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I want some of my vertebrates to have four limbs, and some of them to have six. Is this even possible? Would there be more four limbed creatures, more six limbed, or just an equal amount?

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    $\begingroup$ In this world of ours, we have vertebrates with no limbs (snakes), with two limbs (dolphins and kiwis) and with four. The split between the four- and six-limbed lineages would have to be very old, because it must predate the emergence of the regular cartilaginous and bony fishes. (They have the four-limbed body plan.) $\endgroup$
    – AlexP
    Sep 6 '20 at 1:43
  • $\begingroup$ Intressting link on six appendaged fish, en.m.wikipedia.org/wiki/Placodermi $\endgroup$
    – lijat
    Sep 6 '20 at 9:09
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    $\begingroup$ What reason do you have to believe that it may be impossible? I don't get the motivation for asking the question. Could you include some more information? As the comments have pointed out, trivially yes at the moment. There is no cosmic principle that makes this impossible $\endgroup$
    – Raditz_35
    Sep 6 '20 at 9:30
  • $\begingroup$ There's a James Blish short story in which a space-faring biologist expresses astonishment that a four-limbed vertebrate and a six-limbed vertebrate could evolve on the same planet (and consequently works out that the apparently four-limbed ones have another two limbs under their clothing); but AFAIR, neither the character nor Blish himself ever explained in detail the reasons for the astonishment. $\endgroup$ Sep 10 '20 at 20:58
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    $\begingroup$ Does this answer your question? Vertebrate-like creatures with more than four limbs how about this worldbuilding.stackexchange.com/questions/91000/… $\endgroup$
    – John
    Oct 20 '20 at 13:26
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Yes

It's possible for some vertebrae to be six-limbed - there's nothing restrictive about being a vertebrae that limits itself to only four limbs. The reason the vast majority of the living vertebrae you see today are four-limbed is generally believed to be because they've all evolved from a common four-limbed ancestor. However, should you wish it, there's nothing stopping you from adding a six-limbed ancestor and additionally populating the world with six-limbed vertebrae.

As for which one would have more members - well, that's largely going to be a tale of 'the early bird gets the worm', as in, the first set of species to evolve within an ecological niche will edge out all other species, so it's really up for grabs as to which has more members unless one of the ancestors shows up a good several hundred thousand years before the other.

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The easiest option to achieve that would be to just make all of your animal vertebrate life to descent from the six-limbed ancestor, with some groups of animal lineages losing their third pair of legs for various evolutionary reasons (for example something with an equine-like style of wandering the prairies would probably benefit from ditching a couple of limbs to become more aerodynamic, lightweight and able to run more efficiently with wider strides)

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