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Re: The Enigma of Psittacosaurus



Alessandro Marisa wrote-

> The high number of denticles could be a character that linking P.
xinjiangensis and P.
> mazongshanensis.

That was suggested by Xu (1997), who also grouped the two together based on
"slender secondary ridges of tooth crowns".  I don't see any difference in
secondary ridge width myself, but it would be correlated to denticle number
in any case.  Brinkman et al. (2001), in their description of new P.
xinjiangensis material, found that some P. xinjiangensis teeth have 9-11
denticles, like other psittacosaurs.  Whether this is due to individual
variation, or variation within the tooth row, is uncertain.  Either way, it
makes the character "high number of denticles" problematic.

Steve Brusatte wrote-

> Thanks for his diagnosis.  After reading this I find it very, very odd
that Sereno dismissed this species > in but a single paragraph in his 2001
paper.  He states that it possesses "no clear diagnostic features
> that are absent in other psittacosaur species," and refers it to
_Psittacosaurus_ incertae sedis.  Very
> odd.  I have not seen even photos or illustrations of the type specimen,
but of the above characters,
> the posterior maxillary process and elongate snout with dentary
contribution seem like valid characters > to me.  The others seem
reasonable, but I cannot verify nor deny.

Sereno has a habit of making taxa nomina dubia without much published
analysis, though plenty of others nowadays do so too.  He did write his
dissertation on Psittacosaurus, but still....

>    1. skull rectangular with long antorbital region (0); intermediate (1);
> >rounded with short antorbital region (2).
>
> A potential valid character.  There certainly seems to be significant
differences between both the shape of the skull and length of the snout in
different species.  _P. meileyingensis_ has a rounded skull with a snout
that makes up 27% of the skull length, and other species do show a
rectangular and intermediate dichotomy, IMHO.

Needs to be quantified.

> >2. antorbital fossa very shallow or absent.
>
> Maybe a better way of doing this would be an ordered character, as
Alessandro Marisa mentioned a few days ago.  Something like:
> Antorbital fossa present (0), present as a shallow or "secondary"
depression a la Sereno (1), or absent (2).  With outgroups that all possess
antorbital fossae, its absence would definitely be a derived character.
This is one of the few characters, IMHO, that doesn't suffer from an unknown
polarity.

True, but how shallow does it need to be to be 1 and not 0?  And could you
tell from an illustration or photo?

> >3. ten or less maxillary and dentary teeth.
>
> Bad character, in my opinion.  The work of Coombs and Sereno has shown
that tooth count is extremely variable throughout ontogeny.  It might have
been best if they left this out, as there appears to be no set pattern in
psittacosaur tooth count.

The pattern seems to be P. mongoliensis has more than most other species, so
is most basal.  And if you only used adult specimens, ontogenetic
differences would not matter.

> >4. lateral margin of prefrontal strongly upturned.
>
> I thought this was an apomorphy of _P. mongoliensis_.  Is this shared by
any other psittacosaur species?

You're right.  Don't see why they included it.

> >5. postorbital region broad
>
> Hmmm...another possible bad character.  The breadth (is that the word??)
of the postorbital region might be functionally correlated with the size of
the snout.  More work probably needs to be done on this.  Also, the paucity
of material in some species makes this somewhat difficult to document.

Good observation.

> >6. horizontal ridge strongly developed on postorbital.
>
> Don't know enough about this character, but, based on illustrations, the
ridge appears to be more strongly developed on _P. mongoliensis_ and _P.
youngi_ than on _P. sinensis_ and _P. meileyingensis_.  But, this could be
an artifact of either incomplete material or poor illustration...or both.

Victim to the same problems as the antorbital fossa depth.

> >11. ventrolateral dentary ridge.
>
> Too hard to tell from illustrations.

That's the "dentary flange" you refer to below.  Easy enough to see.

> >I think 5 is useless, as it's just an apomorphy of P. mongoliensis.
>
> Maybe it's an apomorphy of _P. mongoliensis_, but other species appear to
have very similarly broad > postorbital regions.  As I stated above, I think
it is perhaps useless because it might be correlated with > the first
character in Russell and Zhao's list.

Oops.  Meant "4 is useless" of course.

> And, for one final point to end this unbelievably long post, awhile back
it was mentioned on list that
> _Hypsilophodon_, _Iguanodon_, and _Stegoceras_ have coronoids, along with
the neoceratopsians > _Protoceratops_ and _Montanaceratops_, while
_Chaoyangsaurus_ and _Psittacosaurus_ lack them.  > How much potential does
this have as a possible psittacosaurid + _Chaoyangsaurus_
> synapomorphy.

Speaking as someone who's seen oviraptorids coded as lacking coronoids until
one was found in Citipati just last year, I think such a character is prone
to preservational errors.

Mickey Mortimer