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Re: ANKYLOSAURS, STEGOSAURS, AND PSITTACOSAURS



----- Original Message -----
From: "Steve Brusatte" <dinoland@lycos.com>
To: <dinosaur@usc.edu>
Sent: Thursday, July 04, 2002 9:15 PM
Subject: Re: ANKYLOSAURS, STEGOSAURS, AND PSITTACOSAURS

> That is if we recognize _P. sattayarki_ as valid.  Sereno seems to think
>not.  I have no opinion yet, as I have not yet read its description.
>However, if indeed the type specimen comes from two different individuals,
>and if indeed the dentary it fragmented, broken, and worn, then it is very
>difficult to place this fossil in its own species.

The main point and much probably the much important in the resolution of the
validity of  P. sattayaraki is to understand if the material assigned to P.
sattayaraki belong to a fully-grown or juvenile individual. A study in
the ontogenetic changes in Psittacosaurus have been documented by Coombs
(1982) on the basis of juvenile specimen (AMNH 6535 and AMNH 6536) of
Psittacosaurus mongoliensis, but unfortunately no juvenile features are
listed for the dentary, which not seem to have very notable morphological
changes during growth. The dentary assigned to P. sattayaraki is twice the
size of that of P. mongoliensis (AMNH 6536) and three times the size of AMNH
6535, it's 2/3 the size of the type of P. mongoliensis and about the same
size of the type of P. youngi and P. meileyingensis; this seem to prove that
the type-dentary of P. sattayaraki do not belong to a particularly young
individual. Interesting is the fact that in the very small skull of P.
mongoliensis from Mongolia the dentary and predentary are fused together,
which is not the condition in P. sattayaraki, because the predentary is
separated from the dentary at the level of the unfused sutural region. The
number of dentary teeth may also has some relevance in the question of the
individual age of P. sattayaraki because the number increased during growth
in Psittacosaurus as pointed out by Sereno (1990a 1990b). The teeth count
increase during growth from as few as 5 to as many as 12 in P. mongoliensis,
the adult teeth count in Psittacosaurus's species varies from 8 to 12. In P.
sattayaraki we have 7 alveoli which suggest that it not belong to a fully
growth animal. But in this topic, the main question is; are the two main
characters used to define P. sattayaraki affected by ontogenetic change?
>From the figures of Coombs there're no marked curvature of the alveolar edge
and in the description he mention a straight ventral margin, that much
probably imply the absence of a flange. As pointed out by HP Steve Brusatte
in a previous email the development of the flange is quite variable from
species to species, a strong flange in P. meileyingensis and no flange in P.
sinensis and P. youngi, although the three species are of the same size,
this suggest that the development of the flange is not related to the size
and therefore can be used as a differential character.
In this view appears that P. sattayaraki differs from the other
Psittacosaurus species by at least one or two characters of the dentary,
that much probably seem justified its separation at the specie-level. And in
view of the fact that the dentary flange is absent in Neoceratopsia much
probably the presence and the great development of this flange is a derived
condition, in view of this P. sattayaraki is more primitive than P.
meyleingensis and P. mongoliensis, because has a less developed flange, but
is more derived than P. sinensis, P. youngi and P. xinjiangensis because
this three species do not have the flange.

By
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Alessandro Marisa
Via Achille Grandi n°18
Rovereto (TN) ITALY
Tel: 039-0464-434658 Email: amaris@tin.it
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