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Non-Diagnostic Typification

  The latest issue of _Acta Palaeontological Polonica_ also contains the
previously mentioned new descriptive revision of *Hadrosaurus foulkii*,
Linnaean type species for the taxon Hadrosauridae (rank of family) and all
"pararanks" around it.

  Prieto-Márquez, A., D. B. Weishampel, & J. R. Horner. 2006. The dinosaur
*Hadrosaurus foulkii*, from the Campanian of the East Coast of North America,
with a reevaluation of the genus. _Acta Palaeontologica Polonica_ 51(1):77-98.

  Most important in the paper is the determination that the material is
non-diagnostic as a species. The problem with this, however, is that the
authors are quick and capable of differentiating *H. foulkii* from other taxa,
and are capable of informing us that certain taxa, such as *Gryposaurus* and
*Kritosaurus*, are not synonyms of *H. foulkii*. Indeed, the authors go to some
pains to show that this taxon IS distinct from other taxa, only to state:

  "[T]he synonymy of *Hadrosaurus* with *Gryposaurus* is not supported by the
present osteological data, primarily from differences in the humerus and ilium.
It is more likely that ANSP 10005 represents a separate taxon, probably at the
generic level." (pg. 89)

  Though I am not sure how one manages to distinguish *H. foulkii* only at the
generic level, but that is the point, anyway. This would argue the authors CAN
treat the taxon as distinct.

  Yet, they argue "We will show that the holotype of *H. foulkii* lacks
distinguishing characters, but that it shares apomorphies with Euhadrosauria
(sensu Weishampel et al. 1993; the clade formed of Lambeosaurinae and
Hadrosaurinae within Hadrosauridae) and, tentatively, with Hadrosaurinae" (pg.
78) [...] "In conclusion, it is not possible to unequivocally diagnose and
distinguish *H. foulkii* from all other hadrosaurid genera and species.
Therefore, *H. foulkii* must be regarded a nomen dubium." (pg. 87)

  And later, "[W]e recommend preserving *Hadrosaurus* and *H. foulkii* as the
type genus and type species, respectively, of Hadrosauridae. However, because
the membership of *H. foulkii* to the Hadrosaurinae is only tentative and not
fully reliable (see below), we prefer to wait until further data are available
in order to provide a conclusive answer as to whether this taxon can remain the
type genus and species of this clade." (pg. 88)

  So to maintain stability of the name Hadrosauridae, they will _keep_ it,
despite it "lacking distinguishing characteristics," but they are capable of
discerning it from other taxa so clearly it can be placed in a phylogenetic
analysis without the tree collapsing. They code it for 16 characters in a 90
character matrix with 14 taxa, 17% of the available codings (of which _only_
66% of the characters are cranial, yet these comprise the bulk of *H.
foulkii*'s codings), and still manage to produce a semi-coherent tree
preserving known topologies of Lambeosaurinae vs Hadrosaurinae, as well as
finding (though coded for) the *Maiasaura*/*Brachylophosaurus* clade, which Jon
Wagner says we should just call *Brachylophosaurus* and be done with it! In
this, should we know that the character sampling should just include more
postcrania? We have a substantial amount of the fore and hindlimb accounted
for, yet it is not extensively sampled. Nonetheless, the study shows us that,
even though the type specimen appears to lack _autapomorphies_, it is probably
_apomorphic_ and may bear an _apomorphic suite_ which distinguishes it from
other hadrosaurids, and allows its placement within Hadrosaurinae. There is
thus no question of its value as a useful taxon, and one should not regard it
as a _nomen dubium_ because, as it may be, the cranial material is so scarce.

  But this does raise a group of questions, and the purpose of this post:

  At what exact level does a taxon become useless for a type? Are we required
to have autapomorphies when diagnosing taxa or names based on them? Or are
apomorphic suites sufficient and thus indicative of a diagnostic taxon? Are
apomorphic suites useful for distinguishing species?


Jaime A. Headden

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

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