66 - 0 million years ago
The Cenozoic Era (// or //; also Cænozoic, Caenozoic or Cainozoic // or //; meaning "new life", from Greek καινός kainos "new", and ζωή zoe "life") is the current and most recent of the three Phanerozoic geological eras, following the Mesozoic Era and covering the period from 65 million years ago to present day.
The Cenozoic is also known as the Age of Mammals, because the extinction of many groups allowed mammals to greatly diversify.
Early in the Cenozoic, following the K-Pg event, the planet was dominated by relatively small fauna, including small mammals, birds, reptiles, and amphibians. From a geological perspective, it did not take long for mammals and birds to greatly diversify in the absence of the large reptiles that had dominated during the Mesozoic. Some flightless birds grew larger than the average human. These species are sometimes referred to as "terror birds," and were formidable predators. Mammals came to occupy almost every available niche (both marine and terrestrial), and some also grew very large, attaining sizes not seen in most of today's terrestrial mammals.
Climate-wise, the Earth had begun a drying and cooling trend, culminating in the glaciations of the Pleistocene Epoch, and partially offset by the Paleocene-Eocene Thermal Maximum. The continents also began looking roughly familiar at this time and moved into their current positions.
The Cenozoic is divided into three periods: The Paleogene, Neogene, and Quaternary; and seven epochs: The Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. The Quaternary Period was officially recognized by the International Commission on Stratigraphy in June 2009, and the former Tertiary Period was officially disused in 2004 because of the necessity to divide the Cenozoic into periods more like that of the previous Paleozoic and Mesozoic eras.[why?] The common use of epochs during the Cenozoic helps paleontologists better organize and group the many significant events that occurred during this comparatively short interval of time. There is also more detailed knowledge of this era than any other because of the relatively young strata associated with it.
The Paleogene spans from the extinction of the dinosaurs, some 66 million years ago, to the dawn of the Neogene twenty three million years ago. It features three epochs: the Paleocene, Eocene and Oligocene.
The Paleocene ranged from 65 million to 55 million years ago. The Paleocene is a transitional point between the devastation that is the K-T extinction, to the rich jungles environment that is the Early Eocene. The Early Paleocene saw the recovery of the earth. The continents began to take their modern shape, but all continents (and India) were separated from each other. Afro-Eurasia was separated by the Tethys Sea, and the Americas were separated by the strait of Panama, as the isthmus has not yet formed. This epoch featured a general warming trend, with jungles eventually reaching the poles. The oceans were dominated by sharks as the large reptiles that had once ruled went extinct. Archaic mammals filled the world such as creodonts and early primates that evolved during the Mesozoic, and as a result, there was nothing over 10 kilograms. Mammals were still quite small.
The Eocene Epoch ranged from 55 million years to 33 million years ago. In the Early-Eocene, life was small and lived in cramped jungles, much like the Paleocene. There was nothing over the weight of 10 kilograms. Among them were early primates, whales and horses along with many other early forms of mammals. At the top of the food chains were huge birds, such as Gastornis. It is the only time in recorded history that birds ruled the world (excluding their ancestors, the dinosaurs). The temperature was 30 degrees Celsius with little temperature gradient from pole to pole. In the Mid-Eocene, the circum-Antarctic current between Australia and Antarctica formed which disrupted ocean currents worldwide and as a result caused a global cooling effect, shrinking the jungles. This allowed mammals to grow to mammoth proportions, such as whales which ,by that time, were almost fully aquatic. Mammals like Andrewsarchus were at the top of the food-chain and sharks were replaced by whales such as Basilosaurus as rulers of the seas. The Late Eocene saw the rebirth of seasons, which caused the expansion of savanna-like areas, along with the evolution of grass.
The Oligocene Epoch spans from 33 million to 23 million years ago. The Oligocene featured the expansion of grass which had led to many new species to evolve, including the first elephants, cats, dogs, marsupials and many other species still prevalent today. Many other species of plants evolved in this period too, such as the evergreen trees. A cooling period was still in effect and seasonal rains were as well. Mammals still continued to grow larger and larger. Paraceratherium, the largest land mammal to ever live evolved during this period, along with many perissodactyls in an event known as the Grande Coupure.
The Miocene spans from 23 to 5 million years ago and is a period in which grass spread further across, effectively dominating a large portion of the world, diminishing forests in the process. Kelp forests evolved, leading to new species such as sea otters to evolve. During this time, perissodactyls thrived, and evolved into many different varieties. Alongside them were the apes, which evolved into a staggering 30 species. Overall, arid and mountainous land dominated most of the world, as did grazers. The Tethys Sea finally closed with the creation of the Arabian Peninsula and in its wake left the Black, Red, Mediterranean and Caspian Seas. This only increased aridity. Many new plants evolved, and 95% of modern seed plants evolved in the mid-Miocene.
The Pliocene ranged from 5 to 2 million years ago. The Pliocene featured dramatic climactic changes, which ultimately lead to modern species and plants. The Mediterranean Sea dried up for several million years. Along with these major geological events, Australopithecus evolved in Africa, beginning the human branch. Also, with the isthmus of Panama formed, and animals migrated across North and South America, wreaking havoc on the local ecology. Climactic changes brought along savannas that are still continuing to spread across the world, Indian monsoons, deserts in East Asia, and the beginnings of the Sahara desert. The earth's continents and seas moved into their present shapes, and the world map hasn't changed much since. 
The Quaternary ranges from 3 million to present day, and features modern animals, and dramatic climate changes and features two epochs: the Pleistocene and the Holocene.
The Pleistocene lasted from 3 million to 12,000 years ago. This epoch featured the ice ages which was a result of the cooling trend that started in the Mid-Eocene. There were at least four distinct separate glaciation periods marked by advancement of ice to as far south as 40 degrees N latitude in mountainous areas. Meanwhile, Africa experienced a trend of desiccation which resulted in the creation of the Sahara, Namib, and Kalahari deserts. Many animals evolved including mammoths, giant ground sloths, dire wolves, saber-toothed cats, and most famously Homo sapiens. 100,000 years ago marked the end of one of the worst droughts of Africa, and led to the expansion of primitive man. As the Pleistocene drew to a close, one of the largest die-outs caused many megafauna to die off, including some of the hominid species, most notably Neanderthals. All continents were affected, but Africa was not hit quite as hard, which is why it has a fair bit of megafauna left, e.g., hippos.
The Holocene ranges from 12,000 years ago to present day. Also known as "the Age of Man", the Holocene features the rise of man on his path to sentience. All recorded history and "the history of the world" lies within the boundaries of the Holocene epoch. Human activity, however, is being blamed for a die-out that has been going on since roughly 10,000 years ago, but the species going extinct have only been recorded since the Industrial Revolution (1500). This is sometimes referred to as the "Sixth Extinction". 322 species have gone extinct due to human activity since the Industrial Revolution. 
Geologically, the Cenozoic is the era when the continents moved into their current positions. Australia-New Guinea, having split from Pangea during the early Cretaceous, drifted north and, eventually, collided with South-east Asia; Antarctica moved into its current position over the South Pole; the Atlantic Ocean widened and, later in the era, South America became attached to North America with the isthmus of Panama.
The Paleocene–Eocene Thermal Maximum of was a significant global warming event; however, since the Azolla event of , the Cenozoic Era has been a period of long-term cooling. After the tectonic creation of Drake Passage, when South America fully detached from Antarctica during the Oligocene, the climate cooled significantly due to the advent of the Antarctic Circumpolar Current which brought cool deep Antarctic water to the surface. The cooling trend continued in the Miocene, with relatively short warmer periods. When South America became attached to North America creating the Isthmus of Panama, the Arctic region cooled due to the strengthening of the Humboldt and Gulf Stream currents, eventually leading to the glaciations of the Quaternary ice age, the current interglacial of which is the Holocene Epoch.
During the Cenozoic, mammals proliferated from a few small, simple, generalized forms into a diverse collection of terrestrial, marine, and flying animals, giving this period its other name, the Age of Mammals, despite the fact that birds still outnumbered mammals two to one. The Cenozoic is just as much the age of savannas, the age of co-dependent flowering plants and insects, and the age of birds. Grass also played a very important role in this era, shaping the evolution of the birds and mammals that fed on it. One group that diversified significantly in the Cenozoic as well were the snakes. Evolving in the Cenozoic, the variety of snakes increased tremendously, resulting in many colubrids, following the evolution of their current primary prey source, the rodents.
In the earlier part of the Cenozoic, the world was dominated by the gastornid birds, terrestrial crocodiles like Pristichampsus, and a handful of primitive large mammal groups like uintatheres, mesonychids, and pantodonts. But as the forests began to recede and the climate began to cool, other mammals took over.
The Cenozoic is full of mammals both strange and familiar, including chalicotheres, creodonts, whales, primates, entelodonts, saber-toothed cats, mastodons and mammoths, three-toed horses, giant rhinoceros like Indricotherium, the rhinoceros-like brontotheres, various bizarre groups of mammals from South America, such as the vaguely elephant-like pyrotheres and the dog-like marsupial relatives called borhyaenids and the monotremes and marsupials of Australia.
|Wikisource has the text of the 1911 Encyclopædia Britannica article Cainozoic.|
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|Preceded by Proterozoic Eon||Phanerozoic Eon|
|Paleozoic Era||Mesozoic Era||Cenozoic Era|