The Trench Gobbler

For once I will show a complete painting. Well, more or less. The painting in question is part of a two page spread concerning 'Fishes VI'. The six groups of 'Fishes' are part of the hexapod family tree, with Fishes I, II, III and V as the direct ancestors of terrestrial hexapods, and Fishes IV and VI as parallel aquatic groups. Mind you, I wondered about using 'Fish' instead of 'Fishes', as 'Fish' in English can be both singular as well as plural. A singular language, English.  I found that 'Fishes' can be used to describe multiple species, so that seemed the right choice.

In Fishes VI the third, i.e. the last, pair of flippers have fused to form a horizontal fluke, very much like that of whales. The problem with making 'Fish' alien is the high probability that a torpedo-like streamlined shape is rather likely to evolve as a 'universal' feature. I chose to accept that, so 'Fishes' superficially look much like Terran animals. But they share their world with cloakfish, kwals and aquatic wadudu, so there are definitely some odd shapes to be found too. And Fishes VI are not all that 'earthy': after all, they have four jaws, four eyes, their respiratory system is completely separate from the digestive tract, etc.

The painting combines several themes. I will split it in four panels that show various species of Fishes VI in 'powers of 10', meaning each species is 10 times as large as the previous one, starting at 4.5 cm. Each panel will also show the species eating, so food webs can be illustrated as well.

Click to enlarge; copyright Gert van Dijk

This is the Trench Gobbler; I haven't thought of a binomen yet. This painting forms the second panel of the four. The Gobbler is a typical deep sea species. In this biotope, the only light is that produced by lifeforms, and these are scattered far and wide. This is in fact a very barren ecosystem, which is due to the fact that it is almost entirely based on a slow and sparse trickle of organic material from above. Before anyone asks, I do not know whether there are hydrothermal vents. Animals need to conserve energy here. The water is largely still, and there is no need to swim fast habitually. Hence, there are no fast swimmers here, so there is no overriding advantage in streamlining. If the rare opportunity to catch some fresh food presents itself, it must be jumped upon, because there may not be a second chance anytime soon. These two influences together have resulted in very odd shapes, just as on Earth. The Trench Gobbler has elongated lateral jaws to grab anything possibly edible. In this image, it is attacking a tentacled creature, probably some larval Cthulhuoid. The larva has just emitted a cloud of bioluminescent ink to try to escape, a trick that seems to be working.

Click to enlarge; copyright Gert van Dijk

And here is a detail, for once at full resolution. It is fun to paint such structures, in particular the somewhat glassy structures of the teeth and fins.      

(PS: There is something wrong with my access to the main Furaha website, so I cannot update the loading screen for a while. To check for new posts you should check here directly)

The 'lateral fin theory' and mackerel mode

I still have no time for posts that take time to read, think and write; well, more than a hour or two. That is a pity, as some ideas need time to do them justice. For instance, there is a post to write on what happens is photosynthesis is less dramatically imperfect as it is on earth (see here); there is also the final chapter of the 'sight is superior series' (see here), and I have at least one other world builder's creations in mind.   

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).