Which marine reptiles were common in the Mesozoic oceans?

Plesiosaurs were a group of marine reptiles that thrived in the oceans during the Mesozoic era, which spanned from approximately 252 to 66 million years ago. These reptiles are characterized by their distinct body shapes, including long necks and large, flipper-like limbs, which made them well-adapted for life in the water. Plesiosaurs were not fish; instead, they belonged to a different class of vertebrates and are considered a subgroup of reptiles that bore live young instead of laying eggs on land.

In contrast, modern fish, dolphins, and crustaceans were not unique to the Mesozoic period as they existed either later or were different forms altogether. Modern fish evolved and diversified significantly after the Mesozoic, while dolphins are mammals that belong to a completely different lineage that emerged much later. Crustaceans, although present during the Mesozoic, were part of a much broader group of organisms and not predominantly representative of the marine reptiles of that time. Thus, plesiosaurs are correctly identified as the marine reptiles that were common in the Mesozoic oceans, marking a distinct group that contributed to the marine ecosystem of that era.