Complex sentence: The Late Triassic (which can also be referred to as the Triassic-Jurassic extinction) is in the geologic timescale, the third and final of three epochs of the Triassic period, which is one of the five major mass extinction events of the Earth.
Split into simpler sentences: The Late Triassic is the third and final of three epochs of the Triassic Period in the geologic timescale. The Triassic-Jurassic extinction event began during this epoch and is one of the five major mass extinction events of the Earth.