Why did sea monsters have long necks?

From the journal Science: A recently found fossil represents a new species of ancient marine reptile, and researchers think its long neck may have helped it hunt prey stealthily.
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Large Bodies Helped Ancient Sea Monsters With Extremely Long Necks Swim

Sea Serpent Lore

Large bodies helped extinct marine reptiles with long necks swim, study finds

Sea Monster Sightings and the 'Plesiosaur Effect' — Tetrapod Zoology

Plesiosaurs evolved awkward long necks thanks to their big bodies

A new study reveals the secrets of the plesiosaur's neck, a group of predators that thrived after the end-Permian mass extinction.

Bizarre Tully Monster fossil is still a mystery

Long-necked dinosaur-era sea monsters were vulnerable to decapitation by predators, fossil confirms

Reptiles: Paleontologists reveal new findings on 'snaky croc-face' plesiosaurs

Five “Real” Sea Monsters Brought to Life by Early Naturalists, Science

Oldest Antarctic Sea Monster Found

Dinosaurs: Why did seafaring reptiles have longer necks than modern day aquatic carnivores? - Quora