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Re: Plesiosaurs Necks

Dann Pigdon wrote:

Dinosaur World wrote:

This suggests to me that they were active, fast moving animals. What I don't understand is if the necks were somewhat ridged, wouldn't this prevent them from "snaking" their necks through the water in an attempt to catch prey? Were fast moving fish not a big part of their diet?

Recent studies have suggested that bivalves made up part of the
plesiosaur diet.

That's one thing...

The long necks may have been for foraging back and
forth on the sea floor

That's completely another. A single data point showing the use (at one point of time and space) of a structure does _not_ make a functional argument about either (1) the origin or (2) the maintenance (in evolutionary terms) of same structure. For what it's worth (rather little, probably), I don't think that elasmosaurids evolved that neck simply to harvest benthos. The advantages of a long neck for catching agile nekton seem too great to be ignored. But could a long neck be used to pluck bivalves from the sea floor? Apparently so... Remember, the animal that (I presume) you are referring to also had a fair amount of nekton (belemnites) in its stomach content.

(like underwater versions of diplodocids

Unless diplodocids were catching fish and cephalopods, I doubt that....

I imagine you don't have to be all that fast to catch shell
fish either - just faster than anything that might want to eat YOU.

Lots of modern marine tetrapods manage to avoid their predators without being faster than them. You can be clever, too....(or at least, have an an evolution-refined behaviour that looks like cleverness).

What do juvenile elasmosaurids look like? Could they have had relatively
shorter necks

Most likely, given basic patterns of allometry for amniotes.

(hence capable of faster evasive action)?

Maybe, but maybe not. Reynolds numbers are affected by shape and size, and larger animals are relatively more efficient swimmers than smaller ones (as far as my limited understanding of the fluid dynamics goes - please let me know if I've got this, as PB would say, 'bass ackwards').

Perhaps diets
changed from active persuit of prey to more passive bottom feeding as
they grew larger...

According to Joan Wiffen, more likely the other way around:
Wiffen et al., Ontogenetic evolution of bone structure in Late Cretaceous Plesiosauria from New Zealand.Geobios 28, 625-40 (1995).

***************** Colin McHenry School of Environmental and Life Sciences (Geology) University of Newcastle Callaghan NSW 2308 Australia Tel: +61 2 4921 5404 Fax: + 61 2 4921 6925