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Herbivorous diet in an ornithomimid / Re: New ornithomimid?

Dinogeorge@aol.com wrote:

> The following abstract is for an article in tomorrow's NATURE. Anybody know
> if this is a new taxon? Chinese ornithomimids are still pretty few and far
> between...

The article doesn't name or describe the osteology of the specimens beyond a
couple of measurements and the statement that one of the specimens is larger
than the others, which are presumed to be juveniles.  One of the juveniles,
with a femur length of 57 mm, is estimated to have weighed 10 kg when alive.

Figure 1a is a photograph one of the ornithomimid fossils, which is indeed well
articulated, but the on-line photograph (available to subscribers only, sorry
to say) lacks clarity, perhaps because the specimen needs much more prep work.
I can't say for sure.  This photograph is accompanied by a labeled line drawing
of the fossil, Figure 1b.  I don't know if one could positively identify the
species from a photograph such as this, although one could probably use this
image in order to determine what it isn't, as the skeleton is nearly complete,
and the proportions are clear enough.  If I'm not mistaken, the Upper
Cretaceous Ulansuhai Formation in China, where the twelve ornithomimid fossils
were found, is located in the Nei Mongol Autonomous Region.  (Correct me if I'm
wrong, please).  This may aid in tentative identification.

Figure 1c shows a close-up of a juvenile's gastrolith mass, complemented by 1d,
a computed tomography scan of these gastroliths, which gives one a sense of the
volume of the gastroliths.  The close-up photo shows good detail, so the
specimens, when fully prepared, _may_ preserve excellent detail.

The emphasis of this _Nature_ "brief communications" article is on the make-up
and implications of the gastroliths themselves, not the skeletal material.  The
size, angularity, and numbers of gastroliths are described, and these are
compared to the gizzard stones and grit utilized in the gizzards of living
birds.  One juvenile's gastroliths were sonically disaggregated for study, and
yielded over 1,000 grains larger than 0.5 mm (measured along the intermediate
axis).  The authors suggest that the smaller grains, which are described as
being more angular than the larger grains, may have resided in the gizzard for
only a short time.  These smaller grains are judged to be unusually small for
use in gizzards (as compared with avians).  Larger, more rounded grains were
typically over 2 mm.

The ornithomimid specimens' possession of pockets of numerous small silicate
grains (gastroliths),  devoid of bone or insect remains, taken together with
the ornithomimids' edentulous condition and inferred weak jaw muscles, suggest
to the authors that these animals were herbivores which utilized grit held
within muscular gizzards to mechanically break down their food.

As ornithomimids are not considered to be particularly closely related to
_Caudipteryx_ or to birds, their nearest relatives known to possess gastroliths
or gizzards, Kobayashi et al. propose that the ornithomimids' use of
gastroliths represents convergence with these other groups.

It appears that this paper represents the first publication of an
ornithomimid's use of gastroliths, hearkening back to comments voiced by Philip
J. Currie at the 1997 SVP meeting, if I recall correctly.  So, among the
dinosaurs, we now have gastroliths sometimes associated with sauropods, one
species of ornithomimid, one oviraptorosauroid, birds, and _Psittacosaurus_
(the sole ornithischian of the lot).

-- Ralph W. Miller III       gbabcock@best.com

Pig + eons = Pigeons