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Re: Gastroliths

I recommend reading David Gillette's _Seismosaurus, the Earth Shaker_ and the
gastrolith section of Dr. Robert T. Bakker's _The Dinosaur Heresies_ for some
background and a healthy helping of speculation regarding gastroliths.

David Gillette states that five sauropod specimens had been found associated
with gastroliths prior to _Seismosaurus_, three from the Morrison Formation
and two from the Tendaguru beds of Tanzania.  One of the latter two sauropods,
which was found with gastroliths in the neck region, was _Barosaurus_.
Gillette is of the opinion that gastroliths were only used by some sauropods,
because many sauropod skeletons are found without the stones.  He suggests
that gastroliths can be identified on the basis of being collections of
semispherical, rounded, polished stones, some with a pitted surface, being
quartz stones associated with a skeleton but far removed from any similar
stones in the sediment, and of similar size (i.e. not a complete range of
sizes down to very small stones as you would find with river rocks).
Sometimes a gastrolith is found standing on end, which is not what you would
expect in stream-deposited stones.  Gillette believes that _Seismosaurus_ had
"gizzard" stones both in its crop and in its gizzard, the crop being the site
for fermentation, the gizzard being the place where acids further break down
the food.  Gillette points out that the extant folivorous Hoatzin bird is
unique in that it uses grit in both the crop and the gizzard.  Given that at
the time of the book's writing only 240 gastroliths had been found for this
immense animal, Gillette did not feel that the gastroliths -- amounting to
only a ten quart volume of rocks in all -- would be capable of effectively
grinding large amounts of food.  He proposed that the gastroliths facilitated
a stirring and mixing of foods and digestive juices -- like a chemist's
magnetic stirrer -- rather than grinding.  Gillette has also suggested that
the largest gastrolith ingested by the _Seismosaurus_ specimen may have caused
it to choke to death.  No word as to whether this was an accident or a death
wish.   ;^)

Bakker mentions finding "a half dozen brontosaur bodies in the field where
smoothly rounded pebbles were scattered through and around the ribs."  He goes
on to write that gastroliths are also very rarely found in fossil
crocodilians, indicating that a rarity of gastroliths found associated with
sauropod remains should not imply that gastroliths were not routinely utilized
by these animals.  Bakker also mentions the discovery of a tiny dinosaur
skeleton excavated in Mongolia in the 1920's which had gastroliths in place.
I would guess that this refers to _Psittacosaurus_.  Philip J. Currie has
mentioned finding a Chinese ornithomimid with gastroliths (so far
unpublished), as well as the well documented (oviraptorosauroid?)
_Caudipteryx_ specimen from China (see _National Geographic_, July 1998, and
_Nature_, Vol. 323, June 25, 1998).  Regarding extinct avialan dinosaurs,
Bakker also mentions the presence of gastroliths among moa remains.

Bakker believes that the presence of gastroliths within sauropod gizzards
proves that these animals weren't "mush eaters," for extant birds only utilize
gizzards if they eat tough foods (such as seeds and nuts).  Birds which feed
exclusively on soft fruits and nectar do not utilize grit and gizzards in
processing their food.  Bakker also points to tooth wear on the sauropods --
particularly the camarasaurs -- as further evidence that these animals fed on
tough plants.  Dr. Bakker believes that the gizzard stones were employed by
sauropods for grinding up tough vegetation within muscular gizzards.

Bakker mentions the "belched-up gizzard stone" hypothesis for piles of
polished stones in the Morrison, and is not entirely convinced.  He proposes
that the resilient quartz stones were held within an animal's gizzard while it
lived, and that the body decayed and weathered around the stones after the
animal died.  The gastrolith piles remain long after the body disappears,
producing "ghost dinosaur stomachs."  Dr. Bakker bases this notion on the
discovery of moa remains in New Zealand which include a quantity of moa
gizzard stones, but little if any remaining moa.  He also notes the
possibility that some of the accumulations of moa gastroliths may be the
result of moa gizzard stone "belching."

For further information, if you check out the New Mexico Friends of
Paleontology site at <www.nmmnh-abq.mus.nm.us/nmfp/nmfp.html>, you can click
on "1996 Newsletters" and go from this page to "Gastroliths: Enigma Stones."
This brief online article lists a number of the animals which apparently
utilized gastroliths, and includes a close-up photograph of the acid-pitted
surface of a gastrolith.  The acid pitting is a diagnostic characteristic of
gastroliths and enables scientists to distinguish them from river rocks.

Hope this helps,

Ralph W. Miller III  <gbabcock@best.com>