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What major extinction marked the beginning of the Tertiary Period?

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Messinian Stage — Tertiary Period Gypsum cones, which resulted from the evaporation of the Mediterranean Sea during the Messinian Salinity Crisis, in the Sorbas basin, Spain.

The beginning of the Tertiary Period was marked by the K–T extinction (also known as the K–Pg extinction), a global mass extinction event that occurred approximately 66 million years ago.12

This event, which defines the boundary between the Cretaceous and Paleogene periods, resulted in the disappearance of approximately 80 percent of all animal species, including nearly all dinosaurs. The extinction is widely associated with the impact of a large asteroid near the Chicxulub site on the Yucatán Peninsula, which triggered catastrophic environmental changes, such as the cessation of photosynthesis due to a global dust cloud. While some researchers also point to terrestrial factors like the Deccan Traps volcanic activity, the impact remains a primary focus for explaining this dramatic transition in Earth's history.1234