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Three legs?
Topic Started: Apr 25 2011, 06:41 AM (1,372 Views)
magpie
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Ok, so I'm trying to make a small evolutionary tree for a project. The idea is that a single pioneer creature evolves into different animals, depending on the environment, diet etc.

However, here is where I have a problem. I decided that the pioneer creature is a tripod, with two legs at the front and one at the back. What I want to know is why would an animal evolve three legs? I have the ways in which they would evolve (some becoming armoured like tortoises, some becoming ambush predators) but I am wondering if there is a way to develop the idea of being a tripod.

Any advice, criticism or ideas are happily accepted.
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SIngemeister
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Well, I can't remember who it was, but somebody had a tripod have its rear leg evolved from a tail. Or, you could try and evolve it from the pectoral and anal fins or nearest on a fishlike ancestor.
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zaktan
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They could have the 2 back legs fused as one. Or a feeding apparatus as the leg.
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Ook
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SIngemeister
Apr 25 2011, 06:55 AM
Well, I can't remember who it was, but somebody had a tripod have its rear leg evolved from a tail. Or, you could try and evolve it from the pectoral and anal fins or nearest on a fishlike ancestor.
wasnt it a nemo ramjet's terrestrial placoderm?
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SIngemeister
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http://s1.zetaboards.com/Conceptual_Evolution/topic/389788/1/#new
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Empyreon
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Are you plausible?

Nereus is also an example of widespread tripod life. The reason it features three legs is because of the initial threefold radial symmetry of the nereids that came onto land. In short order life developed bilateral symmetry. For your creatures the pioneer species could simply have developed bilateral symmetry earlier.
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Dark-Matter
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Well we evolved limbs(is a jointed, or prehensile (as octopus tentacles or new world monkey tails), appendage of the human or other animal body)from fins,sow by having 4 fins we got 4 limbs.Sow if a type of swimmer have 3 fins you will have a creature with 3 limbs,in other word a tripod.Or as other say evolved from the tail or have t leggs fused.
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Ànraich
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Three legs, while awkward for faster motion, are great for standing still. The extra support from a tripod structure would allow them to also be more stable when they grow tall.
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magpie
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Thank you all for your very constructive advice!

Points on a 3 finned aquatic relative were already considered, and now that others had the same idea, I think that can be used. Paraskay's point on mobility is very good, and for that reason most of the carnivorous decedents are ambush predators: that way they would not need to run at a meal. Empyreon's point supplies a scientific point to this evolution, as they would have threefold radial symmetry.

Oh, and thanks to SIngemeister for the skeletal diagram of Flisch's tripod. Very useful indeed!

One thing that bothers me is that why would an animal with four legs evolve to have three. Surely this would of put them at a disadvantage during the middle (not quite four and not quite three legged) stage, and they would not survive?
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Flisch
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magpie
Apr 26 2011, 03:51 AM
Oh, and thanks to SIngemeister for the skeletal diagram of Flisch's tripod. Very useful indeed!
Yeah, SIngemeister, that's great. I wish I made it, oh wai-

Anyway, yeah you're welcome. :D
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magpie
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I have refined the basis of habitat and adaptations now. Yet again, criticism is welcome.

The planet of the tripods (not named yet, any ideas?), the atmosphere is very rich in gases, after its large oceans evaporated. This rich atmosphere can support a vast amount of specialized life. After evolving from three finned creatures, the tripods move with three legs. sensing the environment is mainly done through sonar, which travels fast through the dense atmosphere, and reaches their elastic receptors. The tripods also support a proboscis instead of a mouth, which is adapted to particular food sources, depending on the species in question.

Some of the species groups (no names yet)
group 1: similar to Earth's sheep and goats, these are closely related to the original pioneer. They feed on ground based plant sap, and the occasional egg
group 2: this group took a more predatory route, evolving a secondary proboscis. This, alongside the sharper, serrated feeding part, is used to ambush prey (think of the proboscises being used like a rhino beetle's horns.
group 3: a more herbivorous group, these ones focus their bodily structure on height (using the idea of paraskay). These would also be the largest of the tripods. They feed on hight fruits and tree sap.
group 4: their proboscises are stronger, used for getting inside hard seed pods that drop to the floor. They also have small armored pads on their backs for defense, as they are slow and would be prey for group 2.

These are possible, but I do not know if they drift too far away from the pioneer.
group 5: filling in the gap of pack hunters, these rarely rely on their third leg, using it more when resting or feeding. They reach high speeds on their two legs, with the third acting like a tail for balance.
group 6: the few airborne tripods, they have membranes between their three legs, in complete symmetry. They are also the smallest group, as they move from tree to tree, hanging upside down when not in flight.
group 7: a sentient group?
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magpie
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Sorry for the continuous posts, I just need to get my ideas written down! These are just 4 points I came up with

1) Would the third leg be used for propulsion, then the front legs be used for balance. This would cause a running style similar to a rabbit of a gazelle
2) Whilst they have proboscises, would it be protected with some form of three way opening mouth? Just thinking, it would be protected better
3) How would specialized limbs work? Earth has mantids claws, the human hand, lobster pincers, so why not a tripod with a hunting arm or grasping appendage (this point leads to question 4)
4) If one group were to become sentient (like neanderthals, so simple tools, housing, teaching younglings etc.), how would they hold tools? I can't imagine a monopod, so would they become 5 limbed, only use 1 arm, or would there be another means of manipulating the environment?
Edited by magpie, Apr 26 2011, 10:13 AM.
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Empyreon
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Are you plausible?

Quote:
 
One thing that bothers me is that why would an animal with four legs evolve to have three.

I'm confused. At what point in the lineage do species have four legs instead of three?

Quote:
 
sensing the environment is mainly done through sonar...
What made these win out over eyesight, which is quite easy to evolve and very useful in a variety of circumstances? Are there even eyes (please say yes)?

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The tripods also support a proboscis instead of a mouth, which is adapted to particular food sources, depending on the species in question.

Be prepared to specialize this proboscis to such a degree that it can't technically be called a proboscis any more. The mechanical strength necessary to break through hard shells or to mash tough plant material may require quite extreme adaptations.

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group 1: similar to Earth's sheep and goats, these are closely related to the original pioneer. They feed on ground based plant sap, and the occasional egg

Sounds good to me. Do they retain any amphibious traits, or have they also adapted for drier climates?

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group 2: this group took a more predatory route, evolving a secondary proboscis. This, alongside the sharper, serrated feeding part, is used to ambush prey (think of the proboscises being used like a rhino beetle's horns.

In some clades this combination of proboscises (where did the second one come from, by the way?) could actually develop into powerful pincers or jaws.

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group 3: a more herbivorous group, these ones focus their bodily structure on height (using the idea of paraskay). These would also be the largest of the tripods. They feed on hight fruits and tree sap.

Giraffe aliens! I always enjoy those. Since they feed on forest produce perhaps their mouths retain much of the proboscis nature of their mouthparts. Conversely, they could develop a sort of pelican technique, gathering large amounts of food in a large distensible craw.

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group 4: their proboscises are stronger, used for getting inside hard seed pods that drop to the floor. They also have small armored pads on their backs for defense, as they are slow and would be prey for group 2.

That second proboscis that Group 2 developed would come in handy here as well, so perhaps you could consider making these two groups related. Just an idea.

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These are possible, but I do not know if they drift too far away from the pioneer.

Too far away from the pioneer? Do you mean that they would be too far removed from the pioneer species to evolve and diversify within the time frame you're imagining? What time frame are you imagining?

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group 5: filling in the gap of pack hunters, these rarely rely on their third leg, using it more when resting or feeding. They reach high speeds on their two legs, with the third acting like a tail for balance.

This is much like my nereid placodromeans or pseudocoles. Taking out that third leg makes locomotion easier to conceptualize, and since you said the third leg is used for resting and locomotion it could also be specialized for other purposes. Spike- or club-tipped swinging limbs are useful for predators.

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group 6: the few airborne tripods, they have membranes between their three legs, in complete symmetry. They are also the smallest group, as they move from tree to tree, hanging upside down when not in flight.

Sigmund Nastrazzurro has done some excellent research into radial flyers with his Furaha project (search for tetropters on his blog) and while he explores four-winged flyers, I discuss with him at length about how three wings could work. I'm eager to see how you incorporate this idea.

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1) Would the third leg be used for propulsion, then the front legs be used for balance. This would cause a running style similar to a rabbit of a gazelle

Nereus also has monopods (bottom two species of that linked page), so I fully believe that this form of locomotion is possible, and it frees up the front two limbs for use as manipulators.

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2) Whilst they have proboscises, would it be protected with some form of three way opening mouth? Just thinking, it would be protected better

I think this is entirely within the realm of possibility. Just figure out how and why this protective aperture developed biologically.

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3) How would specialized limbs work? Earth has mantids claws, the human hand, lobster pincers, so why not a tripod with a hunting arm or grasping appendage (this point leads to question 4)

Why not? Free limbs either find a use somewhere else (as wings, weapons, or manipulators) or they atrophy. But you mentioned a tripod with a grasping appendage. Where did this extra limb come from?

Quote:
 
4) If one group were to become sentient (like neanderthals, so simple tools, housing, teaching younglings etc.), how would they hold tools? I can't imagine a monopod, so would they become 5 limbed, only use 1 arm, or would there be another means of manipulating the environment?

They'll hold the tools with whatever manipulators they have. If this means an elaborate and specialized beak then that's what they'll use. They could even be bipedal and use that third, unused limb (much like Group 5) for both balance and now manipulation. Why can't you imagine a monopod, though? A creature that can balance on one leg can often do it while performing relatively small movements; take a look at a flamingo, for example. And again I ask about the five-limbed option, not that it isn't possible, but that you have to figure out where those extra limbs come from.

Anyway, those are my initial thoughts on what you've posted so far.
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magpie
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Thank you greatly for the feedback. The reason i have a slight problem with monopods is that this would greatly effect their speed. The protective face could come from a mouth and tongue, where the tongue evolved into a feeding apparatus. I will have to explore your ideas of the weapon limbed group 2 and the feeding method of my 'giraffe aliens'. The 'alien' part of that quote would answer your question about the time period- who ever said it was on earth? My main goal now is to somehow incorporate a way that gives the pincers to group 2 and the grasping limbs to the sentient group.
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Empyreon
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Are you plausible?

Quote:
 
The 'alien' part of that quote would answer your question about the time period- who ever said it was on earth?

I in no way assumed or suggested this was on Earth. Even on an alien planet (at least in this universe) time will pass, and the time necessary to develop sapient life will take longer than the time it would take to evolve primitive reptilian analogues. When you expressed concern about some of the alien groups being too far removed from the pioneer species I thought you might be considering a younger cladogram of life, where there had been less time for species to radiate from the pioneer species. By time frame I'm wondering if you're giving your groups 1 million years, 10 million years, or 100 million years to evolve. That considered time frame will effect the diversity of the different groups.
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