Re: More phylogenetic taxonom (was Re: Def. of Ornithischia)

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 01:46 PM 6/12/97 -0400, you wrote:
>   I tried to understand, even drew cladograms on paper based on the
>examples you gave, but I still come up with the same problem (assuming
>Amniota is the node based Sauropsida + Synapsida).  Maybe it's the engineer
>in me but....
>
>   Let's see if I can comprehensibly explain my problem:
>
>   Ornithischia is defined as Triceratops and all taxa sharing a more recent
>common ancestor with Triceratops than with birds.  Dinosauria is defined as
>the most recent common ancestor of Saurischia and Ornithischia.
>
>   The cladogram looks like this (non-proportional font needed):
>
>birds X   Y Triceratops
>  \   \   |   /
>   \   \  |  /
>    \   \ | /
>     \   \|/
>      \ Ornithischia
>       \/
>     Dinosauria
>
>    I use Dinosauria to denote the actual common ancestor of Ornithischia
>and Saurischia and Ornithischia to denote the actual common ancestor of X, Y
>and Triceratops in the following comments:

To prevent confusion, I'll use ur-Ornithischian for the actual common
ancestor of X,Y, and Triceratops, and ur-Dinosauria for the acutal common
ancestor of Ornithischia and Saurischia.

>    Under the definition of Dinosauria, there can be no nodes/ancestors
>between Dinosauria and Ornithischia.

Precisely.  The first species on the line leading to Triceratops that was
not also on the line leading to birds was an (indeed, the first) ornithischian).

>The most recent ancestor of
>Ornithischia is Dinosauria, which is Triceratops's, X's and Y's common
>ancestor with birds.

More precisely, the immediate ancestor of the ur-Ornithischian was the first
species of dinosaur (which was also the immediate ancestor of the
ur-Saurischian).

>The clade Ornithischia is Triceratops and all taxa
>sharing a more recent common ancestor with Triceratops than birds.  The most
>recent common ancestor of Ornithischia and Triceratops is Dinosauria.

Ah.  I see your problem.

AS DEFINED in this argument, the most recent common ancestor of
ur-Ornithischian and Triceratops *IS* ur-Ornithischian.  Ur-Ornithischian is
a legit, honest-to-goodness, true, unquestionable ancestor.

Or, looking at the evolution from the opposite direction, the ur-Dinosauria
species left two decendant species with issue (and perhaps others which did
not have later descendants).  These two species were ur-Saurischian and
ur-Ornithischian.  All descendants of ur-Ornithischian are ornithischians.

>Since
>this is the common ancestor with birds, Ornithischia does NOT share a more
>recent common ancestor with Triceratops than with birds.

Yes, it does: since it shares itself.

>I would think this
>would exclude the ancestor from a stem based clade.

Ur-Dinosauria would be the most immediate ancestor to the clade Ornithischia
which is excluded from the group.

>   Hence my confusion.

Hopefully this helped to de-confuse...

>   (I assume that the two different ways of defining stem based clades are
>equivalent:  "all taxa sharing a more recent common ancestor with X than Y"
>vs. "all taxa closer to X than Y")

Correct.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661