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Theropod Postcranial Isolation
Nick's satirical posts on theropod skeletal stasis prompted me to write
something, dealing with differentiation of observed shapes.
As a visual thinker, I've spent a little time looking into how the brain
perceives objects and the psychology of assigning concepts to these
perceptions, and the philosophy fo accepting or discarding them, since this
leads directly to shape analysis by the brain and how we decide whether
something is distinct, and how to parse this. Yes, character selection. Visual
thinking in the human brain versus analytical thinking tend to involve
abstracting and perceiving of shapes in representing the world and objects
within it. For example, "seeing" words as one speaks (as I do) and often
gesticulating while speaking to describe various ideas of shape, space,
distance, and so forth.
But character selection as a process is often relegated to analytical
thinking, concepting in absolute, mathematical reducible structure, and
communicating in language that tends to offer little, if any, visual
accompaniment. Thus for visual thinking, this form of dscription tends to be
very dry. I do tend to notice pictures in a book more than the text, and thus
must concentrate to read. Thus I prefer books without pictures because it
allows me to concept on my own and not be provided with distractions.
Anatomical papers are different, and probably one of the great problems of the
age has been the dry analytical papers with descriptions of bones,
comparability of bones, and descriptions and distinctions of characters, that
do not bear accompanying shape variation on a visual level. Thus it seems easy
to set aside variation when discussing in absolute terms. Analytical
condensation of form and pattern and divergent spacial observation in visual
thinking cannot meet this way, but both offer a means of determining variation
or the lack thereof.
Looking at a skeleton, we should "see" what it is about that skeleton that
makes it so special it gets a new name. Seeing a new species of barbet from the
Andes with a distinctive pattern of head feathers within a population of such
birds, becomes immediately verifiable, whereas discussing the ATGC variation of
the DNA of that animal in "genotalk" can become boring really quickly and
doesn't actually tell you what you are seeing, since the animal is now just a
series of letters of molecule abbreviations. It would appear that unless
figures accompany text, most anatomical descriptions read like "my
preconceptions on why this shape is not important, but this is, because I can't
perceive that one but see this one, by level of degree." This is inherently
dangerous, because one does not provide much in the way of showing anyone else
WHAT they are seeing.
As an example of dry description, just read the short-form _Nature_ paper on
*Sinosauropteryx*. Indications of what is what is in the description are
lacking. Compare to Senter's thesis and resulting analysis of Diapsid phylogeny
and dromaeosaurid phylogeny, which provide figures of the character states in
his analysis, a practice I wish all systematists working in morphology would
use because it would implicitly indicate the variation from an immediately
visual experience, and allow comparative coding for the test of the analysis'
results. (like, what's the cut off between an "L"-shaped versus a "7"-shaped
lachrymal? Degree of angle? Measured from which point to which point across
which similarly measured axis?
It is my opinion that such dry concepting has impaired testability to a
degree that one must simply redo an analysis with their own characters EVERY
TIME they study a group. This seems especially true of shape concepting, what I
call "aspects" of a bone (aka, the lachrymal is an inverted "L" in aspect when
viewed laterally). Aspect, as a term, derives directly from a perception, and
thus lets you know you are dealing with an arbitrary shape feature, and would
seem a right-brained quality of the observer. Quantifiable characters such as
tooth counts are much more concrete and do not neccessarily benefit from such
concepting due to their analytical, left-brained nature. So it's easy to get
away with the latter type of character than the former type of character.
Yet in many animals, the analytical, quantifiable characters are usually more
prominent (absent/present couplets) versus shape variation and shape
abstraction, thus leading to a weight towards one than the other. At this
point, it is easy to see why groups would look more at the more variable
portions of anatomy (usually skulls) before the less variable (aka, less
selected upon) regions (limbs, tail, trunk, etc.). But proposing that such
prominence of variation in one region supercedes that of another, and thus
permits one to dismiss variation as "too small" or even to prevent them from
_seeing_ it, would seem to be left-brained observation in the fore. It may also
be that describing shape variation becomes excessive and is more difficult to
describe both mathematically, explicitly, and visually, than are analytical
characters. If theropods are so static in body form, then this may be true and
we can get on with discarding postcrania, isolating it from the body of our
work and observations and bring this on par with work that endorses cranial
systematics of many groups. Does a skull, with about 5% of a theropod's
skeletal body mass, truly account for 50-75% of the skeletal variation? IS it
the most selective region and thus under the most pressure to vary and exhibit
more "noticeable" features than, say, the pelvis? For that matter, does size
variation really not count if inherited, and thus size-related features such as
"robusticity" really DO count?
I think, in conclusion, a more complete and collective mode of character
selection, taking time to study shape as well as quantify it and then
illustrate it and variation, will aid systematics more than focusing on regions
one thinks are intuitively more phylogenetically informative.
Jaime A. Headden
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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