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PLESIOSAURS: Ballast stones, necks, and noses

I haven't picked up a copy of _Ancient Marine Reptiles_ yet, so I'm not
sure if these topics are covered within.Basically I'm wondering about a
few traits of plesiosaurs to see what the general concensus is on them...

In 1993 Dr. Michael Taylor suggested that plesiosaurs swallow stones for
ballast. I assume that such stones have been associated with fossil
remains. Anyway, the theory goes that plesiosaurs, which swim by
"underwater flying" (like penguins, sea turtles etc.) would be constantly
heading to the surface from its swimming action. So, it would swallow
stones to counteract this effect. Is this still accepted? I'm not sure if
penguins or sea turtles swallow stones for ballast, but it seems unlikely
plesiosaurs wouldn't have some anatomical trait that allows them to steer
themselves on a proper course, without having to rely on swallowing stones.

Second, is a 1991 study by Arthur Cruickshank et al. (Nature 352 [July 4]:
62-4) on _Rhomaleosaurus megacephalus_. They noticed furrows that ran from
the tip of the snout along the palate into each of its internal nasal
openings, and they suggested that _Rhomaleosaurus_ swam along and allowed
water to run into its mouth; the water would be "channeled" along the
palatal furrows into the internal nasal openings, and then the water woudl
pass through the nose and come out of the external nostrils. Cruickshank
et al. suggested this allowed the creature to smell underwater, and that
the nostrils were more suited to this task than breathing. Respiration
would be accomplished by gulping air through the mouth. (Incidentally, I
don't have the actual Nature report in front of me, I'm am summarizing
from another summary in a popular book). My question is: is this still
accepted for _Rhomaleosaurus_, and do any other plesiosaurs show signs of
similar respiration?

Finally, are the necks of plesiosaurs considered to have been flexible or
rigid? Or did this differ between species.

Sorry for any ignorance, i'm a marine biologist by trade (extant forms!),
but dinosaurs and marinereptiles, as well as general vert paleo, are also
fascinating to me.