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Deriving a Central Arm
Topic Started: Jan 31 2009, 07:31 AM (1,270 Views)
StrandedAlien
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I have a question: How do you design a creature with a central arm? Like Nemo Ramjet's example here, a "Monkey" of Pazattan. Monkey of Pazattan

I am working on a redesign of one of my creatures, which has such a central arm, which has now become necessary, no longer just interesting. (It has a long neck with a beak, and uses its beak and its central arm as its two manipulators. We have a right hand and a left hand, they would have an upper beak and a lower hand/talon.)

Any feedback, much appreciated!
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PousazPower
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Giant Blue Anteater
Feb 2 2009, 08:10 PM
Nemo Ramjet told me that the "monkey" of Pazattan was descended from some wierd animal that resembled a cross between a jumping spider and a springtail. And seeing how the middle arm is a genital organ, it seems to have derived from some jointed genital from the undersides of the Pazattanian's ancestors.

Now, a middle limb, if it were to derive from something like a strange sea cucumber-like animal, then the many tube feet would've fused into a single, frontal middle limb, while two other limbs form from tube feet. This animal would be a tripod, bearing some resemblance to the prismalope from Barlowe's Darwin IV. Given that these tripods could evolve bipedal forms, some may adapt the middle limb, lose it, or even as a grasper.

Now, using a fish-like ancestor, a fins could radially evolve near the head, if the animal is torpedo-shaped with four fin webbings. The extremities evolving from each of the four tail fin webbings may be downward-facing cusps, and propelling the animal foreward. If they evolve land forms, the lateral fins could evolve as legs, while the ventral, medial fin could, like the sea cucumberoid derivative, a third leg, while the dorsal fin could be a sail for warming up or attracting mates. They could free the third limb from the ground some way or another, and one of the adaptations could be a grasper.

However, with only one arm, a sapient species evolving from both of these lineages could have limited technology at the beginning, after all, you cannot do much with one arm, can you? If the single central arm were to splt into two 'hands', then I think things could be a little promising.
If you were thinking of something similar to a coelocanth as an ancestor for the non-tripod, the central arm wouldn't be in a place that would be useful for grasping. A coelocanth has at least five appendages that have the potential to become limbs. First are the paired pelvic and pectoral fins, which gave rise to our limbs (our ancestors weren't exactly coelocanths, but who cares?). Second is the second dorsal fin (the leading dorsal fin is actinous and spiny), and third is the anal fin. The anal fin would be the closest thing to something that could give rise to a central arm, but it's an anal fin; it's in between the anus and the tail. That's not a place you'd want to have an arm. I know someone will suggest that it could migrate towards a more convenient position, but then the anus would migrate with it and who wants to have an anus on their chest? The second dorsal fin is also kind of problematic, not for sanitation regions, but because what use would a hand be if it were behind you?
Posted Image

All that, however, is based on coelocanth anatomy, which would be radically different from GBA's hypothetical fishy ancestor anyway. Besides, a central arm would be very hard to derive from a fish like a coelocanth on earth anyway. Nevertheless, a ventral fin in front, with the proper internal supports, could give rise to a ventral arm, as GBA has suggested.
Was your nose discovered or invented?

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StrandedAlien
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zypher
Feb 3 2009, 09:17 PM
hmmm... the way you have it I don't quite see it...

perhapse draw the head in a different position so I could get a better idea? (nothing fancy, just a quick skecth)
Here's a quick little MS Paint diagram that might show it. I think in the drawing I posted before, the eyestalk was in the way of the "neck".
Hliaen head diagram
Edited by StrandedAlien, Feb 4 2009, 08:51 PM.
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ATEK Azul
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you could make a back fin work as a middle arm by evolving an up side down creature where the spine is in the front of the body and they have evolved a different body plan.
I am dyslexic, please ignore the typo's!
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Viergacht
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Something like an elephant's trunk might develop as a central limb.
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ATEK Azul
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never mind wrong topic sorry.
Edited by ATEK Azul, Feb 7 2009, 02:59 PM.
I am dyslexic, please ignore the typo's!
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Rodge the Linkbot
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I imagine a change in locomotion would do it. Our Reproductive systems changed completely when we became completely bipedal. Why not a limb?
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ATEK Azul
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as for a central arm i think that maybe the reproductive organs of a mammalian creature might be able too turn into a tentacul and then an arm through cartilage structure and eventually bone structures.
I am dyslexic, please ignore the typo's!
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Rodge the Linkbot
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I was just thinking it may have moved around the body like the vagina did when we became bipedal. I was imagining this limb was always a limb, but where would it have been before and what was it used for to make it move to the front of the creature?
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ATEK Azul
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another way for a arm might come from the head of an animal evolving into 2 one with hearing and sight on top and below has the breathing and digestive parts of the body.
I am dyslexic, please ignore the typo's!
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Venatosaurus
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Did someone suggest a reproductive organ (most likely a penis) as a manipulatory organ :D LOL , Imagine shaking hands XD



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ATEK Azul
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yeah i basicly did but come on is it really that hard too think of espesialy for some thing alien, many invertebrates have it any way so its not that big of a deal.
I am dyslexic, please ignore the typo's!
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Giant Blue Anteater
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Some food for thought:

Posted Image

Quote:
 
Tripodelephantoides gracilens is an alien animal from the planet Hawking III of the Hawking System. It is thin, due to the lower gravity of its world, and is about 50 centimeters tall. It is a grazer, using its flexible and extensible proboscis, equipped with a keratinous beak to bite off pieces of yellow plants. Its skeleton is made cheifly of copper rather than calcium on Earth. It also has neurons of copper, which transmit signals much faster than our neurons. It has two, lensed eyes, which is nothing unique to us. Near the back two legs are twin cloacae which deal with waste as well as reproduction. They are hermaphroditic, and mating impregnates both individuals. The mated pair build a nest and then they both bend down to the ground to lay their eggs. They have many chromatophores in their skin that change to a wide variety of colors to communicate with one another or camouflage in the yellow fields when threatened. If the predator sees behind their disguise, they have sharp, keratinous spines which can hurt an attacking marauder.

An early ancestor of the stock of which T. gracilens comes was a marine predator that resembled a strange brush, with the "bristles" being tube-feet. The tube feet were arranged in four circles, the first front three fuse together to form hydraulic limbs, while the back fourth one caved in and internalized, and became a breathing organ, with the tube feet becoming rich in blood vessels and essentially becoming aveoli-like.


While this creature roughly fills the niche of a rabbit or a small deer, imagine a primate-like relative (not realy primate-like per se, but you know what I mean). While you cannot really do much with one arm, how about the muscular proboscis serves as one arm, the middle, boned limb would serve as another.
Ichthyostega

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cdk007
 
Intelligence is awareness of ignorance. Stupidity is ignorance of ignorance. Think about it.
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