The Maastrichtian is the last stage of the Cretaceous period, and therefore of the Mesozoic era. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma (million years ago). At the end of this period, there was a mass extinction commonly referred to as the K/T (or Cretaceous/Tertiary) extinction. At this extinction event, many commonly recognized groups such as dinosaurs, plesiosaurs and mosasaurs, as well as many other lesser known groups, died out. The Maastrichtian is named after the Dutch city Maastricht.