In the geological time scale, Epochs are periods of measurement. Multiple Epochs constitute Periods, which in turn constitute Eras, which in turn constitute Eons. Below, we look at the eight epochs to have occurred since the Late Cretaceous Extinction events. Those events saw the fall of reptilian dinosaurs and many of the other species that had dominated life on Earth in the Creataceous Period.
Paleocene (66-56 million years ago)
The Paleocene Epoch spans the interval between 66 million and 56 million years ago. Most of the evidence of life forms in this epoch are derived from the study of terrestrial rock deposits, as marine rocks from this period are relatively rare. The rock record of North America provides the greatest volume of evidence regarding animal life in the early Paleocene, while evidence of life in late Paleocene Epoch have also been derived from rock deposits in France, Mongolia, and Argentina. One of the most striking features of this Epoch is the complete absence of dinosaurs and other dominant reptilian groups that were so abundant during the preceding Cretaceous Period. However, a rapid proliferation and evolution of mammalian species was observed during the Paleocene. Some examples of the mammals of this period are the opossum and other marsupials, the multituberculates, and condylarths (hoofed mammals evolving towards herbivory). Primates evolved during the middle of this Epoch, and displayed traits overlapping the insectivores and lemurs. The late Paleocene Epoch witnessed the evolution of primitive mammalian carnivores, such as the creodonts, as well. Large herbivores and ancestral rodents also appeared during this period. Marine life is believed to have rapidly evolved and diversified during this time. The highly fossil-rich marine sediments from the Paleocene Epoch, recovered from the coastal plains of North America, prove such facts
Eocene (56-34 million years ago)
The second epoch of the Tertiary Period was referred to as the Eocene Epoch, which spanned the interval between 56 million years and 33.9 million years ago. The early Eocene experienced the highest mean annual temperatures of the Cenozoic Era. However, in the mid-Eocene, the separation of the continental plates of Antarctica and Australia led to the creation of the circum-Antarctic Current in the water passage between these two continents. This event led to a significant drop in mean annual temperatures, and increased seasonality worldwide. This led to replacement of forests by savanna-like vegetation in large parts of the world, and the increased evolution of larger mammals. Two new groups of vertebrates, the perissodactyls (which evolved into horses, rhinos , and tapirs) and the artiodactyls (which evolved into deer, sheep, and cattle) appeared during the early Eocene. Primates, however, declined in number due to their habitats being replaced by the more efficient rodents. This Epoch also marks the development of new marine mammal groups, namely the sirenians and the cetaceans (e.g. modern day whales, dolphins, etc.). The ancient bats and elephant-like creatures, many bird orders, and gastropods also appeared during this period.
Oligocene (34-23 million years ago)
The Oligocene Epoch extended between 34 million years and 23 million years ago. This Epoch is believed to have been the transition period between the archaic world of the Eocene and that of the relatively modern Miocene. The mean annual temperature of this period exhibited a cooling trend, influencing the lives and habitats of many organisms during this period. Organisms capable of withstanding cooler temperatures congregated further away from the equator. The diversification of marine plankton was also slow paced during this period, also largely due to the decreasing temperatures. The tropical and sub-tropical forests were gradually replaced with temperate deciduous ones, and the proliferation of angiosperm plants continued to expand. Open landscapes became more common, allowing animals to grow in size. The equids, rhinos, camelids, and entelodonts of this period achieved the capability to run better in the absence of dense rainforests. Proailurus, the first felid, also originated during the Oligocene Epoch. The separation of South America from Antarctica, and its subsequent isolation from the rest of the world, allowed a completely different set of species to develop in this continent. The pyrothers, liptoterns, terror birds, and carnivorous metatheres becamee the dominant predators on this South American continent. The Oligocene also saw the extinction of the brontotheres, creodonts, and multituberculates. Marine mammals like the baleen whales and toothed whales appeared during this period, while the archaeocete cetaceans began to increasingly decline in population.
Miocene (23-5.3 million years ago)
The Miocene Epoch spanned the time between 23 and 5.3 million years ago, and is notable for the evolution of two major ecosystems, namely the kelp forests and the grasslands. The formation of the grasslands were aided by the rise and fall of global temperatures during this Epoch. The changes in patterns of vegetation led to morphological changes in many animal species as well. The archaic groups of mammals were nearly extinct by this time, and modern terrestrial animals evolved. While interchange of species occurred between the Old and New Worlds in the Northern Hemisphere, South America and Australia remained isolated during this period. Horse diversification occurred in North America, and also the first dogs, bears, and hyenas appeared during this Epoch. This period witnessed the evolution of the famous saber-toothed tigers. In Eurasia, primitive deer, giraffes, and antelopes spread their habitat ranges, and primitive elephants also made their way into this region from Africa. South America developed its own unique sets of species, including the South American monkeys, marsupial carnivores, liptoterns, and endentates. All modern groups of whales, as well as primitive seals and walruses, also evolved during this period. A large variety of birds were present during this time. Primates also rapidly evolved during the Miocene, and evidence of increasingly advanced human-like primates have been found in fossil records sourced from the rocks of this period.
Pliocene (5.3-2.6 million years ago)
The Pliocene epoch stretched from 5.3 million years to 2.6 million years ago. As the prehistoric life continued to adjust to the changing climatic conditions, a number of extinctions occurred during this time. The world temperatures continued to drop, influencing the vegetation patterns upon each of the continents. Two major geographical developments during this period included the connection of North and South America by an isthmus, and the reappearance of the Alaskan land bridge between Eurasia and North America. This allowed for the exchange of fauna between the two countries, and also influenced the temperature of the Atlantic, which was now cut off from the warmer waters of the Pacific. Migration of mammals between the continents led to rapid extinctions of many species, as they faced tough competition from new invasive species. For example, the migration of animals from North to South America wiped out a large number of species in the Southern New World continent. Towards the later stages of the Pliocene, megafauna, such as the Woolly Mammoth, appeared in Eurasia and North America, just as the Giant Sloth and Giant Armored armadillo appeared in South America. The reptilian diversity did not proceed much during this period, and alligators and crocodiles disappeared from Europe due to decreasing temperatures. The Megalodon, the biggest shark that ever lived, was found in the Pliocene's oceans. Whales and pinnipeds flourished during this period as well.
Pleistocene (2.6-0.012 million years ago)
The Pleistocene Epoch is typically defined as the period that began 2.6 million years ago and ended 0.012 million years ago. The period is characterized by the most recent Ice Age, and is also the time when Homo Sapiens (modern humans) evolved and spread into the different parts of the world. By the Pleistocene Epoch, the continents had drifted to the positions they occupy currently, with glaciers covering large parts of the continents of Europe, North America, and South America, and all of Antarctica. The glaciers of this period were not static, but instead retreated and advanced as temperatures cyclically rose and fell. This period also subjected the flora and fauna of the world to significant stress, leading to a major extinction event. Large mammals like mammoths, saber-toothed tigers, ground sloths, and cave bears started disappearing during the Late Pleistocene. Cold-blooded animals and smaller and swifter animals were generally the species that thrived. Neanderthals also became extinct during the Pleistocene, while Homo Sapiens grew in number.
Holocene (12,000-2,000 years ago)
The Holocene is one of the most recent epochs, dating back 12,000 years ago and finishing just 2,000 years back. The rising temperatures during this period again led to the extinction of animals that had adapted to the cold climate of the Pleistocene. Humans, dependent on these large mammals for their food sources, now turned towards agriculture. By 8,000 B.C., wheat, barley, and other food crops were already cultivated across large areas of fertile land in the Indo-European world. Domestication of animals also began during this time. With the innovation of agriculture, man was able to overcome many of the limiting factors that often determine the fate of other species. The human population thus rose rapidly, and by 1800 the population was estimated to be around 1 billion.
Many geologists and environmentalists across the world continue to debate the separation of the human-dominated timeline of the earth into the Holocene and proposed Anthropocene. According to the International Union of Geological Sciences (IUGS), the world is officially in the Holocene Epoch. However, other experts argue that this label is outdated, as modern and ongoing human activities have completely scarred the face of earth and created massive changes that favor the classification of the current period into a completely different epoch, namely the Anthropocene (Greek for "new human era"). The term was popularized by the Nobel laureate Paul Crutzen in 2000, and rapidly gained acceptance across the world. This forced the IUGS to reconsider the division of the most recent period into the epochs of Holocene and Anthropocene. The proponents of this Epoch claim that it will act as a reminder of how human beings are triggering a massive wave of a sixth mass extinction across the world, with their adverse actions affecting ecosystems globally. This can be defined as the Epoch when a single species on earth controlled the entire planet, depleted it of its resources, exploited its forests and biodiversity, and deteriorated its overall climate. Obviously, this "single species", is none other than our own: Homo sapien humans.
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