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.
This is additional taxonomy that helps us with analytics

Is the Loch Ness Monster real? Existence 'plausible' after plesiosaur discovery - BBC Newsround

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Attenborough and the Jurassic Sea Monster, Discovering the Secrets of a Jurassic Sea Monster, Nature

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

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Sea Monsters: Prehistoric Ocean Predators - Australian National Maritime Museum

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Meet Snakey McCrocface! Seabeast with long neck and crocodile-like jaws swam oceans 70m years ago