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

[Jeff Poling <jpoling@dinosauria.com>]

At 12:30 PM 6/12/97 -0400, Thomas R. Holtz, Jr. wrote:
>Hope this helps.

   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:

    Under the definition of Dinosauria, there can be no nodes/ancestors
between Dinosauria and Ornithischia.  The most recent ancestor of
Ornithischia is Dinosauria, which is Triceratops's, X's and Y's common
ancestor with birds.  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.  Since
this is the common ancestor with birds, Ornithischia does NOT share a more
recent common ancestor with Triceratops than with birds.  I would think this
would exclude the ancestor from a stem based clade.

   Hence my confusion.

   (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")

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