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Re: proto-mammals

From: pheret <pheret@pheret.com>

 > hi all.  i am curious about something i saw a while ago.  are there such
 > things as protomammals?

There is a group of fossil organisms known as therapsids that is
generally held to include the ancestors of mammals.  Within this
group a subgroup called the cynodonts is very similar to mammals,
and is probably the group from which mammals arose.

[To put it cladistically, mammals are therapsids, and probably cynodonts].

 >  these creatures that i remember looked like, well,
 > like half dinosaur/half mammal.  is this possible?

Well, some of the therapsids do look bizarre.

And the group from which the therapsid arose contains many forms that
are somewhat "reptilian" in structure. (This older group inlcudes such
things as Dimetrodon).

[ Since Stan doesn't say this explicitly, it should perhaps be pointed
  out that Dimetrodon (a large lizard-ish animal with a big sail fin
  on its back) was not a dinosaur in the sense that paleontologists
  use the word "dinosaur".  I think Stan's taking it for granted that
  we all know this...  -- MR ]

But, no, there is no such thing as a half dinosaur/half mammal.

 >  did mammals evolve from dinosaurs?

Nope.  Not a chance.

 >  i know that sounds stupid, because they must have evolved from
 > something that looked relatively dinosaurish...

Well, I wouldn't really say the pre-mammals looked all that dinosaurish.
The earliest members of the synapsid lineage (the one containing mammals)
looked more like lizards than like dinosaurs.  And overall they got
progressively more mammal-like with time. The synapsids were always
quadrupedal, and usually had relatively large, well-devloped forelimbs.
Dinosaurs and their relatives were hindlimb dominant.

The best model I know of for the origin of mammals derives them from
Mid-to-Late Triassic cynodonts by a process of miniaturization. The
latest non-mammalian cynodonts were smaller than those common eariler,
and the earliest true mammals were shrew-sized (and shrew-like in

Externally most cynodonts actually look quite "mammalian", so this
isn't really much of a change.

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.