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Re: Theropod taxonomy based only on shed teeth (Re: Dromaeosaur questions (long, I think))



Phillip Bigelow (bigelowp@juno.com) wrote:

<Even if one uses a small character set (and assuming my math is correct),
then the total number of possible combinations of traits is huge: over a
million combinations, in fact. Below is my conservative (and probably very
incomplete) list of 15 theropod tooth characters, each character having
between 2 and 4 possible states.  I ran these numbers through the above
algorithm (again, someone *please* comment if I calculated it wrong!).  I
found that there are 1,119,744 possible combinations of the 15 character
states (see below) that a hypothetical theropod tooth could have.  Someone
who is more knowledgeable that I am could find 15 or so different (aka,
"better") tooth characters of their choice, but the resulting number of
combinations should be roughly the same.  I have some further comments
after my list of characters (below).>

  Increase the number by the relative value for EACH tooth in the jaw, so
that for some animal like *Pelecanimimus,* that variability would be
257,541,120, taking into account the some odd 230 teeth. An animal like
*Edmontosaurus annectens* would be in the billions of calculations. NO one
has truly assessed the potential morphometric properties _along_ the
hadrosaurid tooth row and down each column in a single study, nor really
have they done it separately.

  For someone like me who wants to know the utility of a tooth's shape on
taxonomy and diagnosis, and the "observable" variability inherent in
various teeth, such a character list would be long overdue. However, the
variations seen in some of these features is, admittedly, under-expressed.
One point neccessary to make is that not only is tooth morphotype variable
and often mimicking for carnivores, but the arrangement and degree or
absence of heterodonty is also a major factor in distinguishing some teeth
or taxa via teeth.
 
<Crown length:>

  How long is long or short is stubby? A quantification, based on the
maxillary length or an example, can easily be distorted by shorter or
longer maxillae, as occurs with regards to allometric scaling in
tyrannosaurids.

<Cross-sectional appearance:>

  The third state, "Crown has other shape in cross-section" would permit
rather peculiar state associations or lack there-of, as even the other
states are not really easily quantifiable and change along the crown in
other species. The crown of an advanced, *Saurornithoides*-like troodontid
maxillary tooth changes from "dumbell"-shaped at the base and become
rhomboid apically. The morphometry of the cross-sectional shape and
relative crown areas at the base and long the tooth itself, also
potentially add variability, but one sees this as leading into a possible
6-7 state character, if not more, based on degrees of curvature that would
serve to make some taxa less prone to group with others. That being a
problem with some potentially more complex bistate characters out in play
today, which simplify things like size of an object, or "ziphodont" versus
"phyllodont" where we get "conodont" lumped somewhere, loose sight of the
complex of dental shapes like "ctenodont" (as in some lemurs and
thyreophorans, not to mention other primates such as humans) and the
distinctiveness of denticle shape itself. Presence or absence of the
ampulla in the blood pits, relative size of the blood pit between
denticles ....


<Even if one used a slightly revised total of 25 tooth characters with,
say, a *conservative* two states per character, that would mean that there
are a whopping 33,554,432 possible combinations of characters that a
hypothetical theropod tooth could possess!>

  Plus each tooth compared, making *Coelophysis* (of around 100 teeth)
with 3,355,443,200 or so calcs.

<However, not all combinations of tooth character states are beneficial in
an evolutionary sense.  For instance, long, laterally-compressed teeth are
biomechanically worthless because they will easily break. >

  This is not really true, because the benefit of such a tooth is to
increase the mesiodistal strength and anti-bending properties of the
crown, as when pucturing or slicing, or tearing out of the prey; this is
why crocs don't have ziphodont crowns for the most part, but recurved
conodont, based on the neccessity of twisting the teeth. Otherwise, it is
a prevalent condition among theropods and other carnivorous archosaurs to
have strongly ziphodont teeth.

<Similarly, a bi-cusped theropod fang is redundant (and it would probably
be biomechanically weak).>

  As in mammaliaforms and some crocs, such a tooth increases the
puncturing properties of the crown and allows the tooth to more pulverize
than tear, so that the added "tines" and cusps serve to increase the
damage done when the jaw closes without increasing jaw strength.

<Infrequently-occuring character combinations would require an
explaination as to why they are seldom found in the fossil record.  For
example, why are Troodon teeth so distinctive compared to other tiny
theropod teeth?>

  Omnivores are almost always plentiful and successful :)

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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