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Very sorry my last message on plesiosaurs came out completely
garbled. I have problems with the terminal in my office and I
promise to avoid using it until its corrected.
Anyway, as I tried to state, on locomotion, I believe that we
have quite a bit of evidence that sauroptewrygians in general are
turtle/penguin-like subaqueous flyers. A recent paper by
Nicholls and Russell documents the rotational abilities of the
shoulder, which is very much like that of marine turtles.
Also, the heavy ventral skeleton in all sauropterygians appears
to be a turtle-like specialization, and thus, it is parsimonious
to model their locomotion like sea-turtles: potentially fast,
relaxed, and suited for long distance travel.
The short-necked pliosaurs must have been pursuit predators,
rather like dolphins. The plesiosaurs, especially elasmosaurs
with 70+ cervical vertebrae, may also have been fast but as
others have noted, they could probably use the neck as a "boom"
to lower on prey. As I understand, their necks were not
sufficiently flexible to behave in the snakelike manner portrayed
in the old Chas. Knight painting of the Niobrara Sea.
For the record, in my region, Late Cretaceous pliosaur remains
are fairly common, whereas long-necked plesiosaurs are unknown.
It seems the former tolerated coastal and shelf waters, whereas
the long-necked jobs prefered the inland seas such as the
Niobrara and Pierre.