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Could it be useful for a species to evolve a single row of many legs on the underside of the body? The specific organism is amphibious, and has no paired structures on the outside. It does have unpaired fins along its midline, which would be the origin of these legs

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  • $\begingroup$ How many legs? Two? $\endgroup$ Commented Sep 15, 2021 at 20:35
  • $\begingroup$ My suspicion is that they could be made useful, even if the creature might never be a sprinter. The best way to answer it definitively would be (unfortunately) a simulation sophisticated enough to test various configurations and see if any rise above some arbitrary threshold. Get me a $200k grant and I'll get to work on that. $\endgroup$
    – John O
    Commented Sep 15, 2021 at 20:58
  • $\begingroup$ Would it depend on the direction of movement - two legs one front and one back for a creature walking sideways would effectively be the same in most ways as a "normal" bipedal creature in terms of mechanics and stability, especially if the species already has ~360 degree vision. $\endgroup$ Commented Sep 16, 2021 at 14:43
  • $\begingroup$ If your creature live on Arde, or any other planet in the Planiverse, then a single row of legs is the only possible way to do it! $\endgroup$ Commented Sep 16, 2021 at 16:35

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probably...

The biggest issue is stability. Conceptually, it looks like this viewed head-on:

simple head-on view

If it continuously tips over on its side, it's not going to survive long.

Since this creature is described as having "many legs", I'm going to assume it is proportionately long suggesting an eel with legs or a millipede with only half its legs. That form suggests a possible mitigation for the stability issue. If this creature never walks straight forward but always follows a serpentine path, the head-on via changes to something like this:

serpentine head-on

This, unfortunately, introduces an efficiency issue. continuously walking in serpentine path wastes a lot of energy when compared to going straight. So, maybe we just grow a proper foot to widen the base a bit. Now the head-on view looks like this:

I have feet!

To make this work, it'll need to evolve an ankle to handle uneven terrain. That's not impossible so it could work. But there's an arguably easier and more efficient fix. What if it pointed its center-mounted legs to alternating sides? Maybe all of the odd-numbered legs lean right, and all the even-numbered legs lean left. That gives us a head-on view like this:

angled legs

Ok, problem solved. Your amphibian can now walk on land which is a definite advantage and worth doing. It even had two different approaches so they'll probably both exist. Two species for the price of one!

But...

Nothing's that easy. Over time, our angle-leg version is going to find it more efficient to move the legs out from the center. Like this:

two rows of legs, oops!

And now, unfortunately, we've evolved two rows of legs. Oops.

conclusion

This is definitely doable but it's going to face pressure from more efficient designs (bilaterians dominate animal life forms for a reason). If this design is important to your project, I'd recommend making your setting right after a mass extinction event when evolution tries all the crazy stuff.

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YES

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and no.

Such a broad, open ended question deserves at least two hundred different answers! I'll start with the most obvious.

First, it's obvious that such an adaptation is useful. Anyone who has access to the ancient archives of the Mar Somur, the last great empire of Mars, and who has studied the remains of Martian fauna knows of the baalgar, a kind of amphibian megafauna of the equatorial regions.

Ever since the 19th century, exoarchaeologists have been aware of the complex system of canals that criss-cross the Arean landscape, monumental works of civic engineering that carried water from one place to another. It wasn't until the first aetherionauts left Earth and set foot on Mars that the true nature of the canals and creatures that called them home became known.

As the Mar Somurian archives were discovered and translated, a picture of life on Mars in the remotest of ancient times became clear. According to the Annals of Hannunna, a kind of beast had long inhabited the abandoned canals and waterways of the long dead empires of the wastelands. Long and serpentine it was, and it was described as "having but a single row of paddle-like appendages along its ventral surface.

As the beast grew into the baalgar known to the Hannunna of the last days of Mar Somur, its body had "outgrown the grand canals made by the labours of our ancestors; and its paddles were now become heavy clubbed feet." Combined with a "thick carapace of scales along its belly", the baalgar was able to rest its body along the canal walls as it used its single row of appendages to push through the murky waters and mud of the drying water courses in its endless search for sustenance.

Of course, Terran aetherionauts could only find the skeletal remains of the beasts, and were at first perplexed. Bony plates and scales they found along the margins of the canals in the thousands, along with what appeared to be a single row of legs laid neatly to one side or the other of the spinal bones.

Theorising at first that these might be some kind rib or spinous process, it was not until the textual evidence was brought to light that the natural philosophers who have braved the sere deserts that now lay the red planet waste were able to understand the true nature of the baalgar.

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