Geological Time Scale

The Precambrian is the earliest part of Earth's history, spanning from the formation of the Earth (4.6 billion years ago) to the beginning of the Cambrian Period (538.8 million years ago). This period makes up 88% of the Earth's geologic time. It is not formally defined as a single unit, but rather divided into three eons: Hadean, Archean, and Proterozoic. The name "Precambrian" comes from the fact that it precedes the Cambrian period, which was named after the region of Wales where rocks from this age were first studied. Read more...

The Cambrian period is the first geological period of the Paleozoic Era and Phanerozoic Eon, lasting from 538.8 million to 485.4 million years ago. It is unique because of its high number of lagerstätte deposits, which are sites where "soft" body parts of organisms are preserved along with their shells. As a result, scientists have a better understanding of Cambrian biology than some later periods. The Cambrian period marked a significant change in life on Earth, as complex, multicellular organisms became more common and mineralized organisms, which could be fossilized, became abundant. Prior to the Cambrian, most living organisms were small, unicellular, and simple. Read more...

The Ordovician period is a geologic period that spanned from 485.4 million to 443.8 million years ago. It was named after the Welsh tribe of the Ordovices and was defined by Charles Lapworth in 1879 to resolve a dispute between geologists about the classification of rock beds in North Wales. The Ordovician period was marked by the continued flourishing of life, including the dominance of invertebrates such as mollusks and arthropods in the oceans, and the emergence of the first land plants. The period also saw the evolution of fish, including those with jaws, and a significant increase in biodiversity. Read more...

The Silurian period is a geological period that lasted from 443.8 million years ago to 419.2 million years ago. It's the shortest period of the Paleozoic Era and is characterized by the extinction of up to 60% of marine genera. During this period, terrestrial life began to emerge, with vascular plants, fungi, and certain arthropods becoming more complex and diverse. Additionally, jawed fish diversified and became more prominent, including placoderms, acanthodians, and osteichthyan (bony fish), while jawless fish like conodonts and ostracoderms declined. Read more...

The Devonian period, which spanned from 419.2 million to 358.9 million years ago, was a time of significant evolutionary change on land and in the oceans. During this period, land plants underwent a remarkable transformation, with free-sporing and vascular plants spreading across the continents and forming extensive coal forests. The first seed-bearing plants also emerged, marking a major milestone in the evolution of plant life. On land, the first animals to colonize the terrestrial environment were arthropods, such as myriapods, arachnids, and hexapods, which began to thrive in their new environment.

In the oceans, the Devonian period was marked by a explosion of fish diversity, earning it the nickname "The Age of Fishes". Jawed fish in particular reached a high level of diversity, and armored placoderms dominated almost every known aquatic environment. Cartilaginous fishes, including primitive sharks, also became more numerous during this period. Meanwhile, the ancestors of four-limbed vertebrates, known as tetrapodomorphs, began to diverge from freshwater lobe-finned fish. Although they were not yet fully adapted to life on land, these early tetrapods were taking the first steps towards developing limbs and eventually becoming the diverse range of terrestrial animals that we see today. Overall, the Devonian period was a time of rapid evolution and colonization of land and water by various life forms, setting the stage for the next major phase of evolution in the Carboniferous period. Read more...

The Carboniferous period, which spanned from 358.9 million to 298.9 million years ago, is a significant part of the Paleozoic era. The period gets its name from the Latin words for "coal" and "bear", as it was a time when many coal beds formed globally. During this period, life on land was well established, with four-limbed vertebrates (tetrapods) and land plants becoming more diverse. Amphibians, including early amphibians like temnospondyls, became dominant land vertebrates, and the first amniotes (mammals and reptiles) appeared.

The Carboniferous period was also a time of major evolutionary radiation for land arthropods, such as insects and arachnids. Vast forests and swamps covered the land, which eventually became the coal beds characteristic of the Carboniferous stratigraphy that we see today. In the later half of the period, the continents collided to form Pangaea, leading to glaciations, low sea levels, and mountain building. The period ended with a minor extinction event, known as the Carboniferous rainforest collapse, which was likely caused by climate change. Read more...

The Permian period, which spanned from 298.9 million to 251.9 million years ago, was the last period of the Paleozoic Era. During this time, the supercontinent Pangaea was formed, and the world was dominated by a single superocean, Panthalassa. The Permian period saw the diversification of amniotes, including reptiles and mammals, which adapted to the drier conditions that replaced the Carboniferous rainforests.

The Permian period was marked by multiple extinction events, including the Capitanian mass extinction event and the Permian-Triassic extinction event, which was the largest mass extinction in Earth's history. The Permian-Triassic extinction event wiped out nearly 81% of marine species and 70% of terrestrial species, leaving a significant impact on the Earth's ecosystems. It took a long time for life to recover from this catastrophe, with ecosystems taking 30 million years to recover on land. Read more...

The Triassic period, which lasted from 251.902 million years ago to 201.4 million years ago, was a time of great change and transformation on Earth. The period began with a massive extinction event that wiped out many species, leaving the Earth's biosphere impoverished. Despite this, life on Earth slowly began to recover, and during the Triassic, reptiles, especially archosaurs, dominated the terrestrial landscape. Dinosaurs first appeared in the Late Triassic, but it wasn't until the Jurassic period that they would become the dominant species.

During the Triassic, pseudosuchians, relatives of modern crocodilians, were the dominant archosaurs, while pterosaurs, the first flying vertebrates, evolved and thrived. Therapsids, the dominant vertebrates of the previous Permian period, declined during the Triassic, while the first true mammals also evolved during this time. The supercontinent Pangaea dominated the globe during the Triassic, but it began to break apart into two landmasses (Laurasia and Gondwana) in the Jurassic period.

The climate during the Triassic was mostly hot and dry, with deserts covering much of Pangaea's interior. However, as Pangaea began to break apart, the climate shifted and became more humid. The Triassic period ended with another massive extinction event that wiped out many species, including most pseudosuchians, allowing dinosaurs to assume dominance in the Jurassic period. Overall, the Triassic period was a time of great change and upheaval on Earth, setting the stage for the evolution of new species and ecosystems that would characterize the Mesozoic Era. Read more...

The Jurassic period, which spanned from 201.4 million to 145 million years ago, was the middle period of the Mesozoic Era. It was marked by a significant extinction event at the beginning of the period, which was likely caused by the eruption of the Central Atlantic Magmatic Province and the Karoo-Ferrar large igneous provinces. This event led to a period of global warming and oceanic anoxia, also known as the Toarcian Oceanic Anoxic Event.

As the Earth continued to change, the supercontinent Pangaea began to break apart into two landmasses: Laurasia to the north and Gondwana to the south. The climate during this period was warmer than it is today, with no ice caps and forests growing near the poles. The landscape was vastly different from what we see today, with large arid expanses in the lower latitudes.

On land, a significant shift occurred in the fauna. The dinosaurs, which had evolved during the Triassic period, became the dominant species, replacing earlier archosaurs like dinosauromorphs and pseudosuchians. The first birds also appeared during this period, evolving from a branch of theropod dinosaurs. Other major events included the emergence of early mammals, the diversification of lizards, and the transition of crocodylomorphs from a terrestrial to an aquatic life.

In the oceans, marine reptiles like ichthyosaurs and plesiosaurs thrived, while pterosaurs dominated the skies. Modern sharks and rays first appeared and diversified during this period. The flora was dominated by ferns and gymnosperms, including conifers, which made their first appearance during this period. The Bennettitales, a group of ancient plants that is now extinct, also flourished during this time. Overall, the Jurassic period was a time of significant change and evolution on Earth, setting the stage for the next period of geological history. Read more...

The Cretaceous period, which lasted from 145 to 66 million years ago, was the third and final period of the Mesozoic Era. It was also the longest geological period in the Phanerozoic era, spanning around 79 million years. During this time, the climate was relatively warm, with high sea levels and numerous shallow inland seas. These oceans and seas were home to a variety of marine reptiles, ammonites, and rudists, while dinosaurs continued to dominate on land.

The Cretaceous period was a time of significant evolutionary change, as new groups of mammals and birds began to emerge. Flowering plants also appeared and rapidly diversified, becoming the dominant plant group by the end of the period. This marked a significant shift away from the dominance of gymnosperm groups, which had been widespread during the earlier parts of the Mesozoic Era.

However, the Cretaceous period came to an abrupt end with the Cretaceous-Paleogene extinction event, which wiped out many species, including non-avian dinosaurs, pterosaurs, and large marine reptiles. This mass extinction event is widely believed to have been caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous period is marked by the distinct geological boundary known as the Cretaceous-Paleogene boundary (K-Pg boundary), which is characterized by a layer of sediment rich in iridium and other extraterrestrial elements. Read more...

The Paleocene epoch, 66-56 million years ago, was marked by two major events: the K-Pg extinction event, which wiped out 75% of species, including non-avian dinosaurs, and the Paleocene-Eocene Thermal Maximum (PETM), which caused a global temperature spike.

During this time, the continents were still connected, and the Earth had a greenhouse climate without permanent ice sheets. Forests grew worldwide, but with low species richness. Mammals proliferated, and early placental and marsupial mammals appeared. The oceans were different, with downwellings in the North Pacific and water density controlled by salinity. The global average temperature was around 24-25°C (75-77°F). Read more...

The Eocene epoch is a geological period that lasted from 56 to 33.9 million years ago. It is the second epoch of the Paleogene Period and is characterized by the "dawn" of modern fauna. The Eocene began with a brief period of low atmospheric carbon levels and ended with a major extinction event known as the Grande Coupure or Eocene-Oligocene extinction event. The average temperature of Earth at the beginning of the Eocene was around 27°C. The exact dates of the start and end of the Eocene are slightly uncertain, but the strata that define the epoch are well identified. Read more...

The Oligocene epoch, which spanned from 33.9 million to 23 million years ago, is a significant period in the Earth's geological history. The name "Oligocene" comes from the Greek words "olígos" (few) and "kainós" (new), referring to the scarcity of new mollusk species found in marine beds during this time. This epoch is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene.

During the Oligocene, there were significant changes in the Earth's ecosystems. A major extinction event, known as the Grande Coupure, marked the beginning of the Oligocene, with European fauna being replaced by Asian fauna. This event was likely caused by a combination of factors, including changes in climate and sea levels. The Oligocene is also marked by a global expansion of grasslands and a regression of tropical broad leaf forests to the equatorial belt. These changes would have had a significant impact on the evolution of life on Earth.

The end of the Oligocene is not marked by a single global event, but rather by regional changes in climate and ecosystems as the Miocene epoch began. The Oligocene-Miocene boundary is not as well-defined as some other geological boundaries, and its exact timing and characteristics are still a subject of scientific debate. Despite this, the Oligocene remains an important period in the Earth's history, marking a significant transition between two major epochs in the Earth's geological history. Read more...

The Miocene epoch, which spanned from 23.03 to 5.33 million years ago, was the first geological epoch of the Neogene Period. The name "Miocene" comes from the Greek words "meíōn" (less) and "kainós" (new), meaning "less recent" because it has 18% fewer modern marine invertebrates than the Pliocene epoch. During this time, the Earth's climate slowly cooled towards a series of ice ages, with regionally defined boundaries between the warmer Oligocene and the cooler Pliocene epochs.

The Miocene was marked by significant geological events, including the collision of Afro-Arabia and Eurasia, which severed the connection between the Mediterranean and Indian Oceans. This event allowed for a faunal interchange to occur between Eurasia and Africa, including the dispersal of proboscideans into Eurasia. Later, the connections between the Atlantic and Mediterranean closed, causing the Mediterranean Sea to nearly completely evaporate in an event known as the Messinian salinity crisis. The Strait of Gibraltar eventually opened, and the Mediterranean refilled at the Miocene-Pliocene boundary, in an event known as the Zanclean flood.

The Miocene was also a time of significant evolutionary developments. Apes first evolved and diversified during this epoch, with early human ancestors splitting away from chimpanzees towards the end of the Miocene. The Miocene saw the expansion of grasslands and forests, as well as the emergence of modern ecosystems like kelp forests. Mammals, birds, whales, pinnipeds, and kelp were all present during this time. Read more...

The Pliocene epoch is a geological time period that spanned from 5.333 million to 2.58 million years ago. It is the second and most recent epoch of the Neogene Period in the Cenozoic Era. The Pliocene is preceded by the Miocene epoch and is followed by the Pleistocene epoch. The exact start and end dates of the Pliocene are not precisely known, but it is generally defined as the transition from the warmer Miocene to the cooler Pleistocene. The upper boundary of the Pliocene is marked by the onset of the Pleistocene glaciations. Read more...

The Pleistocene epoch, often referred to as the Ice Age, spanned from approximately 2.58 million to 11,700 years ago. During this time, the Earth experienced repeated glaciations, with the most recent one ending just 11,700 years ago. The epoch began with a significant event - the formation of the Isthmus of Panama, which connected North and South America and led to changes in ocean circulation patterns. This marked the onset of glaciation in the Northern Hemisphere.

As the Pleistocene progressed, archaic humans of the genus Homo originated in Africa and spread throughout Afro-Eurasia during the Early Pleistocene. The Mid-Pleistocene Transition marked a significant change in the cyclicity of glacial cycles, making climate variation more extreme. The Late Pleistocene saw the spread of modern humans outside of Africa, leading to the extinction of all other human species. This period also witnessed the colonization of Australia and the Americas for the first time, coinciding with the extinction of most large-bodied animals in these regions.

The Pleistocene epoch was characterized by highly variable climate conditions, with sea levels dropping by up to 120 meters (390 feet) at peak glaciation. This allowed for the connection of Asia and North America via Beringia and the covering of most of northern North America by ice sheets. The Pleistocene came to an end with the end of the last glacial period and the end of the Paleolithic age in archaeology. Read more...

The Holocene is the current geological epoch, which began around 11,700 years ago. It follows the last ice age and is characterized by a period of rapid human growth, technological advancements, and development of civilizations. The Holocene is divided into three distinct ages: Greenlandian (11,700-8,200 years ago), Northgrippian (8,200-4,200 years ago), and Meghalayan (4,200 years ago to present). The ages are defined by changes in climate, including warming, cooling, and drought. The Greenlandian age was marked by a warming period, the Northgrippian age saw significant cooling due to changes in ocean circulation, and the Meghalayan age has been marked by extreme drought lasting around 200 years. Read more...