RE: REALLY good introduction to cladistics!!

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> philidor11@snet.net
> Sent: Tuesday, October 23, 2001 7:59 AM
> To: david.marjanovic@gmx.at
> Cc: dinosaur@usc.edu
> Subject: Re: REALLY good introduction to cladistics!!
>
>
> So they are willing to assume that direct ancestors can be identified
> in the second window discussion, but are unwilling to assume
> an amniotic egg for an animal that far up the amniotic branch?

Actually, they don't say that direct ancestors can be identified.  Here is
the text from the panel in discussion:

"By looking at early relatives of caimans and parrots, we see that they all
have two openings on the side of the skull. These later got lost in the
lineages leading to crocodiles (such as caimans) and birds (including
parrots).

The same is true for humans and hares."

And, the proper coding for the presence or absence of an amniotic egg in _T.
rex_ IS "?", until such time as it is demonstrated.  Yes, we can infer its
presence, but the coding of the matrix is for observations, not inferences
(otherwise you run into circularity).

> One of the characters they use to demonstrate that T. rex's share
> a more recent common ancestor with parrots than with caimans
> is the 'heel knob'.  This they note is unique to crocs.  Unless
> you're looking at the question of whether T. rex descended from
> a croc, I thought unique to a group characters were considered
> bad form.
> Is this correct?

Obviously you've never designed a lesson plan...  The entire point of that
part of the excerise is to demonstrate that autapomorphies and
symplesiomorphies (not named as such, but that's what they are) are not
helpful in phylogenetic reconstruction.  They show that the presence of the
autapomorphic nature of the croc's heel can't help you resolve the
relationships of crocs, parrots, and tyrants.

Also, this is an INTRODUCTORY lesson: it isn't someone's Ph.D. dissertation!
For that matter, it isn't even a review of vertebrate phylogeny.  It is just
to give the basics of the method. They most certainly are not going to deal
with every single relevant character and its distribution on the line from
sharks to _T. rex_, any more than an introductory algebra class is going to
deal with graduate level number theory.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796