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plesiosaurs et al

Last night's PaleoWorld discussed plesiosaurs, icthyosaurs and (was it) 
mososaurs, and mentioned that they had all back-evolved into sea 
creatures. Have any of their terrestrial forerunners been unearthed? Did 
they branch off from archosaur stock or before? Also, it seemed to imply 
that these creatures died off due to gradual changes in the food chain 
because of tectonic drift; any validity to this?
Blaise Considine [bpc.apa@email.apa.org]