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PLESIOSAURS: UNDERWATER PREDATORS



Plesiosaur necks. Here we go:) Jeffrey Wilson wrote..

> Last I heard about elasmosaur feeding  habits, they were fairly slow
> surface-feeding piscivores, sort of a Leatherback turtle in the 
> back-end and a heron in the front-end. They used their long necks to 
> be able to range around and "swoop" down on surface fish - therefore 
> no problems with hydro-drag on necks, at least when feeding.

There are probably as many ideas about plesiosaur necks as there are 
people that work on them, and likewise for how fast and manoueverable 
the animals were. It is probably correct to think that plesiosaur 
necks were moderately, but not overly, flexible. A small area of 
overlap on pre- and postzygapophyses, and the presence of fairly tall 
neural spines, indicates that eloborate twists, bends and s-shapes 
(as per that Russian worker whose names begins with Z) were probably 
not possible. But some articulated necks have gentle curves where the 
head drifted toward the body as far as it could go, showing that the 
neck was not stiff. I have heard of one individual plesiosaur that 
has a neck that does this (where H is the head and B is the 
body).....

          *  *
        *       *
       *          *
      H           *
                   *
                  B

... indicating the flexibility of the neck. Necks were probably used 
in swift lateral or dorsal strikes: not too much dorsal motion, this 
is restricted by the relatively tall neural spines. I don't think 
resistance would be a problem: cormorants, anhingas and other 
fish-grabbing/spearing birds catch fish fine by swift strikes, as do 
sea snakes.

At various times it has been suggested that long-necked plesiosaurs 
could have been stealthy ambush hunters that did not need to move 
their heads or necks that fast: maybe fish were fooled into thinking 
that the fish-sized object among them was not part of a 5 ton 
fish-eating reptile, but just another little fishy. Vladimir Krb 
captured this idea in a very nice painting: the elasmosaur's body is 
in the distance, and the head is foraging amongst a schoal of fishes. 
One could speculate on this idea and imagine that long-necked 
plesiosaurs disguised their heads as pretend fishes, maybe exhibiting 
the same colour patterns as would be seen on the body of whatever 
Mesozoic fish the plesiosaur might be preying on.

Glenn Storrs has argued that, because sauropterygians used ballast 
stones (and pachyostotic bones too in some taxa) to improve their 
negative buoyancy, it is most likely that they cruised at depth, and 
not near/at the surface. 

An opposing view, it's popular in some older literature but I'm 
unsure of anyone advocating it today (you can see it in a recent 
John Sibbick painting), is that long-necked plesiosaurs were 
surface-dwelling predators that moved the neck through the air, 
thereby avoiding water resistance, and plunged the head down through 
the water. I don't believe this at all, for several reasons. As 
mentioned above, plesiosaurs made their bodies *heavy*, and would 
only have done this if they had wanted to stay away from the surface, 
not be near to it. Importantly, what's known of plesiosaur skulls 
indicates that they were well adapted for sensing things via water, 
not via air. For example, all plesiosaurs have this unique 'flow 
through' nasal system, where water runs via palatal channels into the 
internal nostrils, through the olfactory chamber (where scent 
particles are collected and tested), and out via the external 
nostrils (Cruickshank, Small and Taylor 1991). This system was first 
recognised in _Rhomaleosaurus_, but you can see it in all described 
plesiosaur palates and hence it's a good plesiosaur synapomorphy 
(Cruickshank pers. comm. 1998). 

Plesiosaur hearing is not adapted to the detection of airborne 
sounds. Pachypleurosaurs at least have an impedence matching ear, but 
plesiosaurs have lost this and may have relied on bone-conducted 
hearing instead. _Thalassiodracon_ Storrs and Taylor has a fixed, 
rigid stapes that was no good at conducting airbone noises and Ken 
Carpenter reckons that polycotylids and elasmosaurids lacked the 
stapes altogether (he notes that Glenn Storrs is sceptical of this 
however). That plesiosaur skulls were not adapted for hearing in air 
also suggests they were underwater animals. There is no indication, 
incidentally, that plesiosaur ear ossicles were separated from the 
rest of the skull as it thought necessary for directional hearing and 
echolocation.

Also proven by _Thalassiodracon_ is that plesiosaur eyes were 
somewhat flattened (the shape of the sclerotic plates shows this), 
and thus not good for coping with the different densities of air and 
water. Their eyes, instead, were modified for seeing in water - a 
brilliant underwater adaptation and one of the best reasons against 
the idea of hunting from above the air-water interface.

Also, many plesiosaurs have laterodorsally or dorsally directed 
orbits: not exactly helpful if you are supposed to be looking down 
though the air-water interface. This indicates instead that 
plesiosaurs attacked from behind and below, or just from below (cf. 
_Carcharodon_).

Most of my thoughts on plesiosaur lifestyles will be encapsulated
in a _BBC Wildlife_ article that will be published some time next 
year.

"Ready for the saurian limbo"

DARREN NAISH
darren.naish@port.ac.uk