Hexapod evolution in a twist

Just a short post this time, as I am busy painting. The painting in question involves the early evolution of hexapods, something also discussed in a post titled 'The lateral fin theory and mackerel mode'.  

Click to enlarge; copyright Gert van Dijk

Above you see the specimen of 'Fishes II' that was shown in the previous post. It was digitally sculpted and painted in Sculptris. Such sculpts help define the perspective of the undulating fins. Once you have such a shape in your computer, you can go two ways: the first is to  perfect digital sculpting, which at present probably means mastering ZBrush. That road does result in a 2D image, taken as a snapshot of the model, but do do that the models need to be sculpted with much more finesse that the rough ones I produce. Readers of this blog will know the work of Marc Boulay, who does all this at the expert level.

But I chose to stick with regular figurative painting, because there is something about a painterly look that I like. It is not that easy to define 'figurative painting' in such a way that it excludedes digital sculpting. Perhaps it is creating the illusion of a three-dimensional object by placing colours on a two-dimensional surface. This includes digital painting as well as classical painting using oils or water colours or any such technique (I sometimes encounter a resistance against digital paintings in art circles, which must mean they see it something else than it is: just another technique).

In this process, 3D sculpt programs are aids to get the perspective or the lighting right. As with any technique they have their own unique problems. People will accept any perspective on a photograph or computer rendering, but not on a drawing (see here for an explanation).

Click to enlarge; copyright Gert van Dijk

Anyway, I try to produce a painterly effect. The two images above show two versions of the head of the Fishes II species Vexilloscissus. The left one was based on the 3D sculpt. I thought the painting was finished, and suddenly realised that there was no way that the six protojaws seen here could evolve into the typical four jaws of the basic terrestrial hexapod Bauplan. That design involves upper and lower jaws with two rows of teeth each, and two lateral jaws with one row each.  While sculpting I had forgotten that, so I had rotated the ensemble of six jaws incorrectly, with jaws in the midline in the upper and lower positions, and no jaw in the lateral positions. In world building it is hard to keep tracks of all the details, or at least that is my excuse for the mistake.  

So I had to erase the jaws and paint them again in the correct position. The result is at the right. It's a  pity really, as I rather preferred the left one. Oh well, never mind...      

Ballonts in Gas Giants ('Ballonts V')

More ballonts? Well, yes: I had previously explored whether it is possible to produce a fairly small life form floating around using the lighter-than-air mechanism, but there were some loose ends left. As the last one was posted in 2011, it may be wise to recapitulate a bit (or work your way up from here, through here, to this one).

Click to enlarge; copyright Gert van Dijk

The image above show a scene on Earth on sea level at about 20 degrees Centigrade. A default local sophont (let's call him 'Julius') holds a stick indicating two meters. There is also a balloon with a radius of 62.03 cm. Why 62 cm? Because that yields a sphere with a volume of exactly one cubic meter (m^3). The skin is made of a 0.1 mm thick mylar-like material with a mass of 0.5802 kg. The balloon is filled with the lightest possible gas, hydrogen. Hydrogen has a density of about 0.0899 kg/m^3 at 20 degrees, while the air has a density of 1.2019 kg/m^3. So, the 1 m^3 balloon has 0.0899 kg of hydrogen in it, while the corresponding volume of air has a mass of 1.2019 kg. The balloon can therefore lift 1.2019-0.0899 = 1.1120 kg (that is the part needed to understand how balloons work). As the skin masses 0.5802 kg, that leaves 1.1120-0.5802 = 0.5318 kg to build a nice body out of. That is not a nice big body at all; given a body density of 1.1 kg/m^3, which is like our bodies a bit heavier than water, we can hang a spherical body with a radius of just 4.9 cm under our balloon, and the ensemble will then just float. Of course, a real animal would have tentacles and limbs and mouthpieces etc.

As said, I wanted ballonts with a body mass of, say, 10 kg but with only a moderately sized sac. As the example above shows that does not work on Earth. The hydrogen inside the balloon cannot be made lighter, but we can alter the atmosphere outside it; this is speculative biology after all. There are two ways of doing so: the first is to stuff the atmosphere with very heavy gases such as argon, but such elements are quite rare in the universe. The other is to add mass by increasing pressure, as that will squeeze more mass in the same volume. So, let's explore gas giants, where high pressures are easily found.

Click to enlarge; Source: Brian Vanderwende University Colorado

The pictures above show information about 'our' gas giants: the composition of the atmosphere, the temperature and the pressure. Not surprisingly, atmospheric pressure increases the deeper you descend into the atmosphere. For our first try, we should perhaps be a bit conservative and stay with biology in fluid water. A temperature of 20 degree centigrade should not upset Julius; it is the same as 293 degrees Kelvin. For Jupiter, the 293 Kelvin zone results in an atmospheric pressure of some 9-10 times that of Earth, which sounds like a decent start. Instead of jumping in directly, it may be easier to take it in stages, building on the Earth model shown above.

Click to enlarge; copyright Gert van Dijk

The image above shows the first step: Earth's atmosphere is changed to a Jovian one at one atmosphere and 20 degrees centigrade. Internet sources show that the Jovian atmosphere consists of about 86% hydrogen, 14% helium and a smattering of other compounds. Based on the densities of hydrogen (0.0899 kg/m^3) and helium (0.1664 kg/m^3) the density of a 86:14 hydrogen/helium mixture should be 0.1006 kg/m^3. Oops! That is only very slightly denser than pure hydrogen, which we need to fill the ballont with! If you thought Earth air was a bad medium for ballonts, think again. So what are the effects? Well, the liftable mass is 0.1006-0.0899= 0.0107 kg. Remember that the skin had a mass of 0.5802 kg? There's nothing left for a body, so this balloon is not getting off the ground at all.

Click to enlarge; copyright Gert van Dijk

We were aiming for high pressures, so let's increase the pressure to 10 atmospheres. The mass in the balloon will be 10 times higher, and so will the mass of the equivalent volume of air. So the liftable mass also becomes 10 times larger: 10 x 0.0107= 0.107 g. That's still nowhere near the mass of the skin, so this balloon isn't going up either.

Click to enlarge; copyright Gert van Dijk

Let's leave Jupiter and find a ballont-friendlier place. Uranus and Neptune have atmospheric pressures about 50 times Earth's at the 293 Kelvin range. That's better, and apparently the Uranian atmosphere is heavier, with 2.3% methane thrown in. I make the density of its mixture to be 0.1148 kg/m^3 at 1 atmosphere and at 20 degrees C. So, the 1 m^3 balloon can lift 0.1148-0.0899 =0.0249 kg. That is not good enough, but at 50 atmospheres the liftable mass is 50 times that, or 1.2450 kg. Subtracting the skin leaves 0.6648 kg. Finally, a floating balloon! Hurrah!

Or perhaps not 'hurrah', as that is only a tiny bit more than what we had on Earth to start with... Let's go up to 200 atmospheres in Uranus: the liftable mass, skin already subtracted, would be 4.4 kg, and at 500 atmospheres it would be 11.9 kg. Finally we have what we wanted!

Well, not really; these values are not yet adapted for the lower temperature. Julius is left behind, as we need a wholly new biochemistry. The atmosphere is now also so soupy that you would not want to think about the wind or moving in it. Adding even more problems, there is another potential disaster lurking in these gas giants: gravity. The gravity constant for Uranus is nice at 8.85 m.s^-2, a bit less than Earth's at 9.8 m.s^-2. But Jupiter has a value of over 25, so if you thought you could get away with a nice fragile ballont there, waving its slight tendrils through the air and looping in prey with slender tentacles, think again: the animal would need the sturdy limbs befitting a 2.5G environment.

It really does seem as if the universe is trying to sabotage ballonts, doesn't it? Gas giants do have high atmospheric pressures, but their beneficial effects are counteracted by the fact that the atmospheres consist of very light elements. It seems that the only way to get a viable (pun intended) ballont on a Jovian planet is to make the ballont extremely large. But that is where we started... I am beginning to think that there may not be any appreciable advantage in locating ballonts in gas giants, even though science fiction is full of them. They do about as poorly there as they do on terrestrial planets, meaning they can in fact work, but they have to be big, very big. Perhaps gas giants have other advantages for ballonts: there's certainly a lot of atmosphere to play with in them.

Ca I still claim that ballonts are so common in gas giants that they are boring? Yes, but they will be big, as usual; perhaps that's what makes them boring. The best way out for small ballonts seems to be offered by terrrestrial planets with heavy gases and high pressures: Venusian analogues? Perhaps there will be a 'Ballonts VI', one day.   

Evolving another aggie

Click to enlarge; copyright Gert van Dijk

What you see here is a sketch of the 'aggie', the favourite prey of the marblebill. The pages detailing the marblebill in The Book have this to say regarding the aggie:

"The marblebill’s favourite prey is the ‘Aggie’ (Agitator augur), a tree-dwelling fructivore. Once caught, the victim’s feeble attempts at defence have little chance of success against the marblebill’s armoured chest and abdomen. There is little time for resistance anyway, as marblebills usually disarm their victims quickly by snapping its cervical medullae."  

 "A troop of Aggies, admittedly not the brightest of beasts, may suddenly see a branch swaying and a baignac falling. Only when they hear the marblebill’s triumphant howl does it dawn upon them that one of their comrades had just now been sitting on that branch and been munching that baignac."

That's all that is known in the entire universe regarding the aggie. I have started sketching them several times, but was never too happy with the result. The sketch you see here is not the definitive aggie either. This particular one is a brachiator, just like the marblebill. The degree of adaptation of the marblebill to its arboreal brachiating life style suggests that its environment has been around for quite a while. If so, other species could be equally well adapted to an arboreal way of life. That does not necessarily imply brachiation (see here and here); the animal could be a jumper, a climber, or even a glider. But this one is a hexapod brachiator.

Click to enlarge; copyright Gert van Dijk

In previous versions I toyed with the idea of using the second pair of legs as the main. In fact, here is an old very quick and dirty sketch showing that approach (Brynn Metheny also did one once, the 'pygmy esorifleu', which I discussed previously). In the 'Mark I' the body is suspended from the middle limbs, and the front and aft ends hang down. It must have evolved from basic hexapod stock, and it is hard to imagine an ancestral species with six more or less equally-sized legs preferring to grasp branches with its second rather than its first pair of limbs. You can see the 'Mark II' next to it. That sketch was ancestral to the marblebill's design, and they still swing from their front limbs.

Click to enlarge; copyright Gert van Dijk

Still, there may be a way to evolve a brachiator with 'middle limb suspension'; take a typical Furahan neocarnivore, one of those animals exhibiting centaurism. As you may remember their first pair of legs are not used for locomotion but to catch prey. If such an animal started climbing trees, it might keep its weapons intact, and adapt its second and third pair of legs for locomotion among the branches. Its offspring could become either become jumpers of climbers, using four more or less equal limbs, but they could also turn into brachiators. If so, they would swing from their middle limbs and use their front legs as weapons. In my mind, I  see the hexapod evolutionary tree sprouting a new branch even while I am writing this...
Then again, a neocarnivore taking to the trees might use its spears or clubs to hook a branch. Being at the front of the body they are well placed to do so. If these limbs then become brachiating arms they would resume a locomotor function again; I see another evolutionary branch exploding into view with an almost audible 'whoomph'. By the way, that latter branch is also the first official example of 'decentaurism', or the reversal of nonlocomotor limb use to a secondary locomotor purpose.

Anyway, back to the aggie. Have a look at some of its features.

• It sits upright, which may make sense for a brachiator: its body is held vertical while brachiating, and it might easily keep doing so at rest.
• Its limbs are attached to the body with joints that allow three axes of rotation. The brachiating arms are attached to the body through a short bone that ends at the 'shoulder'. Unlike Earth primates, the shoulder girdle is attached through bones to the axial body skeleton rather than through muscles only, but the animal still needs thick muscles to control the position of the body with regard to the arm. The unfortunate result is that the attachment looks much like a primate shoulder girdle; parallel evolution or a limit of my imagination?
• You might just make out the ancestral hexapod toe branching pattern (more about that here). 
• This particular aggie version has a pot belly. While sketching it I had forgotten about it being a fructivore with a preference for baignacs (remind me to show you a baignac one of these days). Fruits usually offer high quality food, so animals does not need many of them. While sketching I had low-grade food in mind, say fibrous leaves, and such food requires a lot of processing and a sizable gut. Specialising on low-grade food has the advantage that there will not be much competition, but the end point might be a slow animal that is not at all energetic: something like an Earth sloth. While sloths are preyed upon by harpy eagles, the dense parts of the forests are probably closed to eagles. But introduce the marblebill, and anything as slow as a sloth has a problem. So, the aggie cannot be too slow. It should probably lose its potbelly and resume a high-energy fruit diet. Of course, it should perhaps be better able to defend itself, or use its social skills, or perhaps...

...never mind; thinking about the aggie has once more led to interesting predators rather than their prey. One of these days I will design the definitive aggie; this is not yet it.