RE: Cladobabble

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

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Matthew Bonnan
>
> Hi All:
>
> I've been outta the loop for a while, but I'm back on the
> dinolist again. =)

Welcome back!

> I suppose to briefly address Josh Smith's appropriate concerns about the
> subjectivity of character data, I would say that many skeletal characters
> have functional significance.  Perhaps by understanding what certain
> characters mean functionally and paleobiologically, we may be
> able to narrow
> down or eliminate certain characters, or even combine them.  Do
> the shape of
> the head of the humerus and the shape of the glenoid constitute
> two separate
> characters or one?  From a functional perspective, the shoulder
> don't work
> well without both parts, so perhaps these two characters should really be
> one.
>
Quite so.  As Brochu mentioned yesterday, one of the serious topics of
research in cladistic methodology is refinement of character concepts, and
within that realm is clarification of the independance (or lack thereof) of
separately coded characters.

I've reviewed a number of manuscripts in recent months that deal (directly
or obliquely) with this issue: indeed, from a couple of these I had to
rethink the character choice and coding in the Gaia MS.  Some things which
seem to be independant characters turn out, on more detailed examination,
turn out not to be separate at all, such as the case you give.

In others, morphometric examination reveals that rather than a binary (or
other discrete) distribution of features, there is a continuous spectrum.
For example, a much-cherished character state of early basal dinosaur
phylogenetics was "tibia/femur length: 0, more than 1; 1, less than 1" (and
sometimes phrased with the opposite polarity for studies of coelurosaurs, by
comparing them to their outgroup).  Studies by myself and others should have
put paid to this concept: tibiae grow with negative allometry with respect
to the femur in a wide variety of taxa, and thus if the animal is
sufficiently large it crosses over the threshold where the tibia becomes
shorter than the femur.   (Of course, the scaling is different for some
different groups of theropods, another issue...).

However, I've seen some folks who've proposed that all characters which
change state at a particular branching point must be functionally linked,
and that this criterion should be used to judge indendence.  I find this
funny, as a gross-level coelurosaur phylogeny would thus find that (for
example) a combined squamosal-quadratojugal prong extending into the
infratemporal fenestra and loss of manual digit III are functionally
linked...

Later,

P.S.: Should anyone have gotten the wrong idea from one of my postings
yesterday: I did NOT say that Barsbold is a bad person, nor that the 1983
paper is useless.  However, the assertion that no significant work in the
understanding of theropod evolution has been done since then is demonstrably
absurd, for the reasons I articulated immediately thereafter.

                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