The Paleogene begins with a clay layer laid down in a few days
In Tunisia, geologists mark the start of the Paleogene, 66 million years ago, at the rusty base of a clay band about 50 centimetres thick that settled in just days. It carries the chemical fingerprint of the asteroid that ended the dinosaurs, and it opens the 43 million years in which mammals, birds and modern fish took over.
The Paleogene is the first period of the Cenozoic Era, running from 66 million to about 23 million years ago and split into the Paleocene, Eocene and Oligocene epochs. Together with the following Neogene it replaces the old term Tertiary, still used informally. Its boundary layer, found worldwide, contains an iridium spike, microtektites, nickel-rich spinel and shocked quartz, all signs of a huge impact whose crater lies at Chicxulub on Mexico's Yucatán Peninsula.
Survivors inherited an emptied world. With non-avian dinosaurs, pterosaurs and marine reptiles gone, mammals expanded from small, simple animals into a range of sizes and orders, reaching the air and the sea by the Eocene. Birds, the one surviving dinosaur lineage, radiated from a handful of groups into a spectacular variety of forms, and percomorph fish, today the most diverse vertebrates, exploded into their modern families.
The climate swung hard. The Eocene opens with the Paleocene-Eocene Thermal Maximum, a burst of warming blamed on the rapid release of frozen methane from seafloor sediments; its carbon signature defines the epoch's base at Dababiya, near Luxor in Egypt. Long cooling followed, and by the start of the Oligocene, marked at Massignano in Italy by the extinction of a group of plankton, permanent ice sheets had appeared in Antarctica.
Continents were rearranging too. The last stages of Pangaea's breakup opened the North Atlantic between North America and Eurasia, while Australia and South America pulled away from Antarctica to create the Southern Ocean. Africa and India pushed into Eurasia, raising the Alpine-Himalayan chains. In the Mediterranean, a northward-jutting African promontory called Adria helped build the Alps and Carpathians, and a roughly 10-million-year pause in convergence coincided with Greenland rifting from Eurasia.
Source: Paleogene
