Evolution of limbs (Cephs to Mammals?)

cuttlegirl said:
But maybe since the ancestral cephalopod only moved in two dimensions, it didn't need 8 appendages. Modern cephalopods navigate three dimensional space.

That's true, but when octos are using their arms for locomotion, they're mostly in contact with the bottom or with whatever rock/cave/reef they're involved with, so they're not really any more 3-d than squirrels or monkeys. Of course, octos can free-swim, too, but they don't use their arms most of the time when they're doing, that-- and squids, who spend their whole lives in the 3-d world, don't use their arms and tentacles for locomotion at all... and they have ten limbs!

Also, most snails can only crawl forward, they can turn and move another direction, but it is still head forward.

wow! I had no idea (although I guess I've never thought about it before.) So if a snail goes into a little box canyon sort of place, the only way it can get out is to crawl up the walls, I guess.
 
Originally posted by Monty
That's true, but when octos are using their arms for locomotion, they're mostly in contact with the bottom or with whatever rock/cave/reef they're involved with, so they're not really any more 3-d than squirrels or monkeys. Of course, octos can free-swim, too, but they don't use their arms most of the time when they're doing, that-- and squids, who spend their whole lives in the 3-d world, don't use their arms and tentacles for locomotion at all... and they have ten limbs!

Ok, maybe it has to do with that cephs are using their appendages for food gathering devices instead of locomotion like quadrupeds and bipeds.

wow! I had no idea (although I guess I've never thought about it before.) So if a snail goes into a little box canyon sort of place, the only way it can get out is to crawl up the walls, I guess.

Well, they might be able to make a u-turn...
 
monty said:
Us bipeds have to consistently be in danger of falling over; in fact, walking is really more like continually falling forward and catching yourself, which is not true of quadropeds. Sometimes there is a slight gain in going to 6 legs in this regard, since you don't have to shift your center of mass so much to take the weight off a leg to step; there's a "hexapod" robot someone made to take advantage of this.
I'm pretty sure this is at the heart of my fear of spiders -- they seem almost overly-equipped and dominant in that they are in full control; moreso than me. And perhaps that doesn't bother me so much with octopuses since they don't have to contend with gravity in the same way.
 
tonmo said:
I'm pretty sure this is at the heart of my fear of spiders -- they seem almost overly-equipped and dominant in that they are in full control; moreso than me. And perhaps that doesn't bother me so much with octopuses since they don't have to contend with gravity in the same way.
That's funny, those are some of the same reasons that I like them....plus, of course, they kill flies and other nasties, and they spin and weave like champs.:smile:
 
Hmm.. This is very interesting. See, in Cosmos, Carl Sagan made an interesting note about how the tetrapod body plan was so prevalent among vertebrates, and how this was probably due to the overall tetra-well, tetra-'fin' body plan of our lobe-finned teleost ancestors. The five-rayed "hand" comes from reduction of the main bony rays of the fins, and so on...

Here's the issue that some find troubling: its probably just by chance that there are "magic numbers". I know we try, unconsciously for some, religiously for others, to find patterns and directions in evolution. We look for an arrow, as if everything is evolving toward some grand design. If those aforementioned rays had been reduced to four, then maybe many things would have been different. Imagine if the events leading to the end-Permian event had been different. It would seem evolution happens a lot by chance.

Monty had an interesting idea with the Hox Genes. See, linkage is an important factor in evolution as well. And entire phenotypes (expressed genes - eye color, hair color, etc.) can change with one gene. This happens because gene effects can cascade - one gene can affect many. Which is why a small mutation can have mostrous, even lethal changes. It also explains why something like punctuated equilibrium, or rapid evolutionary change, is prevalent in the fossil record.

I know this sounds like a cop-out, but I have to think that the evolution of limbs and symmetry is a mixture of chance happenings - environmental & genetic - along the evolutionary line. I mean, yes, octopus-neurology has evolved a remarkable control system for eight legs, but was this due to having eight legs, or was the evolution of eight legs due to the neurology being better adapted for eight legs, or was it both happening at the same time? There's the $10,000 question.

My :twocents:, or $10,000

John
 
We do have tribrachidium from the Ediacaran weird faunas that had three fold symmetry. Not much came of that pattern!

Fujisawas Sake said:
I mean, yes, octopus-neurology has evolved a remarkable control system for eight legs, but was this due to having eight legs, or was the evolution of eight legs due to the neurology being better adapted for eight legs, or was it both happening at the same time? There's the $10,000 question.

So to paraphrase, it's the chicken and the egg again.

The earlier nautiloids from which the octopuses and other coleoids eventually evolved probably had many more arms, akin to Nautilus. Perhaps it was the reduction in the number of limbs that allowed for increasing sophisticated neurology and adaption of each limb? On the other hand, increasing the control and development in each arm may have forced the reduction of arms. Hmm...I see what you mean.
 
Phil said:
We do have tribrachidium from the Ediacaran weird faunas that had three fold symmetry. Not much came of that pattern!



So to paraphrase, it's the chicken and the egg again.

The earlier nautiloids from which the octopuses and other coleoids eventually evolved probably had many more arms, akin to Nautilus. Perhaps it was the reduction in the number of limbs that allowed for increasing sophisticated neurology and adaption of each limb? On the other hand, increasing the control and development in each arm may have forced the reduction of arms. Hmm...I see what you mean.

Does anyone know if the HOX control of development has been studied the same way in nautilus as it was in decapods (I think in an article Um... posted a link to)?

In the decapods, the specific arm pairs had unique HOX sets for each, but in Nautilus, there would seem to be too many arms for that to work combinatorically, and anyway, don't Nautiluses have a non-constant number of arms, presumably controlled by environmental conditions or random chance in development, or something like that?

I suppose I could google that, maybe I'll try that tomorrow.
 
tonmo;72219 said:
I'm pretty sure this is at the heart of my fear of spiders -- they seem almost overly-equipped and dominant in that they are in full control; moreso than me. And perhaps that doesn't bother me so much with octopuses since they don't have to contend with gravity in the same way.

My son absolutely LOVES spiders, and his unearthly fascination with them has made me wonder about the evolution of metamerism (segmentation) in invertebrates.

Certain species of trap-door spider actually have rudimentary segmentation of their abdomens. Spiders follow a basic arthropod body plan, with twelve appendages (eight legs, two pedipalps, and two "fangs" which are not teeth but rather modified appendages).

Given that, maybe there are segmentation genes which are deep in the mollusc genome which affect the number of arms. Has anyone actually mapped the octo-genome? I don't think its that much of a stretch - there is most likely some proto-invert form that links molluscs with segmented bioforms.

Any thoughts?
 
:shock: spider fangs are modified appendages! Zoinks! The deuce, you say!

As far as I know, there is no full genome project for any ceph. PZ Meyers was bemoaning that at some point in his "Pharyngula" blog: http://scienceblogs.com/pharyngula/2006/09/what_no_cephalopod_genome_proj.php
I've been getting very interested in ancestral body plan lineages, but I'm really just starting to try to learn about them... And I keep finding out things like this spider fang thing that turn my brain into a pretzel.

- M
 
Actually there are some absolutely bizarre properties of the Arachnida.

One that has had me a little stumped is the fact that many scorpions fluoresce under UV light. Given that they're nocturnal, this doesn't really make sense to me. The morphological why is probably due to calcium salts in the exoskeleton. But as to WHY this fluorescence happens, I have no clue.

However... This brings up a question. Does this have to do with their aquatic ancestry (think Eurypterids, but even more modern scorpion-like)? How does UV affect the lives of aquatic forms? Can cephs percieve UV or are they UV fluorescent in any way?

Not that this is earth-shattering, just that its extremely cool to go out into the desert at night with a UV light...
 
monty;85439 said:
:shock: spider fangs are modified appendages! Zoinks! The deuce, you say!

.... And I keep finding out things like this spider fang thing that turn my brain into a pretzel.

- M

Yeah, I find it a mind twister too, but it makes sense. Imagine some ancient Onychophoran-like bioform being the ancestor of arthropods. Now imagine 500+ million years of mutation, changing appendages, changing the structure of the cuticle, and even breaking off into aquatic and terrestrial forms. But if you look at the ancestral features of spiders, the origins of the arachnida become clear. Book lungs are just dry-air gills, and the plastcity of insect genes that make metamorphosis possible is just nothing short of a brain-EMP.
 

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