Chapter 17: The History of Life Calif. Science standards 8e

17-1 The Fossil Record

Fossils and Ancient Life Paleontologists are scientists who collect and study fossils. All information about past life is called the fossil record. The fossil record includes information about the structure of organisms, what they ate, what ate them, in what environment they lived, and the order in which they lived. 17-1

Fossils are useful because . . . We can compare ancient organisms with today’s to see if organic evolution has occurred.

The fossil record provides incomplete information about the history of life. Over 99% of all species that have lived on Earth have become extinct, which means that the species has died out.

How Fossils Form Fossils can be as large as a complete, preserved animal, or as small as a fragment. Most fossils form in sedimentary rock. Sedimentary rock forms when exposure to the elements breaks down existing rock into small particles of sand, silt, and clay. 17-1

How Fossils Form Fossil Formation The fossil record provides evidence about the history of life on Earth. Most fossils are formed in sedimentary rock.

How Fossils Form The fossil record provides evidence about the history of life on Earth. Most fossils are formed in sedimentary rock.

The fossil record provides evidence about the history of life on Earth The fossil record provides evidence about the history of life on Earth. Most fossil formed in sedimentary rock.

What are the types of fossils? The fossil record provides evidence about the history of life on Earth. Most fossil formed in sedimentary rock.

1. Petrified wood 17-1

Impressions made in mud which then hardens into rock. 2. Imprint Impressions made in mud which then hardens into rock. Examples: Leaves, footprints 17-1

Dinosaur footprints 17-1

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Interpreting Fossil Evidence Paleontologists determine the age of fossils using relative dating or radioactive dating. What information do relative dating and radioactive dating provide about fossils? 17-1

NO, not dating a relative. Relative Dating NO, not dating a relative.

Relative Dating In relative dating, the age of a fossil is determined by comparing its placement with that of fossils in other layers of rock. Rock layers form in order by age—the oldest on the bottom, with more recent layers on top. 17-1

Relative Dating

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Index Fossils Are used to compare the relative ages of fossils. e.g. Trilobites, marine animals which were distant relatives of insects. Only lived between 500 and 600 million years ago. 17-1

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Absolute Dating = A method to determine how long ago an event occurred and to assign an absolute age to rocks. 17-1

Radioactive dating Some elements are radioactive and steadily break down into nonradioactive elements. They decay at a fixed rate over time 17-1

E=mc2 tells us that a very small mass can be converted into lots of energy! 17-1

Carbon 14 Nitrogen 14

Radioactive dating: the use of half-lives to determine the age of a sample. Half-life: time required for 1/2 the radioactive atoms in a sample to decay. 17-1

Can date objects up to 50,000 yrs old Example: Carbon 14 Nitrogen 14 Half life = 5730 years C14 5730 yrs C14 + N14 Can date objects up to 50,000 yrs old

Half life = 4.5 billion years What if something is older than this? Uranium 238 Lead Half life = 4.5 billion years 17-1

Interpreting the Fossil Record 17-1

Through dating of rocks, scientists have been able to construct a timetable of earth’s history. Geologic time scale

Geologic Time Scale Major divisions = Eras (e.g. “Age of Reptiles”) Eras divided into: Periods/Epochs (e.g. “Jurassic Period”) 17-1

Way older than your teacher. There is good evidence that earth is 4.5 to 5 billion years old. Way older than your teacher.

Eras Geologists divide the time between Precambrian time and the present into three eras: Paleozoic Era Mesozoic Era Cenozoic Era 17-1

Geologic Time Scale The Paleozoic began about 544 million years ago. Many vertebrates and invertebrates lived during this time. 17-1

Geologic Time Scale Permian 290–245 360–290 Carboniferous 410–360 Devonian 410–360 The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods. Silurian 440–410 Ordovician 505–440 Cambrian 544–505

Geologic Time Scale The Mesozoic began about 245 million years ago. Dinosaurs lived during this time. Mammals began to evolve during this era. 17-1

Cretaceous 145–65 Jurassic 208–145 Triassic 245–208 The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods. Triassic 245–208

Geologic Time Scale The Cenozoic began about 65 million years ago and continues to the present. Mammals became common during the Cenozoic. 17-1

Geologic Time Scale The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.

Periods Eras are subdivided into periods, which range in length from tens of millions of years to less than two million years. Many periods are named for places around the world where geologists first discovered the rocks and fossils of that period.

Earth’s history is often compared to a familiar measurement, such as the twelve hours between noon and midnight. In such a comparison, notice that Precambrian Time lasts from noon until after 10:30 pm.

Which of the following statements about fossils is NOT true? Most fossils form in sedimentary rock. Fossils occur in a particular order. Only a small portion of fossils are from extinct organisms. Fossils can be used in relative dating of rock formations. 17-1

The fossil record consistently shows evidence that all forms of life have existed in all geologic eras. living organisms have only been on Earth for a short time. living things have changed over time. ancient life-forms are much the same as forms found living today. 17-1

Index fossils assist paleontologists in dating rocks because they represent species that were widely distributed and existed for a very long time. existed in a single location for a short period of time. were widely distributed and existed for a short time. existed in a single location for a very long time. 17-1

Determining the age of a fossil by comparing its placement with fossils in other layers of rock is called carbon-14 dating. fossil-indexing. relative dating. absolute dating. 17-1

According to the geologic time scale, geologic time begins with Precambrian Time. the Paleozoic Era. the Quaternary Period. the Cambrian Era. 17-1

Patterns of evolution Evidence of mass extinctions: Many fossils are from species no longer living (extinct).  Of all species that ever lived, < 1% exist today! Evidence of mass extinctions: Floods, comets, volcanic activity 17-4

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