Sunday 12 November 2023

Shaping hexapod shapes: high browsers

 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. 

Click to enlarge; copyright Gert van Dijk


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.

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? 

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.

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.


Idle Speculation said...

This head structure opens up many interesting possibilities.

Perhaps some browsers would take on an elephantine shape with the distal/post-cranial neck being much longer than the proximal/pre-cranial one?

I can also imagine a lineage of predators, perhaps on that empty continent you’ve mentioned before, which, in the absence of neocarnivores, have extended their post-cranial neck into a kind of spear-thrower shape like an anhinga to launch serrated jaws deep into prey.

I could imagine them developing centaurism, but, much like tyrannosaurids and very much unlike neocarnivores, doing so solely for speed and reducing the arms, perhaps into display structures or a means of balancing while running, to make space for the larger and more powerful head. I’d try to sketch one such “pilugnath” but I’m unsure where to send it to you.

On an unrelated note, this structure is very reminiscent of DeviantArt user ComplicatedStarman’s art of an alternate timeline where descendants of Opabinia, rather than chordates, were the dominant megafauna. Their facial arms develop similar joints, and some fuse them into club-like structures for combat, while food is passed into the true mouth by centaurized arms.

Idle Speculation said...

I’d also like to mention that some of the animations on the locomotion section of the Furaha website don’t work. The “swimming with tubes” and “walking with 8 legs” pages are good examples…

Keenir said...

You have nothing to apologize for.

This is a great examination of how, with a understanding of how organisms are put together, they can be stretched into new niches...sometimes literally. :D

I really like how many different ways there are for Furahan organisms to reach their food in the heights of the flora (these here, the earlier rusps with reduced mid-limbs, for two sets of examples)

Thank you very much.

Sigmund Nastrazzurro said...

Idle: those are interesting ideas! I could fill a second Book with such ideas...
I know some animations aren't working. This is irritating because they used to work fine: it is the browsers that no longer do their job, not the animations. Sigh...

Keenir: I hadn't even realised that that was a theme in my work! Thanks for that observation.

Akavakaku said...

Regarding asymmetry in vertebrate skeletons, there are a few examples, particularly in skulls: Flatfish have very asymmetrical skulls. Crossbills have asymmetrical beaks so that the beak tips cross each other. Odontocetes have asymmetrical skulls as an adaptation for echolocation. Owls have asymmetrical ears so each ear can hear a different frequency. Asian snail-eating snakes have asymmetrical teeth because they eat asymmetrical snails. And for a postcranial example, the plates of Stegosaurus were asymmetrical and alternating.

Akavakaku said...

(Just leaving another comment so that I can be notified of replies.)

Sigmund Nastrazzurro said...

Akavakaku: those are very good examples. Perhaps the 'rule' here is that symmetry is strongly preserved for locomotor structures but less so for nonmotor structures.

Abbydon said...

It's just a guess but I would imagine that symmetry would be reinforced when the evolutionary pressure(s) linked to that trait are also symmetric. For example, deliberately moving from A to B could be done with an asymmetric body plan through careful orientation, though perhaps with greater difficulty. However, avoiding predators would be reactive and asymmetry would effectively produce a direction where avoidance is harder in the time available. Evolution would presumably therefore act to remove the "weak point" by selecting for symmetry.

Are predators more likely to be be asymmetric than prey animals?

Sigmund Nastrazzurro said...

Abbydon: are predators more likely to be asymmetric? Oh dear... Here are my running comments:
If you are a predator running after prey, it will try to swerve out of the way, and then it would not be good to only catch it when it runs towards the left. Under such circumstances it must pay to be able to be equally agile in all directions.
I had this image of a lion or cat using just one paw to catch prey, which sounds like a poor idea. Even so, cats and dos do have a preference for one side (there is 'pawedness') L
In this case, the asymmetry is one of function, not of anatomy, which ny itslef adds a layer of complexity. Would cats be better hunters if they were ambidextrous? No idea, but they are pretty efficient as they are; perhaps they are 'efficient enough'.
Now upoose a predator sitting on a branch, waiting for a fish to swim by so it can be speared. In that case it would not matter much if the predator had only one spear instead of two, because the chances of success would not be increased significantly by having gtwo spears. P:erhaps the mode of catching prey is moost important here.

Petr said...

Hello again!
I know this is going to seem like a random comment out of nowhere but I still keep coming back to this blog and still enjoy your project thoroughly, especially your commitment to plausibility and real world biomechanics.

This blog has reinvigorated my interest in spec evo and science in general when I had first discovered it and I still love coming back.

I popped back in to see what's new and just wanted to say how incredibly happy it makes me to know that this project is still receiving updated.

Just wanted to say that asymmetry is not the only option if it's not something that you have decided on as a quirk of scalates, but there can be more knobs and bumps on a bone than one, especially on alien bones. And backbone elements are probably the knobbliest bits there are in any skeleton. No reason to restrict yourself to just one unless that's something you specifically want to do.

You could also imagine each neck segment as having a single neural spine in the front and two in the back, and having the one slot between the two in flexion. If the basic structure was inspired by a "ladder" where the sides have fused, the midline neural spine could be explained as further fusion of the left and right skeletal element.

Furaha is still one of my favorite alien biosphere projects and The Book is still on my wishlist.
Good luck and happy 2024!

Sigmund Nastrazzurro said...

Hi Petr! Very good to hear from you. Do you still fold (origami)?

As for the spines, I have in fact sketches of something looking very much like you describe. At one point the distal spine was on the left and the proximal one the right, so they could slide past one another, much like you describe.