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

Plesiosaurs evolved awkward long necks thanks to their big bodies

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

Bizarre Tully Monster fossil is still a mystery

Mystery solved: 240-million-year-old reptile with 'extraordinarily long neck' lived in the ocean - BBC Science Focus Magazine

Sea monster - Wikipedia

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How Sea Monsters Work

The reign of the reptiles: Meet the monsters – Queensland Museum Blog

Sea Monsters: Prehistoric Ocean Predators - Australian National Maritime Museum

Ancient Sea Monster Remains Reveal Oldest Mega-Predatory Pliosaur

Sea monsters more than a legend?

Why did sea monsters have long necks?