Are "advanced" six-limbed animals plausible?

Are "advanced" six-limbed animals plausible. (So I'm not counting insects, spiders and the like?)

I read a while ago about the theory of protoceratops-bones being the inspiration for griffins.

That lead me to wonder if it in an mostly realistic world of mine (that occasionally handwaves cool but wierd things) could be plausible that a mutation could have given a feathered, griffinesque, lizard a pair of extra limbs that could become wings, thus creating a classic winged griffin?

Two ways to go about it, hexapod vertebrates evolve and somehow out-compete tetrapod ones, or the way I'm doing it, insects get bigger. Not that I'm doing Hollywood style giant insects you understand, but aliens with insectoid-style biological setups (compound eyes become multiple sets of almost normal eyes, antenna remain but have grown whip-like, and their respiratory system involves lungs derived of spiracles).

There's no reason in biochemistry why a large animal, or a land-dwelling vertebrate, could not have more than four limbs. The four-limbed body plan that is dominant in our world became that way mostly by accident (i.e. the creatures poised to evolve into the right niches at the right time happened to have four-limbed skeletons). There are large and 'advanced' organisms with more than four, if you count the cephalopods. You'll also notice, for instance, that some vertebrates have evolved substitutes for extra limbs, such as the elephant's trunk and the prehensile tails of monkeys and chameleons.

The main problem with the traditional fantasy griffins isn't so much their body plan, but their size. Due to the square-cube law, the bigger a heavier-than-air flying creature is, the larger its wings need to be in proportion to its body. This eventually hits an upper limit where the entire body has to be wing just to keep the creature in the air. Artificial aircraft are only able to overcome this limitation because their power sources are more weight-efficient than that of an animal. The largest flying organisms known to have ever existed on Earth were the azhdarchids, giant Cretaceous pterosaurs, which were close to the 'entire body is wing' limit at a weight of only about 200kg. Griffins are usually depicted with both heavier bodies and smaller wings than this, and would be unable to fly unless they lived in an atmosphere much denser than that of the Earth.

My Griffins exist in both winged and non-winged species and with lots of sizes varying from a housecat a to tiger. They would compare to the niches of felines, so would this be plausible?

Smaller winged breeds are used for "falconeering", while both winged and wingless ones of mid-size compare to dogs, as they are used as pets by the upper class.

The idea of kids (circus-artists) flying full-sized griffins has to be scrapped then?

And there are also loads of various winged and non-winged griffins living in the wild.

Having both winged and wingless griffins makes little evolutionary sense. Unless the "wingless" one just have non-functional (but still full-sized, or almost so) wings, meaning they could be two clades that branched not long ago, so they still look mostly the same.

Possibly the wings on the "wingless" griffins have turned into something else that's of use to the animal (Penguin wings have turned into flippers, the first wing pair on most beetles has been converted into armor plates...) or atrophied (kiwi wings are pretty much entirely gone, ostrich wings have been reduced to signalling tools...).

Implausible, but not impossible. There's no reason for larger-than-insect animals to need more than four legs to hold their weight up, and those six or more legs are there in insects because Square-Cube is all "I'm going to allow this".

That mutation has to happen, first, and then not get weeded out by Mother Nature before reproduction and ...eventual "standardization".

Once you get past that over a few million years, then you can start looking at flying variants. (And not just vestigial wings - full-functioning ones.)

My usual suggestion for actually getting these flying is by taking those pincers Mantis Shrimp have and applying them en masse to the creature's wings. Surely that'd be enough to get off the ground? But it would make them too noisy in the air (pulsejets aren't terribly quiet yet), so you'd have to deal with that.

Plan B would be to have them be able to glide or "fall slowly" rather than fly, and that would require less mass than full flight-capable wings. This would limit their size to... a large dog at most? We are capable of gradually landing with hang gliders, so...

The biggest thing here is what of the environment necessitated the need for multiple limbs? Is it speed? Dexterity? What in the evolution of the species required multiple limbs?

It could simply be a consequence of the animal's evolutionary path.

The reason insects (and other arthropods) have multiple limb pairs is that they still have traces of their original segmentation.

Velvet Worms are a good example of what the evolutionary stage somewhere between segmented worms (earthworms and other annelids are good examples of that) and fully limbed arthropods must have looked like. Note how the legs are modifications of the segments and identical in each segment. In annelids and their relatives, most segments are broadly the same, which makes them easy to "build" in embryonal development (or even later on in the case of many segmented worms).

Insects have modified this original segmented build plan to the point where it's barely recognizable (for the most part only larvae still show it openly) but they did keep the multiple limbs.

So in essence it wasn't that they suddenly discovered the evolutionary need to grow another pair of legs...it was that they kept reducing the number of legs until they reached something that was efficient enough to last.

The reason so many terrestrial animals have four limbs is because the very first fish to walk on land had four fins in the right places. To get animals with a different number of limbs, they need only develop from a creature that had a different number of proto-limbs in the right places. That could include insects (or more likely crabs) but any fish with six fins could theoretically do.

What about planets where conditions are different? Creatures with more than 4 limbs could come from planets different than ours.

Everything collapses under its own weight, if said weight gets heavy enough; that's how beached whales die, besides dehydration and drowning. The point is that those tiny (relative to the size and mass of what they support) segmented legs you see so often in the insect world are effective only because the weight they support is so small.

But they don't need six of them to get by.

How well does Bizarre Alien Biology excuse multi limbed animals?

edited 2nd May '15 9:41:15 AM by EchoingSilence

Fairly well I should think given what just life on this planet comes up with. Also, two 'r's in 'bizarre'.

The square-cube law requires more supportive limbs (or larger ones) the bigger you get. In the case of arthropods, exoskeletons are just much less efficient at supporting weight than internal skeletons. (Also, their breathing organs can't be scaled up either).

They can however be scaled down, allowing sizes as low as below 0.2mm (for Megaphragma Caribea).

Regardless, there is nothing to say that a creature with an internal skeleton can't have six limbs.

That's why I brought up the fish.

Couldn't make head or tails of how to introduce winged, flying griffons into an Earth-like setting. And then I talked about it with my wife. Her solution was...

1) Alien travelers dropped by Earth, and thought it'd be a good place for their livestock ("griffons").

2) Something happens, and they're all like "aw HELL nah" and leave. (She suggested "dumb locals" or "oh god HUMANS".)

3) ...but forget about the griffons.

We also talked about how the act of flying in a creature of that size (in the range of a lion or rhino) would require an impressive cardiovascular system, maybe even incorporating two hearts like those really old theories about sauropods (more here), as well as a high metabolism and readily available source of food to survive it.

At this point, I figure that a high-gravity origin would explain how they're able to fly on Earth and require less food than you'd expect of them (less energy required to move around means less food intake).

...Or maybe there's something "magic" about Earth that sustains them to some extent. The rays of a yellow sun, perhaps.

I've sort of part-developed a fantasy setting where gryphons are basically dragons 'gone native', while dragons started out as basically reptiles, before evil mages got their hands on them.

Of course, that's a fantasy world, in my sci-fi the six-limbed aliens evolved from the local equivalent of insects, over a period of about several hundred million years.

edited 10th Dec '15 10:51:45 PM by MattII

Slight threadomancy, but...

What about there being two origins for griffins? Both with four limbs originaly

• Winged breeds: Wings + two legs. Mutations have given them two extra legs over the years.
• Four-legged breeds: Four legs.