Chapter 21 – Earth’s History 21.1 – The Rock Record

Why study Earth’s history? Scientists organize geologic time to help them communicate about Earth’s history.

Organizing Time Fossils = the remains, traces, or imprints of ancient organisms. By studying rock layers and the fossils within them, geologists can reconstruct aspects of Earth’s history and interpret ancient environments

Geologic Time Scale A record of Earth’s history from its origin 4.6 billion years ago (bya) to the present. The geologic time scale enables scientists to find relationships among the geological events, environmental conditions, and fossilized life-forms that are preserved in the rock record

http://geology.com/time/geologic-time-scale-550.gif

Visualizing Geologic Time Eons = is the largest of these time units and encompasses the others Divided into 4 different eons = Hadean, Archean, Proterozoic, and Phanerozoic The three earliest eons make up 90 percent of geologic time, known together as the Precambrian. During the Precambrian, Earth was formed and became hospitable to modern life. Eons are further divided into eras

Eras Usually tens to hundreds of millions of years in duration. Divided into 4 different eras = Precambrian, Paleozoic, Mesozoic, Cenozoic Eras are divided into periods

Periods Tens of millions of years in duration Each period in the geologic time scale contains epochs. Epochs = hundreds of thousands to millions of years in duration

http://www. habitat. adfg. state. ak http://www.habitat.adfg.state.ak.us/geninfo/kbrr/coolkbayinfo/kbec_cd/html/image/figures/geotime.gif

In-Class Assignment/Homework Your Geologic Time Scale

21.1 – Notes Continued

Succession of Life-Forms Phanerozoic Eon - multicellular life began to diversify Paleozoic Era - the oceans became full of many different kinds of organisms http://www.palaeos.org/Paleozoic

Succession of Life-Forms Trilobites = Small, segmented animals among the first hard-shelled life-forms. They dominated the oceans in the early part of the Paleozoic Era. Land plants appeared later, followed by land animals.

Succession of Life-Forms End of the Paleozoic is marked by the largest mass extinction event in Earth’s history In a mass extinction, many groups of organisms disappear from the rock record at about the same time At the end of the Paleozoic, 90 percent of all marine organisms became extinct

Succession of Life-Forms Mesozoic Era is known for the emergence of dinosaurs Many other organisms appeared, including large predatory reptiles and corals in the oceans http://media-2.web.britannica.com/eb-media/50/8050-004-55077BBF.jpg

Succession of Life-Forms During the Mesozoic, water-dwelling amphibians began adapting to terrestrial environments Insects, some as large as birds, lived Mammals evolved and began to diversify Flowering plants and trees emerged

Succession of Life-Forms End of the Mesozoic is marked by a large extinction event Many groups of organisms became extinct, including the non-avian dinosaurs and large marine reptiles

Succession of Life-Forms Cenozoic Era - mammals increased both in number and diversity Human ancestors, the first primates, emerged in the epoch called the Paleocene, and modern humans appeared in the Pleistocene Epoch

In-Class Assignment/Homework 21.1 WKT (Transparency & Study Guide)