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Re: "Wonderful Life" - repeated.



On Fri, 15 Nov 1996, Thomas R. Holtz, Jr. wrote:

> {Not to divert too much attention from dinosaurs, but I thought I should
> clear up something here...}
> >     By saying that only the lobe-finned fish were capable of
> >developing walking appendages he is truly a slave to his thesis.
> >Nearly every other group put representatives on land: arthropods,
> >molluscs, worms.
>
> Hardly!!  In fact, the three groups mentioned are just about the ONLY
> representatives of the vast diversity of marine animal life which made it
> onto land:  Several branches of arthropoda, pulmonate gastropods alone among
> the molluscs (no fully terrestrial prosobranch or opisthobranch gastropods,
> bivalves, cephalopods, scaphopods, chitons, etc.), only a few branches of
> the annelids, a few other "worms", and that is just about it (other than
> tetrapods).  Where are the land cnidarians? Ctenophores? Sponges? Bryozoans?
> Brachipods?  These are all very common (some less so now than in earlier
> periods), but (for various reasons) they never made it onto land.

        How could you DARE forget the onychophores? ;)
It is interesting how the invasions are confined. Molluscs- If I remember,
once for snails, and slugs are a separate branch? Then arths represented
by the spiders, insecta (+millipedes and centipes, probably), and isopoda
(pillbugs) and then tons of kind of half-@ssed invasions by the crabs and
hermit crabs with varying degrees of success. But arthropods seem the best
suited to it. Perhaps the skeletons work well to prevent them from drying
out, and also give them support without water. You could also argue that
there have probably been (and there probably have been) a lot of invasions
that didn't make it over the however many hundred million years things
have been moving onto land. Some groups hit it big, but others, like
isopods and onychophores, are less diverse, and still others, like
mudskippers and coconut crabs and land crabs, only get halfway there. It
may be that back when the land environment was less saturated, more things
tried going ashore, and it may be that some of the things that did
succeed are no longer with us.

[While I can't help but applaude the direction this thread is taking, it is
probably geting a little extreme for the vertebratocentrics out there, so
try and bring it back on topic - Chris]