You Gotta Know These Geologic Time Periods
- The Hadean Eon (4600 million years ago to 4000 mya) stretches from the formation of the Earth to the formation of the oldest known rocks. The Hadean is so named because of its harsh conditions, such as intense leftover heat from planetary accretion, the absence of a stable atmosphere, and the frequency of impacts. Key events during the Hadean include the Moon’s formation (4500 mya), likely due to the impact of a small planet with the young Earth, and liquid water ocean formation (4400 mya).
- During the Archean Eon (4000 to 2500 mya), the Earth cooled enough for stable continents to form. There is contrasting evidence as to whether a period of increased impact events called the Late Heavy Bombardment (LHB, 4100 to 3900 mya) during the early Archean actually occurred. The Archean Eon also saw the emergence of life, the first definitive evidence of which comes from stromatolites—fossilized microbial mats that formed in shallow coastal areas.
- The Proterozoic Eon (2500 mya to 541 mya) contained the Great Oxidation Event (GOE, 2400 to 2000 mya), during which oxygen levels rose significantly due to oxygenic photosynthesis performed by cyanobacteria. During the late Proterozoic, the Earth underwent multiple glaciation events, possibly even resulting in a fully frozen “Snowball Earth.” The last period of the Proterozoic Eon was the Ediacaran Period, during which the first known multicellular organisms with specialized tissues—mostly simple, sessile, and flat marine animals—evolved. The Proterozoic was followed by the Phanerozoic Eon—the current eon, characterized by the abundance of plant and animal life.
- The Cambrian Period (541 mya to 485 mya) is the first period of the Phanerozoic Eon and Paleozoic Era. The key event of the time period was the Cambrian Explosion, during which almost all modern animal phyla appeared. The Cambrian Explosion is an example of adaptive radiation, an event by which organisms rapidly evolve and diversify to take advantage of new resources or fill empty ecological niches. The cause of the Cambrian Explosion is unknown, but could be due to environmental changes or evolutionary breakthroughs such as eyes and/or animals that were exclusively predators. Evidence for the Cambrian explosion can be found in the Burgess Shale, a fossil deposit in British Columbia. This period also contained the Cambrian Substrate Revolution, in which the evolution of burrowing animals allowed water and oxygen to penetrate deep into ocean sediments and permanently alter their chemistry.
- The Ordovician Period (485 mya to 444 mya) contained the Great Ordovician Biodiversification Event. Invertebrates flourished, with marine filter-feeders dominating the shallow seas of the period. The Ordovician saw the first complex life on land, as plants colonized the continents. The end of this period was marked by the Ordovician Mass Extinction, the second largest mass extinction in Earth’s history, which killed around 85% of marine species. The mass extinction is thought to have been caused by the Late Ordovician glaciation, which caused sea levels to fall and eliminated many of the shallow seas that housed most organisms of the time.
- The Silurian Period (444 mya to 419 mya) was much more stable and warm than the Ordovician. The Silurian saw the expansion of terrestrial life. Vascular plants—which have specialized tissues containing lignin for transporting water and minerals—emerged, as well as terrestrial fungi and three groups of terrestrial arthropods: myriapods (centipedes and millipedes), arachnids (spiders), and hexapods (insects). Jaws and bones evolved for the first time in fish, which were beginning to rapidly diversify.
- The Devonian Period (419 mya to 359 mya) is sometimes known as the Age of Fishes, as they came to dominate marine environments. Some fish even moved beyond the ocean; during the Devonian, the ancestors of four-limbed vertebrates, or tetrapods, began to adapt to life on land as the first amphibians. Land plants also underwent an adaptive radiation sometimes known as the Devonian Explosion, as evolutionary innovations including the first leaves, roots, and seeds allowed them to rapidly colonize swaths of dry land. These plants may have also indirectly caused the Late Devonian Mass Extinction by absorbing huge quantities of atmospheric carbon dioxide, leading to global cooling and marine anoxia (low-oxygen conditions). The extinction primarily affected marine organisms; at least 75% of species went extinct, including almost all of the formerly dominant trilobites.
- The Carboniferous Period (359 mya to 299 mya) can be divided into two halves: the Mississippian (359 mya to 331 mya), and the Pennsylvanian (331 mya to 299 mya). The Carboniferous is named for the abundant coal deposits dated to that time. Receding seas led to the prevalence of lowland swamps, which were filled with lignin-rich trees and numerous ferns. These extensive forests sequestered carbon dioxide, leading to an increase in atmospheric oxygen levels to about 35%, as compared to 21% today. High oxygen levels may have led to gigantism in insects; millipedes grew to over 8 feet long and dragonflies had wingspans of 30 inches. The Carboniferous is also known as the Age of Amphibians, as tetrapods became the dominant terrestrial animals and began to diversify. The first amniotes appeared, as well as the first reptiles. Landmasses including the southern continent Gondwana and the northern continent Laurasia collided to form the supercontinent Pangaea, which was surrounded by the Panthalassic Ocean.
- The Permian Period (299 mya to 252 mya) saw a decline of the swamps of and rainforests that dominated the Carboniferous and an increase in deserts throughout the interior of Pangaea. Amniotes were able to cope with the prevalent dry climates, and two amniotic groups—the synapsids (ancestors of modern mammals) and sauropsids (reptiles)—greatly diversified. The end of the Permian was marked by the Permian Mass Extinction (also known as the Great Dying), the largest mass extinction in Earth’s history; over 90% of marine species and 70% of terrestrial vertebrate species went extinct. The extinction is thought to have been caused by an eruption of the Siberian Traps, a large igneous province that released massive amounts of lava and noxious gasses. The outgassing of carbon dioxide led to severe global warming, ocean acidification, and anoxia. All large synapsids went extinct, leaving an ecological void.
- The Triassic Period (252 mya to 201 mya) was the first period of the Mesozoic Era. A specialized group of reptiles called archosaurs underwent adaptive radiation to fill the voids left by the Permian Mass Extinction. The dominant group of archosaurs was the pseudosuchians, ancestors of modern crocodiles. Another group of archosaurs, the dinosaurs, first appeared, as well as the first true mammals. The climate of the Triassic was hot and dry, but became humid as Pangaea began to break up near the end of the period. The period ended with the Triassic Mass Extinction, which wiped out about 75% of species. Marine invertebrates were hit particularly hard, but large pseudosuchians and amphibians also went extinct, paving the way for the rise of the dinosaurs.
- The Jurassic Period (201 mya to 145 mya) was the first to be dominated by dinosaurs. The climate was warm; there were no ice caps and forests stretched to the poles. Many of the most well-known dinosaurs from the Jurassic Period are known from the Morrison Formation in Colorado, including Allosaurus, Stegosaurus, Brontosaurus, Diplodocus, and Apatosaurus. Archaeopteryx, a transitional species between dinosaurs and birds, was discovered in the Jurassic-age Solnhofen Limestone in Germany. The skies were ruled by flying reptiles called pterosaurs, and the oceans were dominated by marine reptiles, including ichthyosaurs and plesiosaurs.
- The Cretaceous Period (145 mya to 66 mya) was similar to the Jurassic. The climate was still warm, with numerous shallow inland seas. Dinosaurs continued to dominate, and mammals and birds continued to diversify. Angiosperms, or flowering plants, appeared for the first time and quickly became the dominant group of plants on Earth. North America was split lengthwise by the Western Interior Seaway, which was dominated by marine reptiles called mosasaurs. Some of the most well-known dinosaurs that walked the land included Tyrannosaurus, Velociraptor, and Triceratops. The Cretaceous Period, and the Mesozoic Era, ended with the Cretaceous-Paleogene Mass Extinction. A worldwide layer of iridium dated to 66 mya sparked the Álvarez Hypothesis, which posited that the extinction was caused by a huge impact event. This idea was strongly supported by the discovery of the massive Chicxulub crater on the Yucatán peninsula. 75% of all species went extinct, including all non-avian dinosaurs and large terrestrial animals.
- The Cenozoic Era (66 mya to present) is the current era, sometimes known as the Age of Mammals. During the initial Paleogene Period (66 mya to 23 mya), mammals diversified and grew in size to become the dominant land animals, filling the void left by the previous mass extinction. Temperatures were high at the beginning of the period, peaking at the Paleocene-Eocene Thermal Maximum (PETM), but cooled down during the Oligocene epoch as Antarctica froze. During the Neogene Period (23 mya to 2.6 mya), landmasses settled close to their modern positions. The current period is the Quaternary Period (2.6 mya to present). The period’s Pleistocene Epoch (2.6 mya to 11,700 ya) was marked by cyclical ice ages. It saw the evolution of megafauna including mammoths, saber-tooth tigers, and giant sloths and armadillos, as well as the first appearance of Homo sapiens. The current epoch is the Holocene epoch, which is characterized by the growth and impacts of the human species.
This article was contributed by ÎÞÓǶÌÊÓƵ writer Justin Duffy.