Palaeogene Period

Palaeogene Period, in geology, division of the geological timescale, older of two geological periods comprising the Cenozoic Era, the last of the three eras comprising the Phanerozoic Eon. The Palaeogene extends from 65 to 24 million years ago and the Neogene from 24 million years ago to the present day. The Palaeogene is divided into three epochs, the Palaeocene (65-55), Eocene (55-34), and Oligocene (34-24 million years ago).

Palaeogene rocks occur worldwide and are predominantly sedimentary rocks developed on lowland plains. Although global sea level fell gradually throughout the Cenozoic, globally fluctuating rises and falls resulted in the deposition of extensive interdigitated (interleaved) marine through to continental sedimentary deposits on the continental margins of Europe, the Middle East, North Africa, the eastern United States south to Mexico and southern Australia. However, two of the volumetrically greatest volcanic outpourings known also occurred in the early Palaeogene. The first was the extensive deposition of flood basalts (500,000 cubic km; 119,000 cubic mi) in central India as it passed over a deep mantle plume (the Deccan Traps at the Cretaceous-Palaeocene boundary), the second was explosive volcanism in the northern Atlantic (east Greenland, Scotland) later in the Palaeocene. The latter was associated with the initial rifting between Greenland and Scandinavia to form the Norwegian-Greenland Sea that had connected the north Atlantic with the Arctic Ocean by the Early Eocene. Other major tectonic events included the separation of Antarctica and Australia in the Late Palaeocene, and the Early Oligocene connection of the southern Atlantic and Pacific through the Drake Passage.

A number of great mountain ranges have their origins in the Palaeogene. The still continuing underthrusting of the Eurasian Plate by the northwards moving Indian Plate has produced the Himalaya. By the Late Eocene this collision had almost closed the Tethyan seaway. As a result of the northwards collision and subduction of Africa in the Late Eocene through to the Oligocene, the Alpine Orogeny produced the Alpine mountain chains, the Pyrenees and Carpathians of southern Europe, and the Atlas Mountains of north-west Africa. In the western United States the Cordilleran Orogeny (Late Cretaceous to Eocene) produced deeply downwarped intermontane basins that filled in the Palaeocene and Eocene with up to 15 km (9 mi)thickness of fluvial and lacustrine sediments in the area of the present-day Rocky Mountains.

There was a long-term decline in global temperatures throughout much of the Palaeogene. Temperatures worldwide peaked during the Early Eocene and warm climates extended from pole to pole with a latitudinal gradient of only 5°C compared with 25°C today, enabling subtropical faunas to live in the Canadian Arctic. Plate movements increasingly divided the world ocean and led to less efficient latitudinal energy transfer through the Palaeogene to the present day. Global cooling intensified at the Eocene-Oligocene boundary, when Antarctic glaciation was initiated.

The end-Cretaceous mass extinction had a major effect on marine ecosystems: a number of prominent and diverse invertebrate groups, such as ammonite and belemnite cephalopod molluscs, became extinct together with many organisms comprising the planktonic food chain (coccolithophorids and planktonic foraminiferans had catastrophic declines worldwide). Among marine vertebrates, large predators such as ichthyosaurs, plesiosaurs and mosasaurs all became extinct. Surviving groups of marine organisms rapidly rebounded and through the Palaeogene most major groups of marine vertebrates, such as teleost fish, and invertebrates, including gastropod and bivalve molluscs, bryozoans and reef corals, continued to diversify. This extensive radiation of groups which have come to dominate modern marine faunas gave Palaeogene communities a much more modern appearance and composition than those characterizing the latest Cretaceous.

In terrestrial ecosystems the flora, as today characterized by highly diversified angiosperms (flowering plants), continued to radiate through the Palaeogene, having not suffered large-scale extinction at the end of the Cretaceous. In contrast, the demise of the dinosaurs led to a concomitant explosive radiation in mammals, which radiated to fill now vacant ecological niches. Their diversity rapidly increased in the Early Eocene and more than 20 orders became established, including early perissodactyls (rhinoceroses, horses), artiodactyls (deer, antelopes, camels), and rodents, and these extended throughout Eurasia and North America. Whales evolved in the Eocene and pinnipeds (seals, sea-lions, walruses) evolved in the Late Palaeogene, both from terrestrial ancestors. By the Oligocene, falling temperatures had led to a decline in tropical forests and to the development of extensive open forests and grasslands—landscapes which became more prevalent still in the Neogene. Grasses originated in the Late Palaeocene but grasslands and prairies only developed at the end of the Palaeogene.