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Re: Paronychodon teeth are NOT homalocephalid fangs - killing an internet rumor



Mickey Mortimer (mickey_mortimer111@msn.com) wrote:

<An odd possibility was suggested by Olshevsky (DML, 1997), that some 
Paronychodon specimens, including the holotype, may be anterior dentary 
fangs of "homalocephalid" pachycephalosaurs (cf. Goyocephale).>

...

<The utter lack of "homalocephalids" in well sampled strata like the Dinosaur
Park Formation is particularily telling. Also notable is that each
"homalocephalid" only had eight fang-like teeth, but had around sixty-six
leaf-shaped teeth. So we would expect more pachycephalosaur teeth by a factor
of 8:1 or so, but Baszio (1997) showed this is not the case. For instance, he
recorded 12 Paronychodon teeth from the Dinosaur Park Formation, and only 16 
pachycephalosaur teeth. Similarly, Baszio recorded 84 Paronychodon teeth 
from the Milk River Formation, but only 16 pachycephalosaur teeth. Finally, 
Zinke and Rauhut (1994) described paronychodontid teeth within a theropod 
dentary fragment, though these differ from Paronychodon in some details.>

  I doubt this shows as much clear data as described, though somewhat
misleading in the extent that it would skew results toward a particular taxon,
when the tooth crowns recovered demonstrate a type recovery versus sampling.

  Tooth type, rather than taxon type, tends to differ in that some taxa has
similar types of teeth despite being wildly different. Historic confusion among
predatory archosaur teeth throughout, say, the Jurassic and Triassic has led to
some amusing confusing uses of "teratosaurid" ("rauisuchian") teeth for
dinosaurs, among others. Finding what are conventionally a predatory morphology
in some non-predaceous-seeming taxa, such as recurved, ziphodont teeth in basal
"prosauropods" and basal ornithischians, skews the assumption bias based on
isolated teeth. Applying these assumptions to mass collections without
extensive  comparisons to morphological variation in even a single jaw (e.g.,
assuming as has been historically popular that "reptiles" are homodont [bearing
a single dental morphology]) causes such a biased skew towards types.

  Two problems then arise given the quoted message:

  1) Assumption of normality in the tooth sampling biases, so that they would
be consisent when looked upon later, and

  2) Assumption of normality in the teeth of one taxa to all related taxa.

  Here's my basic response:

  *Goyocephale*'s dentary teeth are worn, from the apex basally down the
margins and onto the lingual as well as the labial surfaces, causing a pair of
longitudinal wear surfaces and the formation of not one, but three lingual
"ridges", one of which lies between the two wear facets and is produced more
distinctly as a result, and one "ridge" at each external boundary to these
facets. This is not a "normal" morphology of the tooth which would never be
found unworn or intact, due to diet. This is also not normal for a vegatarian.
One has but to look at tusked vegatarians to examine the functionality of
elongated "canine" teeth, though this begins to cause issues that arise when
one makes comparisons between reptiles and mammals.

  *Stegoceras* is not a representative pachycephalosaur, but is abundant in the
DPP to some degree, this is largely due to its dome. Collections in the DPP
have produced numerous other taxa, some with intact jaws (e.g.,
*Sphaerotholus*), but with a strong bias towards domes. Some flat-headed taxa
are known from overlying levels, such as *Dracorex* which presumably has
premaxillary teeth, and the undescribed "Sandy" specimen which may or may not
be related or like *Pachycephalosaurus* (which also has premaxillary teeth).
However, these teeth differ in morphology from those of *Goyocephale*, and
*Homalocephale* is incomparable as it lacks this region of the jaws.
Additionally, no pachycephalosaur jaw has been recovered to date that compares
to *Goyocephale*, upper or lower, in development of the mesial dentary
dentition or the distal premaxillary dentition. It then seems curious why it is
possible to use this data of known pachycephalosaur material to rate
comparisons to one, argually "basal" taxon? The sampling bias also seems
curious if given in relation to the dietary functionality of pachycephalosaur
teeth, but also in the referal of both the collection to particular taxa or no
particular taxa (junkbin taxa), and the arguably testy concept of making
general morphology referals to particular taxa, even particular species. A
valid criterion or two would be helpful when evaluating tooth-based taxa before
one starts dismissing a referal, or supporting it, for that matter.

  So far, body fossils of a number of curious dental taxa are unknown, despite
extensive sampling, which suggests that we do, in fact, have body fossils for
these taxa, or that these are more on the line of predators in which dentition
is left more frequently than body parts, or that the animals in question are
not lowlands/watershed dwellers and don't normally arrive at the regions in
which their teeth arrive sedimentologically or geologically.

  We don't know sh*t about tooth-based taxa.

  Cheers, 

Jaime A. Headden
http://bitestuff.blogspot.com/

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


       
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