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Understanding geology to
understand fossils
The Hadean, Archean and Proterozoic Eons are all in the Precambrian. Fossils from this time are primarily unicellular with the beginning of multicellular organisms developing, The Phanerozoic Eon is comprised of the Cenozoic, Mesozoic and Paleozoic Eras, with 11 periods, with fossils of hard bodied organisms including exoskeletons (external protective coverings) and eventually endoskeletons (internal skeletons) with bony structures.
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For an interactive experience of what the Earth looked like over the eons, check out the Dinosaur Database.
The true rock(s) of ages in the geologic timescale
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In geology, the ages of rocks are based on a stratigraphic concept of "superposition" which states that deeper rock layers are older than those closer to the surface. This and age-dating techniques have allowed geologists to assign the ages of rocks.
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The geologic timescale is broken into subdivisions called eons, periods, epochs and ages.
Phanerozoic Eon
541 million years ago to Present
QUATERNARY PERIOD
Neanderthal and modern man appear
Modern man evolves during this time. Present day landforms and topography develop. Mammals and marine species continue to evolve. The Quaternary Ice Age begins.
Holocene Epoch:
1.8 million years ago to present day
Pleistocene Epoch:
2 to 1.8 million years ago
CENOZOIC ERA
Pliocene Epoch:
5.3 to 2.6 million years ago
Miocene Epoch:
23 to 5.3 million years ago
NEOGENE PERIOD
Grasses replaced ancient forests. Horses antelope, and bison arose
Due to a global cooling, forests become grasslands. Ruminants like camels, bison, sheep and giraffe developed to digest the grass. Cats and dogs with powerful teeth evolved to take down prey. New shark species appeared, including the Charcharodon Megalodon. At the end of the Pilocene, the earth enters an ice age.
Oligocene Epoch:
33 to 23 million years ago
Eocene Epoch:
56 to 33 million years ago
Paleocene Epoch:
66 to 56 million years ago
PALEOGENE PERIOD
Wide diversification of modern birds and small mammals like dogs and cats and the first primates
With the extinction event at the close of the Cretaceous which killed dinosaurs and large reptiles, many small, mammals that are familiar to us evolved, including dogs, cats, whales, rodents and the first primates. The evolution of birds took flight, including flightless birds. Open woodlands and grasslands abounded, which supported grazing animals.
After the Cretaceous Period, about 66 million years ago, there is strong scientific evidence of a large meteorite impact to the Earth, which deposited a layer of Iridium. No dinosaur fossils have ever been found above this Iridium layer which exists at the K-Pg (Cretaceous-Paleogene) Boundary (previously known as the K-T Boundary.) This has led scientists to theorize that this meteorite impact caused a mass extinction event which killed all non-avian dinosaurs and up to 75% of all species on Earth.
Late Cretaceous Epoch:
100 to 66 million years ago
Early Cretaceous Epoch:
145 to 100 million years ago
CRETACEOUS PERIOD
Hadrosuarus, ankylosaurus, T. rex, sharks; mammals and
birds; leafy trees
Spanning 79 million years, and including the early and late Cretaceous, at this time, deciduous trees proliferate and flowering plants evolve. Dinosaurs hold center stage as principal land animals, with Tyrannosaurs, triceraptops and edmontosaurus roaming the earth. Marine reptiles such as mosasaurs swim the seas and pterosaurs fill the sky. Modern orders of mammals appear. The period ended with one of the greatest mass extinctions on the planet, exterminating dinosaurs, marine and flying reptiles and many marine invertebrates.
201 to 145 million years ago
JURASSIC PERIOD
Brachiosaurus, stegosaurus, pterosaurs, plesiosaurs, modern squids, ammonites
Pine and other conifer forests cover the land. The sauropod dinosaurs like Diplodocus, Stegosaurus and Allosaurs were abundant. Icthyosaurs and Plesiosaurs swim the seas. Crocodiles and turtles exist along the shoreline and mammals became lastingly established on land. Insects diversified.
252 to 201 million years ago
TRIASSIC PERIOD
Ichthyosauria, pterosaurs and very small, early mammals
Land animals grew, seeing the first bi-pedal form of dinosaurs and the first small mammals. Icthyosaurs swam the oceans and early forms of flowering plants diversified. At the end of the Triassic, another mass extinction occurred, particularly among
primitive mollusks.
MESOZOIC ERA
PALEOZOIC ERA
PALEOZOIC ERA
PALEOZOIC ERA
298 to 252 million years ago
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PERMIAN PERIOD
Sponge, coral, sharks; seed-bearing plants; sauropsids
Palm trees and other conifers populate the woods. Insects undergo rapid evolution. Reptiles arise and become carniverous. Mollusks greatly diversify and ammonites dominate the seas. At the end of the Permian, the largest mass extinction on the planet occurred when an estimated 90 percent of marine invertebrate species and 70 percent of land animals died out.
PALEOZOIC ERA
PALEOZOIC ERA
Pennsylvanian Epoch:
323 to 298 million years ago
Mississippian Epoch:
358 to 323 million years ago
CARBONIFEROUS PERIOD
The age of amphibians, terrestrial invertebrates
such as ants
Large spore-bearing trees and ferns covered the landscape, and their decomposition formed the carbon in carboniferous. Four-legged vertebrates, including early reptiles evolve. Crinoids, bryozoans and brachiopods dominate the seas. New fish and shark species develop. Pangea is formed.
PALEOZOIC ERA
PALEOZOIC ERA
419 to 358 million years ago
DEVONIAN PERIOD
The "Age of Fishes" First amphibians and vertebrates proliferate; ferns and trees appear
Thousands of fish species develop. Bony fish called lobe-fins develop which heralded the evolution of amphibians with legs. The first ammonites emerge. Ancient sharks evolve and flourish. Vertebrates first appear on land. Land plants, including ferns and the first trees cover the earth in dense forests. The Devonian ended with a mass extinction of marine animals.
PALEOZOIC ERA
PALEOZOIC ERA
443 to 419 million years ago
SILURIAN PERIOD
Coral, fish and mollusks in the seas and the first terrestrial life on land
Corals proliferate and become existing reefs. Early fish with jaws and bones swam the seas. ​Arthropods (such as ancestors of scorpions and millipedes are the first terrestrial animals and become the first to colonize land.. Land plants form.
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PALEOZOIC ERA
PALEOZOIC ERA
485 to 443 million years ago
ORDOVICIAN PERIOD
Trilobites dominate the seafloor and the first vertebrate organisms evolve, such as fish.
The sea covers most of North America. Marine invertebrates become abundant. Trilobites develop new species and cephalopods become dominant predators. Brachiopods develop. Algae, sponges and corals appear.
PALEOZOIC ERA
PALEOZOIC ERA
541 to 485 million years ago
CAMBRIAN PERIOD
The Cambrian explosion created a diversity of life forms, some of which evolved hard shells that more easily mineralized into fossils than their soft-bodied ancestors.
The "Cambrian Explosion" occurs when the Earth saw a huge diversification in invertebrates in the seas including mollusks, worms and sponges. 100 types of trilobites are one of the first animals to have eyesight. One-celled algae proliferate.
Hadean Eon
4.6 billion to 4 billion years ago
Archean Eon
4 to 2.5 billion years ago
Proterozoic Eon
2.5 to 541 million years ago
PALEOZOIC ERA
PRECAMBRIAN ERA
Stromatolites, algae,
multicellular organisms
The Precambrian represents more than 80 percent of the total geologic record. Primitive life-forms existed approximately 3.5 billion years ago, These include algae, cysts of flagellates and stromatolites. In the late Precambrian multicellular organisms evolved and sexual division developed.
4.6 billion years ago to
541 million years ago
Dating as per The Geologic Society of America's Geologic Time Scale
