First of all, I apologise for taking so long to post something. I have been much too busy writing large reviews. Some of you probably hoped that I was busy dealing with publishers to shape The Book. If only! At present it seems that sending manuscripts to publishers in the way they want you to has as much apparent effect as shouting in a vacuum. Do you remember the slogan 'In space no-one can hear you scream?' It's like that. If you didn't know the slogan, it's from the first Alien film (1979). Rather like that particular alien I intend to keep coming back, though.
Nevertheless, I did toy with the directions evolution might take the hexapod design into. The fun of doing something like this is that you must follow the rules, and these state that the basic anatomy is a given, exactly like in real biological evolution. In this case, the challenge was to evolve a high-browsing animal, something like a giraffe or possibly a sauropod. One way to get an animal's mouth high up into the air is of course to enlarge the entire animal but that is rather boring. Other solutions would be to stretch all animal parts vertically, or to only stretch parts close to the mouth, such as the mouth parts themselves, the skull and and the neck. I chose somewhere in between the last two choices: legs are elongated, but not as much as the neck. Or necks.
Hexapods do not have vertebral columns in the sense of one chain made out of a large number of short bones. Instead, they have two parallel chains together forming a 'scala' (ladder). In the neck the original two series of bones forming the stiles of the ladder merged into one structure while the rungs disappeared altogether. This happened early on, when hexapods were sea dwellers, as an obvious way to expand the range of the mouth at little cost. With a long mobile neck you can increase the 'feeding envelope', which is the volume the mouth can reach while the body stays still. The same idea applies to sauropod necks and rusp mouthparts (see here). As you may have noticed from earlier sketches (here and here), hexapods first have a proximal neck of two segments, then a sensocranium, a distal neck of two segments and finally the jaw apparatus.
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| Click to enlarge; copyright Gert van Dijk |
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| Click to enlarge; copyright Gert van Dijk |
Here are two sketches in which I played with such a high browsing hexapod scheme. I also stretched the sensocranium. Normally that is just a bulb sitting above where proximal and distal necks approximate one another. Here the neurocranium is truly a part of the chain of segments. If you look closely, you will see some curved lines gradually following the angled neck bones. That is the asymmetrical oesophagus, situated on one side of the animal, not in the middle below the bones. See here for more on that odd feature.
Elongating the neck in this way involves elongating the individual bones. I felt that such animals should be able to hold their necks horizontally, not just vertically. Such a horizontal posture will stress such long bones though. Here's why: take one such long bone and assume that the proximal end (near the body) is fixed. All bones attached at the distal end then act as a weight to pull that distal end down. This weight tends to bend the bone down, which stretches the top margin of the bone while simultaneously compressing the bottom margin of the bone. Bone tissue is usually better at withstanding compression than tension, while tendons have opposite characteristics. A typical vertebrate trick to solve these stresses is to string strong tendons along the upper surface of the backbone to take care of those tensile forces, leaving the bones themselves to deal with compression. The trick works better when the tendons are at some distance from the centres of the vertebrae, and that is what neural spines, the bone projections sticking out of the top of vertebrae are for: they keep the tendons at a distance.
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| Click to enlarge; from Preuschoft & Klein 2013 |
The image above shows a scheme with vertebrae, spines and a big tendon in place for a sauropod. In the drawing, the neck vertebrae are fairly long, so we are getting close to hexapod anatomy. But do extra-long bones pose additional problems?
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| Click to enlarge; copyright Gert van Dijk |
I am still considering that and can at present only offer some thoughts. Let's start with one spine on each bone; there are tendons running from the spine that bridge the joint on either side (A). That means the tendon is attached at a sharp angle to the bone, whereas it would work better if the attachment were nearly vertically. That can be done by making the spine longer, or, in this case, by placing it near a joint (B). That works well for the closest joint but worse for the farther one, so that tendon now attaches to the bone itself and no longer crosses a joint. It still looks as if the bones would have to withstand lots of bending forces, which we do not want. Very well, let's duplicate the 'spine & tendon structure' so we have a spine at each end of the bone (C). But just to be safe we may add an additional ligament crossing all elements (D). So there we are.
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| Click to enlarge; copyright Gert van Dijk |
A new problem now may be that flexing the neck might bring one spine into contact with the other, preventing the movement. That can be solved by moving the spine away from the joint again, but there is another solution. We already allowed the oesophagus to be asymmetrically placed to one side of the animal. Could we also place the distal and proximal spines off-centre, so one is displaced towards the right and the other towards the left? That is shown in the image above.
Do the rules allow that? We are all used to the fact that vertebrate skeletons are nicely symmetrical when our intestines are not all that symmetrical. I wonder why; anyway, there are skeletal exceptions, such as narwal teeth and crabs that have one big and one small claw. My guess is that large anatomical asymmetries between walking legs would make both the mechanics and the neurological control of walking extremely difficult, without offering any advantages whatsoever. (Mind you, quite a few neurological functions are already asymmetrical even when the underlying anatomy is nicely symmetrical. Humans have handedness, but also 'footedness' and 'eyeness'; bees have 'antennaness', and I could continue). Anyway, anatomical asymmetry in a nonlocomotory part should not cause any major problems. So perhaps we can add a new rule stating that skeletal asymmetry occur in Hexapods. I must think some more about that and will try to find out whether anyone has already solved the riddle why vertebrate innards are more asymmetrical than their skeletons.
