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On Earth, land vertebrates generally have four limbs. Other creatures such as insects can have more limbs, but those creatures tend to be small. I'm thinking of creating an alien ecosystem with relatively large, vertebrate-like creatures (larger than insects and with some kind of endoskeleton). Is it realistic for those creatures to have more than four limbs (six is good enough) and if so, what conditions would make it more likely for such a thing to evolve? To be clear, I am not interested in four-limbed creatures evolving extra limbs (from what I've read, that would be quite difficult, perhaps impossible), but creatures that have six or more limbs from their common ancestor.

I have seen some information on this topic, such as:

Why would an animal need six legs?

tvtropes.org/pmwiki/pmwiki.php/Main/VertebrateWithExtraLimbs

planetfuraha.blogspot.com/2010/02/avatars-walking-with-hexapods-dont-walk.html

www.xenology.info/Xeno/11.3.2.htm

What seems to be missing much of the time, however, is numerical calculations. I would like to have, as much as possible, hard data on aspects such as stability (especially under different gravity conditions), a nervous system's ability to control multiple limbs, and the energy cost of extra limbs. Another issue that seems to be dealt with even less often is the number of fins that is useful for a fish or fish-like creature. As land-dwelling vertebrates on Earth evolved from a four-finned creature, and some similar process may happen elsewhere, especially on Earth-like planets, data about the usefulness of more fins to a fish-like creature should be useful as well.

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    $\begingroup$ Asking for hard science on an evolution question might take a long time for someone to come along with a suitably rigorous answer, and I'm not sure that some of the hard numbers you're asking for exist even for species that we're all familiar with, let alone a hypothetical xenoevolutionary tree... $\endgroup$ – Joe Bloggs Mar 6 '17 at 7:37
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    $\begingroup$ Yes, you'd need to remove the hard science tag to allow answers (since nature hasn't given us any six legged vertebrates yet). You might also want to look at this hexapod question as it might contain answers to your questions. This one discusses the conjectured evolution of hexapods. $\endgroup$ – user10945 Mar 6 '17 at 7:49
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    $\begingroup$ Unless you specifically require scientific citations for claims about the stability of a vertebrate-like six-legged alien species, replacing [hard-science] with [science-based] should be good enough here. $\endgroup$ – a CVn Mar 6 '17 at 10:29
  • $\begingroup$ At least in fantasy, dragons and gryphons are two examples of "six-limbed" creatures that come to my mind. Oh, there is also some variations of flying sphinxes. And the pegasus. And centaurs. And, of course - angels and demons. Now that I think of it, slapping wings to creatures seem to be the fastest way to create a six-limbed being. $\endgroup$ – T. Sar - Reinstate Monica Mar 6 '17 at 14:08
  • $\begingroup$ I intended to write an answer to the question and then realized that OP is asking about a bit too many aspects of a hexaped creature. Answering stability and gravity aspects alone would take up 6-8 paragraphs, let alone CNS and all the other discussion. $\endgroup$ – Youstay Igo Mar 6 '17 at 14:17
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Hexapod mobility is something that's currently being studied in robotics. Ding et al. published a paper on the topic in 2010, where they discussed a variety of advantages that they saw in hexapod locomotion, such as:

  • Increased static stability
  • Increased efficiency while walking
  • Ability to remain stable while using some legs as manipulators
  • Ability to remain mobile after losing a leg by changing their walking pattern

A hexapod vertebrate would have similar advantages over a tetrapod vertebrate. For slow moving creatures, hexapod gaits would likely require less neural mass, since they're very statically stable. Faster moving creatures with more developed brains would likely rely on quadrupedal locomotion for speed, using the remaining two limbs as manipulators.

Beyond the question of feasibility in a terrestrial creature, the other pertinent question is whether six-limbed fish (or fish-like ancestors) would evolve. The most similar creatures to an ancestral fish, in terms of size, body plan, and ecological role, are likely the eurypterids, or sea scorpions. Like early vertebrates, eurypterids were mostly bilaterally symmetric free swimming creatures that used their limbs as paddles. Free swimming eurypterids generally had a single pair of primary swimming paddles, plus several pairs of crawling legs for moving along the bottom. Some eurypterids had an additional pair of grasping claws for manipulating their environments.

Interestingly, a similar gait has evolved in a modern fish: trigloporus lastoviza, though they've evolved a set of rays for locomoting across the ocean floor, rather than using their primary limbs. Ancestral hexapods would likely evolve on similar lines, but using primary limbs for sea floor locomotion instead of rays: four limbs for locomotion across the bottom of the ocean, an additional pair for swimming, and possibly an extra pair of limbs for manipulating their environments. Such an arrangement might even make it easier for them to transition to living on land, needing to transition only from crawling on the sea floor to crawling across the surface, rather than from swimming to crawling. The swimming limbs would likely either evolve to be used for locomotion or manipulation, or else be lost, leaving their descendants with four locomotion legs and two manipulators.

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  • $\begingroup$ That's interesting information. But I wonder why early aquatic vertebrates didn't have legs while some early arthropods, such as the eurypterids you mentioned, did. Already having legs to crawl on the seafloor would make a transition to land easier. Is it a coincidence that vertebrates had no legs before moving to land, or is there a deeper reason? $\endgroup$ – Inflationary_Bubble Mar 7 '17 at 15:31
  • $\begingroup$ Vertebrates probably evolved from lancelet-like creatures, which didn't have any limbs, with fins evolving later, possibly from gills. Early fins were pretty simple structures, which likely would have made for poor limbs. We ultimately evolved from a pelagic group of fishes, so my guess would be that our ancestors, which were limbless (discounting fins), had little reason to evolve limbs, prior to our movement to land. $\endgroup$ – ckersch Mar 7 '17 at 16:49
  • $\begingroup$ That's a good explanation for why it didn't happen on Earth. But is there a basic structural reason why protovertebrates simply couldn't have legs, unlike arthropods, or is it just chance - and perhaps, on another planet, there could be a creature with both the traits that became vertebrate qualities such as endoskeletons and traits that let aquatic arthropods have legs? $\endgroup$ – Inflationary_Bubble Mar 7 '17 at 21:18
  • $\begingroup$ There probably existed some such species on Earth! One example is Pikaia, which was similar to a lancelet, but had many small pairs of legs and a pair of small tentacles in addition to a notochord. If such a creature had evolved to use its legs to aid in swimming and catching food, it could be a candidate for "something that could evolve into a vertebrate, but with more legs." Its descendant's limbs would likely look different from ours, having evolved from very different structures, though I'm not entirely sure how. $\endgroup$ – ckersch Mar 7 '17 at 23:13
  • $\begingroup$ Sorry, but as far as I can tell from my reading and pictures, Pikaia did not have legs. $\endgroup$ – Inflationary_Bubble Mar 7 '17 at 23:48
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First a definition of limb from merriam-webster.com

a : one of the projecting paired appendages (such as wings) of an animal body used especially for movement and grasping but sometimes modified into sensory or sexual organs

The penis is definitely projecting, more or less. Does it qualify as a limb? According to the above definition, yes. Except it is not paired. You are skeptical. Then what if this sexual limb contained a rigid bone, like the arm? Bigger than an arm. A big bone. Like this walrus baculum, or "penis bone".

enter image description here

That is a sweet shirt, Dr Rowe.

A projecting fleshy part with a rigid bone inside of it is a limb by any measure, I think. Many (male) vertebrates have bacula and so qualify as 5 limbed, though most do not use this 5th limb for locomotion except in cases of extreme need. Could evolution further modify such a structure so that it might be useful for functions other than mating and possibly emergency locomotion? Might such a limb become prehensile, allowing the organism so endowed to, say, tip its hat in greeting? If it can happen to a tail or a finger, why not? The possibilities of the prehensile penis are ripe for exploration.

If I may add: calculations, schmalculations. Impossible in this circumstance. What are the numerical calculations relevant to evolving a tail? It is not just how big it is, but what you do with it, and what you choose to do depends on circumstance, ability, need and other variables. This changes not only generation to generation but minute to minute. It is not a process reducible to a single calculation.


※ I was hoping elephants, by virtue of the trunk, would qualify as 6 limbed. But like humans, elephants lack a baculum.

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  • $\begingroup$ There's a big difference between "appendages" and actual "limbs"; your quote even clearly states that "limb" is a subset of "appendage". A penis is an appendage, not a limb by any stretch of the term. The "sometimes modified into sexual organs" thing doesn't refer to penises, but rather to such things as a shark's claspers or a millipede's gonopods, which are actually modified legs (or in the shark's case, the pelvic fins, which are homologous to the rear/lower limbs in tetrapods). And the trunk of an elephant is simply a highly modified nose that became a muscular hydrostat (a la the tongue). $\endgroup$ – MarqFJA87 Jun 22 '18 at 2:43

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