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Re: Dinosaurs versus mammals



At 10:25 AM 5/23/97 -0400, Jeff Poling wrote:
>   According to the recent AMNH book about their collection, it's the 
>other way around:  true mammals evolved before true dinosaurs.  Is 
>there any consensus on this?

Odd, I have never heard this.

The earliest "true" mammals I know of are the Haramiyids, Morganucodontids
and Kuehneotheriids of the Late Triassic, specifically the Late Norian (or
Rhaetian, if one accepts that time division). This is substantially alter
than the earliest known dinosaur, which are the early forms from the
?Carnian of South America.  Mammals certainly do not become really diverse
and widespread until the earliest Jurassic.

If one allows that the first mammals to predate the earliest records, one
must likewise allow that the first dinosaurs preceded *their* first
records, so that is a wash.  One is probably talking an early to mid-Norian
origin, at the earliest, for mammals, and probably a mid-Triassic origin
for dinosaurs.

Now, the real weakness of this exclusionary model is that the later
cynodonts were very similar to mammals, especially the Traversodonts,
Tritylodontids, and Thrinaxodontids.  And these occur rather earlier than
the mid-Norian - in fact Traversodonts are known from earlier than
dinosaurs, dating from the end of the Early Triassic.

Whatever went on, it looks like it was fairly subtle and complex.

--------------
May the peace of God be with you.         sarima@ix.netcom.com
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