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Arthropods and tetrapods on Earth, though they differ in many areas, both have their legs attached on their underside. This arrangement has been slightly changed in some species to have legs coming out of the sides, but never from the back. Is there any reason why a creature would not be able to colonise land with legs attached at the back?

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  • $\begingroup$ because then the back would become the new front? $\endgroup$
    – L.Dutch
    Aug 29 '21 at 8:40
  • $\begingroup$ @PcMan What would be competing with them? If we assume that the organism can't flip itself for whatever reason (such as the breathing holes being on the top of the head), then the entire alt-fishapod lineage would be of the dorsal-legged variety $\endgroup$ Aug 29 '21 at 8:55
  • $\begingroup$ your real problem is the more they rotate the legs to the side the greater their range of motion and the longer the legs are functionally (or they can get shorter for the same function) so there is a huge evolutionary advantage to move the legs to the side and eventually all the way around to the underside. $\endgroup$
    – John
    Aug 29 '21 at 15:47
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There aren’t because it burdens the joints.

Legs on the back can work for tiny creatures but as they get larger the strain on their back leg joints, the equivalent of their hips, would get more and more unbearable. This is why large things have legs underneath. Reptiles having legs on the side also works but giant sauropods still have legs underneath to carry their massive bodies. The legs on top arrangement works if the legs are strong enough and firmly attached by ligaments to carry the body like a grocery bag. You can do the opposite and have them hang on tree branches. Apes carry themselves with no problem, though they still walk with legs on land.

The problem has to be ease of movement

With straight legs underneath the body is stable, to move forward shift the weight to fall forward and catch your step, very little energy expended. For legs on the back they have to carry the whole weight of a creature at all times every single step. So really, hanging like a monkey and swinging from branch to branch seems like the best option.

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No

  • The first basic rule of evolution is that, given enough time, pretty much anything can evolve from or into anything.

  • The second basic rule of evolution is, if it's useful, it'll probably evolve.

  • The third basic rule of evolution is that need drives change.

  • The fourth basic rule of evolution is that life uses what's available to achieve the need.

Given those four rules, you have a problem. What's the point of legs on your back? A fish swimming in water that finds itself regularly trapped by low tides on soggy land would want a way to get back into the ocean. The result is that adaptive changes that facilitate using fins to move outside of water would be favored for continued life. Those fins eventually become legs.

But why would fins on the back do that when there's perfectly good fins on the front to do that?

So, maybe... yes

Let's imagine a fish that doesn't use its tail for primary motivation. In fact, let's imagine a fish that uses a pair of lovely, almost angelic fins on the "top" (aka, the "back") of the fish for primary motivation. When trapped on land due to low tides, the fish has only one option... to flip itself over and use those fins to push itself back into the ocean.

As change favors that ability, I can imagine the slowly strengthening fins developing into defensive and offensive limbs. They might even have started that way, which would underscore their plausible development into legs.

But...

But what you have is a creature that on land is vulnerable because that soft underside is exposed to the sky as it walks around. This means it would either evolve (as @L.Dutch mentions in a comment) to turn the "front" into its "back" (in which case, you have evolution as seen today, nothing actually changed), meaning eyes would move, ears would develop to reflect the position of the legs, etc.

Or the creature doesn't evolve at all as whatever develops wings first would find it a remarkably tasty and easy-to-catch meal.

Which begs the question... what are you really asking?

If you're asking if the creature in question can evolve on your fictional world, the answer is always "yes." So asking if it can seems kinda silly to me. What are you really asking? At a guess, you're asking what things would promote the evolution and what things would demote the evolution, which is why I answered the way I did.

What's your actual goal for the question?

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  • $\begingroup$ I think the question was pretty straightforward. Ichthys King just want's to know if there's a disadvantage or fatal flaw in the body plan. I doubt they thought that far when asking the question, It's a quick reality check I think. $\endgroup$ Aug 29 '21 at 19:05
  • $\begingroup$ The creature would not be belly-up, but would instead simply have the shoulders/hips higher up (further from the ground) on the body $\endgroup$ Aug 29 '21 at 20:24

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