What major event marks the beginning of the mesozoic era?

The Mesozoic Era begins with the Triassic Period, which follows the mass extinction event at the end of the Paleozoic Era, known as the Permian-Triassic extinction event. This event, occurring around 252 million years ago, is the most severe extinction in Earth's history, leading to the loss of approximately 90% of marine species and 70% of terrestrial vertebrates. The recovery during the Triassic paved the way for the dominance of dinosaurs and the evolution of mammals in the subsequent periods of the Mesozoic.