Chapter 21 Section 1Chapter 21 Section 1

The Rock RecordThe Rock Record

Review Vocabulary

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

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

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

I. Geologic Time ScaleI. Geologic Time Scale

• Record of history of Record of history of EarthEarth

• Based largely on the Based largely on the fossils contained within fossils contained within rocksrocks

II. Visualizing the Geologic Time Scale

A.Begins with Earth’s formation 4.6 bya

B.Geologists organize Earth’s history according to groupings called

eons

1. Each eon contains eras

2. Eras contain periods

3. Each period contains epochs

Section 21.1

The Rock Record

Page 591

Earth’s History as a Clock• Fossils found in rock layers tell the history

of life on Earth. The history of life can be compared to 12 hours on a clock.

- The Fossil Record

Precambrian Time and the Paleozoic Era

Mesozoic and Cenozoic Eras

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.

1.1. Precambrian Time – 90% ofPrecambrian Time – 90% of geologic time geologic time

a. Hadean a. Hadean

b. Archean – life beganb. Archean – life began

c. Proterozoic - some organismsc. Proterozoic - some organismsmight have been able might have been able

toto move in complex move in complex waysways

2. Phanerozoic 2. Phanerozoic

A. Eons - largest time unitA. Eons - largest time unit

1.1. Tens to hundreds of millions ofTens to hundreds of millions of years in duration years in duration

2.2. Defined by life-formsDefined by life-forms

3.3. Named for relative agesNamed for relative ages

a. Paleozoic (greek for old)a. Paleozoic (greek for old)

b. Mesozoic (middle)b. Mesozoic (middle)

c. Cenozoic (recent)c. Cenozoic (recent)

B. ErasB. Eras

1. Some named for geographic regions

C. Periods

1. Most complete

2. Less time for weathering and erosion

3. Holocene began 11,000 years ago

D. Epochs

A. Phanerozoic Eon (greek for visible life)

1. multicellular life began to diversify

2. oceans became full of many different kinds of organisms

III. Succession of Life-Forms

1.1. TrilobitesTrilobites

a. Small, segmented animalsa. Small, segmented animals

b. first hard-shelled b. first hard-shelled

2. Land plants appeared later,2. Land plants appeared later, followed by land animals followed by land animals

B. Paleozoic Era

C. Carboniferous Period1. Provides plants for

today’s coal deposits

1. Largest

2. The end of the Paleozoic

3. 90% of all marine organisms became extinct

D. Mass extinction - many groups of organisms disappear from the rock record at about the same time

The era following the Paleozoic—the The era following the Paleozoic—the Mesozoic—is known for the Mesozoic—is known for the

emergence of dinosaurs, but many emergence of dinosaurs, but many other organisms also appeared other organisms also appeared

during the Mesozoic, including large during the Mesozoic, including large predatory reptiles and corals in the predatory reptiles and corals in the

oceans. oceans.

E. The age of dinosaursE. The age of dinosaurs

During the Mesozoic, water-dwelling During the Mesozoic, water-dwelling amphibians began adapting to amphibians began adapting to

terrestrial environments. Insects, terrestrial environments. Insects, some as large as birds, lived. some as large as birds, lived.

Mammals evolved and began to Mammals evolved and began to diversify. Flowering plants and trees diversify. Flowering plants and trees

emerged.emerged.

The 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.

During the era that followed the Mesozoic—the Cenozoic—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.

F. The rise of mammals

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

Scientists organize geologic time into eons, eras, periods, and epochs.

Scientists divide time into units based on fossils of plants and animals.

The Precambrian makes up nearly 90 percent of geologic time.

The geologic time scale changes as scientists learn more about Earth.