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Re: Tiny dinosaur on verge of swearing off meat



Wow, didn't know Azendohsaurus turned to be so basal, most actualized
thing I had was "The Dinosauria, 2nd ed" claim that it can be not
dinosaurian, citing some few characters. Do you know if more than that
abstract (which do not mention phylogenetically interesting
characters) has been published on Azendohsaurus or its Malagasy
relative?

2008/10/27, Jaime A. Headden <qilongia@yahoo.com>:
>
>   Except that these perpendicularly oriented denticles are present despite
> other teeth displaying modifications of the crowns. Note that in
> *Goyocephale*, the cheek teeth are cingulate, possess a distinct lingual
> vertical ridge, and the denticulations are oblique to the carina, and very
> coarse, in effective being phyllodont in form. In *Heterodontosaurus*, the
> cingulum is less distinct but the vertical ridge still distinct, and the
> denticulations are restricted to the apical half of the crown with oblique
> orientations, with orthal processing helping produce a tooth with primarily
> apical and transverse wear instead of wear oblique to the crown axis,
> causing the teeth to take on a chisel-like appearance. This is a
> specialization, and as such the retention of the premaxillary form seems not
> in keeping with the specialization of the cheek teeth, especially as basal
> taxa, including *Azhendohsaurus*, have "coarse" oblique denticulations of
> the known crowns,

But that some teeth especialize and evolve too much does not
necessarily imply that all teeth have to be equally derived and
modified (a mechanism of obligatory covariance should need to be
proposed, which can be later conveniently broken. At least, it is more
parsimonious to hypothesize that serrations continued in anterior
digits from the ancestral condition instead of assuming they changed
with the other teeth to latter revert to the primitive condition. Now
Azendohsaurus turned out to have little to do with dinosaurs.

>  and the basal ornithischian condition, as can be inferred from taxa like
> *Scutellosaurus*, *Agilisaurus*, and *Lesothosaurus*, agree that the
> oblique, phyllodont condition is the primitive condition for all teeth. This
> means that the premaxillary variation seen in Heterodontosauridae and
> Marginocephalia are unique to them as a clade. It is also important to note
> that that the basal condition for onrithischian denticulation density and
> orientation is coarse and oblique, rather than fine and perpendicular; it is
> then arguable that *Heterodontosaurus* developed the premaxillary
> modifications for the sake of rendering meat to some degree. They may even
> belong to the same clade as a result of this (although this seems to have
> gone under testing and been rejected, at least by Peyer and Butler).

I do not know if there is a phylogeny of Ornithischia more updated
than this: ^ a b Butler, Richard J.; Upchurch, Paul; and Norman, David
B. (2008). "The phylogeny of the ornithischian dinosaurs". Journal of
Systematic Palaeontology 6 (1): 1–40.

There, Heterodontosaurus forms a polytomy at the base of Ornithischia;
its position, together with that of other "heterodontosaurids" is more
basal than that of Lesothosaurus or Scutellosaurus, at least putting
in doubt their relationship to Marginocephalia.

The "predatory" hand characters of Heterodontosaurus, such as the
retractor pits in metacarpals and phalanges, elongate penultimate
phalanges and recurved unguals turn out to be Herrerasaurus-like
plesiomorphies, according to paper by Butler et al. (2007) on
Eocursor, so that the "predatory" features of the canine, similar to
those of theropods (i.e., lack of basal constriction, fine serrations
perpendicular to edge) may be most parsimoniously considered as
similar retentions if accepting the basal position of
heterodontosaurids that Butler et al (2008) indicate. In this I
however admit the enlargement of the "canine" as truly apomorphic, and
perhaps defensive.