Geologic Time

Geologic Time Scale The geologic time scale is a record of the life forms and geologic events in Earth’s history. First developed by studying rock layers and index fossils worldwide. Different divisions of the geologic time scale are separated by events in the history of life on Earth. (aka – major life changes)

Divisions of Geologic Time Geologic time is broken down into four main divisions of time – Eons, Eras, Periods, Epochs. Eon – the longest division of time. 4 eons to date Last eon (Phanerozoic) started 544 million years ago (mya) Era – A subdivision of an eon. 3 eras to date Last era (Cenozoic) started 66.4 mya

Divisions of Geologic Time (cont.) Period – Subdivision of an era. 11 periods to date Last period (Quaternary) started 1.8 mya. Epoch – subdivision of a period. Last epoch (Holocene) started about 10,000 years ago. Are sometimes divided into Ages (smallest time periods)

Precambrian Precambrian time starts when Earth was created 4.6 billion years ago. Scientists think Earth began as a ball of dust, rock, and ice in space that was pulled together by gravity. During Precambrian Time the atmosphere, oceans, and continents all began to form. Oceans form and cover Earth about 4 billion years ago.

Precambrian (cont.) Earth’s second atmosphere formed, making ozone layer. Life is thought to have developed during Precambrian time, about 3.5 billion years ago. Fossils similar to modern day bacteria 2.5 Billion Years ago first Photosynthesis by organisms First rocks form about 4 billion years ago Continental drift begins First multi-cellular organisms develop in late Precambrian. Jellyfish-like animals

Paleozoic Era (Cambrian Period) 544 – 505 mya “Cambrian Explosion” LOTS of different organisms evolved. All animals lived in the sea Most were invertebrates (no backbone) Some had shells (trilobites) Most of land covered by shallow seas

Paleozoic Era (Ordovician Period) 505 – 438 mya Invertebrates dominate the oceans Jawless fish evolves First vertebrates (animal with a backbone) to appear. Ancestors of modern octopus and squid appear

Paleozoic Era (Silurian Period) 438 – 408 mya Coral Reefs start to develop Plants become abundant Ancestors of ferns Fish with jaws develop Insects first appear

Paleozoic Era (Devonian period) 408 – 360 mya Known as “Age of Fishes” Sharks first appear, fish develop scales, and fish develop bony skeletons Fish developed Jaws Animals begin to invade land First vertebrates to crawl on land were lungfish Amphibians evolved from lungfish

Paleozoic Era (Carboniferous Period) 360 – 286 mya Divided into two segments (Mississippian 360 – 320 mya and Pennsylvanian 320 – 286 mya) Appalachian Mountains begin to form. Winged insects evolved Cockroaches and dragonflies Small reptiles developed Giant ferns and cone bearing plants form swamps, making “Coal forests” Where we get our coal from now

Paleozoic Era (Permian Period) 286 – 245 mya Pangaea forms (~ 260 mya) All continents together as one land mass Deserts expand and ice covers poles Reptiles rule the land First warm-blooded reptiles appear First mass extinction occurs. Trilobites, most marine organisms

Mesozoic Era (Triassic Period) 245 – 208 mya Beginning of Mesozoic Era (sometimes called Age of the Reptiles) Some fish, reptiles, insects and conifers (cone bearing plants) survived first mass extinction First dinosaurs appear First turtles and crocodiles appear First mammals appear Very small (mouse)

Mesozoic Era (Jurassic Period) 208 – 144 mya Dinosaurs dominant land Mostly large dinosaurs Pangaea begins to break apart First flying reptiles appear First birds appear Archaeopteryx: means “ancient wing thing” Thought to be link between birds and dinosaurs

Mesozoic Era (Cretaceous Period) 144 – 66 mya First flowering plants appear Like modern day plants – have a fruit to protect seeds Flying reptiles go extinct after competition with “new birds” Second mass extinction eliminates over half of all plants and animals on Earth at end of Cretaceous (~ 65 mya) including all dinosaurs. Most think it was due to an asteroid hitting Earth

Cenozoic Era (Tertiary Period) 66 – 1.8 mya Often called Age of Mammals Continents move into modern day positions First grasses appear Let grazing animals thrive (ancestors to cattle, sheep, deer etc.) Modern day like groups appear Horses, elephants, bears, primates, whales, dolphins

Cenozoic Era (Quaternary Period) 1.8 mya – present Earth’s climate cooled during a series of ice ages 20,000 years ago climate began to warm Coral, fish, algae, and mammals thrive in ocean Smaller land mammals become common Cats, dogs, cattle, humans Giant mammals go extinct when ice age ends about 10,000 years ago. Modern humans may have evolved as early as 100,000 years ago.