FW: Somerset Plesiosaur

["Craven, David" <David.Craven@bolton.gov.uk>]

Colin McHenry wrote:

>Yes, it certainly reminds me of P. macrocephalus...  Most vertebrates
>have larger heads when juvenile, so it's not unreasonable to suppose 
>that plesiosaurs did as well.  If this is a juvenile, then it is 
>probably a young Rhomaleosauus - like thing?

>However, 1.5 metres long is very large for a juvenile reptile to be
>displaying such 'un-adult' proportions.  By the time a crocodile or 
>turtle is that sort of size, it has essentialy the same proportions as 
>the adults, and has had for quite some time.  The only time that crocs 
>and turtles show such 'juvenile' body proportions  is when they're just 
>out of the egg.

>So the thought's just occurred to me....if this thing is a young 
>version
>of some Liassic rhomaleosaur, then could it constitute very suggestive 
>(although indirect) evidence for live birth?.  The logic being that to 
>see juvenile proportions it would have to be very young - and to be so 
>young and so large, it would have been way too large to have hatched 
>from an egg and would necessarily have been born live. And what a baby - 
>1.5 metres is a pretty significant fraction of a (guessing) 5 metre 
>mother pliosaur!

>Live birth in plesiosars? who would have thought it.....

R.zetlandicus can reach 8m or so, so 1.5m would represent a specimen that
was still fairly young. I don't know if anyone has done any research into
Plesiosaur growth rates.

However I suspect the angle of the picture increases the apparent size of
the skull. I think some better scaled and clearer pictures are needed before
any conclusions can really be made about the implications of the specimen.

Given the evidence for live births in Ichthyosaurs, I'd personally have
thought live births in plesiosaurs was quite conceivable.

David Craven
Bolton Museum

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