Chapter 21

Fossils & the Rock Record

Major milestones in life• Crawling, walking, potty training (still in progress?), first day of school,

first kiss, first gf/bf, first love/break up, starting high school, driving a car, graduating, full-time job, parenting, retiring

Are there any categories into which these events can be placed?- Could include infancy, toddlerhood, childhood, adolescence,

adulthood

How do these categories help people communicate and analyze their life histories?- Not everyone reaches the same milestone at the same time. It is using to have references for communicating events

21.1 The Rock Record

Objectives Explain why scientists need a geologic time scale Distinguish among eons, eras, periods, and epochs Characterize the groups of plants and animals that dominated

eras in Earth’s history

Main Idea: Scientists organize geologic time to help them communicate about Earth’s history

Unknown vocab: fossil—the remains, trace, or imprint of a once-living plant or animal

Organizing TimeThese multicolored layers of rock are called strata• Some of these layers contain

fossils

By studying such rock layers and the fossils within them, geologists can reconstruct aspects of Earth’s history and interpret ancient environments

To help in the analysis of Earth’s rocks, geologists have divided history of Earth into time units• Based largely on the fossils contained within the rocks• Time units are part of the geologic time scale—a record of Earth’s history from its origin

(4.6 bya) to the present)

Some units have remain unchanged for centuries, while others have been reorganized based on new evidence

Geologists organize Earth’s history according to grouping called eons• Each eon contains era, which contain periods, which contain periods, which contain

epochs E + P = EP

Geologic Time Scale

Enables scientists to find relationships among geological events, env. conditions, and fossilized life-forms preserved in rock recordEons:• Largest of time units and contains all other units of time• Consist of Hadean, Archean, Proterozoic, and Phanerozoic

Eons• 3 earliest eons make up 90% of geologic time, known as

Precambrian During Precambrian, Earth was formed and became hospitable to modern life

Simple life-forms began in Archean Eon and that by end of Proterozoic Eon, life had evolved to point that some organisms might have been able to move in complex ways

Precambrian

Most fossils from this eon were soft-bodied organismsOthers had bodies with rigid partsAll life-forms had soft bodies without shells or skeletons

Era time units are defined by the different life forms found in the rocksSome periods are named for the geographic region in which the rocks or fossils characterizing the age were first observed and described

Eras, Periods, & Epochs

Eras All eons made up of eras—next largest unit of time• Usually tens to hundreds of millions of years

Periods All eras divided into periods• Generally tens of millions of years in duration

Epochs All periods are divided into epochs• Generally hundreds of thousands to millions of years in duration• Rocks & sediments from epochs of Cenozoic Era are most complete due to less time

for weathering & erosion to remove evidence

Succession of life-forms

During Phanerozoic Eon, multicellular life began to diversify• First era of this eon—Paleozoic—oceans became full of many different kinds of

organisms• Small, segmented animals called trilobites were among first hard-shelled life-forms• Dominated oceans in early part of Paleozoic Era• Land plants appeared later, followed by land animals

• End of Paleozoic is marked by largest mass extinction event in Earth’s history

• During a mass extinction, many groups of organism disappear from rock record at about same time

• At the end of the Paleozoic, 90% of all marine organisms became extinct

Conditions toward end of Paleozoic eraPangea had been in its final stages of formation• Conditions became dry on Pangea• Even though ocean covered most of Earth, Pangea was so big that the

interior did not benefit from the ocean waters • Swamp lands dried up—so many plants died out as well as amphibians

that depended on the swamps

other theories include • Increased volcanic activity• Comets/meteors

Whatever the cause, life on Earth would never again look as it had during the Paleozoic Era

Age of dinosaurs

Followed the Permian extinction Era known as the Mesozoic Large predatory reptiles and corals in the oceans also

appeared during this era Mammals continued to evolve and diversify Flowering plants & trees emerged as well End of the Mesozoic is marked by a large extinction that

eliminated many groups of organisms—including the non-avian dinosaurs and large marine reptiles

Rise of mammals

• Era the followed the Mesozoic—known as the Cenozoic

• Mammals increased in both number & diversity

• 1st primates, emerged in epoch called the Paleocene

• Modern humans appeared in the Pleistocene Epoch