Upper limb disposition for a mammal-like alien with radial symmetry?

Question

I am working on an alien species with mammal-like features and characterized by radial symmetry, in opposition to bilateral. They have four lower limbs arranged in a square-like disposition, with the palms all facing outwards(they have ape-like hands), sustaining a central torso which grows out of the junction; the animal is meant to have a pretty heavy build.

In order for it to really be able to functionally operate with a 360 degrees angle, I need the head and arms to be able to move independently of the central torso, given that full torso rotational capacity is out of the question, and it's not even that pretty.

The head problem is solved by owl-like adaptation, only amped up. But I am not sure how to go about the upper limbs matter.

I have thought of simply giving them four arms, and possibly of extending that to having two sets of forward facing eyes on the "front" and "back" of the head, which would allow the functionality provided by radial symmetry to be maintained with a "static" configuration, rather than one requiring active movement on the part of the animal.

Yet, that is not really the concept I had started with, and I would like the animal to only have 6 limbs overall, since with eight I feel it's already losing too much of the mammalian "feel"; in the end it will come down to how much I want to adhere to "science", but I want to listen to this "side of the equation" and consequently I am very interested about your opinions : would it be possible for the upper limb junctions of this animal to roate in such a way that full functionality is always maintained?

The only thing I can think of is that it may perhaps be able to fully rotate its arm at the shoulder junction, and given that the alien has its five fingers pentagonally arranged around its palm, the problem of the human thumbs changing place is not there.

I am not asking about the likelihood of such a being evolving given our current knowledge of the evolution of life on one single planet, yet that's where we must start, of course. It is clear, however, that out-of-the-box thinking has to be used in these matters.

And besides, would an arachnid-like leg structure (and consequent gait) be feasible for such an animal (7-15 tons range)?

Any insights?

Edit : I would also add, I mean the upper limbs to be articulated. I have thought of the possiblity of them having proboscis-like limbs, which would indeed solve the problem of rotation given that there would be no junction, but the fact of the matter is that, again, it doesn't fit with the "feel" I want the alien to stimulate. Besides, in the same story there are already creatures which use such a tentacular configuration, and I'd rather avoid redundancy.

Answer 1

Given the constraints, I might suggest looking at it a bit differently.

The creature is large and heavy, so needs trunk like supporting limbs, If all six limbs are used to support the ancestral beast, then the individual limbs can be smaller, which allows for more flexible limbs and gives evolution a better starting point for articulation.

So we start with a circular body plan with six limbs arranged radially around it, placed underneath to support the bulk of the animal. Since you already want it to be radially symmetrical and move in all directions, the limbs will need some unusual articulation and flexibility, which is good, since the "hip" joint will need to have a wide range of motion. The "feet" will need to have toes which can evolve to become fingers, which is going to be difficult since they have to support a great deal of weight. On earth large animals have either shrunk their toes or evolved hooves, but we can go by the example of the great apes and have the fingers/toes rolled up in motion so the creature walks on its knuckles.

The complex articulation of the "hip" joints allows the creature to lift one or perhaps two limbs at a time above the center line so the hand can come into view and be used. Initially, this would be for behaviours like grooming and feeding (bringing food to the mouth), but as the creature evolves finer motor control and more adept articulation of the fingers can take place to allow for simple and then complex tool usage, hand signing and other complex behaviours. While it may be possible for this to happen on all six limbs, specialization reduces the amount of resources that the creature has to devote to this (we already need a lot of brainpower for intelligent behaviour, the amount of brainpower to control six fully articulated limbs/hands and be able to balance and move might be overwhelming, leaving the creature vulnerable to other ill effects ranging from mental illness to being so focused on limb use that it is vulnerable to predation to being less capable of hunting or foraging).

So the body plan would be generally disc or hemispherical in shape, with six very complex hip joints equally spaced around, allowing the limbs to swing up from a pillar like standing stance to over the centerline and reach the top part of the animal for grooming and feeding.

Answer 2

Keep in mind several things:

• Square-cube law - Spider-like limbs are going to be increasingly brittle and unable to hold weight the bigger the creature gets, which is likely why we don't see many creatures with such limbs beyond a certain size. If you look at elephants, they are incapable of jumping and they have padded feet to soften each step.
• Features generally serve a purpose from an evolutionary standpoint. This isn't to say that there aren't things which serve no purpose (like our appendix for instance), however they also don't get in the way. A radial symmetry would likely serve some purpose. Perhaps such a creature can move faster this way. If this is the case, it would likely move like a wheel, and its limbs would end in claws to grip into the dirt to propel it forward.
• Correllary to the point above, you don't see features which aren't useful for a creature specialized in a certain role. If this creature can move quickly, they aren't going to use a carpace (think of a cheetah). If it is an herbivore, then it moves fast to run away from predators, and inversely if it is a carnivore, then it moves fast to catch prey. In other words, don't give an herbivore pointy teeth.
• Symmetry for creatures on earth is external, meaning that we don't always see two of every type of organ in our bodies. This implies that evolution doesn't care much about symmetry beyond balance and durability, so this creature doesn't have to be radially symmetric in every way.

Given this, I don't think you would see a torso attached to a wheel-type lower body with limbs without a matching part on the other side to counter-balance. You would likely see eyes or sensor organs on both sides as well. You wouldn't see a torso attached to a wheel-type lower body with the torso on top, because there'd be no reason for radial symmetry. This would present a problem, because it isn't a "do nothing" feature. It would get in the way of movement.

I hope that helps.

Answer 3

And besides, would an arachnid-like leg structure (and consequent gait) be feasible for such an animal (7-15 tons range)?

No. The legs need to be straight vertical under the body, as with an elephant.