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.