What animals are related to plesiosaurs?

Plesiosaurs were closely related to, but distinct from, pliosaurs, contemporary marine reptiles possessing thicker torsos, shorter necks, and longer heads. The eponymous member of the pliosaur family was – you guessed it – Pliosaurus.

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.