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RE: Ceratops (was RE: Glishades ericksoni, ...)
Jaime Headden wrote-
> This is actually not favorable towards *Ceratops montanus* in the sense that
> you argue. Simply having a taxon named on the basis of a relatively low
> percentage of the skeleton does not validate the practice of naming taxa on
> such a basis. Virtually all taxa named from one, or two bones, even if they
> are cranial, reduces the comparability of the taxon used.
> We have two ways to compare percentage of the skeleton: Valuing the total
> bone mass, and subtracting the material not preserved; and valuaing the bone
> preserved as though it were complete (both approaches are covered in ).
> Mannion and Upchurch  add a value for sauropods where the gross number of
> definable characters relative to the body region/skeleton can also be used,
> and it represents a third and novel manner in which we evalutate the
> completeness of a skeleton. We can use these numbers to help us determine
> whether a specimen is "diagnostic," and assign a threshhold value for
> taxonomic nomenclature. We don't and so far very few people have attempted to
> do so. Taylor  recently valued the number of characters by which the
> brachiosaurs *altithorax* and *brancai* differed, and this represents a
> particularly interesting starting point from which to discuss the value of
> ranks in the bias of taxonomic nomenclature (and make no mistake, there are
> more than just a few biases involved).
> Most of the taxa you list above are rendered difficult to associate with
> others simply because of their incompleteness. You need to have particular
> portions recovered in order to support these taxa generally, although in the
> specific one _could_ make the argument that gross differentiation is all you
> need. The problem with this argument, though, and one of the things that
> makes me like the idea of making it so that all these taxa are nomina dubia
> and should not be made the basis of further taxonomy is that this material is
> directly differentiable only through a temporal gap in our knowledge: we will
> eventually find more out there, will eventually conflate different taxa from
> different formations based on some hypotheses, and will eventually merge taxa
> on the basis of representing different parts of the same skeleton -- not just
> the same type of organism, or taxon, but even the same original individual.
Sure comparability is limited, but that's an unfortunate reality of vertebrate
I think any attempt to assign a threshold value for taxonomic nomenclature is
naive. A single fragment could preserve an autapomorphy while much larger
sections of the skeleton could be indistinguishable from two other similar
taxa, given our current knowledge. And that latter qualifier is an important
argument against Mannion and Upchurch's value where the "gross number of
definable characters relative to the body region/skeleton can ... be used",
since this is dependant on the extent of published studies at any given time.
Take tyrannosaurid pes characters for instance. Looking at any published
matrix or most published descriptions, you would get the impression that within
Tyrannosauridae, the pes structure is basically useless for distinguishing taxa
besides larger genera having more robust bones. Yet as part of his
reexamination of Alectrosaurus, Carr (2005) found 163 pedal characters that
varied among tyrannosaurids. All of a sudden, there's hope for "Ornithomimus"
grandis whereas before everyone would say "it's just a metatarsal III - instant
nomen dubium!" Maybe ceratopsid orbital horn cores are similar if someone
examines them more closely, maybe they're not.
Your objection that the taxa above are only temporarily diagnostic, pending the
discovery of two or more specimens that both show their supposed
autapomorphies, could be applied to any taxon. Say we found two Gorgosaurus
specimens that preserved an amazing amount of skin impressions that indicated
they were different species. Suddenly Gorgosaurus libratus is a nomen dubium
because its holotype doesn't preserve skin impressions, despite being a
complete skeleton. Or if Djadochta Velociraptor turns out to be two taxa that
differ in several pelvic and hindlimb characters. There goes Velociraptor
mongoliensis, since the holotype is just a skull and two manual phalanges.
That taxonomy will inevitably change as we discover more specimens is no reason
to stop trying our best now with the data at hand. Might as well just call a
moratorium on phylogenetic analyses until we have discovered all the preserved
specimens and dedicated several centuries to teasing out all their variable
data and perfecting phylogenetic algorithms. After all, the trees we'll make
then will make today's pale in comparison.
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