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Re: mammalian reptiles

From: MFHanson@aol.com
 > To: Multiple recipients of list <dinosaur@lepomis.psych.upenn.edu>
 > What specific details determine that a fossil is from a reptilian mammal?

Well, presuming you mean mammal-like reptile, the most obvious
is the structure of the skull.  In mammal-like reptiles the
skull generally has a single opening behind the eyes, placed
high on the skull.

In mammals proper this opeining has enlarged greatly - becoming
the occipital ridge above and the zygomatic arch below.

The lineage including lizards, crocodiles, and dinosaurs originally
had two openings behind the eyes, one high and one low.  In lizards
and snakes these opening have become modified in a way that makes
them hard to locate - the enclosing bars of bone have been lost,
leaving very little indeed of the original holes.

The lineage containing turtles never had any holes behind the eyes,
but rather it has a solid skull.

There are, of course, other differences, but they are less obvious.
(For instance differences in the structure of the various limb joints).

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

The peace of God be with you.