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

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

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

Flat vector design of mauisaurus. Sea monster with long neck and tail. Marine animal. Prehistoric underwater creature Stock Vector

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A Sea Monster Rising from the Water, with Its Towering Head and Long Neck in Full View. Stock Illustration - Illustration of wildlife, generative: 273844255