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"Mammal-Like Reptile" systematics [was Re: feathers, hair and Compsognathus]



At 08:58 AM 10/23/96 -0600, you wrote:
>
>>         The origional Class Reptilia (Linneaus) was paraphyletic, as it
>> includes early synapsids but not the mammals, and early Sauropsids, but not
>> birds.  As it stands now, Reptilia has been redefined so that it is
>> monophyletic, but excludes the "mammal-like reptiles" and includes the
>> birds.  For some perspective, see the Tree of Life homepages (sorry, URL not
>> handy, e-mail off-list me if you can't get it).
>
>      Does this mean that mammal like reptiles are NOT descended from
>animals with reptilian traits that got these reptilian traits from a
>same common ancestor that reptiles got thier reptilian traits from?
>Exactly why are synapsids excluded?

        I am afraid that I am not up-to-date on the exact reasons, but I
believe that the idea was to cladistically define Reptilia to be as close as
possible to the origional idea of the group, yet still be monophyletic.  If
the synapsids were included, all of the mammals would be included, and you'd
have the clade Amniota.
        "Mammal-like reptiles" (poo poo on said terminology :) ) got their
"reptilian" traits from the same place we did, the birds did, the dinosaurs
did, etc., that is, from the common ancestor of all amniotes.  These traits
are apamorphies of the Amniota, and thus are symplesiomorphies for clades
derived from Amniota.  Thus, the traits which caused scientists to group
"mammal-like reptiles" as members of Reptilia are not useful for determining
phlyogenetic relationships, since all primitive amniotes should share them,
regardless of where their clades were "going" evolutionarily.  What we need
are synapomorphies (derived traits), and "mammal-like reptiles" share more
derived traits with mammals than they do with Reptilia (sensu Gauthier et
omnia cladistica(?)).  Hence they are members of the Synapsida, the sister
stem to Sauropsida (upon which Reptilia resides), and which includes the
Mammals.
        So all amniotes did indeed descend from an animal with "reptilian
traits", but the Reptilia, as defined today, includes only some of these
animals, based on a different group of derived states which unit the
Diapsids and the Anapsids (including the turtles), if I am not mistaken.
Recall, however, that a clade is neither defined by it's constituant taxa,
nor by the characteristics they posses (although it can be defined by shared
derived characters, this is potentially destabilizing, and should be
discouraged (Holtz 1996, and the references therein)).  A Clade is defined
as the most recent common ancestor of two (is it always two?  I need to read
up a little more...) animals and all of it's decendants (a node-based
clade), or all animals more closely related to one taxon than to another.
        So, while they look like "reptiles" as we know them today, the
"mammal-like reptiles" are a group of amniotes which exhibit some
plesiomorphic characters, but are more closley related to us than they are
to any modern "reptile" or "bird" youcare to mention.
        Hope that helps...  wince...
        :)
        Wagner
+--------------------------------------------------------------------------+
| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
| Lubbock, TX 79409                               and if they don't clade, |
|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
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