Centaurism II

A while ago I discussed the goumoun, a six-limbed animal. I thought that freeing a pair of limbs for another purpose definitely occurred more than once in evolution, but that this principle sadly had no name. So I took it upon myself to call it 'centaurism'. Do not get me wrong: I am not claiming eternal fame for this invention, nor do I think evolution biology will suffer at all if it does not spread beyond this page.

Just treat it as an intellectual game. The original centaurs in Greek mythology had the body of a horse, but where its neck and head should be you would find the torso, arms and head of a human. Ancient Greeks were notoriously vague about the evolutionary history of their mythological beings, so we know very little about protocentauroids. Nothing, in fact. Did they walk on six limbs? The Greeks were probably more inclined towards a form of creationism: 'Poof!', leading to 'here's one I made earlier'.

On Furaha, the ancestors of neocarnivores did walk on six limbs in the typical hexapod fashion. There is nothing wrong with walking on four limbs, as tetrapods on Earth show, and indeed on Furaha. There was therefore ample room for evolutionary experimentation with front limbs in hexapod Furahan animals. They accordingly evolved into clubs, spears, lances, hatches and even nets (in the Microraptoria). If you want to see a few examples, simply travel to the land page on the Furaha site and have a look. Here's one of them:

Copyright Gert van Dijk

But centaurism is not confined to the goumoun or to Furaha. In fact, there are quite a few examples on Earth. Let's start with animals that started with four legs: are there any that stopped walking on all fours and got up on their hind limbs? Definitely; there are ostriches and other ratites, kangaroos, predatory dinosaurs and people, to name the most obvious.

What do they all do with their freed front legs? Some animals use them for grasping purposes at some times but to walk with at other times. Examples are the giant panda, lots of primates, as well as the kangaroo. If kangaroos move very slowly, they move on five limbs: the hind legs swing forwards while the body rests on the front legs and the tail.
Other animals do not appear to do very interesting things with their front legs, such as ostriches. I have seen a male ostrich flapping it to attract females, but that is about it. In fact, some Furahan analogues do exactly that, but, having freed not just two but four limbs, they look even odder (the Grec on the land page is a perfect example). The same probably goes for the front limbs of Tyrannosaurus. I know that its arms were less weak then they look, and am aware of theories saying the beast used its arms to help stand up from a lying position. But all in all these puny arms do not impress me at all. Faced with animals that do not do anything interesting with their freed front legs, perhaps the definition of centaurism should include a new non-locomotory purpose for the freed legs.

That thought reduces the list: primates are still in, as well as those predatory dinosaurs that used their front limbs to do something interesting with. That group goes under the name of Maniraptora. One of the most intriguing examples would be Therizinosaurus, here shown as seen in the BBC documentary 'The Giant Claw', a 'Walking with Dinosaurs special'. Not that much is known about the animal apart from its front legs with their huge claws, so the claws are to the right scale, but the rest is educated guesswork.

BBC Walking with Dinosaurs Specials: The Giant Claw
Click to enlarge

There is no reason to limit the principle of centaurism to animals starting with four walking limbs. Crabs are decapods ('ten-leggers'), but walk on eight legs. The front pair, the claws, are modified legs. In fact, the modifications are not that extensive, as is shown on an excellent animation on the internet from the University of Alberta and explaining how crabs' claws evolved and work. Here is a non-moving fragment to entice you to go and visit the original.

Click to enlarge

There are many other arthropod examples. Here is another one: Amblypygi or whip scorpions are spiderlike, but no longer walk on eight legs. Instead, one pair has evolved into long whiplike sensory organs. The next two photographs are from 'Life in the Undergrowth', another BBC documentary. It has some excellent footage of these animals. The whips are the thin limbs between the pedipalps and the walking legs. If you need more, check up on Uropygids in Wikipedia.

BBC Life in the Undergrowth
Click to enlarge

In conclusion centaurism is fairly common. The modified legs are front pairs in all cases. I have not found any examples of a hind pair of legs becoming centaurised. That is probably because of 'cephalisation': the front end of a body commonly has the most sensory organs, food goes in there, etc. Once you have cephalisation, any manipulatory limb is best placed at the front. But if anyone can think of a sound evolutionary way to free hind limbs from locomotion in an animal that is already thoroughly cephalised, AND that might fit on Furaha, I will consider making a sketch of one.