[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

The Mystery of *Echinodon*



*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!!!!

__________________________________________________
Do You Yahoo!?
Make international calls for as low as $.04/minute with Yahoo! Messenger
http://phonecard.yahoo.com/