Those will have to wait; instead, here is a short post on where the six limbs on Furahan hexapods came from. On the 'real life' level the answer is easy: 'six limbs will look exotic and therefore help create an alien ambiance'. Within the Furahan world, the logic of science fiction demands an answer that fits within the concept.
Click to enlarge; copyright Gert van Dijk
The sketch above is an old one, and the first that showed the first steps in hexapod ancestry. By now, many of the anatomical features are being overhauled, so the number of eyes is incorrect. The overall scheme is still there though: it all starts with a less than impressive little elongated tube with broad fins at its sides. This 'ULF' (unassuming life form) swims by waves that pass from front to back along the fins. Nothing particularly spectacular here: undulating fins may well be a constant throughout the universe. From that start I assumed that the fins might be divided gradually, to provide greater control and flexibility (you cannot suddenly move a part in one direction if it is fixed to parts in front and behind of it). This greater need for manoeuvrability evolved together with jaws; you cold also say that the jaws and the fins helped one another's evolution: without jaws, there is no speed, and without a better propulsion, the jaws do not provide that much benefit. The third stage shows an animal with fully separated fins, just not necessarily six of them; the reduction to six came afterwards.
The origin of hexapod fins therefore lay in a lateral membrane that split up. Many years later I wondered where Earth vertebrate limbs originated. To my very large surprise, the first explanation I came across was something called the 'lateral-fin theory'.
Click to enlarge; Coates MI. The origin of vertebrate limbs. Development 1994; Supplement 169-180
The images above show an example of what a hypothetical vertebrate ancestor was supposed to look like: it already had unpaired fins along its back and belly, and lateral (sideways) fins along its sides. The theory, apparently first formulated in 1877, states that these lateral fins later gave rise to limbs. I was first a bit irritated, but later pleased that I had stumbled on a principle that apparently was not altogether fictional. That was until I went back for a closer look at current theories regarding vertebrate limbs. It would appear that the lateral-fin theory is now out of date. Other theories held that limbs evolved out of gill branches, which seemed to make sense as the first forelimbs were attached directly to the skull. That theory apparently also now belongs in the dustbin of history.
Click to enlarge; Coates MI, Cohn MJ. Fins, limbs, and tails: outgrowths and axial patterning in vertebrate evolution. BioEssays 20:371–381, 1998
The image above shows an illustration from a recent paper on limb development. It shows that that unpaired fins in the midline ('median fins') existed well before vertebrates had jaws or lateral fins/limb. When lateral fins appeared, the first to appear were the front pair, with as yet no trace of hind limbs. The two pairs did not evolve together, which you would think, given their similarities. Modern discoveries in the field of 'evo-devo' ( embryonic development in light of evolution) centres on hox genes as a sort of overall conductors of embryo formation. A recent theory holds that the genes responsible for limb formation were co-opted from a previous use, one that involved formation of the gut through the 'lateral plate mesoderm'. The paper from which the image above was taken mentioned the possibility of a third pair of limbs in vertebrates, the kind of nice exotic happening that we like in speculative biology. Here is what Coates and Cohn wrote:
"Finally, the absence of vertebrates with more than two sets of paired appendages has often been used as an illustration of evolutionary constraint. Developmental mechanisms responsible for this anatomical limitation remain unclear. Arguably, the nearest approach to a third pair of lateral appendages may be the lateral caudal keels of certain fishes, such as tuna and various sharks."
So there are in fact three pairs of lateral, well, outgrowths in vertebrates? Fascinating. But the test continues:
"Even the most elongate lateral fins of primitive fishes terminate in front of the anal level. Clearly, lateral caudal keels can and do emerge, but articulated endoskeletal paired appendages require the lateral plate mesoderm, and this is linked intimately to the extent and pattern of the gut."
Curiouser and curiouser. It does not look as if vertebrates will surprises us by evolving a third pair of legs, though. For three pairs of legs, you need to turn to insects, and for big hexapods, there is always the fictional universe.
But does all this mean I should give up on my 'lateral fin theory'. Actually, I see no reason to do so. In fact, it is rather nice that the lateral fin theory remains in place on Furaha, as the explanation of the origin of six legs in Furahan hexapods.
Click to enlarge; copyright Gert van Dijk
To celebrate that I stole another two hours and used Sculptris to sculpt two quick ULFs. The first is shown above: no jaws, four eyes, two lateral undulating membranes and two long gill tubes running along the belly connected to the sea by a number of spiraculae. I still need to name it, and I think I need something that does justice to its pivotal position in evolution. Suggestions are welcome. Latin or Greek only though, please.
Click to enlarge; copyright Gert van Dijk
And here is its successor. As you can see, the membrane has developed indentations and the animal is longer and bigger. It has six claspers in front that can already deal with soft prey quite well. Note that the body is stiff, very unlike the very flexible body in the old sketch. The body can flex up and down, but sideways movement are almost impossible, thanks to he two stiffening rods that lie buried in the body at the root of each lateral fin. The stiffness is a consequence of this early body plane, and is a feature of all later hexapods.
Click to enlarge; copyright Gert van Dijk
I could not resist quickly daubing one in Sculptris with colour to show one in 'mackerel mode' (I still prefer painting, but the 3D process certainly is a very quick way of producing an illustration).
7 comments:
Interesting- I like the explanation that the ULFs' undulating fins gradually split into six separate fins. It is a very nice, plausible explanation for the Furahan hexapods. :-)
Speaking of exotic life forms, though, what do you suppose is the limit on the number of limbs an animal may possess? Six works, but what about eight or ten? Would twelve limbs be too many? At some point, our attempts to create exotic alien creatures may reach the land of silliness...
Hello Christopher,
That is hard to say. On Earth the only really large animals are vertebrates, and their number of walking legs is two or four (well, perhaps five f you count the kangaroo's tail). A large number makes jumping difficult if not impossible, but very large animals (elephants, probably sauropods) do not jump anyway. As I hypothesised earlier in my discussion on rusps, a large number offers some protection against injuries. Beyond (perhaps) eight legs that additional insurance is ensured, so more legs probably have little added value.
For small animals the cost of legs are probably relatively less, so the rules are probably different there.
The subject of this post has got me thinking about the spine of a creature, specifically how its orientation may affect the physiology of the lineage. In the pictures, we can see that the ULF and its descendants undulate vertically as they swim, using their lateral fins from side to side; the example of Earth lineage expresses a lateral undulation, moving the body-- or primarily the tail-- to the left and right in order to move forward.
The question then arises of how those differing arrangements may influence the behavior of the different lineages. For example, due to the vertically aligned structure of its skeleton, an Earth fish can more easily turn to the left or right to avoid danger than up or down, which take a little more effort from maneuvering fins to go in those directions. Does the horizontally aligned Furahan model have similar limitations, but in the reverse? Does a ULF-derived swimmer have no problems swerving up or down, but because of whatever structures may be in place to support and power the lateral fins, it struggles to turn left or right as easily? Perhaps the best way to investigate whether or not this is the case is to look at Earth's examples of swimmers that use vertical undulation, such as dolphins and whales...
But presuming such a hypothesis is true, and the creature's spinal orientation affects other aspects of its physiology, we can start to extrapolate. For example, we see that Earth fish have their eyes set on either side of their head, which possibly lets them more clearly see which way they want to turn. Furahan swimmers have four eyes, which may help solve problems of seeing where to go, but would we not potentially see a "pairing" of eyes above and below the mouth? Or would we see a more terrestrial arrangement, with a "pairing" on either side of it? Or, ultimately, is my hypothesis flawed at some point and irrelevant to a creature's biology and behavior?
So far as naming the ULF is concerned, I think a lot of it depends on exactly what the focus is. Is it the lateral fins? If so, it could be called something like gemellopinna or didymopteryga. If there's some other feature that is innovative, like the multiple spiracles, then trypanapna might work. Of course, you're more familiar with Greek and Latin grammar than myself, so you may recognize some incorrect usage in those names. In any case, that's the direction I would go as far as names for the ULF.
Hello Evan,
Your line of reasoning is what set me on the path of hexapod evolution. Starting with two parallel chains of bones at the bases of the lateral fins, you wind up with animals with structures equivalent to two paired spinal columns. As the limbs are attached to these structures I considered them unlikely to be replaced by a single column. The chains are connected side to side, so the result is a bit like a rope ladder, but with bone segments instead of ropes. You can easily bend the structure up and down, and can twist it lengthwise as well, but it does not bend readily from side to side. No hexapods are therefore good at bending laterally.
Does that matter much? Not in the sea, and probably not that much on land either. At nay rate is is a fixture of hexapod evolution.
The big difference with the spinal column is that on Furaha the structure is limited by its dual nature. On Earth there is one spinal column, and that can evolve to bend laterally, dorsoventrally of in both direction. Many fish seem to be limited to sideways movements, and interestingly the spine in many large mammals is either very stiff or it does NOT move sideways. As far as I know whales descended from creatures that like otters swam with their hind legs in unison; that up and down motion lended itself well to the development of a fluked tail. In contrast, seal ancestors used a kicking movement and that got translated into sideways leg/hind flipper movements.
As far as the names are concerned, I like Trypanapna and may use it, or a derivative. I will get back on that.
Ah, I see. That ladder-like twin spine structure is something that I've considered exploring (though not in Nereus) and I'm interested to see how that's played out on Furaha. Does a twin spine system have any neurological implications, or is Furahan neurology not so closely related with spinal columns as it is in vertebrates?
And the limitation on lateral movement is really the only problem with such a skeleton, and like you said it's really not that much of a problem. If a species really needed to turn left and right with ease they might be able to orient themselves on their sides, like how a flounder picks a side and its body adapts to that new orientation. Of course, I can't really think of any situation where a Furahan creature would need to make such an adaptation, but I think the possibility is there.
Evan: that is right. I did think of some predatory Furahan Fishes that might rotate to one side, bend their body in that direction, and rotate back gain to facilitate very fast turns, but generally that is not necessary.
The stiffness also explains why Fishes VI have a tail (fused flippers) that beats up and down.
The nervous system is not enclosed in the spinal column analogues. I have always wondered what the mechanical consequences would be of not having to design a vertebral column with a tube running down its length.
Wow. Such an amazing post. I am so sorry I couldn't visit this website regularly, school has intervened.
All I wanted to say is that I love your project and I really enjoy seeing it grow and expand. =)
I'll now proceed to read the more recent posts, I feel like it's early christmas for me!
=)
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