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Re: Phytodinosauria status
I think this interpretation may be incorrect. Note, e.g., that Carnotaurus
retains four short ceratosaurian digits in an otherwise pretty useless
(whatever that means) forelimb. I think digits are lost only when they are
>positively detrimental<, not just useless. In other words, a forelimb
three digits will, when reduced, generally retain the three digits.
Here you are making an assumption about the evolutionary process: that
natural selection only acts to remove features that are detrimental, not
just neutral ("useless").
In tyrannosaurids and _Carnotaurus_ alike, the forelimbs became very small,
and a certain point (when the forelimbs ceased to have any grasping ability)
the number of fingers (digits) became totally unimportant. Why
tyrannosaurids have two fingers and _Carnotaurus_ has four may have nothing
to do with the selective advantage of javing two digits, and more to do with
the number (and/or position) of mutations that occurred in the lineage
leading to tyrannosaurids.
To put it another way, once the forelimbs were reduced to a certain size,
and were no longer used for grasping, the exact number of digits was removed
from selective forces. A certain mutation pops up in the gene that governs
the number of fingers in tyrannosaurids, with the result that the hand has
one less finger. No big deal - the mutation is not harmful to the animal,
so the two-fingered mutation persists. This mutation (or equivalent
mutations) may not have occurred in _Carnotaurus_ nor its ancestors.
To couch this in neo-Darwinian terms, "bad" traits are removed by selection,
and "good" traits are retained by selection - but neutral mutations (those
which are neither good nor bad, and have no effect on the organism's
fitness) may be retained through drift.
Timothy J. Williams
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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