Evolution of limbs (Cephs to Mammals?)

sorseress

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#21
Ok,THAT one will engender some heavy thinking. :hmm:
 

Phil

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#22
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.
 

monty

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#23
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.
 

Fujisawas Sake

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#24
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?
 

monty

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#25
: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
 

Fujisawas Sake

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#26
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...
 

Fujisawas Sake

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#27
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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