Cretaceous–Paleogene boundary Cretaceous–Paleogene boundary The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K-T) boundary, is a geological signature, usually a thin band of rock. "K", the first letter of the German word "Kreide" (chalk), is the traditional abbreviation for the Cretaceous Period and "Pg" is the abbreviation for the Paleogene Period. The K–Pg boundary marks the end of the Cretaceous Period, the last period of the Mesozoic Era, and marks the beginning of the Paleogene Period, the first period of the Cenozoic Era. Its age is usually estimated at around 66 Ma (million years ago), with radiometric dating yielding a more
Early Triassic have revealed that temperatures rose in the first 3 million years of the Triassic, ultimately reaching sea surface temperatures of in the tropics around 249 million years ago. Large and mobile species disappeared from the tropics, and amongst the immobile species such as molluscs, only the ones that could cope with the heat survived; half the bivalves disappeared. On land, the tropics were practically devoid of life. Big, active animals only returned to the tropics, and plants recolonised on land when temperatures returned to normal around 247 million years ago. Early Triassic The Early Triassic is the first of three