What would a hexapodal skeleton look like?

Question

Having looked at some of the questions regarding creatures with more than four limbs, I've found some useful info, like the idea that additional limbs increase stability while requiring more brain power to coordinate. This helps me imagine how a fictional group of hexapodal animals could evolve.

However, I'm having trouble picturing what the basic structure of a vertebrate with additional limbs would look like. I'm not talking about specifics like a dragon's wings or centaur's hooves. I mean basic things - assume we're taking about a common hexapodal-vertebrate ancestor here, not the ins and outs of specific mythological creatures here.

I know tetrapodal vertebrates have a pelvis and shoulder blades, which function differently because the forelimbs and hind limbs serve different purposes and take different amounts of impact. So I assume hexapods would have similar features... But with three sets of limbs instead of two I'm not sure what changes. Are the middle limbs attached to a pelvis, to shoulder joints, or to something else entirely? Where would the ribs go in relation? I'm sure there would be other things needed that I'm not thinking of because I'm not overly familiar with anatomy to begin with (and please forgive any errors because of that).

Answer 1

You would really have to rewind the tape of evolution in order to do this properly.

All Earthly animals are bilaterally symmetric and have 4 limbs because they are descended from fish. Fish are bilaterally symmetric and have 4 fins because that is hydrodynamic (indeed, we know this is the "ideal" shape because sharks, ichthyosaurs and whales have all independently evolved much the same shape, despite being from different branches of the tree fo life, and in the case of ichthyosaurs and whales, returned to fully aquatic life from a land based ancestor.

So you would have to go even farther back in time, perhaps to the Cambrian Explosion 500 million years ago, and preferentially select the "base" animal as something which does not start out like that. The Burgess Shales tell us that early animals had all kinds of bizarre and wonderful layouts, as almost any kind of combination that was possible was being tried out. Many creatures in the Burgess Shales have no obvious connection to current life forms, essentially because they eventually died out.

Opabinia, one of the more unusual creatures from the Burgess Shales

Modern animals are all descended from chordates, which are bilaterally symmetric and have a primitive spinal cord, the architecture from which fish, and ultimately we are all descended.

For your answer, we will need to assume that at some point a six limbed creature evolved from one of the Chordate family, and it was successful enough to thrive in the environment, and its descendants ultimately inherited the Earth. It would likely have pairs of limbs equally spaced along the body, in order to prevent mutual interference. Early versions would be much like fish or sharks, and not have strongly defined structures to attach the fins to the spines. As they evolved towards land dwelling, they would also evolve more defined structures to transmit mechanical loads to the spine and attach muscles. A likely development might be for two sets of "shoulder" girdles to develop for the front two sets of limbs, as flexible limbs for grasping or controlling difficult manoeuvres in the water for hunting or evasion would be advantageous. A bottom dwelling and "walking" creature might develop three sets of hip like structures instead.

It all starts here

The key is that evolution will change and adapt already existing structures in order to provide the best possible chance to exploit a niche and survive. Who would think creatures as diverse as a giraffe, flying squirrel or anteater all come from a common ancestor? You will also find examples of convergent evolution when creatures exploit similar niches. A Greyhound and Cheetah actually have similar limb structures and articulation of the spine, because they are evolved to do explosiive sprints, despite one being a dog and another a cat.

Answer 2

In my reply to this question of Anatomically correct multiarmed humanoids I suggested the following:

A similar arrangement of limbs on a mammalian/endoskeletal frame would likely entail the sternum bifurcating and descending to replace the costal margin (and secondary scapulae over the false ribs). This would require significant changes to the respiratory system as the lower ribcage and upper abdomen wouldn't be as flexible; diaphragm-based breathing might not even be possible without further changes to the ribcage.

So without diverging too much from the established tetrapod model (though not its evolutionary history; this would be a significant and unlikely deviation) of vertebrate life on Earth, you might reimagine the sternum as an anchorage for an additional pair of anterior limbs. The larger and more robust these limbs are, the more rigid the rib cage supporting the sternum would need to be.

Answer 3

Evolutionary Development and Scenarios

A simple solution would be a bug. They are hexapods. Their legs are underneath their body, which is possible and stabilized because they have an exoskeleton.

With vertebrates this is different. They have an endoskeleton. The rib cage protects the main organs, heart, lungs and intestins (also reproductive organs).

Body / Skeletton Shape 1: Merged Full Bodies

Imagine as an example, two horses in a row, where the second one has no head but is the continuation of the first body.

This would come down to a doubling of all the primary organs and some really strange needs to connect the frontal organs with those at the back - air, food, blood, nervous system, everything has to be "piped" back.

This would imply evolution had a dire need for additional organs in one and the same body. To make sense doubling that, is to assume that they are prone to "lose" half of the body but have to still be able to live on. Like a lizard sacrificing their tail to flee from predators that caught them there.

So, maybe the primary, frontal part is more buffed and the back part is far weaker. Also there is maybe a biological predetermined breaking line in the middle, allowing body "splitting". After such a predatory attack and "splitting" of the formerly "merged horse", it would look like the tetrapodal horse we are used to.

A major concern is the reproduction process. Mating would probably be performed on the rear half, which was intended to be the weaker and disposable one and therefore unsuitable for bearing the offspring. One solution coming to mind is, that female hexapods perform the "splitting" of the body prior to their first mating season.

Body / Skeleton Shape 2: Larger Pelvis

In a different evolutionary scenario, maybe vertebrates (or at least of one branch) developed the need for being better "jumpers" thus needing more muscular strength at the hind legs.

This resulted in an enlarged pelvis which gave room for an additional pair of legs. The rest of the anatomy is almost identical to what we know (as a horse for example) except for the rear end where some organs and muscles have to adapt to a small degree (certainly smaller and therefore easier and plausible than body shape 1).

Conclusion

Maybe there are better anatomical solutions which provide some clear evolutionary advantage over tetrapodal vertebrates. For now, it seems to have a good reason why there are no hexapodal vertebrates around.

Answer 4

Form Follows Function:

I think that before you envision what the skeleton looks like, you need to answer the question "What do these limbs DO?" The biology of large animals demands that structures take energy to grow, and anything not actively useful will gradually degenerate (like blind fish).

The exact structure of the bones will likely resemble existing animals, but there are literally tens of thousands of possible arrangements to be had. Start with an animal with a long thick tail and highly flexible limbs that look similar front and back - like lizards. One of these sets will get duplicated, but I would have the placement of an additional set of limbs posterior to the rib cage - so probably a second hip, which would require the least interference with respiration. Bodies make multiple identical things easier than inventing completely alien parts, and since hips are independent of vital organs, a second set would be relatively unobtrusive. The starting body plan should extend more-or-less unchanged past the legs - think of the solid construction of an alligator.

More importantly, however, WHY does your species need more limbs? It's fairly simple to imagine a relatively simple organism with less specialized hips and shoulders having an extra set of back legs. But WHY is that an advantage? Do they need extra grip on a shifting environment like sand? Do they need to distribute weight over a broad area (and why doesn't a flat body or claw do a better job?)

For example: imagine your creature has evolved six limbs as a duplicate set of hips. the animal can still walk like a quadruped if the function of two of the limbs begins to mutate. So then the front limbs can become smashers, specialized diggers, lobster claws, hands, etc. To distribute weight, there would be a tendency for organs to push back and the forebody to lift in order to maximize the use of these limbs and place the extra weight on the mid-hips (so a bit like a centaur?).

But maybe you want a different set of limbs to be exotic. Maybe the front and back limbs are standard, but the middle ones have a special job. Perhaps your creatures pick up their prey, arch their back, and can still run like a quadruped. Or the wrestle with prey and the mid-limbs are for disemboweling. Of course, there is always wing development in a species that needs to be able to crawl into small spaces to hunt, but also needs to range widely in search of their hiding prey. Compared to a classic "dragon" morphology, the wings would then be behind the ribcage instead of being packed in around the shoulders.

If the rear limbs specialize, you are likely to get either specialized mating limbs or powerful legs with a very heavy tail to help achieve a more bipedal stance (like a dinosaur). This does once again beg the question of what you NEED four arms for. Front limbs might be heavier and allow quadrupedal motion in emergencies, or they might be finer so as to minimize mass in front of the mid-hips and also allow fine manipulation near the mouth and sensory array on the head.