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I need to add a bit of descriptive detail on a species from a brief description. Essentially add a bit of finer detail to someone else's broad-strokes worldbuilding. An amphibious species' body layout is described as having

one pair of hands, one pair of legs, and a pair of intermediate limbs that could be used for either walking, swimming, or manipulation, as circumstances demanded.

Assuming the species' general layout is horizontal, and that it spent some time in the water and some on land, from the point of view of plausible evolutionary and other biological considerations, what are the plausible orders of these limb pairs?

At first I thought that the limbs are listed in a head-to-tail order, but the placement of the most versatile limbs near the tail, and the proposition of manipulation-capability on rearmost limbs but not the central ones, made me wonder if that's not how it should be. Are there good biological/evolutionary/etc. arguments in favour of either the listed order or some other order of limb placement?

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  • $\begingroup$ Horizontal animals, like you call them, don't show a distinction between hands and feet $\endgroup$ – L.Dutch - Reinstate Monica Jun 20 '19 at 8:45
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    $\begingroup$ @L.Dutch In that case what would be a proper term for an animal like a raccoon or elephant, that uses four (or more) limbs for support/mobility when moving quickly, and has a horizontal spine/chord, but either has a separate limb for manipulation (elephant) or uses its front limbs for it when it slows down (raccoon)? $\endgroup$ – vicky_molokh Jun 20 '19 at 8:48
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Limbs can be used either for manipulation or for locomotion.

Manipulation is better carried out near eyes and mouth, as it serves for either inspection/interaction or alimentation. This poses a constraints on having the manipulation appendices in the front, if with front are identify the part where the head is.

From this it follows that the locomotion is left for the middle/hinder section. Think of something like a crab, a spider or an an insect.

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The most logical order (assuming the limbs are roughly the same size and are analogous to the limbs of current animals) is to have the manipulation capable limbs near the mouth and the intermediate (capable of water propulsion) limbs at the back, leaving the leg analogues in the middle.

The reasons for this are simple: flippers are most efficiently used when ‘waved’ like the fluke of a whale’s tail. To do that well you need them to be free of obstruction, hence at the rear.

Manipulation is a very useful trait for accessing food but requires decent sensory input to be effective. Food is (I presume) ingested at the head end, so having the manipulators near the eyes/nose/mouth combo also makes sense. It’s much simpler to get a potential food item, manipulate it to liberate the food and then eat the food if your hands are near your face.

As evolution is strongly affected by energy efficiency (more energy for mating/killing if you don’t waste it elsewhere) it will tend to favour energy efficient solutions. Hence: fins at the back, hands at the front.

But since this is evolution we’re talking about you can posit damn near anything you want and justify it. If your creatures use their tails for propulsion and flippers for incredible directional control (like penguins) then hands/flippers/legs might make more sense. If they had an intermediate stage where the hands were used to feed their young in a pouch then legs/hands/flippers might make more sense. If they evolved over a long time scale then anything goes, really. Humans have some nerves that do loop-the-loops because our evolutionary path was long and complicated, and our anatomy is flat out wrong for bipedal living, yet here we are.

So really: whatever order suits you. Justify it later.

EDIT: just noticed the intermediated are also manipulation capable. My answer remains the same, but now you treat the creature more as a chimp than a humanoid. The legs are probably going to end up vestigial unless they’re essential for balance. The exact details very much depend on the complexity of the things they’re manipulating vs the amount and style of swimming they do though, so the second part of my answer is even more relevant.

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