Eurycleidus is an extinct genus of plesiosaur. The type species is E. arcuatus, which lived during the Hettangian age (earliest Jurassic). Like other plesiosaurs, Eurycleidus probably lived on a diet of fish, using its sharp needle-like teeth to catch prey. Its shoulder bones were fairly large, indicating a powerful forward stroke for fast swimming.

 Ромалеозавры (Rhomaleosauridae) — примитивные, относительно длинношеие плиозавры преимущественно раннеюрского возраста, дожили до середины мела.

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