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Re: MEGA-PLIOSAURS REVISITED (AGAIN)



> Date: Thu, 1 Mar 2001 00:05:59 +1100
> From: "Colin McHenry" <cmchenry@westserv.net.au>
> 
> My final suggestion on the subject is that a 9 metre (~ 9,000 kg)
> pliosaur [...] was probably very manoevurable and fast.

Interesting.  In his excellent book _Dynamics of Dinosaurs and Other
Extinct Giants_ (more details at
http://www.miketaylor.org.uk/dino/books/index.html#dod
for those who aren't familiar with it), McNeill Alexander argues
persuasively that plesiosaurs couldn't swim very fast at all.  I'm
sure others will be able to correct any mistakes, but the argument
basically goes as follows:

First all all, plesiosaurs most likely used the "underwater flight"
method of swimming (like marine turtles) rather than "rowing" (like
freshwater turtles).  I can't remember the reasoning for this offhand,
but since it's more efficient than rowing, we can assume it anyway for
the purposes of working out top plesiosaur speeds.

Now there are two modes of underwater flight.  In both of them, the
thrust is produced by lift on the flippers during the power stroke.
The first mode is the horizontal analogue of the way that fish use
their vertical tails: both the upward and downward strokes are power
strokes, so both produce thrust.  In the second mode, by contrast,
only one of the strokes produces thrust, and the other is a recovery
stroke.

Alexander claims from analysis of plesiosaur skeletons that while
there are good anchor points for the down-stroke muscles, there is
nowhere for strong up-stroke muscles to anchor.  So he concludes that
in plesiosaurs, the down-stoke produced the power, and the up-stroke
was purely for recovery.

Now here's the bit where I don't follow Alexander's logic: he says
that in this asymmetric mode of underwater flight, the flippers must
move down and backwards together in the power stroke, and forwards and
upwards together in recovery.  He further says that during the power
stroke, the flippers can't be moving backward relative to the body any
less quickly than the body is moving forwards through the water -- in
other words, the net motion of the flippers during the power stroke
must be backwards relative to the water.  (Unfortunately, I don't have
the book here, so I can't explain why this should be so -- can someone
else?)

if we accept Alexander's reasoning so far, then the problem becomes
one of how many cycles per second the flippers can achieve.  Imagine a
large (8-9m) pliosaur.  Say its flippers are about 2m long, so that
their center of force is about 1m out from the body.  If the pliosaur
can manage a beat frequency of, say three cycles per second (which
sounds like _a lot_ to me), then in each second, it's spending about
0.5 seconds in those three power strokes (and the other 0.5s in
recovery, obviously.)  In each of those three 1/6s bursts, the
flippers' center of force moves backwards say 1m (this doesn't seem
too unreasonable, assuming as it does a sixty-degree arc of movement),
so if it's achieved the "Alexander maximum" where the flippers are
exactly stationery relative to the water, then the plesiosaur's body
equally moves forwards 1m during each of those three cycles, and
perhaps during the recovery stroke too (due to momentum) -- yielding a
maximum speed of 6m/s.

Now that's a lot better than I could do (:-) but it certainly doesn't
seem fast compared to the performance of many fish, and the likely
speed of other contemporary taxa such as the ichtyosaurs.

So: first of all, my apologies to Alexander for what is no doubt a
horrible mangling of his original chain of reasoning.  I hope it's
close enough to what he actually wrote that it doesn't upset anyone
too much.

Secondly, are there flaws in this reasoning?

And thirdly, if I have learned anything at all on the DML, it's that
"everything you know is wrong."  Alexander's book is now twelve years
old, which is a long time in politics.  Has the whole shape of the
plesiosaur world changed since then?

Yours breath-batedly,

 _/|_    _______________________________________________________________
/o ) \/  Mike Taylor -- <mirk@mail.org> -- http://www.miketaylor.org.uk/
)_v__/\  "You can't have free speech without responsibility, and anyone
         in such a high profile position has to know the difference
         between saying what you want and saying what you ought" --
         Geoff Thompson, acting FA chairman, shows his ignorance.