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The Structure of Geologic Time

The Geologic Periods of Time

Eons > Eras > Periods > Epochs

The Geologic Eras are large interval units of time that encompass smaller intervals of time called Periods. Each of the three Eras of the Phanerozoic Eon are comprised of several Periods that are each subject to the Law of Superposition and the Law of Faunal Succession.

The Paleozoic Era

The Paleozoic Era has seven Periods beginning with the Cambrian Period.

The Cambrian Period extended from 540 Million to 490 Million Years Ago. Life-forms in the Cambrian Period were sponges, segmented worms, trilobites, and brachiopods.

The Ordovician Period extended from 490 Million to 443 MIllion Years Ago. Living things in the Ordovician Period were early plants, trilobites, jawless fishes, bivalve mollusks, nautiliae, and gastropods.

The Silurian Period extended from 443 Million to 417 MIllion Years Ago. Living things in the Silurian Period were crinoid stems, trilobites, jawless fish, brachiopods, fish with laws, freshwater fish, nautiliae, gastropods, and coral reefs.

The Devonian Period extended from 417 Million to 354 MIllion Years Ago. Living things in the Devonian Period were ammonites, starfish, corals, crinoid stems, armored fish, sharks, early bony fish, and early amphibians.

The Mississippian Period extended from 354 Million to 323 MIllion Years Ago. Living things in the Mississippian Period were snails, amphibians, tree ferns, sharks, freshwater clams, trilobites are becoming rare. The armored fish became extinct in the Mississippian Period.

The Pennsylvanian Period extended from 323 Million to 290 MIllion Years Ago. Living things in the Pennsylvanian Period were spiders, cycads, amphibians, early reptiles, flying insects, and cockroaches.

The Permian Period extended from 290 Million to 248 MIllion Years Ago. Living things in the Permian Period were conifers, terrestrial reptiles, beetles, bugs, freshwater bony fishes, and ammonids. At the end of the Permian Period one of Earth's greatest extinctions occurred. The fossil record changed at the end of the Permian Period, and a new Era, the Mesozoic, began with the Triassic Period.

The Mesozoic Era

The Mesozoic Era was comprised of three Periods of Time.

The first Period was the Triassic Period that extended from 248 Million to 206 Million Years Ago. Life-forms in the Triassic Period were arthropods, turtles, crocodilians, true lizard, bony fishes, sea urchins, early dinosaurs, marine reptiles, and the therapsids, the first mammal-like reptiles appear.

The Jurassic Period came next and extended from 206 Million to 144 Million Years Ago. Life-forms in the Jurassic Period were flying reptiles, dinosaurs, earwigs, conifers, redwood, cypress, yew, cycads, ginkgoes, and the early mammalian dicynodonts.

The Cretaceous Period was the final Period of the Mesozoic Era and extended from 144 Million to 65 Million Years Ago. Life-forms in the Cretaceous Period were dinosaurs, flying reptiles, marine reptiles, amphibians, lizards, crocodilians, snakes, early birds, and early mammals.

The Cenozoic Era

The Cenozoic Era is comprised of two Periods derived from an earlier 1900's Geologic Time Chart that separated the rock layers into a fourfold division named Primary, Secondary, Tertiary, and Quaternary. Only the Tertiary and Quaternary intervals of Time are utilized in the modern Geologic Time Chart.

The Tertiary Period extended from 65 Million to 2 Million Years Ago. Modern life-forms continued to diversify and inhabit the land, the sky, and the oceans. Whales, dolphins, and manatees evolved in marine habitats. Amphibians, lizards, crocodilians, snakes, early deer, antelope, pronghorns, and horses also evolved in the Tertiary Period. The first elephant-like mammals appeared, as did dogs cats, pigs, and weasels.

The Quaternary Period began approximately 2 Million Years Ago and continues into the Present. The notable event in the Quaternary Period were the ice ages that began 2 Million Years ago and remained until 10,000 Years Ago. Life-forms during the Quaternary Period included the giant ground sloths, giant beavers, bison, mammoths, mastodons, sabre-tooth cats, and human beings.