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

[NJPharris@aol.com]

In a message dated 97-06-13 03:27:28 EDT, jpoling@dinosauria.com (Jeff
Poling) writes:

>    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).  

Just to be on the safe side, maybe we'd better have a stem-based Amniota (all
tetrapods closer to mammals [or birds, or crocodiles] than to
lissamphibians)--There are certainly animals with an amniotic egg that
predate the split between synapsids (or theropsids) and sauropsids.


>     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.

...and all descendants of that most recent common ancestor...


>     The cladogram looks like this (non-proportional font needed):
>  
>  birds X   Y Triceratops
>    \   \   |   /
>     \   \  |  /
>      \   \ | /
>       \   \|/
>        \ Ornithischia
>         \/
>       Dinosauria

Except that this cladogram makes Ornithischia look like a node-based taxon!
 In reality, the label should go just below the first forward slash above
Dinosauria:

       \   \|/
        \  /
         \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:

...but that's not how Ornithischia is defined.  What you are defining here is
some node-based taxon *within* Ornithischia.

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

True.  Saurischia and Ornithischia together account for all dinosaurs except
the most recent common ancestor of the two.


>  The most recent ancestor of
>  Ornithischia is [located at the node labeled] Dinosauria, which is
Triceratops's, X's >  and Y's common ancestor with birds.  

No, the most recent common ancestor of Triceratops and all other
ornithischians is located one mathematical point *above* "Dinosauria" on the
ornithischian side.  As a stem-based taxon, Ornithischia includes animals not
shown on your cladogram (indeed, it includes animals that have not yet been
discovered--nor are likely ever to be).

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

Yes.


>  The most recent common ancestor of Ornithischia and Triceratops is
Dinosauria.  

No.  That first forward slash between "Dinosauria" and "Ornithischia" on your
original cladogram is shorthand for a whole string of species.  The first of
these is the MRCA in question here.


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

I'm not sure what you're saying here.  All ornithischians must share a more
recent common ancestor with Triceratops than with birds.  That is what makes
them ornithischians.


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

To which ancestor are you referring?  If you are referring to the ancestor at
node "Dinosauria", then no, that ancestor is not included in Ornithischia (or
in Saurischia).  If you are referring to the MRCA of all ornithischians, then
yes, this ancestor is included in Ornithischia, as it shares a more recent
common ancestor with Triceratops than with birds--namely, itself!


>     (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")

That is my understanding.

Whew.  I was starting to confuse myself there!  Hope this helps.

Nick Pharris.