Re: mammal evolution

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

 From: John Rosloot <john@cs.uregina.ca>
 > 
 > I was looking through another book though, 'The MacMillan Illustrated
 > Encyclopedia of Dinosaurs and Prehistoric Animals', and a diagram there
 > of mammal evolution indicates that the line leading to primates branched
 > off in the early Cretaceous, maybe 130 million years ago.
 > 
 > So which is right? Or is the history of mammal evolution still that iffy?

The details are still iffy.
The question is twofold, first, which branching occurred first,
and then when the branchings happened.

Now, I think the rodents lineage probably *did* diverge first,
but I am not current on the latest data on that, so I cannot say
for sure.

As to dating of the branching, that is difficult.  The earliest known
primate is about 66 million years old, but the earliest known rodent
is *much* younger.  The question then is, how long before the first
fossil did the various groups exist.

 > I thought I had read of a genetic test that allows you to determine how long
 > ago two life forms had a common ancestor. Wouldn't it be simple just to run
 > comparative tests on members of all the existing orders of mammals, in order
 > to get a more accurate picture?

This has probably been done, but it is only an approximation, and
is subject to various sources of error, especially when only a few
lineages are being compared.   Thus, by itself it does not resolve
the issue.
 > 
 > One more question, 'The Rise of the Mammals' mentions a now extinct group
 > of mammals I had not previously heard of, the multituberculates. The book
 > doesn't go into great detail about them, though it mentions they probably
 > had pouches like marsupials. What are some of the unique characteristics of
 > this group that separates them from marsupials?

Many.  The multituberculates where a group of superficially
*rodent*-like mammals.  However, in skull structure, tooth structure,
and other details of their anatomy they are distinct from almost
all living mammals, except *perhaps* the monotremes (like the
platypus). I cannot easily summarize the details, since to do
it in less than a few pages requires that you already know
the detailed anatomical terms (do you know what the allosphenoid is?)

Except for the monotremes, all living mammals share a common ancestor
about 100-120 million years ago (Early Cretaceous), and are placed
in the group called "therian mammals".  Most of the Jurassic mammals,
the monotremes, and a few long lasting groups like the multi-
tuberculates are called "non-therian mammals".

Multituberculates filled the "rodent" niches on into the Eocene,
when they were finally ousted by modern rodents.

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

The peace of God be with you.