Was the plesiosaur a predator

None. Elasmosaurus was a herbivore. It probably was part of the diet of the more famous Permian finback, Dimetrodon

I made a stupid mistake...it all comes of not reading the question and my answer a bit more carefully.

Elasmosaurus was a plesiOSaur which, while not a dinosaur, did live in Mesozoic seas as a contemporary, and preyed mainly on fish, I expect. I quite mindlessly confused Elasmosaurus with EDAPHOSAURUS. Edaphosaurus was a synapsid reptile that lived in the late Paleozoic (late Pennsylvanian to the end of the Permian) and was indeed an herbivorous animal, probably a part of the menu of its contemporary, Dimetrodon. Both of these reptiles are considered 'mammal-like' reptiles and were dominant, in terms of numbers, many millions of years before the dinosaurs came onto the scene.

They were obliterated in a mass extinction that was more catastrophic and sweeping than the one that brought the age of dinosaurs to an end.