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
Triassic amphibians, the first Lissamphibia, progenitors of first frogs, are known from the Early Triassic, but the group as a whole did not become common until the Jurassic, when the temnospondyls had become very rare. Other survivors the Chroniosuchia and Embolomeri were more closely related to amniotes than temnospondyls. Those became extinct after some million years. Most of the Reptiliomorpha, stem-amniotes that gave rise to the amniotes, disappeared in Triassic, but two water-dwelling groups survived; Embolomeri that only survived into the early part of the period, and the Chroniosuchia, which survived until the end of Triassic. Archosauromorph reptiles, especially archosaurs, progressively