3.5 Geologic time units

The geologic time scale divides Earth's 4.6-billion-year history into distinct intervals based on major events in its evolution. It's crucial for paleontologists to place fossil discoveries in context and study patterns of evolution, extinction, and environmental change over time.

Understanding geologic time units helps us grasp Earth's development and life's progression. From eons spanning billions of years to epochs lasting millions, these divisions provide a framework for studying Earth's history and the intricate story of life's evolution.

Geologic time scale overview

• The geologic time scale is a system used by geologists and paleontologists to describe Earth's history and the timing of events
• It divides Earth's 4.6-billion-year history into distinct time intervals based on major events in Earth's physical, chemical, and biological evolution
• Understanding the geologic time scale is crucial for paleontologists to place fossil discoveries in their proper temporal context and study patterns of evolution, extinction, and environmental change over time

Relative vs absolute dating

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• Relative dating determines the order of events without specifying exact ages, based on principles like superposition (older layers below younger ones) and cross-cutting relationships
• Absolute dating assigns specific ages to events or materials using techniques like radiometric dating (measuring decay of radioactive isotopes), providing a chronological framework
• Combining relative and absolute dating methods allows paleontologists to construct a detailed timeline of Earth's history and the evolution of life

Principles of stratigraphy

• Stratigraphy is the study of rock layers (strata) and their succession, which records the chronology of Earth's geologic history
• The principle of original horizontality states that sedimentary layers are deposited in nearly horizontal sheets, so tilted or folded strata indicate later deformation
• The principle of lateral continuity holds that layers extend laterally in all directions until they thin out or reach the edge of the depositional basin
• The principle of cross-cutting relationships asserts that a geologic feature that cuts across another must be younger than the feature it cuts, establishing relative ages
• The principle of inclusions states that rock fragments included in another rock must be older than the host rock
• The principle of faunal succession recognizes that fossil organisms succeed each other in a definite, recognizable order that can be identified over wide areas, enabling correlation of strata

Eons of geologic time

• Eons are the largest divisions of geologic time, spanning hundreds of millions to billions of years
• The four eons of Earth's history are the Hadean, Archean, Proterozoic, and Phanerozoic, representing major stages in the planet's evolution
• Each eon is characterized by distinct conditions, events, and life forms, providing a broad framework for understanding Earth's development and the evolution of life

Hadean eon

• The Hadean eon (4.6 to 4 billion years ago) represents Earth's earliest history, named after Hades (the Greek underworld) due to its hellish conditions
• During the Hadean, Earth was heavily bombarded by asteroids and comets, experienced intense volcanism, and had a reducing atmosphere devoid of oxygen
• The Hadean crust was likely recycled by plate tectonic processes, leaving no known rocks from this eon, although some zircon crystals have been dated to the late Hadean

Archean eon

• The Archean eon (4 to 2.5 billion years ago) saw the formation of the first continents, the emergence of primitive life, and the gradual oxygenation of the atmosphere
• Archean rocks, including banded iron formations and pillow basalts, provide evidence for the earliest known life forms (prokaryotic microbes) and the operation of plate tectonics
• The Archean atmosphere was still largely reducing, but photosynthetic cyanobacteria began to release oxygen as a byproduct, setting the stage for the Great Oxidation Event

Proterozoic eon

• The Proterozoic eon (2.5 billion to 541 million years ago) witnessed the rise of oxygen in the atmosphere, the evolution of eukaryotic cells, and the appearance of multicellular life
• The Great Oxidation Event occurred around 2.4 billion years ago, when atmospheric oxygen levels rose significantly due to photosynthetic activity, dramatically altering Earth's surface chemistry
• The Proterozoic saw the formation of several supercontinents (Columbia, Rodinia, and Pannotia), as well as multiple glaciations, including the Snowball Earth events

Phanerozoic eon

• The Phanerozoic eon (541 million years ago to present) is marked by the proliferation of complex life forms, including the evolution of animals, plants, and fungi
• The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic, each characterized by distinct faunas and floras
• Major events in the Phanerozoic include the Cambrian Explosion, the colonization of land by plants and animals, mass extinctions, and the evolution of modern ecosystems

Eras of the Phanerozoic

• The Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic
• Each era represents a significant chapter in the evolution of life and is characterized by unique faunas, floras, and environmental conditions
• Understanding the characteristics and events of each era is essential for paleontologists studying the history of life on Earth

Paleozoic era

• The Paleozoic era (541 to 252 million years ago) saw the rapid diversification of marine invertebrates, the colonization of land by plants and animals, and the evolution of early tetrapods
• Major Paleozoic events include the Cambrian Explosion, the Great Ordovician Biodiversification Event, the Devonian rise of forests, and the late Paleozoic ice age
• The Paleozoic ended with the Permian-Triassic extinction, the largest known mass extinction in Earth's history, which wiped out over 90% of marine species and 70% of terrestrial vertebrate species

Mesozoic era

• The Mesozoic era (252 to 66 million years ago) is known as the "Age of Reptiles" due to the dominance of dinosaurs, pterosaurs, and marine reptiles
• The Mesozoic saw the breakup of the supercontinent Pangaea, the evolution of flowering plants (angiosperms), and the rise of modern insect groups
• The Mesozoic ended with the Cretaceous-Paleogene extinction, which eliminated non-avian dinosaurs and many other groups, paving the way for the age of mammals

Cenozoic era

• The Cenozoic era (66 million years ago to present) is the "Age of Mammals," characterized by the adaptive radiation and diversification of mammalian lineages
• The Cenozoic saw the global expansion of grasslands, the evolution of humans, and the onset of Pleistocene glacial-interglacial cycles
• The Cenozoic is divided into three periods: the Paleogene, Neogene, and Quaternary, each marked by distinct climatic and biotic events

Periods of the Paleozoic

• The Paleozoic era is divided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian
• Each period is characterized by unique faunas, floras, and environmental conditions, reflecting the progressive evolution of life and changes in Earth's climate and geography
• Understanding the key events and characteristics of each Paleozoic period is crucial for paleontologists studying the early history of complex life on Earth

Cambrian period

• The Cambrian period (541 to 485 million years ago) is marked by the "Cambrian Explosion," a rapid diversification of animal phyla and the appearance of most modern animal body plans
• Cambrian faunas are dominated by arthropods (trilobites, crustaceans), sponges, brachiopods, and early chordates, with the first appearance of mineralized skeletons
• The Cambrian saw the development of complex ecosystems, the evolution of predator-prey relationships, and the establishment of the marine food web

Ordovician period

• The Ordovician period (485 to 444 million years ago) witnessed the "Great Ordovician Biodiversification Event," a significant increase in marine biodiversity
• Ordovician faunas are characterized by diverse invertebrates, including graptolites, trilobites, brachiopods, and cephalopods, as well as the first appearance of jawed fishes
• The Ordovician ended with a mass extinction, possibly related to glaciation and changes in sea level and ocean chemistry

Silurian period

• The Silurian period (444 to 419 million years ago) saw the continued diversification of marine life, the appearance of the first vascular plants on land, and the evolution of jawed fishes
• Silurian faunas are dominated by brachiopods, crinoids, corals, and trilobites, with an increasing diversity of fish and the first evidence of terrestrial arthropods
• The Silurian experienced a series of extinction events, including the end-Ordovician and Lau/Kozlowskii extinctions, which affected marine communities

Devonian period

• The Devonian period (419 to 359 million years ago) is known as the "Age of Fishes" due to the diversification of jawed fishes, including placoderms, sharks, and bony fishes
• The Devonian saw the rise of forests, with the evolution of tree-sized lycophytes, ferns, and progymnosperms, as well as the appearance of the first tetrapods (four-limbed vertebrates)
• The Devonian ended with a series of extinction pulses, collectively known as the Late Devonian extinction, which severely affected marine communities, particularly reef-builders and shallow marine organisms

Carboniferous period

• The Carboniferous period (359 to 299 million years ago) is named for the extensive coal deposits formed during this time, reflecting the abundance of swamp forests
• The Carboniferous saw the diversification of tetrapods, including early amphibians and amniotes (reptiles and synapsids), as well as the evolution of insect groups like cockroaches and dragonflies
• The Carboniferous experienced a transition from a warm, humid climate to a cooler, drier climate, with the onset of the late Paleozoic ice age and the formation of the supercontinent Pangaea

Permian period

• The Permian period (299 to 252 million years ago) saw the continued diversification of reptiles and synapsids, the evolution of conifers and cycads, and the widespread formation of desert environments
• Permian faunas are characterized by the dominance of synapsids (mammal-like reptiles), the appearance of herbivorous reptiles, and the diversification of marine invertebrates like brachiopods and ammonoids
• The Permian ended with the Permian-Triassic extinction, the most severe known mass extinction, which eliminated over 90% of marine species and 70% of terrestrial vertebrate species, setting the stage for the Mesozoic era

Periods of the Mesozoic

• The Mesozoic era is divided into three periods: the Triassic, Jurassic, and Cretaceous
• Each period is characterized by the dominance of different reptilian groups, the evolution of new plant and animal lineages, and significant changes in Earth's climate and geography
• Understanding the key events and characteristics of each Mesozoic period is essential for paleontologists studying the "Age of Reptiles" and the rise of modern ecosystems

Triassic period

• The Triassic period (252 to 201 million years ago) saw the recovery of life after the Permian-Triassic extinction, with the diversification of reptiles, amphibians, and marine invertebrates
• Triassic faunas are dominated by archosaurs (ruling reptiles), including early dinosaurs, pterosaurs, and crocodylomorphs, as well as marine reptiles like ichthyosaurs and plesiosaurs
• The Triassic witnessed the breakup of Pangaea into Laurasia and Gondwana, the evolution of the first mammals, and the appearance of modern conifer groups

Jurassic period

• The Jurassic period (201 to 145 million years ago) is characterized by the dominance of dinosaurs, the evolution of birds from theropod dinosaurs, and the diversification of marine reptiles and cephalopods
• Jurassic faunas showcase a wide variety of sauropod, theropod, and ornithischian dinosaurs, as well as the first appearance of lizards, turtles, and crocodilians
• The Jurassic saw the continued breakup of Pangaea, the evolution of the first flowering plants (angiosperms), and a warm, humid climate with high sea levels

Cretaceous period

• The Cretaceous period (145 to 66 million years ago) witnessed the peak diversity of dinosaurs, the dominance of angiosperms, and the evolution of modern mammal, bird, and insect groups
• Cretaceous faunas are marked by the presence of iconic dinosaurs like Tyrannosaurus, Triceratops, and Velociraptor, as well as diverse marine reptiles, bony fishes, and sharks
• The Cretaceous saw the final breakup of Pangaea, the formation of the Atlantic Ocean, and a warm, greenhouse climate with high sea levels and reduced temperature gradients
• The Cretaceous ended with the Cretaceous-Paleogene extinction, which eliminated non-avian dinosaurs, pterosaurs, ammonites, and many other groups, paving the way for the age of mammals in the Cenozoic

Epochs of the Cenozoic

• The Cenozoic era is divided into three periods: the Paleogene, Neogene, and Quaternary, each further subdivided into epochs
• The Paleogene includes the Paleocene, Eocene, and Oligocene epochs; the Neogene comprises the Miocene and Pliocene epochs; and the Quaternary consists of the Pleistocene and Holocene epochs
• Each epoch is characterized by distinct mammalian faunas, climatic conditions, and evolutionary events, providing a detailed framework for understanding the development of modern ecosystems and the evolution of human ancestors

Paleogene period

• The Paleogene period (66 to 23 million years ago) saw the recovery and diversification of mammals, birds, and other groups after the Cretaceous-Paleogene extinction
• Paleogene faunas are characterized by archaic mammals (e.g., condylarths, creodonts), the appearance of modern mammal orders (e.g., primates, rodents, artiodactyls), and the evolution of early whales from land-dwelling ancestors
• The Paleogene witnessed a gradual cooling trend, the formation of the Himalayas, and the opening of the North Atlantic Ocean

Paleocene epoch

• The Paleocene epoch (66 to 56 million years ago) represents the initial recovery of life after the Cretaceous-Paleogene extinction, with the diversification of mammals, birds, and plants
• Paleocene faunas are dominated by archaic mammal groups, including condylarths (early ungulates), multituberculates (rodent-like mammals), and plesiadapiformes (early primates)
• The Paleocene saw the appearance of the first true primates (plesiadapids), the evolution of large flightless birds (gastornithids), and the diversification of flowering plants

Eocene epoch

• The Eocene epoch (56 to 33.9 million years ago) is characterized by a warm, greenhouse climate, the evolution of modern mammal orders, and the diversification of birds and insects
• Eocene faunas showcase the appearance of horses, rhinoceroses, camels, and whales, as well as the diversification of primates, rodents, and bats
• The Eocene witnessed the Paleocene-Eocene Thermal Maximum (PETM), a rapid global warming event, and the formation of vast inland seas in North America and Europe

Oligocene epoch

• The Oligocene epoch (33.9 to 23 million years ago) saw a global cooling trend, the expansion of grasslands, and the evolution of more modern mammal faunas
• Oligocene faunas are marked by the appearance of elephants, cats, and dogs, the diversification of horses and camels, and the evolution of early apes and monkeys
• The Oligocene experienced the Grande Coupure ("Great Break"), a faunal turnover event in Europe, and the formation of land bridges between continents

Neogene period

• The Neogene period (23 to 2.6 million years ago) witnessed the continued evolution of modern mammal, bird, and plant groups, as well as significant climatic and environmental changes
• Neogene faunas are characterized by the diversification of horses, elephants, and cats, the evolution of early hominins (human ancestors), and the spread of grassland ecosystems
• The Neogene saw the uplift of the Tibetan Plateau, the formation of the Mediterranean Sea, and the onset of Northern Hemisphere glaciation

Miocene epoch

• The Miocene epoch (23 to 5.3 million years ago) is marked by the evolution of grasses and the expansion of grasslands, the diversification of horses and antelope, and the appearance of early apes
• Miocene faunas showcase the evolution of larger mammals like mammoths, mastodons, and ground sloths, as well as the diversification of birds, including the first appearance of modern bird families
• The Miocene witnessed the Miocene Climatic Optimum,