The
Cretaceous–Paleogene (K–Pg) boundary, formerly known as the
Cretaceous–Tertiary (K–T) boundary, is a
geological signature, usually a thin
band. It defines the end of the
Mesozoic Era, and is usually estimated at around 66
Ma (million years ago), with more specific
radioisotope dating yielding an age of 66.043 ± 0.011 Ma.
K is the traditional abbreviation for the
Cretaceous Period, and
Pg is the abbreviation for the
Paleogene Period. The boundary marks the end of the Cretaceous Period, the last period of the Mesozoic Era, and marks the beginning of the Paleogene Period of the
Cenozoic Era. The boundary is associated with the
Cretaceous–Paleogene extinction event, a
mass extinction which is considered to be the demise of the non-
avian dinosaurs in addition to a majority of the world's Mesozoic species.