There were two major extinctions during the Mesozoic era:
Triassic–Jurassic extinction:
Around 201 million years ago, the Triassic–Jurassic extinction happened. 75% of all species lost, leaving dinosaurs with little terrestrial competition.
The Cretaceous–Paleogene extinction:
The end of the Cretaceous period is marked by the Cretaceous–Paleogene extinction around 65 million years ago. 75% of the plant and animal species on Earth—including all non-avian dinosaurs.
Source: Carl Sagan, Cosmos (New York: Random House, 1980)
Image by Charlie Sanabria under CC BY 4.0 license
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!