Cenozoic Era

Cenozoic Era, third and final era of the Phanerozoic Eon, extending from 65 million years ago to the present day. It is preceded by the Mesozoic Era and is subdivided into the Tertiary Sub-Era (65 million to 1.8 million years ago) and the Quaternary Period or Sub-Era (1.8 million years ago to the present day). The Tertiary is divided into the Palaeogene Period (65 million to 24 million years ago) and the Neogene Period (24 million to 1.8 million years ago). These two periods are further subdivided into five epochs: Palaeocene, Eocene, and Oligocene in the Palaeogene, and Miocene and Pliocene in the Neogene. The Cenozoic is a fascinating era of geological time, revealing the progressive development of the modern world through plate tectonic movements and mountain building (see Orogeny), changes in climate and oceanic circulation patterns, and the evolution of the modern biota.

At the start of the Cenozoic the world map was substantially different to that of the present day. The Atlantic was a relatively narrow ocean, tapering to the North, where Greenland was still attached to Europe. There was a continuous equatorial seaway (Tethys) running between North and South America, between Europe and Africa, and between India and Asia. Antarctica was almost in its modern position but still connected to South America and Australia. Through the Cenozoic the Atlantic progressively widened, while South America, Africa, and especially India moved northwards, closing the Tethys Sea and creating the Alps and Himalaya. Concurrently, the southern continents separated, with the deep-water Drake Passage opening between South America and Antarctica, while Australia detached from Antarctica and drifted 30° in latitude to the north.

The major climatic theme of the Cenozoic was progressive cooling from warm equable “greenhouse” conditions in the earlier part of Palaeogene to the “ice-house” glaciations of the Quaternary. This climatic deterioration appears to have been a stepwise process from a climatic optimum about 50 million years ago. The most important shifts occurred during the Late Eocene, about 40 to 35 million years ago, and towards the end of the Pliocene, about 2.5 to 2 million years ago. Both steps are thought to be linked to major changes in oceanic circulation, driven ultimately by plate tectonics. The cooling of 40 to 35 million years ago coincides with the formation of the Southern Ocean, due to the northward movement of South America and Australia away from Antarctica. This allowed formation of circum-Antarctic currents that isolated Antarctica and led to the formation of the massive Antarctic ice sheet. The cooling of 2.5 to 2 million years ago is linked to formation of the Isthmus of Panama. This disrupted equatorial circulation, and led to formation of the Gulf Stream which transported warm water to the North Atlantic. As a result, precipitation increased in the Arctic leading to northern hemisphere glaciation.

The Cenozoic began badly for life on Earth following the end Cretaceous mass extinction caused, at least in part, by the impact of an asteroid in Chixculub, Mexico (see Meteoric Impacts on the Earth). This mass extinction eliminated some of the most characteristic groups of the Mesozoic, notably dinosaurs, marine reptiles and ammonites, and severely reduced several other groups. The Cenozoic Era is ending in much the same way with widespread extinctions, especially of large vertebrates, caused by a combination of climatic change and human activities. In between times the prime characteristic of evolution during the Cenozoic has been the radiation and rise to prominence of the groups which dominate the modern biota. These include the flowering plants (angiosperms), land and sea mammals, birds, gastropod molluscs, scleractinian corals and diatoms. This progressive radiation was interrupted by episodes of more rapid change, most notably at the Palaeocene-Eocene boundary, about 55 million years ago, and during the Late Eocene-Oligocene climatic deterioration, 40 to 35 million years ago. The Late Eocene-Oligocene change occurred gradually in response to climate change but had profound effects with numerous groups of mammals that had evolved in the warm climates of the Eocene being replaced by new groups. The Palaeocene-Eocene changes were, by contrast, very abrupt, with major climatic change including a global warming of about 4°C that occurred within a few thousand years or less. This 'Late Palaeocene Thermal Maximum' event is associated with significant extinctions, most noticeably in oceanic benthic (sea-floor) foraminifera. The event has attracted much recent research, and geochemical evidence suggests that neither meteorite impact nor volcanic activity are likely causes but rather that it was probably triggered by a massive release of the greenhouse gas methane from gas hydrate deposits on continental shelves.

Thus the overall pattern of the Cenozoic is a combination of progressive change punctuated by episodes of relatively rapid change. The episodes of rapid change are all related to environmental change but with variable causes: asteroid impact, methane hydrate degassing, glaciation, and the destructive influence of humans. In each case our perception of the pattern and process has been revolutionized by detailed research over the past two decades. Our knowledge is still, however, very incomplete and it is certain that continuing research will develop a richer understanding of the interplay between plate tectonics, climatic change, and evolution which have given rise to the modern world and which are ever more rapidly changing it.