested. If one can build models of pterosaurs to see if they can fly, then one can certainly build models of Pachyrhachis or Haasiophis and see if these can dig with their hindlegs.

So, if your model(s) are unable to dig, does that necessarily falsify your story? What about competing stories, should they all be tested? For instance... the guy across the hall thinks that Pachyrhacis used their hindlegs during mating and that Haasiophis used theirs as an aid in locomotion, the people down in the fish range think that both taxa used their hindlegs to kick predators, our secretary thinks they were used to attract mates, and my five-year old daughter thinks that they were simply maintained over time and offered no selective advantage to either species. All of these are of course plausible, and the last one, cannot be falsified by any model.

Certainly! To give you an analogy. Larry Martin and others (including some cladists) have studied and shown that Archaeopteryx cannot swing its legs forward at the hip joint. It is therefore no cursor. It falsifies the reconstruction of Archaeopteryx as a cursorial, feathered theropod.

Perhaps not all of them need to be tested. For example, the hypothesis that Pachyrhachis used its hindlegs to stomp on grapes during wine making can safely be dismissed.

If you reread my original post, you will find that I did NOT reject this hypothesis.

How?

Certainly possible and in no way refutes my hypothesis that these snakes used their legs to dig.

The last hypothesis is unlikely. Hindlimbs and pelvic girdles have been lost in varying degrees between and within different lineages of snakes. Hence they have been lost multiple times. No living snake has anything more than vestiges of their hindlimbs. The loss of the hindlimbs appear to have been adaptive for living snakes because multiple lineages have lost them independently and most snakes have lost them without a trace. Therefore it does not appear that hindlimbs aid in locomotion on land or in the water. As I also pointed out, whales, manatee and dugong have no problem moving without any hindlimbs. Newts, as I also pointed out, fold their forelimbs and hindlimbs tightly against their bodies when they need to move quickly by swimming.

OTOH, forelimbs are very useful for locomotion on land. Some amphisbaenians, those enigmatic reptiles of unknown origin (though possible closely related to the squamates) retain their forelimbs but not their hindlimbs. Amphisbaenians also share a morphological feature with the caecilians: externally segmented body, which undoubtedly aids in burrowing. Snakes, OTOH, have no obvious burrowing adaptations and lack the external body segments of these two other groups of fossorial, elongated tetrapods. Snakes, therefore, are highly unlikely to have been fossorial in origin. If snakes evolved from a fossorial lizard ancestor, then one would expect the earliest snakes to retain their forelimbs (like some amphisbaenians) and possibly evolve an externally segmented or annulated body. Instead, the earliest known snakes are marine species, without an annulated body and also any trace of forelimbs, but with hindlimbs and the ability to consume large prey (evidence that their jaws are more mobile than the scolecophidian snakes). Hence snakes most likely evolved from an aquatic, oviparous animal that used its hindlimbs to dig holes to lay its eggs on land.

Why? To dismiss any hypothesis would indicate that you have some biological understanding of the two taxa. Inasmuch as we weren't around to witness their habits, you must be relying on the bits of information you've collected over years of studying extant squamates. However, there is a better way. The comparative method... plot the character/behavior of interest on a cladogram, try to ascertain if you're looking at a true adaptation or simply it's maintenance, then examine the behavior under the circumstances in which it arose. A cladogram is a hypothesis of relationship upon which futher applications may be tested. The appropriate means to study adaptation, is though the reciprocal illumination of examining traits on a cladogram.

That's because you can't... you'll never know.

To push around through the rocks catching squid.., to steer with..., to flap and swim with... it really doesn't matter, because all they amount to are untestable stories.

Then, can the hindlegs serve multiple purposes. What if you model fifty different hypotheses and non are falsified. Do you give all fifty explanations? Haasophis use their hindlegs to swim, climb, dig, walk, attract mates, position mates, steer, and stomp grape. Well... probably not all of these (at least at the same time), but you get the point. If they can't be falsified, they can't be ruled out and therefore there is no rigor involved in selecting among the competing hypotheses. The comparative method is a better way.

None of this means that hindlimbs on these two snakes aren't historical baggage. However unlikely you believe it to be, the hypothesis still remains.. and can be tested via the comparative method.

Some people may not be able to construct a cogent argument without the aid of a cladogram (and one may even argue that they cannot do so because of their blind adherence to inane cladograms), but not all people are similarly challenged. In fact, some people rely on cladograms so much that it is apparent they are incapable of independent thinking. If a cladogram says, for example, that feathers evolved twice, then that is what they will believe. If a cladogram says that Pachyrhachis and Haasiophis must have reacquired legs from a limbless ancestor, then that is what some will believe as well. It is of course anyone's prerogative to perform a cladistic analysis and then treat the cladogram as some sort of divine revelation of the truth; they have the right to occupy their 'philosophical mountain.' But your 'recommendation' that we all perform cladistic analyses and treat the results similarly is the problem, to paraphrase Dowling (1993, Herpetol. Rev.).

Not exactly untestable. There are lots of animals living in the water. How many of them need legs to push around through the rocks to catch squid? Do we have evidence that Haasiophis and Pachyrhachis push themselves around through the rocks to catch squid? Is is necessary to have legs to catch squid? These questions are all answerable by empirical data.

Absolutely. Our hindlegs did not evolve to stomp grapes or kick balls or step on gas pedals but they can be adopted for these multiple uses. The primary purpose of our hindlegs, however, is locomotion. It is locomotion that has shaped the evolution of the human hindlimbs.

What I am interested in is not how many uses Haasiophis and Pachyrhachis have for their hindlegs, but why do they retain their hindlegs. IOW, why are their hindlegs indispensable even though their front limbs appear to be unneeded. One can point out that the inability to reproduce without the hindlegs (i.e. the inability to dig holes to bury one's legs) would mean that any mutant that loses the hindlimbs would be eliminated from the gene pool. Just as pre-industrial age humans would not be adaptive without hindlimbs (because of lack of mobility), Haasiophis and Pachyrhachis might not be able to reproduce without an ability to dig holes to lay their eggs.

Many animals (including living sea snakes) can perform all of these functions, with the exception of stomping grapes, without any rear limbs. Living sea snakes, fish-like ichthyosaurs and equally fish-like cetaceans, all of which lack hindlimbs, cannot come ashore to dig holes to lay their eggs. The cetaceans, ichthyosaurs and some sea snakes are ovoviviparous/viviparous; they do not need to lay eggs or dig holes. Some sea snakes are oviparous but they have to lay their eggs in crevices or caves, not in holes that they dig themselves. They are also small compared to the larger Haasiophis and Pachyrhachis, which would have a much harder time finding crevices to lay their eggs. A more parsimonious solution for Haasiophis and Pachyrhachis is to retain their rear limbs (as opposed to re-evolving them from a limbless ancestor) for digging holes for egg laying even if they are unnecessary for practically any other function, including mate manipulation or attraction, since modern sea snakes have no problem mating or attracting mates without legs.

As I pointed out, modern sea snakes can perform all of these functions without legs. Therefore it appears that none of these functions can explain the retention of hindlimbs in Haasiophis and Pachyrhachis. OTOH, the fact that living sea snakes cannot dig their own holes to lay their eggs and the fact that some of them have to evolve ovovivipariy/ viviparity is proof that hindlegs are essential for those needing to dig their own holes for egg laying. Large marine serpents such as Pachyrhachis and Haasiophis would most likely need to dig their own holes, since it would be hard to find crevices small enough for them to creep inside.

Even if we follow your suggestion and use the 'comparative method,' there are cladists who, following their cladogram, claim that Haasiophis and Pachyrhachis re-evolved legs from a limbless ancestor! It is hard to imagine why any animal would re-evolve 'historical baggage.' Such a scenario would indeed falsify Darwin's theory of natural selection if it were true. Hence the hindlimbs of Haasiophis and Pachyrhachis must have been functional and adaptive for some cladists to have argued that they re-evolved. And such an argument also implies that the loss of the hindlimbs is also either adaptive or selectively neutral. I propose that it is indeed adaptive for many snakes to lose their hindlegs (if they can find a way to reproduce without needing their hindlimbs to dig holes), but it is also adaptive for Haasiophis and Pachyrhachis to retain theirs. I also propose that Haasiophis and Pachyrhachis did not need their hindlegs to stomp grapes or to perform any of the functions that living snakes can perform without hindlegs, including attracting and manipulating mates. They need their hindlegs to perform a task that no living sea snake can perform: dig their own holes to lay their eggs.

Of course I meant digging holes to bury one's eggs. Sorry for the

Many modern snakes bury their eggs despite their lack of hind legs, so hind legs are not necessary for a snake to reproduce.

Btw, I like the grape-stomping theory. If the snake in the garden of Eden was just trying to get Eve interested in the possibilities of viticulture and oenology so it could get work as a farmhand, it was certainly unfair for God to take its legs away. This new view should make fundamentalists rethink their hatred of snakes, but Adam's sniveling excuse 'It's not my fault, she told me to!' hasn't caused them to rethink their misogyny, so it probably won't.

Note that manatees, dugongs and whales have retained their forelimbs, and never leave the water. Diving birds like ducks hold their forelimbs close to their bodies and propel themselves with their feet. On land they walk on their hindlimbs. Penguins swim by 'flying' through the water with their forelimbs, and walk on their hindlimbs on land. Otters use four legs on land and move through water by undulating their whole bodies. Crocodylians do like the newts. Beavers swim like ducks underwater, but walk on all four limbs or on their hindlimbs on land. Humans swim with four limbs and walk on two. You just have to pick your examples right.

See examples above. Hind limb bipedalism is widespread and has recurred many times. The only other forelimb bipeds I can think of are the sirens which use their rudimentary forelimbs to pick their way across the bottoms of their aquatic habitats but swim and move over land by undulation.

Loss of all limbs has occurred at least 50 times among the lizards, mostly among skinks but in several other families as well. None of these legless lizards have external segmentation either. Some of them have immovable and/or transparent eyelids, and some have partially closed ear openings.

Neither trait is seen in any of the numerous legless lizard lineages.

Lots of modern snakes have no problems burying their eggs (or themselves).

These animals also lack the slender, elongated bodies of Haasiophis and Pachyrhachis; they could not swim as Haasiophis and Pachyrhachis did. The elongate body allows the snake to swim and to steer by undulating their bodies laterally. Snakes do not need forelimbs to steer or to swim. Neither do the newts, which fold their forelimbs against the body while swimming.

IOW, just like the newts. It points out that forelimbs can slow down a swimming animal, especially a weak swimmer.

Yes, birds are bipedal. Turtles too, swim mostly with their forelimbs. Haasiophis and Pachyrhachis are most likely not bipedal, hence they do not need hindlimbs to walk on land, unlike birds.

Yes. Otters apparently are not as specialized for an aquatic existence as, say, the pinnipeds. Morphologically, they differ little from their closest terrestrial relatives.

All of these examples show that there are a variety of swimming styles and adaptations. Animals that cannot swim by undulating their bodies laterally usually propel themselves throught the water with their limbs or tails. Since snakes such as Haasiophis and Pachyrhachis can undulate their elongate bodies, they do not need any limbs to propel themselves through the water. The loss of the front limbs suggest that these earliest known snakes are primarily aquatic animals since they would be quite awkward on land with only two rear limbs. The fact that they retained (or according to some re-evolved) the rear limbs means that the rear limbs are functional. I propose that they function as shovels for digging holes so the snakes can lay eggs.

It is highly unlikely that Haasiophis and Pachyrhachis were bipedal, and it is just as unlikely that their limbs can help them move about on land, since their bodies are elongated and snake like. They probably can slither around faster than they can move with their hindlimbs alone. OTOH, their skull show no digging specializations, so they probably needed to retain their hindlimbs to perform this vital function.

'Bachia trinasale possesses only two small forelegs, the hind limb rudiments being entirely internal. Members of this genus are burrowers, digging tunnels in the forest soil beneath leaf-litter.' (Cogger and Zweifel, 1998 Encyclopedia of Reptiles and Amphibians).

Again, it shows that the retention of the front limbs but not the hind limbs and an annulated body are both fossorial adaptations that have evolved multiple times in reptiles. Yet the earliest known snakes have neither of these 2 types of fossorial adaptations. Therefore the fossorial origin of snakes is contradicted by the morphology of the earliest known snake fossils.

Both of these characters are equally consistent with either an aquatic or a fossorial existence. Fish have fixed transparent eyelids and no tympanum. So these two characters support equally the aquatic or fossorial origin of snakes. Since water transmits sound more efficiently than air, the tympanum is probably more of a hindrance than a necessity. In fact, recent research shows that 'Cellular hypertrophy of inner ear support structures (stria vascularis, spiral ligament, basement membrane, etc.) in all whales may mean whales have inner ear mechanisms that decrease the potential for acoustic trauma' according to the web site below:
http://www.auditory.org/asamtgs/asa94aus/3aAO/ 3aAO7.html

In layman's terms, all whales have evolved stronger inner ears to prevent damage from loud noises. Hence it may be adaptive for an aquatic animal to lose the tympanum, since the greater efficiency of sound transmission in the water may damage a tympanum designed to detect airborne sounds. Besides, a tympanum may not even be necessary to hear underwater, if snakes can hear on land without a tympanum, as Sean Barry painstakingly point out on more than one occasion.

Au contraire, as I pointed out, some burrowing lizards do have forelimbs but no hindlimbs. Besides, many of the legless lizards are not true burrowers because they dwell in the loose leaf litter or other crevices as opposed to digging their own holes.

Yes, these snakes have obvious digging adaptations such as a skull that is highly modified for different styles of digging. No such fossorial specializations are evident in either Haasiophis or Pachyrhachis. It is far more likely that Haasiophis and Pachyrhachis have never lost their hindlimbs than having re-evolved hindlimbs from a limbless ancestor, since its limbless ancestor could have reproduced by either live-bearing or laying eggs in crevices rather than digging its own hole to lay eggs. Pachyrhachis and Haasiophis could have done the same, so that there is really no need to re-evolve hindlimbs. But if the hindlimbs were retained ancestral characters, Haasiophis and Pachyrhachis could have used them to dig holes on land to lay their eggs. Digging holes for egg-laying can certainly explain the retention of hindlimbs in the earliest known snakes far better than any alternative explanation, including bipedal walking and grape stomping.