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Re: The Mystery of *Echinodon*

I agree, but other genera also seem to be in this same small taxonomic area of divergence, especially Jeholosaurus and perhaps Agilisaurus.
Are Echinodon and Jeholosaurus enough alike that Echinodon could be a stem-thyreophoran and Jeholosaurus a stem-cerapodan, both of which branched off soon after the thyreophoran-cerapodan split? Or perhaps Echinodon is on the cerapodan stem, followed by Jeholosaurus (or vice versa?). Either way (or with Echinodon preceding the split) could Jeholosaurus be viewed as a sort of intermediate form between Echinodon and Agilisaurus?
Or does incompleteness for the fossil material and/or homoplasy make such questions impossible to answer at this time?
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

  Where the Wind Comes Sweeping Down the Pampas!!!!

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