Vertebrate tripods, pentapods, hexapods, etc...

[Xyph]

Hypothetically, would there be any environments that would be conduicive to the evolution of vertebrate land-based animals with more than four limbs or asymmetric numbers of limbs? Is having four limbs almost always inherently more advantageous, evolutionarily, or is it just a case of tetrapods gaining the advantage early on, when things could have turned out differently?

[Mokele]

It seems mostly to be a matter of developmental constraints. Most fish have "4 limbs" (the pelvic and pectoral fins), and the only lineage that had more died out a long time ago. Why this is the case is probably that, for some reason, mutations to developmental genes that result in more than 4 limbs also do other things which negatively affect the organism (the same reason 6 fingers never caught on, though in that case we know that the downside is that it comes with genital malformations).

 

A lot of evolution is actually constrianed by the organism's development. For instance, hoofed mammals have independently evolved multiple times, but never in marsupials, because marsupial babies need well-developed, dextrous forelimbs to climb into the mother's pouch. The developmental pathway constrains the possible evolution of the taxon. That's also why anthropods have done so well: they seem to be able to vary just about any segment of their body and any part of the limbs without much affect on the rest, giving them a mix-and-match body that's allowed them to become the most speciose phylum on the planet.

 

Mokele

[Xyph]

Hm, so, generally, once a reasonably successful species has emerged with a certain number of limbs, it's descendants will adhere to this without any significant variation?

 

Does that mean, then, that had whatever creature is ancestor to all today's tetrapods been a tripod (for example), the vast majority of today's vertebrates would be tripods as well?

[insane_alien]

the tripods could have died out and another tetrapod evolved and dominated the wourld. but in theory yes.

 

You can make yourself a tripod just cut off a limb.

[LucidDreamer]

Hm' date=' so, generally, once a reasonably successful species has emerged with a certain number of limbs, it's descendants will adhere to this without any significant variation?

 

Does that mean, then, that had whatever creature is ancestor to all today's tetrapods been a tripod (for example), the vast majority of today's vertebrates would be tripods as well?[/quote']

I would say that a tripod would be far less likely, mostly because it would be non-symmetrical. Can you imagine a fish trying to swim with 3 appendages?

[Xyph]

Hmm, yeah, I suppose that could be a problem. What about a hexapod?

[LucidDreamer]

Hmm, yeah, I suppose that could be a problem. What about a hexapod?

Much more likely.

[Mokele]

There was actually a group of fishes (whose name I have forgotten) from the devonian period who had more than 4 fins (eight to ten, iirc), and, had these made it onto land, we might well have vertebrate decopods. However, they died out before anything but bugs and a few plants were on land, so they never got to give it a shot.

 

Mokele

[Rasori]

An odd number of legs could be symmetrical. A tripod, for example, could have one leg on each side and then one in the rear (or front, I suppose. Seems more advantageous to have the third leg in the rear, however). Only advantages I can see to it are jumping (a third leg gives you 150% the jumping power over a two-legged animal). Running could be a problem, but then you could have the two side legs pull back while the third leg jumps forward to be equal with them. Kind of ungainly, but surely workable.

[LucidDreamer]

An odd number of legs could[/i'] be symmetrical. A tripod, for example, could have one leg on each side and then one in the rear (or front, I suppose. Seems more advantageous to have the third leg in the rear, however). Only advantages I can see to it are jumping (a third leg gives you 150% the jumping power over a two-legged animal). Running could be a problem, but then you could have the two side legs pull back while the third leg jumps forward to be equal with them. Kind of ungainly, but surely workable.

It could work. Monkey's tails are a sort of pseudo-appendage.

[cambrian_exp]

Well there is an imaginary hexapod vertebrate on national geogrophic channel's Extraterristial programe..

 

see it http://www.nationalgeographic.com/channel/extraterrestrial/index.html

 

look whole site u will find it