From: "Jaime A. Headden" <qilongia@yahoo.com>
Reply-To: qilongia@yahoo.com
To: dinosaur@usc.edu
Subject: The Mystery of *Echinodon*
Date: Wed, 22 Aug 2001 12:01:08 -0700 (PDT)
*Echinodon becklesii* Owen, 1861, represents a taxonomic enigma. It has
been referred to the
Fabrosauridae, removed upong disbandment of that taxon, allied to the basal
ornithopods,
heterodontosaurids, and basal thyreophorans, on the basis of the
fragmentary type cranial
material. These comprise most of the maxilla, and nearly all of the dentary
and premaxilla. There
are 11 maxillary and 10 dentary teeth, the first maxillary tooth of which
is fanglike, whereas all
other teeth (aside from the premaxilla) are phyllodont, and strictly so; no
tooth is recurved, or
chisel-shaped, and the wear facets are confined to the tips of the primary
ridges, and not the
entire distal edge of the tooth. These features, along with the
articulation of the predentary to
the dentary (predentary is bifurcated caudally), lack of fanglike
premaxillary and anterior
dentary teeth, indicate that *Echinodon* does not pertain to the
Heterodontosauridae or the
Marginocephalia. The teeth lack any form of cingulum, but the primary
ridges of the teeth are
confluent with the bases in large expanded platforms which are distinctly
separated from the rest
of the surface of the crowns; the denticles have ridges on the surface of
the crown that extend to
the base, and the denticulate ridges are completely vertical without a
fan-like radiation. The
strong rostral taper of the dorsal and ventral margins is plesiomorphic,
while the reduced
dentition count coupled with relative maturity of the specimen (wear facets
are well-developed),
first maxillary fang, apparent diastema with slender and shallow caudal
premaxillary process and
large external naris, suggest these are autapomorphies of the species.
There is no lateral recess
of the diastema, as in *Heterodontosaurus* and even fangless
*Abrictosaurus*, and *Goyocephale*,
suggesting the diastema is not comparative in form or function. The
premaxillary teeth are small,
almost blade-like, but well-spaced and not close together. These features
indicate that the
species is neither ornithopod (form of teeth), basal to the Genasauria
(form of teeth, premaxilla
form), higher thyreophore (form of premaxilla, form of premaxillary teeth),
heterodontosaurid (see
above), and leaves the animal as either the unknown form at the base of
Cerapoda, base of the
Genasauria or more advanced than *Lesothosaurus* [maybe barely], or a basal
thyreophore. Except
for the fang, diastema, and lack of a cingulum, and the rostral edentulous
portion of the dentary,
the teeth and jaws compare very well to *Scutellosaurus*, and I personally
see this as an
unresolved taxon in a relatively small space of taxonomic divergence (basal
genasaurian,
thyreophore or cerapodan).
=====
Jaime A. Headden
Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
Where the Wind Comes Sweeping Down the Pampas!!!!
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