Why are all vertebrate land animals tetrapods?

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    • #1659

      Is there an inherent evolutionary advantage to having four limbs? I can see how this might be the case, being, perhaps, the best compromise between balance and mobility without tripping over surplus legs, but could there, hypothetically, be a situation in which vertebrate tripods, pentapods or hexapods, for example, might evolve?

      For that matter, why are land based invertebrates so much smaller than land based vertebrates? Once again, I can see how it might be that moluscs just wouldn’t be mobile enough to escape carnivores who might find their newfound size more appealing, but why are all land based insects and arachnids so much smaller than land based vertebrates? What evolutionary advantages are there for them to be so small? Could a situation arise, hypothetically, where this wouldn’t be the case?

    • #28374

      There are situations when the four limbs of the land vertebrates are useless, so evolution favours the economy in energy:snakes. But i can not think of a situation that will favour the appereance of hexapods. As for tripods or pentapods, the genetic structure will need to change drastically since all vertebrates are simetric.
      As for the invertebrates and the advantages of being that small. There are some advantages to being small, but the main reason why they did not grow larger and why you do not see 3 meter high spiders is that the growth of invertebrates is limitated by their locomotory sistem and their respiratory sistem. Unlike vertebrates, invertebrates have an exoskeleton which covers their body. At a larger size, this exoskeleton would become too heavy for the animal to move. Also, they relly on trachey for breathing, and oxygen can not diffuse farther than 1 inch
      A similar discussion on our forum: viewtopic.php?t=1510&highlight=insects

    • #29786
      February Beetle

      Invertabrates and advantage of being small, I remembered learning about the exoskeleton advantage. I was thinking if another advantage could have to do with a large number of offspring. The smaller you are the less enegry you must consume to live, breed, etc… so more food to go around the smaller you are! With a short life span a great number is very important. I don’t remember the huge number that insects have compared to humans, population wise.

    • #29852

      Four limbs seems to be the most evolutionary beneficial number. Over this, I doubt mobility or speed gain anything. The closest example could be some arboreal vertebrates. Their prehensile tails act as an extra limb as it gives better mobility when climbing. However, I also cannot see a situation where extra limbs would be benficial.

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