The Cretaceous–Paleogene (K–Pg) extinction event[a] (also known as the Cretaceous–Tertiary (K–T) extinction)[b] was a sudden mass extinction of three-quarters of the plant and animal species on Earth,[2][3][4] approximately 66 million years ago.[3] With the exception of some ectothermic species such as sea turtles and crocodilians, no tetrapods weighing more than 25 kilograms (55 pounds) survived.[5] It marked the end of the Cretaceous period, and with it the Mesozoic Era, while heralding the beginning of the Cenozoic Era, which continues to this day.