Re: T-Rex predator vs loafer

[Stan Friesen <swf@ElSegundoCA.ATTGIS.COM>]

>From: larrys@zk3.dec.com
 > 
 > >[The cannonical example being the replacement of multituberculates
 > >by rodent, which took about a third of the Tertiary].
 > 
 > "The rodents replaced multituberculates?  Wow!
 >  I didn't even know they were endangered!"  :)
 > 
 > What's multituberculate, what distinguishes it from a
 > rodent,

A multituberculate is a member of a group of mammals that
became extinct in the Eocene.  They were very similar to
rodents in terms of ecological adaptations, even down to
having the classical buck-tooth enlarged incisors.

However, they were part of a totally different lineage of
mammals.  Rodents, like humans, dogs, cattle, and so on,
are part of the group of mammals called placentals.  The
next most common group of living mammals, the marsupials,
are close cousins to the placentals, and are joined with them
in the group clled the "therian mammals" (a polyglot silliness
that means "mammalian mammals").

The multi's were NOT therian mammals.  Thus their closest
living relatives are *probably* the echidna and platypus.
The features that place them outside the therian group are
details of the skull structure, particularly the brain case
and ear capsule. (I doubt most of the readers would even
understand a discussion of allospenoids and the like, so I
will leave the details out).  They also had many ridges,
or tubercles, on their teeth, which is where they get their
name.

Now it is *likely* (but by no means certain) that multi's
        - laid eggs
        - had a naked, scaly tail
        - had a pouch for their young

In the Late Cretaceous they were one of the most diverse groups
of mammals (much like rodents today, and for similar reasons).

 > how did the rodents get the drop on 'em,

We don't know for sure.  Perhaps luck, or perhaps placental
style birth of "competent" young, rather than the minimal
embryonic young of non-placental mammals, may have been of
some slight advantage in taking over incidentally vacant
niches.

 >and
 > why is this the canonical example???

Because it is about the only one that is well documented in
the fossil record.


And it apparently took so long because, even given their destiny
for success, rodents did not often invade a niche currently
occupied by a multi.  Rather, their spread seems to have happened
by taking advantage of the occasional extinction, or local
disappearance, of a multituberculate species.  Some of the
biggest jumps in rodent diversity happened across stage boundaries,
corresponding to minor jumps in extinction rates.

swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.