Geologic Timescale/History of Life

Eons, Eras, Periods, and Epochs

Hadeon

EON: 4.6 – 4 BYA

Begins with the formation of the Earth and ends with cooling of the crust of the Earth.

Hell-like climate, high volcanic activity, cosmic bombardments, raging storms and high, sometimes rock-melting temperatures.

No fossils, but some organic carbon––evidence of earliest life

Archean

EON: 4 – 2.5 BYA

Begins with the cooling of the Earth’s crust

Atmosphere is mostly carbon dioxide.

First time life flourished, mats of microbes in the sea.

Stromatolites/Stromatoliths = fossilized microbial mats.

Proterozoic

EON: 2.5 BYA – 541 MYA

Atmosphere oxygenates.

Cyanobacteria rise and anaerobic life declines. First eukaryotes appear and multicellular organisms like Charnia and Dickinsonia.

Phanerozoic

EON: 541 MYA – present

When life became more obvious.

Paleozoic

ERA: 541 – 252 MYA, Phanerozoic

Begins with Cambrian explosion, diversification of visible life, and ends with The Great Dying, most severe extinction in Earth’s history.

Index fossils: Trilobites

Pangaea formed

Land populated by plants then arthropods, then amphibians, then ancestors of reptiles in desert of Pangaea.

Mesozoic

ERA: 252 – 66 MYA, Phanerozoic

Starts after The Great Dying and ends with the Cretaceous-Paleogene (K-Pg) Extinction Event.

Age of reptiles = dinosaurs, pterosaurs, and marine species. Small mammals, birds, and other animals lived in shadows of big reptiles.

Index fossils: Non-avian dinos, only in Mesozoic

Cenozoic

ERA: 66 MYA - present, Phanerozoic

Starts after the K-Pg Extinction

Climate warms and world filled with jungles, then Earth cooled creating prairies, then the last Ice Age, and then modern day.

Cambrian

PERIOD: 541 – 485.4 MYA, Paleozoic, Phanerozoic

Begins with the Cambrian Explosion and ends with the Cambrian-Ordovician Extinction––sudden crash in O2?

First of many kinds of animals seen today

Ordovician

PERIOD: 485.4 – 443.8 MYA, Paleozoic, Phanerozoic

Begins with the Great Ordovician Biodiversification Event (GOBE) and ends with the Ordovician-Silurian Extinction Event (OSEE)

Continents were on the move, boost of oxygen and changes in sea level lead to many new species

Very first land plants, like modern mosses, took in so much CO2 that Earth’s temperature plummeted –> ice age, causing the OSEE.

Silurian

PERIOD: 443.8 – 419.2 MYA, Paleozoic, Phanerozoic

Starts after the Ordovician-Silurian Extinction Event (OSEE) and ends with a series of minor extinction events.

Climate gradually warms

Plants colonize the land and first fossils of vascular plants and fungi

Dropping sea levels cause many bottom-dwellers, like cephalopods, to decline.

Devonian

PERIOD: 419.2 – 358.9 MYA, Paleozoic, Phanerozoic

Earliest sharks, placoderms appear

Arthropods diversify on land, and trees appear, amphibians appear

Ends with a mass extinction, Late Devonian Extinctions, caused by drops of O2 levels in the sea

Carboniferous

PERIOD: 358.9 – 298.9 MYA, Paleozoic, Phanerozoic

Rise in oxygen levels and humid warm climate leading to dense forests and swamps.

HUGE arthropods

Amniotes lay eggs with shells on land

Pangaea formed, wiping out most of the forests

Amniotes spread and split into the reptiles and synapsids

Permian

PERIOD: 298.9 – 251.902 MYA, Paleozoic, Phanerozoic

Synapsids and reptiles diversify

Climate got hotter and drier, amniotes forced to adapt

Ends with Permian-Triassic Extinction Event, caused by a combination of factors AKA The Great Dying

Triassic

PERIOD: 251.902 – 201.3 MYA, Mesozoic, Phanerozoic

Pangaea almost fully come together

At first, only the survivors of the Great Dying are present

Ichthyosaurs, therapsids (mammal ancestors) appear, as well as the Archosaurs = clade of reptiles originating in Paleozoic that includes dinosaurs, pterosaurs, and croc-like phytosaurs.

Archosaurs had lighter skulls and deep-set in teeth. Unique respiratory system that was suited to the low-oxygen atmosphere at the time. Spread and diversified. First dinosaurs appear, Saurischians––pubis down and forward.

Dinos diverged into sauropods and therapods. Pterosaurs take to the sky.

Ichthyosaurs and Pleisiosaurs rule the sea.

Pangaea breaks apart causing volcanic activity that led to the End-Triassic Mass Extinction, wiping out many of the Paleozoic-era animals.

Jurassic

PERIOD: 201.3 – 145 MYA, Mesozoic, Phanerozoic

Dinosaurs develop and acquire different body plans. Ornithischians’ pubis is reversed, pointing down and backwards allowing for an expanded gut cavity for eating tough plants.

Mammals may have appeared in the Triassic but diversified in the Jurassic. Swimming and gliding mammals.

Paravian Dinosaurs, feathered dinos

Pangaea continues to break-up and sea levels rise leading to a complex series of extinction events

Cretaceous

PERIOD: 145 – 65 MYA, Mesozoic, Phanerozoic

Most extreme changes in flora and fauna: appearance of flowers, feathered Coelurosaurs grew even larger, titanosaurs becoming HUGE, pterosaurs got HUGE––largest animals ever to fly

Continents continued to drift apart, dinos grew more distinct across different regions

K-Pg Extinction, volcanic eruptions and asteroid hit, but mammals and flowers survived

Paleogene*

PERIOD: 66 – 24 MYA, Cenozoic, Phanerozoic

Still recovering from the K-Pg Extinction. Almost all large land vertebrates vanished, as well as most terrestrial plants and giant marine reptiles.

Paleocene

EPOCH: 66 – 55 MYA, Paleogene, Cenozoic, Phanerozoic

Surviving dinosaurs, birds, diversify.

Ungulate-like mammals take over, without predators, but Creodonts soon arrive. Primate-like species arrive.

Earth warms by 5 - 8 ºC, known as the Paleocene-Eocene Thermal Maximum (PETM)

Eocene

EPOCH: 55 – 33.9 MYA, Paleogene, Cenozoic, Phanerozoic

Earth warmed even more, great for reptiles (Titanoboa, Carbonemys=giant carnivorous turtle).

First true primates appear.

Warming trend shifted, caused at least in part by the Azolla Event, moss-like plant grew in arctic and took up lots of CO2 causing cooling. Causing many groups to vanish.

Oligocene

EPOCH: 33.9 – 23 MYA, Paleogene, Cenozoic, Phanerozoic

Ancestor of old world monkeys, Aegyptopithecus, appears.

Ice sheets form in Antarctica and lead to new extinction event.

Grande Coupure, mostly extinction in Europe due to drop in temperatures

Carnivores and artiodactyls

Grassland habitat appeared, fibrous grass necessitated adaption from herbivores giving way to the ruminants

First new world monkeys appeared. Old world monkeys start to have broader diets and spend more time on the ground.

Neogene

PERIOD: 23 – 2.5 MYA, Cenozoic, Phanerozoic

Changes in microscopic fossils of algae and foraminifera.

Miocene

EPOCH: 23 – 5.333 MYA, Neogene, Cenozoic, Phanerozoic

Continental plates shifting and building mountains

First apes evolved from old world monkeys with notable traits: lack of tail, more flexible shoulders, flat rib cage, and shorter spine. Easier to swing through trees.

World continues to cool and grasslands spread. Grazing animals rise.

Animals closer to what exists today.

Pliocene

EPOCH: 5.333 – 2.58 MYA, Neogene, Cenozoic, Phanerozoic

Brief period of warming before even faster cooling, caused by South America bumping into North America cutting the Atlantic from the warm currents circling the equator.

Hominids take over grassland, Australopithecus. Homo genus appeared later.

Quaternary

PERIOD: 2.58 MYA – present, Cenozoic, Phanerozoic

Last ice age to the rise and spread of hominids