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Geologic time

8th Grade

+Trilobites

n  Trilobites are small, hard-shelled animals that lived in ancient seas. Trilobites are considered to be index fossils.

n  Index fossils lived over vast regions of the world during specific periods of geologic time. Index fossil help scientist figure out geologic time and other problems. (more to come later in notes)

+Geologic Time scale

n  The appearance or disappearance of types of organisms throughout Earth’s history marks important events in geologic time.

n  Paleontologists, scientists who study the prehistoric world, divide Earth’s history into time units based on life-forms that existed only during certain periods.

n  This division of Earth’s history is known as the geologic time scale. Sometimes few fossils remain from a period. Then paleontologists use other methods to define a division of geologic time.

+Subdivisions of Geologic Time

n  The fossil record is used to divide Earth’s history into geologic time periods. There are four major subdivisions of geologic time—eons, eras, periods, and epochs.

n  Eons are the longest subdivision and are based on the abundance of certain fossils. Eons are divided into smaller time periods called eras.

n  Eras are marked by major worldwide changes in the types of fossils present. For example, at the end of the Mesozoic Era, many kinds of invertebrates, birds, mammals and reptiles became extinct. Eras are subdivided into periods.

+Div Geo

n  Periods are units of geologic time during which certain types of life-forms existed all over the world. Geologic periods are divided into epochs.

n  Epochs are also characterized by differences in life-forms, but these may vary from continent to continent. Epochs may be given names, like those in the Cenozoic Era or may be called simply early, middle, or late

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n  There is a limit to how finely geologic time can be subdivided. It depends on the kind of rock record that is being studied.

n  Sometimes it is possible to distinguish different layers of rock that formed during a single year. In other cases, there is little information to help scientists subdivide geologic time.

+Organic Evolution

n  The fossil record shows that species or types of life-forms, have changed over geologic time. Organic evolution is the change in species over geologic time.

n  Most scientists believe that changes in the environment affect an organism’s survival. Organisms that do not adapt to the changes are less likely to survive or reproduce.

n  Over time, the disappearance of individuals that are not adapted to the new conditions can cause changes to species of organisms.

+Species

n  There are many ways to define the term species. Life scientists often define a species as a group of organisms that normally reproduce only with other members of their group.

n  For example, horses generally reproduce only with other horses. Sometimes, members of different species can mate and produce offspring.

n  Horses sometimes mate with donkeys and produce mules. However, the offspring of two different species are often sterile.

+Darwin- Natural Selection

n  Between 1831 and 1836, Darwin sailed around the world, carefully observing plants and animals. He collected samples of life-forms and studied them to learn how they were related.

n  Darwin defined natural selection as the process by which organisms with characteristics suited to a certain environment have a better chance of surviving and reproducing than organisms that do not have these characteristics

+Darwin and Natural Selection

n  All organisms compete for resources, such as food and living space. Individuals within a species could be different, or show variation. An individual’s differences might help or hurt its chances of surviving in a changing environment.

n  Some organisms that were well suited to their environment lived longer and had a better chance of producing offspring. Organisms that were poorly adapted to their environment produced few or no offspring.

n  Because many characteristics are inherited, the characteristics of organisms that are better adapted to the environment get passed on to offspring more often. According to Darwin, this can cause a species to change over time.

+Natural selection with-in a species

n  A new characteristic becomes common in a species only under two conditions. First, some members must already have that characteristic (animals do not morph or change into the desired characteristic). Second, the trait must increase the animal’s chance of survival.

+Artifical Selection

n  Humans have long used artificial selection to breed domestic animals. Animal breeders carefully choose individuals animals with desired characteristics to mate.

n  Their offspring also have the desired characteristics. In this way, animal breeders have created many different breeds of cats, dogs, cattle, and chickens.

+Evolution

n  Natural selection explains how characteristics change and how new species arise. If a species migrated to a different area to survive, then they may have reproduced in the new area, developing different characteristics creating a new evolved species.

+Trilobites

n  A trilobite is an ancient organism with a three-lobed exoskeleton. An exoskeleton is a hard outer skeleton. These ancient animals were given the name trilobite because they have a three-part shell. The three parts, called lobes, run the length of its body. Trilobites also have a head (cephalon), a middle that is segmented or divided into sections (thorax), and a tail (pygidium)

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n  Trilobites lived in Earth’s oceans for more than 200 million years. All through the Paleozoic Era, some species of trilobites became extinct and other species of trilobites evolved. Different periods of the Paleozoic Era had different species of trilobites. Each species of trilobites had its own particular characteristics that were different from all other species.

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n  Paleontologists use the differences in trilobite species to explain how trilobites evolved over geologic time. These changes tell how different trilobites from different periods lived. The changes also tell how trilobites responded to changes in their environment

+The Eyes have it

n  Trilobites may have been the first organisms on Earth with complex eyes. Trilobite eyes are the result of natural selection. The position of an organism’s eyes tells how it lived. If its eyes are in the front of its head, it likely swam actively through the sea. If its eyes are located toward the back of its head, it likely lived on the bottom of the ocean. Most species of trilobites had eyes that were midway between the front and the back of the head. This clue tells us that trilobites were adapted to both active swimming and crawling on the ocean floor.

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n  Over time, the eyes in some trilobites changed. Gradually, the eyes of many trilobite species became smaller and smaller. Eventually, their eyes disappeared completely. These blind trilobites might have burrowed into sediments on the ocean floor. Or they lived in a part of the deep ocean where there was no light.

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n  Not all trilobite species lost their eyes. Some trilobite species developed highly complex eyes. One species of trilobite had compound eyes—eyes with many individual lenses. These trilobites had excellent vision. Still other trilobite species developed complex eyes on stalks that extended from their head. They also could see their world very well.

Different types of eyes

+Changes in body

n  The trilobite body also changed over geologic time. Some early trilobite species had many segments in the middle part of the body. Later trilobites had fewer segments.

+Plate Tectonics

n  Earth’s crust is made up of several plates. These plates are in slow but constant motion. This motion, called plate tectonics, caused continents to split apart or to collide. At the time the trilobites dominated Earth’s seas, Earth’s plates were moving together. When all the continents collided, they formed a single, enormous continent known as Pangaea

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n  . Pangaea was one giant landmass, or supercontinent. When Pangaea was forming (formed at the end of the Paleozoic Era), sea levels were dropping. Because trilobites lived in the ocean, they could not survive in the changed environment. At the end of the Paleozoic Era, trilobites became extinct

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n  . Pangaea was one giant landmass, or supercontinent. When Pangaea was forming (formed at the end of the Paleozoic Era), sea levels were dropping. Because trilobites lived in the ocean, they could not survive in the changed environment. At the end of the Paleozoic Era, trilobites became extinct

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n  Some scientists do not accept that the formation of Pangaea caused the extinctions at the end of the Paleozoic Era. Changes in the climate or other conditions may have led to the Paleozoic extinctions. As in all scientific debates, evidence must be considered carefully, and conclusions must be drawn based on the evidence.

+Question

n  Which of these geologic time subdivisions is longest?

n  A. era

n  B. eon

n  C. epoch

n  D. period

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n  The answer is B. Eons are the longest subdivisions of geologic time and are based upon the abundance of certain fossils.

+Section 2- Precambrian time

n  During the first billion years of Earth’s history, the land was covered with volcanoes. Over the next 3 billion years, simple life-forms began to live in the oceans. Precambrian time is the longest part of Earth’s history. Precambrian time lasted from about 4.5 billion years ago to about 544 million years ago.

+Early life

n  Little is known about the organisms that lived during Precambrian time. Most Precambrian rocks are buried deep within Earth where they have been changed by heat and pressure. Few fossils can survive these conditions. Most Precambrian organisms had soft bodies. These organisms did not have hard body parts that leave fossil imprints in rock.

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n  One clue to early history of life is found in ancient stromatolites. Stromatolites are layered mats formed by colonies, or groups, of cyanobacteria. Cyanobacteria are blue-green algae thought to be one of the earliest life-forms on Earth.

+Early life and atmosphere

n  Cyanobacteria first appeared on Earth about 3.5 billion years ago. Cyanobacteria contained chlorophyll and used photosynthesis.

n  This is important because during photosynthesis, they produced oxygen, which helped change Earth’s atmosphere. Following the appearance of cyanobacteria, oxygen became a major gas in the atmosphere.

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n  Also of importance was that the ozone layer in the atmosphere began to develop, shielding Earth from ultraviolet rays. It is hypothesized that these changes allowed species of single-celled organisms to evolve into more complex organisms.

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n  Near the end of Precambrian time, invertebrates appeared. Invertebrates are animals without backbones. Early invertebrates had soft bodies, so few were preserved as fossils. Because of this, many Precambrian fossils are trace fossils. Examples of trace fossils are tracks, trails, or burrows. Trace fossils provide information about how organisms lived and behaved.

+Unusal life

n  In the late Precambrian, a group of animals lived that were similar to some animals today. These soft-bodied animals looked like modern jellyfish and worms. The first fossils of these earliest invertebrates were found in the Ediacara Hills of Australia. This group of organisms became known as the Ediacaran animals

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n  . They have been found on every continent, except Antarctica. Ediacaran animals lived on the bottom of Precambrian seas. Some scientists think these animals may have had tough outer coverings on their bodies. Trilobites may have outcompeted the Ediacaran animals and caused their extinction. However, no one knows for sure why the Ediacaran animals disappeared.

+Paleozoic

n  In the Paleozoic Era, animals with shells and other hard body parts began to appear. Because hard body parts are well preserved in fossils, it is easier to find traces of life in this era. The Paleozoic Era, or era of ancient life, began about 544 million years ago and ended about 248 million years ago.

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n  During most of the Paleozoic Era, warm, shallow seas covered much of the planet. Many life-forms lived in the oceans. Trilobites were common, especially early in the Paleozoic. Other organisms developed shells. As a result, the fossil record of this time contains many shells.

+Verebrates

n  Animals with backbones are called vertebrates, they evolved during this time. The earliest vertebrates were fishlike creatures without jaws. During the Devonian Period, fish with strong jaws evolved. These huge fish, armed with heavy protective coverings on their bodies, could eat large sharks. By the Devonian Period, forests began to grow on land.

n  Some vertebrates adapted to the land environment.

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n  Most fish, both ancient and modern, breathe through gills. But in the Devonian Period, many fish also had lungs. Because of their lungs, they could live in water that had low levels of oxygen and swim to the surface to breathe air.

n  One kind of ancient fish with lungs also developed fins that were like legs. These leglike fins were used to swim and to crawl around on the ocean floor. Paleontologists hypothesize that today’s amphibians might have evolved from these fish. Modern amphibians, such as frogs, live both in water and on land. All amphibians have one thing in common—they all lay their eggs in water or in a moist place.

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n  By the Pennsylvanian Period, some amphibians evolved eggs that were covered by a protective coating. The coating helped prevent the eggs from drying out. As a result, these animals, called reptiles, did not need to lay their eggs in water. Reptiles also have skin covered with hard scales. The scales prevent water loss from their bodies. These adaptations allow reptiles to live farther from water and in dry climates where many amphibians cannot live.

+Mountain Building

n  During the Paleozoic Era, there were great changes on Earth’s surface. Several mountain ranges formed during this time. One example is the Appalachian Mountains in the Eastern United States. Mountain building occurred in several stages.

+End of Paleozoic Era

n  At the end of the Paleozoic Era, more than 90 percent of all ocean species and 70 percent of all land species died. Perhaps as the supercontinent Pangaea formed, changes in the ocean and land caused species to die. Another hypothesis is that erupting volcanoes changed Earth so much that organisms could not survive. Perhaps an asteroid hit the planet and destroyed its environments.

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n  Which was the longest period of geologic time?

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n  Precambrian

+Mesozoic

n  Dinosaurs lived during the Mesozoic Era. The Mesozoic Era, or era of middle life, occurred between 248 and 65 million years ago and was a time marked by rapid movement of Earth’s plates.

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n  At the beginning of the Mesozoic Era, all the continents were joined and formed one landmass called Pangaea. During a part of the Mesozoic Era called the Triassic Period, Pangaea began to break up. The continents began to drift apart as Earth’s plates moved away from each other. Pangaea split in two (Laurasia and Gondwanaland). Over time, the continents we know today formed. Some species, such as reptiles, survived the tremendous changes and mass extinction that occurred at the end of the Paleozoic Era.

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n  In the early Mesozoic Era, the climate became drier. The reptile’s scaly skin kept in moisture, so reptiles could live in this drier climate. Reptile eggs are protected by a shell, so their young survived as well. Reptiles became the most abundant animals of the Mesozoic Era.

+Continental Changes

n  . Some species, such as reptiles, survived the tremendous changes and mass extinction that occurred at the end of the Paleozoic Era. In the early Mesozoic Era, the climate became drier. The reptile’s scaly skin kept in moisture, so reptiles could live in this drier climate. Reptile eggs are protected by a shell, so their young survived as well. Reptiles became the most abundant animals of the Mesozoic Era.

+Dinosaurs

n  Dinosaurs came in all shapes and sizes. Some were less than 1 m tall. Others, like Apatosaurus and Tyrannosaurus, were enormous. The first small dinosaurs appeared in the Triassic Period. Larger dinosaur species evolved during the Jurassic and Cretaceous Periods. Throughout the Mesozoic Era, some dinosaurs became extinct as others evolved.

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n  Findings indicate that some dinosaurs were likely active warm-blooded, fast-running animals, similar to modern mammals and birds. They have found that some dinosaur footprints were far apart, indicating that these species were fast runners.

n  Scientists figured out that some dinosaurs could run up to 65 km/h—as fast as a modern racehorse. And that Dinosaur bones don’t have growth rings. They are similar to mammal bones. The bones of cold blooded animals have growth rings.

n   The fossil record shows that some dinosaur species fed and took care of their young. These dinosaurs also traveled in herds, so the young were protected.

+Bird- Like Creatures

n  Some paleontologists believe that modern birds evolved from small, meat-eating dinosaurs. The earliest bird-like dinosaur known, Archaeopteryx, had both wings and feathers. Since Archaeopteryx had some features different from modern birds, it is not a direct ancestor of today’s birds

+First mammals

n  The first mammals appeared in the Triassic Period. These tiny, mouse-like animals were warm-blooded and covered with fur. The females produced milk to feed their young. Because of their furry coat and milk production, mammals where able to survive many changing environments.

+Gymnosperms

n  Gymnosperms first appeared during the Paleozoic Era. By the Mesozoic Era, gymnosperms dominated the land. Gymnosperms are plants that produce seeds in cones, like pine cones. They do not produce flowers. There are many species of gymnosperms on Earth today, including pine trees and ginkgo trees.

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+Angiosperms

n  Angiosperms are flowering plants that evolved during the Cretaceous Period. Angiosperms produce seeds with hard outer coverings. Because their seeds are covered and protected, angiosperms can survive in many environments. Angiosperms are the most diverse and abundant plants on Earth today. Today’s magnolia trees and oak trees first evolved during the Mesozoic Era

+End of Mesozoic

n  The Mesozoic Era ended about 65 million years ago, when most land and ocean species became extinct. The dinosaurs disappeared. Many paleontologists hypothesize that this mass extinction was caused by an asteroid that collided with Earth. The impact put a huge cloud of dust and smoke in the air, blocking sunlight.

n  Without sunlight, plants died. As a result, the animals that fed on plants died. Some organisms managed to survive. They are the ancestors of the many species on Earth today.

+Cenozoic Era

n  The Cenozoic Era began about 65 million years ago and continues today. During this time, mountain ranges in North and South America formed. In the late Cenozoic, the climate cooled and ice ages occurred. The early Cenozoic Era is called the Tertiary Period. The present time is part of the Quaternary Period, which began about 1.8 million years ago

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n  Many mountain ranges formed during the Cenozoic Era as Earth’s plates moved and collided. These include the Alps in Europe and the Andes in South America. Many people think the growth of these mountains helped create cooler worldwide climates.

+Mammals Evolve

n  During the Cenozoic Era, grasslands expanded. As a result, grazing mammals like horses, deer, and elephants survived and grew larger. Horses evolved from small animals with many toes into the large, hoofed animals of today.

n  Some mammals evolved to live in the sea, such as dolphins and whales. As the continents continued to move apart, some species became isolated. For this reason, animals like kangaroos and koalas evolved in Australia and are not found anywhere else on Earth.

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n  Modern humans, Homo sapiens, probably first appeared about 140,000 years ago. The appearance of early humans may have caused the extinction of many other mammals. Humans competed for food that other animals ate and also hunted animals.

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n  The era of recent life is the __________.

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n  The Cenozoic Era began about 65 million years ago.