RE: Ceratops (was RE: Glishades ericksoni, ...)

[Jaime Headden <qi_leong@hotmail.com>]

Mickey Mortimer wrote:

<Lots of valid taxa are based on single elements- Kemkemia, Ozraptor, 
Becklespinax, Iliosuchus, Kakuru, Rapator, Unquillosaurus, Caenagnathasia, 
Pneumatoraptor, Richardoestesia, Itemirus, Urbacodon, the famous 
Xenoposeidon...>

  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 [1]). 
Mannion and Upchurch [1] 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 [2] 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.

[1] Mannion, P. D. and Upchurch, P. 2010. Completeness metrics and the quality 
of the sauropodomorph fossil record through geological and historical time. 
_Paleobiology_ 36:283-302.
[2] Taylor, M. P. 2009. A Re-evaluation of *Brachiosaurus altithorax* Riggs 
1903 (Dinosauria, Sauropod) and its generic separation from *Giraffatitan 
brancai* (Janensh 1914). _Journal of Vertebrate Paleontology_ 29(3):787-806.
                                          
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