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Re: Synapsids weren't reptiles?



On Mon, Jun 30, 2008 at 8:37 PM, Dora Smith <villandra@austin.rr.com> wrote:
> Whoa, first of all.
>
> Reptiles did not evolve from amphibians?

It depends on what you mean by "amphibian". That word gets used in a
lot of different ways.

The "classic" meaning is something like: all apo-tetrapods which are
not amniotes. Other meanings are: 1) the total group including modern
amphibians, and 2) the crown group including modern amphibians
(assuming that doesn't include amniotes).

Reptiles and other amniotes did evolve from "amphibians" under the
"classic" sense of that term, but not under the total group or crown
group senses.

> You telling me that fish just
> one day independently grew lungs and legs, climbed out and started walking
> around?

Well, mudskippers are on their way there.... But, no I am *not* saying
that modern amphibians and amniotes are convergently terrestrial.

> Second.   What did the actual ancestor of reptiles and synapsids look like.
> Not, what did his skull look like.   What did he look like?   And please
> don't tell me he had fins - especially since he as a tetrapod.

(Hey, some tetrapods have fins: whales, ichthyosaurs....)

No, the ancestral amniote would not have had fins.

A decent model might be something like _Solenodonsaurus_ (a
stem-amniote): 
http://upload.wikimedia.org/wikipedia/commons/e/e5/Solenodonsaurus1DB.jpg

Or _Hylonomus_ (an early sauropsid):
http://upload.wikimedia.org/wikipedia/commons/5/56/Hylonomus_BW.jpg

Or _Eothyis_ (an early synapsid):
http://upload.wikimedia.org/wikipedia/commons/f/f6/Eothyris_BW.jpg

> What was he anyhow, if he wasn't a reptile, adn wasn't an amphibian?   A
> fish?

No, just an amniote. As the ancestral amniote, it couldn't be
classified with any more detail than that.

> You seem to be saying there were land bound tetrapods who were neither
> amphibians nor reptiles - what were they?

Of course! I'm one. (Well, mostly land-bound.)

Seriously, though, if we ignore the "classic" usage of "Amphibia", and
only look at the cladistic usages, the following are land-bound
apo-tetrapods which are neither amphibians nor sauropsids:

- some stem-tetrapods (e.g., _Seymouria_)
- stem-amniotes (e.g., _Diadectes_, _Solenodonsaurus_)
- the ancestral amniote (aforementioned)
- synapsids/theropsids/pan-mammals (all names refer to the mammalian
total group)

Additionally, if we use "Amphibia" for the crown group (the last
ancestor of frogs, salamanders, and caecilians and all of its
descendants -- assuming that that doesn't include amniotes), then you
can add "stem-amphibians" to the list.

> One other thing.   How do you know that reptiles did not evolve from 
> amphibians?

This is more of a nomenclatural issue than anything. Again, it depends
on what you mean by "amphibian". (And what you mean by "reptile", for
that matter.)
-- 
T. Michael Keesey
Director of Technology
Exopolis, Inc.
2894 Rowena Avenue Ste. B
Los Angeles, California 90039
http://exopolis.com/
--
http://3lbmonkeybrain.blogspot.com/
http://dragabok.blogspot.com/