CH. 9.2 Life and Geologic Time

Mr. Perez

Important VocabularyAdaptationGeologic time scaleEpochEraPeriodEonTrilobite

Geologic Time ScaleThe appearance or disappearance of types of

organisms throughout Earth’s history marks IMPORTANT occurrences in geologic time.

The geologic time scale was made by paleontologists.This time scale divides Earth’s history into time units

based on the life-forms that lived only during certain periods.

The time scale is not only based on life-forms living in certain periods– sometimes, life-forms are not present, so certain time periods of the geologic time scale are based on other information/criteria

Geologic Time Scale If we were to create a geologic time scale based on the

life time of a human, it might look something like this…

Life Eon

Non-Talking Era Talking Era

Crawling Period

Teething Period

School Period Work Period Retirement Period

First tooth Epoch

First molar Epoch

Elementary Epoch

Secondary Epoch

College Epoch

Bus Boy Epoch

Manager Epoch

CEO Epoch

Subdivisions of Geologic TimeThe oldest rock on Earth contain NO fossils. Then, for millions of years after the first appearance

of fossils, the fossil record remained sparse.Later, in Earth’s history came an explosion in the

abundance and diversity of organisms.The FOUR subdivisions of geologic time are:

1. Eons2. Eras3. Period4. Epochs

Subdivisions of geologic timeEons- longest subdivision based on the abundance

of certain fossilsExamples- Hadean eon, Archean eon, Proterozoic

eon, Phanerozoic eon,

Era- second longest subdivision based on the major, striking, and worldwide changes in the types of fossils presentExamples- Paleozoic era, Mesozoic era, Cenozoic eraEras are divided into periods

Refer to p. 259 Figure 12

Subdivisions of Geologic TimePeriods- units of geologic time based on the types

of life existing worldwide at the time.Examples- Cambrian, Ordovician, Jurassic,

Cretaceous, Triassic, DevonianPeriods are divided into smaller groups called epochs

Epochs- units of geologic time based on differences in life-forms, but some of these differences can vary from continent to continentExamples- Paleocene, Eocene, Oligocene, Miocene,

HoloceneRefer to p. 259 Figure 12

Identify when each development in Earth’s history occurred

Event Eon Era (if identified)

Period (if identified)

First life Archean n/a n/a

First trilobites

Phanerozoic Paleozoic Cambrian

First flowering plants

Phanerozoic Mesozoic Cretaceous

Refer to p. 259 Figure 12

TrilobitesSmall, hard shelled organismsCrawl on the seafloor or swam

through water2cm – 7cm in length, 1cm – 3cm in

widthConsidered to be index fossils

because they lived over vast regions of the world during specific periods of geologic time.

Trilobites Body has three lobes along the length

Head= cephalon

Segmented middle section = thorax

Tail = pygidium

Changing Characteristics of TrilobitesTrilobites lived on Earth for over 200 millions

years. Throughout the Paleozoic Era, some trilobite species became extinct and other new species evolved, showing different characteristics.

Trilobite EyesTrilobites might have been the first organisms that

could view the world with complex eyes…The position of eyes on an organism gives clues as

to where it must have livedEyes on the back of the head indicate a bottom

dwellerEyes on the front of the head indicate an active

swimmer

In most species, trilobite eyes were located midway on the headThis indicates that trilobites could both crawl on the

ocean floor and swim through the water

Trilobite EyesAs time passed, the eyes in trilobites

changed. In many species, the eyes became

smaller Some even lost their eyes completely—

these species probably lived underground and did not need eyes

In other species, the eyes became more complex Aeglina, a species of trilobite, developed

large compound eyes (like insects) Other trilobites developed stalks that held

the eyes upward

Trilobite BodiesTrilobite bodies and

tails also underwent changes in form through time

Olenellus: Lived during the

Early Cambrian Period

Had an extremely segmented body

It is thought to have been a primitive trilobite

AdaptationsAdaptations are characteristics of an organism

that increase its chances of survival in its environment

According to Charles Darwin’s natural selection, a species of animal, overtime, will adapt to its environment…The trilobite’s changing body, eyes, and tail are an

example of an adaptation Charles Darwin studied the beak color and size of

finches in the Galapagos islands

ResourcesFlorida Science Grade 7

Glencoe Science & McGraw Hill PublishingGoogle Images