Which of the following reptiles is a hallmark of the Mesozoic Era?

The plesiosaur is a hallmark of the Mesozoic Era, specifically known for its marine adaptations during this time. This group of large marine reptiles thrived in the oceans and is often associated with the age of dinosaurs, which spanned from approximately 252 to 66 million years ago. Plesiosaurs are characterized by their long necks, small heads, and large bodies, and they represent a significant component of the diverse marine ecosystems that existed during the Mesozoic.

In contrast, while alligators, Komodo dragons, and sea turtles are reptiles, they are not distinctive for the Mesozoic Era. Alligators evolved much later and are primarily associated with the Cenozoic Era. The Komodo dragon is part of a lineage that developed after the Mesozoic, during the Cenozoic Era as well. Sea turtles, while ancient, have representatives that existed before the Mesozoic, but they also continued to evolve after this era, making them less emblematic of the Mesozoic itself. Thus, the plesiosaur serves as a key representative of the Mesozoic marine reptiles and stands out in terms of its evolutionary significance and presence during that time period.