Scientists have devised the Geologic Time Chart so that Time can be understood from the largest and most general intervals, or units, to the smallest, most specific intervals.
The Eons are the largest interval units into which Geologic Time is divided and represented on the chart. There are four Geologic Eons. The first three, the Hadean, Archean, and Proterozoic Eons are frequently lumped together and referred to as the Precambrian.
See illustration at right. The first Eon of Time is the Hadean Eon. Most of them were aquatic; the most famous examples are trilobites, small arthropods creatures with exoskeletons whose distinct fossils are still being discovered today.
During the Ordovician period, fish, cephalopods, and corals first appeared; over time, these creatures eventually evolved into amphibians and dinosaurs. During the Mesozoic era, which began about million years ago, dinosaurs ruled the planet. These creatures were the largest to ever walk the Earth.
Titanosaur, for example, grew up to feet long, five times as long as an African elephant. The dinosaurs were eventually wiped out during the K-2 Extinction, an event that killed about 75 percent of the life on Earth.
Following the Mesozoic era was the Cenozoic, which began about 66 million years ago. This period is also known as the "Age of Mammals," as large mammals, following the extinction of the dinosaurs, became the dominant creatures on the planet.
In the process, mammals diversified into the many species still present on the Earth today. Early humans, including Homo habilis , first appeared about 2. These enormous changes to life on Earth have taken place over a period of time that, compared to geologic history, is relatively small.
Human activity has transformed the planet; some scientists have proposed a new epoch, the "anthropocene," to describe this new period of life on Earth. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile.
Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. The oldest fossilized layers of these cyanobacteria—called stromatolites fossilized microbial mats, Fig. Prokaryotic bacteria were the only life on Earth for at least 2 billion years—almost half of the age of the planet Fig.
More complex organisms called eukaryotes first appeared in the fossil record around 2. Eukaryotes are organisms that have cells containing a nucleus and other structures enclosed within membranes Fig. All living organisms other than bacteria are eukaryotes. Some eukaryotic species, such as protists, live as single-celled organisms. All true multicellular life—organisms comprised of many different living cells—are eukaryotes. These include algae, plants, fungi, and animals.
The first multicellular eukaryotic life appeared in the fossil record between 2 and 0. Single-celled organisms can live in close colonies or even form filaments or microbial mats like cyanobacteria Fig. However, the first multicellular organisms could develop specialized tissues or groups of cells with specific properties and tasks. For example, a plant might have root tissue with cells very different from the cells in its leaf tissue.
The arrival of multicellularity led to the relatively rapid evolution of new forms of life. The Cambrian explosion is the name of the event marking the rapid diversification of many new groups of multicellular organisms from a single common ancestor. The Cambrian explosion began around million years ago and continued for another 20 million years. This event marked a major turning point in the evolution of life on Earth, particularly for animals.
After the Cambrian explosion, many new types of complex multicellular organisms and most major groups of animals began to appear in the fossil record Fig. The time that followed the Cambrian explosion is divided into three geological eras: Paleozoic, Mesozoic, and Cenozoic.
The Paleozoic era spanned from to million years ago mya and represents the first era of diverse, complex life on Earth. As the Cambrian explosion continued, almost all of the major groups of modern animals evolved from shared common ancestors.
The ancient world ocean brimmed with invertebrate animals such as sponges, corals, and crabs Fig. The first animals to begin to colonize dry land were likely arthropods Fig. Arthropods are a group of invertebrate animals with jointed arms and hard shells including crabs, shrimp, spiders, and insects. By the end of the Paleozoic era—almost mya after the Cambrian explosion—arthropods, amphibians, and some primitive reptiles dominated the terrestrial environment Fig.
The cause of this extinction event remains unresolved, however scientists have proposed several possible scenarios including asteroid impacts and catastrophic volcanic events. This extinction event is one of many throughout the history of life on Earth. Some names vary from state to state and country to country, especially for more localized rock layers.
Rock units are often named for a town or other feature related to where they were first discovered. Geologic Time Scale for Kansas pdf. International Commission on Stratigraphy. Murphy and Amos Salvador.
Geologic time: A metaphor Geologic periods in Kansas Eons and eonothems?
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