|
Gunter Van Acker wrote-
> Since recent psittacosaur discoveries include skeletons
associated with gastroliths, it is also
> suggested that they may have been omnivorous.
Well, described in 1924 is kind of recent :-) . Who
suggested lithophagy was associated with omnivory?
> 1. pachycephalosaurs have been considered as omnivores,
since their teeth are almost identical to
> troodontid teeth (the latter are certainly meat-eaters,
and even possible omnivores).
Stegoceras premaxillary teeth are rather similar to some
Troodon teeth. More basal pachycephalosaurs like Goyocephale are said to
have canine-like premaxillary teeth. Pachycephalosaur cheek teeth are
quite different from those of Troodon, being much shorter and not nearly as
recurved, more similar to basal ornithischians or ankylosaurs.
> 2. heterodontosaurids, having a complex array of
teeth
Perhaps a better reason for omnivory in heterodontosaurids
would be the serrated premaxillary fangs, although similar structures are
present in the herbivorous Muntiacus.
> 3. some basal sauropodomorphs ("prosauropods") were
probably omnivorous.
As shown by a lizard skeleton in the stomach of a Nova Scotia
specimen, yes, they were.
> On the other hand, there are several other dinosaur
genera that have remains associated with
> gastroliths;
Syntarsus, Poekilopleuron and Lourinhanosaurus (the
allosauroid, not Lourinhasaurus the camarasaurid) have been found with
gastroliths. In fact, a wide range of animals are known to be
lithophagous, including obviously carnivorous species like Protorosaurus and
certain snakes.
> My conclusion is that gastroliths are not necessarily an
indication for an omnivorous diet.
Good conclusion, but why was there ever any
other?
> I would like to see conclusive evidence for the reported
association of bones in psittacosaur stomach
> region (as mentioned in L. Rey's book).
Now this is interesting, but should not be connected to the
presence of gastroliths. Any details?
> Let's assume that these integumentary structures were at
least keratinous, what could have been their > function ?
Well, as every reptile has keratinous integumentary
structures, that's a pretty safe assumption :-) . As the information is
embargoed and you know nothing about the structures other than they exist
however, hypothesizing on possible function is meaningless. I don't
believe Psittacosaurus will bring any relief to those wanting coelurosaur-like
integument on ornithischians, but I can't say more.....
> On a related matter; why did psittacosaurs have such a
short temporal (Aptian to Albian) and
> geographic range (Mongolia & northern China) ? Were
they outclassed as dominant omnivores by
> ornithomimids and troodontids (fast runners and more
intelligent).
This would be because Asian stratigraphy is very uncertain, so
people use biological markers, not radiometric dates, to estimate the age of
most formations. Thus, people assign an Aptian-Albian age to those
sediments with Psittacosaurus in them. Also, Psittacosaurus was only a
single genus, so it wouldn't be expected to last very long. The most
recent estimates place Psittacosaurus-bearing formations in the Barremian-Albian
range (Lucas and Estep, 1998).
> If I recall correctly, another new non-theropod Yixian
specimen (?a basal ornithopod) with
> integumentary structures was reported a while ago on the
DML. The fact that only theropods were
> found during the initial years of quarrying, gave us
sufficient evidence to prove that at least several
> non-avian theropods had feathers, but it also meant that
our view of scaly non-theropods continued to > exist.
Jeholosaurus has no integumentary structures reported- not
scales, not filaments, not feathers. Psittacosaurus has been reported to
have tubercular scales along its arm
(http://www.cmnh.org/fun/dinosaur-archive/2001May/msg01018.html).
The only other confirmed Yixian ornithischian is Jinzhousaurus, which I do not
yet have the description of (but would like to, if someone has the pdf :-)
). There is no published evidence for anything other than tubercular
scales or dermal scutes on any ornithischian.
Mickey Mortimer
|