Cenozoic

Late Eocene (Priabonian Age, latest Paleogene) mammals, showing a mixture of persisting archaic (Brontotherium, Hyaenodon) and early modern genera. In the distance are giant Brontotherium, now called Menodus, among the largest land animals of their time. Just in front of them is one the the first true rhinoceros, Subhyracodon, and in front of it is a group of Merycoidodon herbivorous oreodonts. Next to them, in the center of the frame, is a Protapirus, the first true tapir, that in turn is above a group of Poëbrotherium, small ancestral camels. Two Hyaenodon fight over a Glyptosaurus lizard, while a saber-tooth cat, Hoplophoneus lies in the bushes in the center foreground. Moving across the front from left to right a series of small animals are seen: the primitive rabbit Paleolagus, the insectivore Ictops, three kinds of ruminants, Hypisodus, a couple of Leptomeryx, and on the far right lower corner, a group of Hypertragulus. Artwork and most of the caption is from the the 1964 Time-Life book, The Land and Wildlife of North America - original page

Introduction

The Cenozoic era is the most recent era in the history of life, spanning from 65 million years ago to the present day. In terms of orders of magnitude, which we are using here as a way to conceptualise Deep Time, more than three-quarters of the entire Cenozoic or Age of Mammals can fit in the 50 million year interval, about the average time for a geological period.

The Cenozoic is therefore the shortest of the three main geological eras, shorter even then the Jurassic and the Cretaceous alone. This era, which followed after the extinction of dinosaurs, saw the rise of mammals as the dominant species on Earth, and the tremendous diversification of bird and mammal groups.

The Paleocene epoch, lasting from 65.5 to 56 million years ago, was marked by the recovery of life after the mass extinction event that wiped out the dinosaurs. During this time, mammals started to diversify and take over many ecological niches left by the dinosaurs. There were several warm intervals during the Paleocene, allowing for the spread of tropical and subtropical flora and fauna.

During the Eocene epoch, from 56 to 33.9 million years ago, mammals continued to diversify and evolve into a wide range of forms, including early primates, and ungulates, one line of which became whales. This was also a time of warm temperatures, with the Earth's poles being covered in forests. The Eocene is known for having some of the highest atmospheric carbon dioxide levels in the past 65 million years. The end of the Eocene and the start of the Oligocene marked the extinction of many archaic mammals types, including the predominant Brontothere megaherbivores on land, and early whales ((archaeoceti) which at the time had rules the seas.

The Oligocene epoch, from 33.9 to 23 million years ago, saw the cooling of the Earth's climate and the spread of grasslands. This cooling trend set the stage for the formation of the Antarctic ice cap, which eventually led to further cooling of the Earth's climate. During the Oligocene, mammals such as horses, rhinoceroses, and primates continued to diversify.

The Miocene epoch, from 23 to 5.3 million years ago, was a time of significant cooling and drying of the Earth's climate. This led to the formation of the savanna and the expansion of grasslands, which allowed for the evolution of grazing mammals such as antelopes and giraffes. During the Miocene, primates also diversified, leading to the evolution of apes and eventually, hominids.

The Pliocene epoch, from 5.3 to 2.6 million years ago, saw the continued cooling of the Earth's climate, leading to the formation of the large ice sheets that still exist today. During this time, hominids continued to evolve, including the evolution of early human ancestors such as Australopithecus, leading to the emergence of Homo habilis, the first member of the genus Homo. During this time, the Earth's climate became more seasonal and the planet saw the formation of the large ice sheets that covered much of the northern hemisphere. The Pliocene also saw the evolution of modern species of mammals, including elephants, horses, and whales.

The Pleistocene epoch, from 2.6 million years ago to 11,700 years ago, was a time of repeated glaciations and interglacials, leading to significant changes in the Earth's climate and ecosystems. During this time, the genus Homo continued to evolve, leading to the emergence of Homo erectus and eventually, Homo sapiens. The Pleistocene was also a time of significant extinction of large mammals, such as mammoths and sabre-toothed cats.

The Holocene epoch, from 11,700 years ago to the present, is marked by the end of the last glacial period and the beginning of the modern climate. During this time, Homo sapiens have spread across the world, leading to the development of civilizations and the rise of human culture. The Holocene is also marked by significant changes in the Earth's climate, including the Little Ice Age and the current period of global warming.

For the fanciful equivalent projected into the future, see the long term far distant future.

Timeline

The last fifty million years, from Wikipedia

50 Mya. Peak diversity of dinoflagellates and nannofossils, increase in diversity of anomalodesmatan and heteroconch bivalves, brontotheres, tapirs, rhinoceroses, and camels appear in the fossil record, diversification of primates.
40 Mya. Modern-type butterflies and moths appear. Extinction of Gastornis. Basilosaurus, one of the first of the giant whales, appeared in the fossil record.
38 Mya. Earliest bears.
37 Mya. First nimravid ("false saber-toothed cats") carnivores — these species are unrelated to modern-type felines. First alligators and ruminants.
35 Mya. Grasses diversify from among the monocot angiosperms; grasslands begin to expand. Slight increase in diversity of cold-tolerant ostracods and foraminifera, along with major extinctions of gastropods, reptiles, amphibians, and multituberculate mammals. Many modern mammal groups begin to appear: first glyptodonts, ground sloths, canids, peccaries, and the first eagles and hawks. Diversity in toothed and baleen whales.
33 Mya. Evolution of the thylacinid marsupials (Badjcinus).
30 Mya. First balanids and eucalypts, extinction of embrithopod and brontothere mammals, earliest pigs and cats.
28 Mya. The giant hornless rhino Paraceratherium, the largest terrestrial mammal. First pelicans.
25 Mya. Pelagornis sandersi appears in the fossil record, the largest flying bird that ever lived.
25 Mya. First deer.
24 Mya. First pinnipeds.
23 Mya. Earliest ostriches, trees representative of most major groups of oaks have appeared by now.
20 Mya. First giraffes, hyenas, and giant anteaters, increase in bird diversity.
17 Mya. First birds of the genus Corvus (crows).
15 Mya. Genus Mammut (mastodons) appears in the fossil record, first bovids and kangaroos, diversify Australian megafauna.
10 Mya. Grasslands and savannas are established, diversity in insects, especially ants and termites, horses increase in body size and develop high-crowned teeth, major diversification in grassland mammals and snakes.
9 Mya. First platypuses.
7.246 Mya. Messinian age (latest Miocene) begins
6.5 Mya. First hominins (Sahelanthropus). Hominin lineage splits from the chimp lineage
5.96 Mya The Messinian salinity crisis (MSC), also referred to as the Messinian event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation. ~6 Mya. Australopithecines diversify (Orrorin, Ardipithecus).

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