The boundary clay shows unusually high levels of the metal iridium, which is more common in asteroids than in the Earth's crust. In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. 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. Most other tetrapods weighing more than 25 kilograms (55 pounds) also became extinct, with the exception of some ectothermic species such as sea turtles and crocodilians. The event caused the extinction of all non-avian dinosaurs. The Cretaceous–Paleogene ( K–Pg) extinction event, also known as the Cretaceous–Tertiary ( K–T) extinction, was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago.
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