What significant event marked the end of the Mesozoic Era?

The end of the Mesozoic Era is best marked by a mass extinction event, often referred to as the Cretaceous-Paleogene (K-Pg) extinction event, which occurred approximately 66 million years ago. This event led to the abrupt disappearance of a significant number of species, including most of the dinosaurs and many marine reptiles, which dramatically reshaped the biodiversity of the planet.

This mass extinction sets a clear boundary between the Mesozoic Era, characterized by the dominance of dinosaurs and other reptiles, and the subsequent Cenozoic Era, which saw the rise of mammals and birds in the ecological niches left vacant. The impact of this event was profound, leading to a complete restructuring of ecosystems and paving the way for the evolutionary paths that followed, making it a significant and defining moment in Earth's geological history.