PLESIOSAUR REPRODUCTION

[darren.naish@port.ac.uk]

This has just been sent to the vrtpaleo list, and I'm re-posting it 
here on the off chance that some of you might find it interesting. 
Apologies if you therefore get it twice. Mike: my previous fwd of the 
message to you didn't work because I used the wrong address. Sorry.

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Glad to see plesiosaur reproduction raising its ugly head, via Barry 
Kazmer's questions. Good views thus far from David Brown and Mike 
Everhart. 

How plesiosaurs reproduced is still a real sticky problem because of 
the astounding lack of evidence. A supposed bunch of plesiosaur 
embryos turned out to be probable shrimp burrows, a plesiosaur egg is 
probably a concretion, and there are no (published) cases of 
embryonic plesiosaurs found within pregnant females.. *very* odd when 
you think about all the ichthyosaurs there are with embryos. 
Personally I suspect that plesiosaurs were fully marine and 
viviparous, but I'm not 100% sure of this and I see pros and cons to 
both sides of the debate. Here are some bits of evidence you can use 
to support either view....

View 1) Plesiosaurs laid eggs on land and could haul themselves up 
beaches.

 - No plesiosaur embryo has been discovered (but see below), which we 
would expect if they were viviparous (but see below). A 
pachypleurosaur baby looks like one that was aborted far from land 
(and thereby supporting vivipary in sauropterygians more primitive 
than plesiosaurs), but other interpretations are possible (i.e. the 
baby could just as well have come from inside an eggshell). Other 
sauropterygian babies found far from land have also been reported, 
but they don't prove anything about reproduction.

 - That plesiosaurs were very well braced ventrally (with massive 
ventral limb girdles and tightly interlocking gastral baskets, plus 
associated paddle musculature) might mean that they were powerful 
enough to shift their bodies on land. Ralph Molnar has suggested such 
in print.

 - By laying eggs on land plesiosaurs may have avoided predation on 
hatchlings. However, the Mesozoic was not exactly devoid of 
terrestrial predators.

View 2) Plesiosaurs were viviparous and fully aquatic.

 - No plesiosaur nest or egg has been discovered (but see above). 
Suggestions that, if plesiosaurs were viviparous, embryos MUST, by 
now, have been found operate under a Mesozoic bias. Yes, ichthyosaur 
embryos are well represented. But other viviparous fossil marine 
tetrapods (namely cetaceans) hardly ever reveal fossil embryos (BTW, 
is anyone aware of any fossil cetacean embryo?).

 - Plesiosaurs have 4 wing-shaped paddles, and the hind pair are good 
wings like the front pair. Sea turtles, which fly like plesiosaurs, 
never evolved their hind flippers into wings and instead exhibit 
shorter, stumpier limbs that still have claws. They use these for 
digging with. The same has also been asserted for metriorhynchids 
(i.e. hind flippers are stumpy and equipped with claws.. however, 
metriorhynchids probably did not fly (have to see what Dino Frey and 
Steve Salisbury will say about this at SVPCA '98 though)). Maybe 
then, marine tetrapods that do dig nests on land retain stumpy clawed 
hind flippers. Plesiosaurs did not.. so is this evidence that they 
did not dig nests on land? Dunno.

-  Plesiosaurs grew to gigantic sizes: there are fragments of 
pliosaurids that may have been more than 15 m long and may have 
weighed about 15-20 tons or more. Sea turtles and pinnipeds, 
constrained to terrestrial breeding, never got that big. Thus 
giantism in plesiosaurs >suggests< that they became fully aquatic.

 - Plesiosaurs may be related to lepidosauromorphs, a clade in which 
vivipary evolved multiple times. This might mean that plesiosaurs 
could have evolved vivipary as well unlike chelonians and crocs 
which, according to Samuel Tarsitano at least, are constrained to 
ovipary because their embryos derive calcium from the egg shell.

 - Plesiosaur limb girdles are not braced against the backbone. 
Welles and Bump used this as an argument for full-time aquatic 
lifestyles in plesiosaurs. Other marine forms that can move on land 
(chelonians, pinnipeds, ?metriorhynchids) have limbs braced against 
the vertebral column. However, plesiosaurs may have compensated for 
this with their massive ventral bracing and musculature. Also, even 
if their weight when on land squashed their lungs, they could still 
have held their breath. Beached whales often do not breath.

Well that's my thoughts on it.. make your own decision. 

DARREN NAISH
Palaeobiology Research Group
Dept. of Geology / S.E.E.P.S.
Uni. of Portsmouth
PORTSMOUTH, ENGLAND.