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THESCELOSAURUS AND ORNITHOPOD SYSTEMATICS



Recently, Jaime Headden posted an idea that _Thescelosaurus_ formed a clade
with _Dryosaurus- and _Tenontosaurus_.  Most of the characters he cited seem
to be based on the general large size and robust nature of both
_Thescelosaurus_ and _Tenontosaurus_.

In truth though, Thescelosaurus_ and _Tenontosaurus_ differ quite a bit in the
details of their skeletons, especially their teeth.  The teeth of
_Tenontosaurus_ are certainly those of a euornithopod: they have a strong
central ridge, very fine denticles, and a very wide occlusal surface.
_Tenontosaurus_ also lacks any premaxilary teeth.

Thescelosaurus_, by contrast, has teeth that are very similar in rough profile
to those of _Lesothosaurus_, having fewer, larger denticles.  There are also
numerous ridges in a  concentric pattern converging at the cingulum.
Additionally, _Thescelosaurus_ has 5 premaxillary teeth like most non-
iguanodintian ornithopods.

If anyone is interested, here is my phyologeny of the Ornithischia, with
special emphasis on the basal Ornithopoda.  It needs to be viewed in a
monospaced font:

+--+--Pisanosaurus mertii
   +--+--Lesothosaurus diagnosticus
      +--+--THYREOPHYRA
         +--+--+--HETERODONTOSAURIDAE
            |  +--MARGINOCEPHALIA
            +--+--+--Agilisaurus louderbacki
               |  +--Bugenasaura infernalis
               +--+--Thescelosaurus neglectus
                  +--+--+--+--+--Drinker nisti
                     |  |  |  +--Othnielia rex
                     |  |  +--"Hecaicursor" multidens ("Yandusaurus"
multidens)
                     |  +--+--Zephyrosaurus schaffi
                     |     +--Orodromeus makelai
                     +--+--+--Hypsilophodon foxii
                        |  +--Parksosaurus warrenae
                        +--+--"Tenontosaurus" dossi
                           +--+--Tenontosaurus tilettorum
                              |--Muttaburrasaurus langdoni
                              +--+--Gaspirinisaura cincosaltensis
                                 +--DRYOMORPHA

Peter Buchholz
Tetanurae@aol.com

If the Pilgrims woulda had Jay Leno's chin, they coulda plowed their fields
with it.