How did plesiosaurs differ from living marine reptiles?

Plesiosaurs were a distinct group of extinct sauropterygian marine reptiles that have no modern-day analogs. While they shared an aquatic lifestyle with some living reptiles, they differed significantly in their anatomy and locomotion.1

Key differences include:

• Body Structure: Plesiosaurs possessed a broad, stout trunk with a "basket" of interlocked ventral ribs that made the torso relatively inflexible. They featured either long, snakelike necks or, in the case of pliosaurs, elongated heads with shorter necks.23
• Locomotion: Unlike modern marine reptiles that may use body undulation or paddling, plesiosaurs utilized four long, narrow flippers to "fly" through the water in a manner similar to sea lions or penguins.32
• Physiology and Behavior: Plesiosaurs were known to swallow stones, likely to manage buoyancy or aid in digestion. While most modern reptiles are ectothermic, some extinct groups, such as certain dinosaurs, evolved endothermic physiology, whereas plesiosaurs remain distinct from the specialized adaptations seen in today's turtles, crocodiles, or squamates.214