The Paleogene Period, spanning approximately 66 to 23 million years ago, marks the initial period of the Cenozoic Era, following the profound Cretaceous-Paleogene (K-Pg) extinction event that led to the disappearance of non-avian dinosaurs. This timeframe was characterized by a global recovery of life and significant environmental transformations. The Paleogene is subdivided into three epochs: the Paleocene, Eocene, and Oligocene.
Earth’s Shifting Landscape and Climate
During the Paleogene, continents continued their gradual movement towards present-day configurations. The Indian subcontinent began its collision with Asia, initiating the uplift that would eventually form the Himalayan mountain range. Australia started to separate from Antarctica, and the Mid-Atlantic Ridge continued its spreading activity.
The early Paleogene was marked by a warm, humid global climate, with tropical and subtropical conditions extending widely. Late Paleocene temperatures were 7 to 13°C warmer than today. There were no permanent ice sheets at the poles during this warmer interval.
A gradual cooling trend began later, notably during the Eocene-Oligocene transition. This cooling culminated in the formation of permanent ice sheets on Antarctica. The opening of the Drake Passage, between South America and Antarctica, allowed the formation of the circum-Antarctic current, contributing to polar cooling.
Life’s Remarkable Diversification
Following the K-Pg extinction, life underwent substantial recovery and diversification, filling ecological spaces left vacant by extinct groups. Mammals experienced extensive adaptive radiation, evolving from smaller, generalized forms into diverse modern varieties. This diversification allowed mammals to occupy diverse terrestrial, marine, and even airborne environments.
Birds also diversified significantly, taking over niches previously occupied by pterosaurs. Many modern bird orders, including owls, hawks, ducks, penguins, and vultures, emerged and diversified during the Eocene. Large, flightless predatory birds, sometimes referred to as terror birds, became prominent in certain regions, particularly in South America.
Flowering plants (angiosperms) continued their expansion, becoming dominant across global landscapes. Grasses, important for future ecosystems, first appeared early in the Paleogene. Marine ecosystems also recovered, with sharks becoming more abundant and teleost fish diversifying to dominate the seas.
Major Transitions and Evolutionary Milestones
A notable climatic event during the Paleogene was the Paleocene-Eocene Thermal Maximum (PETM), occurring approximately 56 million years ago. Global temperatures rose 5 to 8°C over a few thousand years, remaining elevated for 100,000 to 200,000 years. The PETM is associated with a massive release of carbon into the atmosphere, which further warmed the planet.
The PETM coincided with the rapid appearance and dispersal of several modern mammalian orders. This includes the earliest known primates, artiodactyls (even-toed ungulates such as deer and cattle), and perissodactyls (odd-toed ungulates like horses and rhinoceroses). These groups likely originated in Asia and then spread across northern continents via temporary land bridges.
While some mammalian lineages reduced body size during the PETM, possibly due to plant nutrition changes, the broader trend after the K-Pg extinction was toward increased mammalian body size. Within the first million years after the extinction, surviving mammals rapidly increased in size and occupied new ecological roles. The expansion of grasslands towards the end of the Paleogene provided abundant food sources, supporting the evolution of larger grazing animals and their predators.