Fish > Amphibians > Reptiles ?

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CA - 10 Jun 2004 22:39 GMT

Got a question for those interested.

I'm trying to follow this evolutionary chain.

Fish:
-scales
-impermeable skin (correct me if I'm wrong)

Amphibians:
-scale-less skin
-permeable skin

Reptiles:
-scales
-impermeable skin

I've read that Ichthyostega and its kin were amphibians, but there is
no recognized affinity between amphibians of that age and modern
amphibians.

So here's my questions:
1. Upon what basis has it been determined that Ichthyostega was an
amphibian?
2. What evolutionary pressures would force crossopterygians to shed
their scales and impermeable skin? It hardly seems adaptive towards
hauling yourself from one pool across dry land to another pool (given
that pools don't dry out except in very dry/drought conditions)?
Indeed, as modern lungfish demonstrate, scales are no hindrance.
3. What evolutionary pressures would push amphibians to re-develop
scales and impermeable skin? Again this does not seem adaptive to a
very wet world. Amphibians are more active in cooler temperatures
whereas the reptile must wait in the sunlight for a lengthy period of
time before becoming active.

I admit I'm stymied. I can't figure out a positive feed-back loop(s)
that would lead to an evolutionary transition from one form to the
other.

I appreciate any assistance in this matter.

CA

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John Harshman - 11 Jun 2004 03:08 GMT

> Got a question for those interested.
>
[quoted text clipped - 3 lines]
> -scales
> -impermeable skin (correct me if I'm wrong)

Yes, you're wrong. Fish skin is quite permeable. They have to keep
pumping ions out (salt water) or in (fresh water) to maintain the ionic
balance of their body fluids, for just that reason. And they get rid of
nitrogenous wastes by converting them to ammonia, which they can manage
only because it leaves the body so quickly through the skin. We can't
afford the water loss required to get rid of ammonia, so we have to use
the much less toxic urea.

> Amphibians:
> -scale-less skin
> -permeable skin
>
> Reptiles:
> -scales

We call them both scales, but they bear no more than superficial
resemblance to each other. Fish scales are bones. Reptile scales are
keratinized skin.

> -impermeable skin
>
> I've read that Ichthyostega and its kin were amphibians, but there is
> no recognized affinity between amphibians of that age and modern
> amphibians.

Not true. There is just as much recognized affinity between amphibians
of that age and modern amphibians as between amphibians of that age and
moder amniotes.

> So here's my questions:
> 1. Upon what basis has it been determined that Ichthyostega was an
> amphibian?

"Amphibian" is a lifestyle, not a taxonomic group. Ichthyostega had an
amphibian lifestyle on several grounds. First, its branching from the
tree the last common ancestor of modern amphibians and amniotes
(determined by phylogenetic analysis). Since being an "amphibian" is the
primitive state in tetrapods, Ichthyostega is an amphibian. (Strictly
speaking, it's not a tetrapod by the commonest modern definition. It's a
stem-tetrapod. Tetrapodomorpha?) Also, it's clearly aquatic. It has a
lateral line, fins, and legs that wouldn't hold it up on land. It's a
fair inference that it didn't lay shelled eggs on land.

> 2. What evolutionary pressures would force crossopterygians to shed
> their scales and impermeable skin?

Scales are heavy and expensive. If not needed, they go. There are in
fact lots of fish without scales. The impermeable skin part is just a
misunderstanding on your part.

> It hardly seems adaptive towards
> hauling yourself from one pool across dry land to another pool (given
[quoted text clipped - 5 lines]
> whereas the reptile must wait in the sunlight for a lengthy period of
> time before becoming active.

I have my doubts that amphibians are any different from reptiles in
terms of the temperatures they can be fast at. But I've never seen a
fast amphibian either. However, the impermeable skin comes in very handy
when you leave a wet environment.

> I admit I'm stymied. I can't figure out a positive feed-back loop(s)
> that would lead to an evolutionary transition from one form to the
> other.
>
> I appreciate any assistance in this matter.

Did that help?

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CA - 11 Jun 2004 04:24 GMT

>> I appreciate any assistance in this matter.
>
>Did that help?

Yes. Thanks :-)

CA

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Natural Science Forum / Biology / Paleontology / June 2004

Fish > Amphibians > Reptiles ?