RE: Sereno's (2005) new definitions

["Jaime A. Headden" <qilongia@yahoo.com>]

Mickey Mortimer (mickey_mortimer111@msn.com) wrote:

<It doesn't matter for the analysis.  That merely drops the "premaxilla
toothed" and "premaxillary teeth not D-shaped" characters, and it still ends up
as a troodontid.>

  For such a comprehensive analysis, it doesn't seem to state much for
explication variation along the tooth row, which is certainly available for
many taxa. Not all "D-shaped" premax teeth are shaped like tyrant D's and
lumping them ignores this variation, and likely causes homoplasy. But that's
not the issue I brought up. To compare the tooth to another taxon, you MUST
have a tooth in the same position to correctly evaluate it. Spatial
relationships among tooth rows are NOT quantified for many taxa, especially of
value those using tooth-based types, and recent studies where taxa ARe
quantifies show that while variation is uniform to a taxon, there is still
substantial variation and a confidence of around 5-10 positions is about as
close to a single placement in mostly isodont taxa as you can get. In
heterodont taxa like troodontids, tyrannosaurs, this confidence narrows to 3-4,
or maybe even 2-3, but these taxa, as in ceratosaurs and dromaeosaurids, can
have identical uppers and lowers. The diagnostic nature of the *Troodon
formosus* type depends on its distinctiveness in a phenetic sense, which has a
high value, but its relevance requires comparability, which is low, despite
being distinct, because of its isolated nature.

  Furthermore, finding the tooth as a troodontid is largely irrelevant, since
no one has doubted this placement, and in fact Troodontidae will always contain
*Troodon* as a matter of correspondence and definition. The issue of gross
limited tooth characters is related to a lack of explicit understanding of
tooth-based variation, convergence, and its relevance to phylogeny, which is
assumed under the banner of "whole data inclusion". This same banner will allow
us to use robusticity indeces and size-related characters again. It is likely,
after all, that if tooth-size relationship is valuable, and animal size is
inheritable, then gross size measurements are phylogenetically informative,
even if homoplasious.

  Cheers,

Jaime A. Headden

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

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