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

Re: Tiny dinosaur on verge of swearing off meat



ups, said "digits" in one place where I tried to mean "teeth". I think
the recurved anterior teeth of Lesothosaurus and other basal taxa may
be also primitive retentions (would need to count required steps under
each assumption of character evolution however, perhaps not if more
basal taxa do not have recurved anterior teeth).

2008/10/27, Augusto Haro <augustoharo@gmail.com>:
> 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.
>