Diagnostic Types

[Jaime Headden <qi_leong@hotmail.com>]

  In their recent appeal to the ICZN, Paul and Carpenter (2010) argue among 
other things that the holotype of *Allosaurus fragilis* (YPM 1930) is not 
diagnostic, requiring a neotype to be designated for *A. fragilis* to preserve 
historic association.  Their other reasons are here irrelevant to whether the 
material is diagnostic, however.

  Paul, G. S. & Carpenter, K. 2010. *Allosaurus* Marsh, 1877 (Dinosauria, 
Theropoda): proposed conservation of usage by designation of a neotype for its 
type species *Allosaurus fragilis* Marsh, 1877. _Bulletin of Zoological 
Nomenclature_ 67(1):53-56.

  Paul and Carpenter write: "The holotype of A. fragilis consists of a tooth, 
two dorsal centra, and a proximal phalanx of right pedal digit III." (p.53)  
Then further "Because a tooth, a pair of dorsals and a toe element are 
inadequate to diagnose a genus, much less a species, YPM 1930 can only be 
identified to family level." (pg.54)  However, according to Chure's PhD thesis, 
Madsen (1976), and as depicted in Glut's Dinosaur Encyclopedia, the material is 
more complete. Specifically, Chure writes:

  "YPM 1930: Incomplee posterior dorsal centrum, anterior caudal centrum (may 
be a smaller dorsal, bottom is rounded), fragment of ?caudal centrum (broken 
and still in matrix in two pieces), rib fragment (possibly last dorsal), 
midshaft of right humerus, right pedal phalanx III-1, lateral tooth crown, with 
past of root in a separate block of matrix."

  A topotype specimen is known from where YPM 1930 was collected, USNM 4735; 
this material includes a partial forelimb (including humerus) as well as a 
pubis and an ischium.  Paul (1988) referred to this (incorrectly) as a 
paratype, and Paul and Carpenter (2010) request that the ICZN set aside YPM 
1930 for USNM 4735 as the holotype.  In Madsen's *Allosaurus* monograph, he 
noted that *Allosaurus* appeared to be diagnostic in the original material due 
to a humeroradial scar on the medial surface of the humeral fragment and a 
potential foramen in this scar.  Madsen also felt that a subsequent specimen, 
DINO 2560, should be designated the neotype, but Chure (thesis) invalidates 
this actt due to the continued existence of YPM 1930.

  So the real issue here is that blasé argument that YPM 1930 is undiagnostic 
(despite citing Madsen, 1976) coupled with the incomplete citation of the 
material belonging to YPM 1930 (presumably based on Marsh not fully noting the 
number of pieces belonging to the specimen -- not an uncommon practice, 
actually). No technical details surround the dismissal of YPM 1930 as 
undiagnostic, but it seems to be couped with the idea that so little material 
can be readily be diagnostic. As the ICZN does not regulate the definition of 
"diagnostic" nor do the authers seem willing to apply a definition of their own 
by which to compare the material, it seems questionable of this work to make 
the appeal to a replacement on those grounds.

  Can anyone elaborate to me any more concrete argument for "diagnostic" versus 
"undiagnostic"?
                                          
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