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Some good information.  However, you said:
"Sauropod teeth are often heavily worn. This is one indication that 
they were _not_ grabbing mouthfuls of soft, aquatic vegetation. Also,

when feeding at ground level, or on aquatic plants, animals 
supposedly ingest bits of grit and dust which leave microscopic scars

on the teeth. Tony Fiorillo determined that the absence of these on 
diplodocid teeth suggested that they fed on vegetation growing well 
away from the ground. We should keep this mind when discussing 
behaviours and ecologies in possibly 'stiff-necked' sauropods."

While several people on this have suggested various reasons for
sauropods feeding in trees or at a higher level than the ground, I
think there are some points to consider here.  For one, extensive wear
on teeth just tells one that either occlusion or tough foods are being
eaten.  You have done an excellent job by bringing up Tony Fiorillo's
work on tooth wear and about the apparent absence of grit wear on the
teeth. However ...

1. Have grit-wear studies been done on herbivorous reptiles?  I would
suspect most have been done on mammals, and while I am not suggesting
that the structure of tooth enamel is radically different in dinosaurs
and mammals, I am suggesting that perhaps there is grit wear on
reptilian teeth, but it happens in a different way.

2. Weak as my first point may be, we have to consider that there is
at least one type of low-lying plant, near and in water, that could
have caused extensive wear on sauropod teeth -- horsetails. 
Horsetails are tough, they contain silica (as far as I'm aware,
although I'm no botanist), and they are plentiful in the Jurassic.  If
a diplodocid were grazing by the waterside, it may well have ingested
large quantities of horsetails.  In fact, I know from my own
experience in a Late Jurassic deposit in Western Colorado
(Mygatt-Moore Quarry) that horsetail pieces are found with diplodocid

3. Even if you don't like horsetails, there's always cycads to
consider, which are low, growing, have tough casings on their fruits,
and have coarse leaves.  Just another suggestion from a non-botanist.

4. Conifer needles are not very nutritious.  I am not aware of too
many things that eat them.  There is a vole and a flying squirrel I am
aware of, but does anyone on this list know of a large vertebrate that
eats conifers?  And of course, since it would probably be a mammal, it
might not help us much anyways.

5. The energy it takes to rear up for low nutritive food may be too
high a cost, even if sauropods could do it.  I can already hear the
argument about large vertebrates being able to eat lots of low-quality
vegetation, but why rear up when you can snack out on the ground for
much less energy?

But we definitely need more data on the sauropods themselves, the
sedimentology (which is still debated), and paleobotany.

Matt Bonnan
Dept. Biological Sciences
Northern Illinois University