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Chapter: Geologic Time
Table of ContentsTable of Contents
Section 3: Middle and Recent Earth History
Section 1: Life and Geologic Time
Section 2: Early Earth History
• Trilobites are small, hard-shelled organisms that crawled on the seafloor.
Geologic Time
Life and Geologic TimeLife and Geologic Time
11
• They are considered to be index fossilsbecause they lived over vast regions of the world during specific periods of geologic time.
• Paleontologists have been able to divide Earth’s history into time units based on the life-forms that lived only during certain periods.
• This division of Earth’s history makes up the geologic time scale.
The Geologic Time Scale11Life and Geologic TimeLife and Geologic Time
• Four major subdivisions of geologic time are used— eons, eras, periods, and epochs.
• The longest subdivisions— eons—are based upon the abundance of certain fossils.
11Life and Geologic TimeLife and Geologic Time
Major Subdivisions of Geologic Time
• Next to eons, the longest subdivisions are the eras, which are marked by major, striking, and worldwide changes in the types of fossils present.
11Life and Geologic TimeLife and Geologic Time
Major Subdivisions of Geologic Time
• Periods are units of geologic time characterized by the types of life existing worldwide at the time.
• Periods can be divided into smaller units of time called epochs.
• Eras are subdivided into periods.
11Life and Geologic TimeLife and Geologic Time
Major Subdivisions of Geologic Time
• Epochs are also characterized by differences in life-forms, but some of these differences can vary from continent to continent.
• Sometimes it is possible to distinguish layers of rock that formed during a single year or season.
• In other cases, thick stacks of rock that have no fossils provide little information that could help in subdividing geologic time.
Dividing Geologic Time11Life and Geologic TimeLife and Geologic Time
• The fossil record shows that species have changed over geologic time.
• This change through time is known as organic evolution.
Organic Evolution11
• Organisms that are not adapted to changes are less likely to survive or reproduce.
• Over time, the elimination of individuals that are not adapted can cause changes to species of organisms.
Life and Geologic TimeLife and Geologic Time
• Life scientists often define a species as a group of organisms that normally reproduces only with other members of their group.
Species11Life and Geologic TimeLife and Geologic Time
Natural Selection11Life and Geologic TimeLife and Geologic Time
• Charles Darwin was a naturalist who sailed around the world from 1831 to 1836 to study biology andgeology.
Natural Selection11Life and Geologic TimeLife and Geologic Time
• In his book, he proposed that natural selection is a process by which organisms with characteristics that are suited to certain environment have a better chance of surviving and reproducing than organisms that do not have these characteristics.
• Because many characteristics are inherited, the characteristics of organisms that are better adapted to the environment get passed on to offspring more often.
Natural Selection11
• According to Darwin, this can cause a species to change over time.
Life and Geologic TimeLife and Geologic Time
• A new characteristic becomes common in a species only if some members already possess that characteristic and if the trait increases the animal’s chance of survival.
Natural Selection Within a Species11Life and Geologic TimeLife and Geologic Time
• By carefully choosing individuals with desired characteristics, animal breeders have created many breeds of cats, dog, cattle, and chickens.
Artificial Selection11
• Natural selection explains how characteristics change and how new species arise.
Life and Geologic TimeLife and Geologic Time
• The exoskeleton of a trilobite consists of three lobes that run the length of the body.
Trilobites11
• The trilobite’s body also has a head (cephalon), a segmented middle section (thorax), and a tail (pygidium).
Life and Geologic TimeLife and Geologic Time
• Paleontologists can use these different characteristics to demonstrate changes in trilobites through geologic time.
Changing Characteristics of Trilobites
11Life and Geologic TimeLife and Geologic Time
Changing Characteristics of Trilobites
11
• These changes can tell you about how different trilobites from different periods lived and responded to changes in their environments.
Life and Geologic TimeLife and Geologic Time
• Trilobite eyes show the result of natural selection.
Trilobite Eyes11Life and Geologic TimeLife and Geologic Time
Trilobite Eyes11Life and Geologic TimeLife and Geologic Time
• In most species of trilobites, the eyes were located midway on the head—a compromise for an organism that was adapted for crawling on the seafloor and swimming in the water.
• Over time, the eyes in trilobites changed.
• In many trilobite species, the eyes became progressively smaller until they completely disappeared.
Trilobite Eyes11Life and Geologic TimeLife and Geologic Time
• Blind trilobites might have burrowed into sediments on the seafloor or lived deeper than light could penetrate.
• In other species, however, the eyes became more complex.
Trilobite Eyes11Life and Geologic TimeLife and Geologic Time
• One kind of trilobite, Aeglina, developed large compound eyes that had numerous individual lenses.
• Some trilobites developed stalks that held the eyes upward.
Trilobite Bodies11Life and Geologic TimeLife and Geologic Time
• The trilobite body and tail also underwent significant changes in form through time.
• It is thought that Olenellus, and other species that have so many body segments, are primitive trilobites.
Fossils Show Changes11Life and Geologic TimeLife and Geologic Time
• Trilobite exoskeletons changed as trilobites adapted to changing environments.
• Species that could not adapt became extinct.
Plate Tectonics and Earth History
• Plate tectonics is one possible answer to the riddle of trilobite extinction.
• By the end of the Paleozoic Era, sea levels had dropped and the continents had come together to form one giant landmass, the supercontinent Pangaea.
11Life and Geologic TimeLife and Geologic Time
Plate Tectonics and Earth History
• Because trilobites lived in the oceans, their environment was changed or destroyed.
• Not all scientists accept this explanation for the extinctions at the end of the Paleozoic Era, and other possibilities—such as climate change—have been proposed.
11Life and Geologic TimeLife and Geologic Time
11Question 1
Which of these geologic time subdivisions is longest?
A. eraB. eonC. epochD. period
Section CheckSection Check
11Section CheckSection Check
Answer
The answer is B. Eons are the longest subdivisions of geologic time and are based upon the abundance of certain fossils.
11Question 2
What is organic evolution?
Section CheckSection Check
Answer
Organic evolution is the change of species over time.
11Question 3
__________ is the process by which organisms with specific characteristics have a greater chance of surviving and reproducing than organisms that do not have those characteristics.
A. Artificial selectionB. Natural selectionC. Organic evolutionD. Predominance
Section CheckSection Check
11Section CheckSection Check
Answer
The answer is B. Charles Darwin wrote about the theory of evolution by natural selection in his book, The Origin of Species.
Precambrian Time• Precambrian time is the longest part of
Earth’s history and includes the Hadean, Archean, and Proterozoic Eons.
Early Earth HistoryEarly Earth History
22
Precambrian Time
• Precambrian time lasted from about 4.5 billion years ago to about 544 million years ago.
Early Earth HistoryEarly Earth History
22
Precambrian Time
• Although the Precambrian was the longest interval of geologic time, relatively little is known about the organisms that lived during this time.
• One reason is that many Precambrian rocks have been so deeply buried that they have been changed by heat and pressure.
Early Earth HistoryEarly Earth History
22
Precambrian Time
• In addition, most Precambrian organisms didn’t have hard parts that otherwise would have increased their chances to be preserved as fossils.
Early Earth HistoryEarly Earth History
22
Early Life
• Many studies of the early history of life involve ancient stromatolites.
• Stromatolites are layered mats formed by cyanobacteria colonies.
22Early Earth HistoryEarly Earth History
Early Life• Cyanobactreia are blue-green algae thought
to be one of the earliest forms of life on Earth. They contained chlorophyll and used photosynthesis.
• During photosynthesis they produced oxygen, which helped oxygen become a major atmospheric gas.
22Early Earth HistoryEarly Earth History
Early Life
• Animals without backbones, called invertebrates, appeared toward the end of Precambrian time.
• Because these early invertebrates were soft-bodied, they weren’t often preserved as fossils. Because of this, many Precambrian fossils are trace fossils.
22Early Earth HistoryEarly Earth History
Unusual Life-Forms• A group of animals with shapes similar to
modern jellyfish, worms, and soft corals was living late in Precambrian time.
• This group of organisms has become known as the Ediacaran fauna.
22Early Earth HistoryEarly Earth History
• Ediacaran animals were bottom dwellers and might have had tough outer covering like air mattresses.
• The Paleozoic Era, or era of ancient life, began about 544 million years ago and ended about 248 million years ago.
The Paleozoic Era• An abundance of organisms with hard parts,
such as shells, marks the beginning of the Paleozoic Era.
22Early Earth HistoryEarly Earth History
Paleozoic Life• Many of the life-forms scientists know about
were marine, meaning they lived in the ocean.
22Early Earth HistoryEarly Earth History
• Trilobites were common, especially early in the Paleozoic.
• Other organisms developed shells that were easily preserved as fossils.
• Vertebrates, or animals with backbones, also evolved during this era.
Paleozoic Life
• Armored fish with jaws lived during the Devonian Period.
22Early Earth HistoryEarly Earth History
• By the Devonian Period, forests had appeared and vertebrates began to adapt to land environments, as well.
Life on Land• Paleontologists know that many ancient fish
had lungs as well as gills.
22Early Earth HistoryEarly Earth History
• Lungs enabled these fish to live in water with low oxygen levels—when needed they could swim to the surface and breathe air.
• One kind of ancient fish had lungs and leglike fins, which were used to swim and crawl around on the ocean bottom.
Life on Land
• Paleontologists hypothesize that amphibians might have evolved from this kind of fish.
22Early Earth HistoryEarly Earth History
• Today amphibians live in a variety of habitats in water and on land.
• They all have at least one thing in common, though. They must lay their eggs in water or moist places.
22Early Earth HistoryEarly Earth History
Life on Land
• By the Pennsylvanian Period, some amphibians evolved an egg with a membrane that protected it from drying out.
• These animals, called reptiles, no longer needed to lay eggs in water.
22Early Earth HistoryEarly Earth History
Life on Land
• Reptiles also have skin with hard scales that prevent loss of body fluids.
• This adaptation enables them to survive farther from water in relatively dry climates.
• Several mountain-building episodes occurred during the Paleozoic Era.
22
• The Appalachian Mountains, for example, formed during this time.
Early Earth HistoryEarly Earth History
Mountain Building
• The first mountain-building episode occurred as the ocean separating North America from Europe and Africa closed.
• Several volcanic island chains that had formed in the ocean collided with the North American Plate.
22
• The collision of the island chains generated high mountains.
Mountain Building
Early Earth HistoryEarly Earth History
• The next mountain-building episode was a result of the African Plate colliding with the North American Plate.
• When Africa and North America collided, rock layers were folded and faulted.
22
• Sediments were uplifted to form an immense mountain belt, part of which still remains today.
Mountain Building
Early Earth HistoryEarly Earth History
• At the end of the Paleozoic Era, more than 90 percent of all marine species, and 70 percent of all land species died off.
22 End of an Era
Early Earth HistoryEarly Earth History
• Near the end of the Permian Period, the continental plates came together and formed the supercontinent Pangaea.
22
• Mountain-building processes caused seas to close and deserts to spread over North America and Europe.
• Many species, especially marine organisms, couldn’t adapt to these changes, and became extinct.
End of an Era
Early Earth HistoryEarly Earth History
• During the late Paleozoic Era, volcanoes were extremely active. If the volcanic activity was great enough, it could have affected the entire globe.
22
• Perhaps a large asteroid or comet collided with Earth some 248 million years ago.
• Perhaps the extinction was caused by several or all of these events happening at about the same time.
Other Hypotheses
Early Earth HistoryEarly Earth History
22Section CheckSection Check
Question 1
Which was the longest period of geologic time?
A. Paleozoic EraB. Pennsylvanian PeriodC. Phanerozoic EonD. Precambrian Time
22Section CheckSection Check
Answer
The answer is D. Precambrian Time was the longest period of geologic time, lasting about 4 billion years.
22Section CheckSection Check
Question 2
_________ are blue-green algae and are thought to be one of the earliest forms of life on Earth.
A. Amphibious plantsB. CyanobacteriaC. DickensoniaD. Trilobites
22Section CheckSection Check
Answer
The answer is B. Cyanobacteria are photosynthetic organisms containing chlorophyll.
22Section CheckSection Check
Question 3
Animals without backbones are called __________.
A. exvertebratesB. invertebratesC. neovertebratesD. nonvertebrates
22Section CheckSection Check
Answer
The answer is B. The proper term for animals without backbones is “invertebrates”.
• The Mesozoic Era, or era of middle life, was a time of many changes on Earth.
33
• At the beginning of the Mesozoic Era, all continents were joined as a single landmass called Pangaea.
Middle and Recent Earth HistoryMiddle and Recent Earth History
The Mesozoic EraThe Breakup of Pangaea
• Pangaea separated into two large landmasses.
33
• The northern mass was Laurasia, and Gondwanaland was the southern landmass.
Middle and Recent Earth HistoryMiddle and Recent Earth History
The Mesozoic EraThe Breakup of Pangaea
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• Reptile’s skin helps it retain bodily fluids.
33
• This characteristic, along with their shelled eggs, enabled reptiles to adapt readily to the drier climate of the Mesozoic Era.
Middle and Recent Earth HistoryMiddle and Recent Earth History
The Mesozoic EraThe Breakup of Pangaea
• Reptiles became the most conspicuous animals on land by the Triassic period.
33
• Dinosaurs ranged in height from less that 1 m to enormous creatures like Apatosaurus and Tyrannosaurus.
• Throughout the Mesozoic Era, new species of dinosaur evolved and other species became extinct.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Dinosaurs
33
• Some dinosaur tracks indicate that these animals were much faster runners than you might think.
• Gallimimus could reach speeds of 65 km/h.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Dinosaurs Were Active
33
• Some studies also indicate that dinosaurs might have been warm blooded, not cold blooded like present-day reptiles.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Dinosaurs Were Active
• Slices through some cold-blooded animal bones show rings similar to growth rings in trees.
• The bones of some dinosaurs don’t show this ring structure.
33
• The fossil record also indicates that some dinosaurs nurtured their young and traveled in herds in which the adults surrounded their young.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Good Mother Dinosaurs
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• One such dinosaur is Maiasaura.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Good Mother Dinosaurs
• This dinosaur built nests in which it laid eggs and raised its offspring.
• Nests have been found in relatively close clusters, indicating that more than one family of dinosaurs built in the same areas.
• Some fossils of hatchlings have been found near adult animals, leading paleontologists to think that some dinosaurs nurtured their young.
• Birds appeared during the Jurassic Period.
33
• Some paleontologists think that birds evolved from small, meat-eating dinosaurs.
• The earliest bird, Archaeopteryx, had wings and feathers.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Birds
• Mammals first appeared in the Triassic Period.
33
• The earliest mammals were small, mouselike creatures.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Mammals
• Mammals are warm-blooded vertebrates that have hair covering their bodies.
33
• The females produce milk to feed their young.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Mammals
• These two characteristics have enabled mammals to survive in many changing environments.
• During most of the Mesozoic Era, gymnosperms dominated the land.
33
• Gymnosperms are plants that produce seeds but not flowers.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Gymnosperms
• These include pines and ginkgo trees.
• Angiosperms, or flowering plants, first evolved during the Cretaceous Period.
33
• Angiosperms produce seeds with hard outer coverings.
Middle and Recent Earth HistoryMiddle and Recent Earth History
Angiosperms
• Because their seeds are enclosed and protected, angiosperms can live in many environments.
• Angiosperms are the most diverse and abundant land plants today.
• The Mesozoic Era ended about 65 million years ago with a major extinction of land and marine species.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
End of an Era
• Many paleontologists hypothesize that a comet or asteroid collided with Earth, causing a huge cloud of dust and smoke to rise in the atmosphere, blocking out the Sun.
• Without sunlight the plants died, and all animals that depended on these plants also died.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
End of an Era
• All the organisms that you see around you today are descendants of the survivors of the great extinction at the end of the Mesozoic Era.
• The Cenozoic Era, or era of recent life, began about 65 million years ago and continues today.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
The Cenozoic Era
• The Cenozoic Era is subdivided into two periods.
• The first of these is the Tertiary period.
• The present-day period is the Quaternary Period. It began about 1.8 million years ago.
• Many mountain ranges formed during the Cenozoic Era.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
Times of Mountain Building
• These include the Alps in Europe and the Andes in South America.
• The Himalaya formed as India moved northward and collided with Asia.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
Times of Mountain Building
• The collision crumpled and thickened Earth’s crust, raising the highest mountains presently on Earth.
• Throughout much of the Cenozoic Era, expanding grasslands favored grazing plant eaters like horses, camels, deer, and some elephants.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
Further Evolution of Mammals
• Many kinds of mammals became larger.
• Not all mammals remained on land.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
Further Evolution of Mammals
• Ancestors of the present-day whales and dolphins evolved to live in the sea.
• As Australia and South America separated from Antarctica during the continuing breakup of the continents, many species became isolated.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
Further Evolution of Mammals
• They evolved separately from life-forms in other parts of the world.
• Evidence of this can be seen today in Australia’s marsupials.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
Further Evolution of Mammals
• Marsupials are mammals such as kangaroos, koalas, and wombats that carry their young in a pouch.
• Your species, Homo sapiens, probably appeared about 140,000 years ago.
33Middle and Recent Earth HistoryMiddle and Recent Earth History
Further Evolution of Mammals
• Some people suggest that the appearance of humans could have led to the extinction of other mammals.
• As their numbers grew, humans competed for food that other animals relied upon.
33Section CheckSection Check
Question 1
During which time period did Pangaea separate?
A. DevonianB. MississippianC. PermianD. Triassic
33Section CheckSection Check
Answer
The answer is D. Pangaea separated into two large landmasses, Laurasia and Gondwanaland, during the Triassic Period.
33Section CheckSection Check
Question 2
The era of recent life is the __________.
A. Cenozoic EraB. Mesozoic EraC. Paleozoic EraD. Precambrian Time
33Section CheckSection Check
Answer
The answer is A. The Cenozoic Era began about 65 million years ago.
33Section CheckSection Check
Question 3
What were the dominant land animals of the Mesozoic Era?
A. DinosaursB. GymnospermsC. Predatory fishD. Trilobites
33Section CheckSection Check
Answer
The answer is A. Dinosaurs were the dominant land animals of the Mesozoic Era. Gymnosperms were the dominant plants.
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