Chapter 5 The Fossil Record Preview Section 1 Geologic HistoryGeologic History Section 2 Looking at FossilsLooking at Fossils Mr. Hall’s thoughts are in.

Chapter 5 The Fossil Record

Preview

Section 1 Geologic History

Section 2 Looking at Fossils

Mr. Hall’s thoughts are in Orange.

Chapter 5

Objectives

• Compare uniformitarianism with catastrophism.

• Describe how the science of geology has changed over the past 200 years.

• Contrast relative dating with absolute dating.


Chapter 5

• In Theory of the Earth (1788), James Hutton introduced the idea of uniformitarianism.

• Uniformitarianism assumes that geologic processes that are shaping the Earth today have been at work throughout Earth’s history.

The Principle of Uniformitarianism, continued


Key word is “assumes”. Science shouldn’t assume.

Chapter 5 Section 1 Geologic History

Chapter 5

• Uniformitarianism Versus Catastrophism During Hutton’s time, most scientists supported catastrophism, the principle that all geologic change occurs suddenly.

• Supporters of catastrophism thought that Earth’s mountains, canyons, and seas formed during rare, sudden events called catastrophes.



Empirical evidence supports this!

Chapter 5

• Most people also believed that Earth was only a few thousand years old.

• Hutton’s work suggested a very different reality.

• According to his theories, Earth had to be much older, because gradual geologic processes would take much longer than a few thousand years.



Really another “belief”

Chapter 5

• A Victory for Uniformitarianism Catastrophism remained the guiding principle of geology in the early 19th century.

• But uniformitarianism became geology’s guiding principle after Charles Lyell reintroduced the concept in his Principles of Geology (1830-1833).



Arguable in a BIG way!

Chapter 5

• Using Hutton’s notes and evidence of his own, Lyell successfully challenged the principle of catastrophism.

• He saw no reason to doubt that major geologic change happened at the same rate in the past as it happens in the present—gradually.



HUGE assumption! = Bad Science!

Chapter 5

Modern Geology—A Happy Medium

• During the late 20th century, scientists such as Stephen J. Gould challenged the principle of uniformitarianism.

• They believed that catastrophes sometimes play an important role in shaping Earth’s history.

• Neither theory completely accounts for all geologic change.


All of this is speculation! Not based on anything that can be measured &

quantified

Chapter 5

• Most geologic change is gradual and uniform. This is not able to be empirically verified!

• But catastrophes that cause geologic change have occurred during Earth’s long history.

• Asteroid and comet strikes to Earth, for example, have caused rapid change. So do volcanoes, hurricanes, tsunamis, etc…

Modern Geology—A Happy Medium, continued


Chapter 5

• Some scientists think an asteroid strike 65 million years ago caused the dinosaurs to become extinct.

Modern Geology—A Happy Medium, continued


Chapter 5

• Scientists can use two methods to determine the age of objects in sedimentary rocks.

• One of those methods is known as relative dating.

• Relative dating examines a fossil’s position within rock layers to estimate its age.

Relative Dating


This is a highly speculative way to date!

Chapter 5

• The bottom layers of rock are usually the oldest, and the top layers are usually the youngest.

• Scientists can use the order of these rock layers to determine the relative age of objects within the layers.

• For example, fossils in the bottom layers are usually older than fossils in the top layers.

Relative Dating, continued


Chapter 5

• The Geologic Column To make relative dating easier, geologists combine data from all of the known rock sequences around the world.

• From this information, geologists create the geologic column—an ideal sequence of rock layers that contains all of the known fossils and rock formations on Earth.

• These layers are arranged from oldest to youngest.

Relative Dating, continued



Chapter 5

• Scientists can use absolute dating to more precisely determine the age of a fossil or rock.

• In absolute dating, scientists examine atoms to measure the age of fossils or rocks in years.

• Atoms are the particles that make up all matter.

Absolute Dating


This method requires a HUGE assumption!

Chapter 5

• Some atoms are unstable, and will decay over time.

• When an atom decays, it becomes a different and more stable kind of atom.

• Each kind of unstable atom decays at its own rate.

Absolute Dating, continued


Chapter 5

• The time it takes for half of the unstable atoms in a sample to decay is known as the half-life of that atom.

• Scientists can examine a sample of rock or fossil, and look at the ratio of unstable to stable atoms.

• Since they know the half-life, they can determine the approximate age of the sample.



Only if they know the size of the original sample. And assuming there weren’t any daughter elements present.


Chapter 5

• Uranium-238 has a half-life of 4.5 billion years. Scientists can use uranium-238 to date rocks or fossils that are millions of years old.

• Assuming none of the daughter elements were present at the origin of the sample.

• Carbon-14 has a half-life of only 5,780 years. • Only works on organic material – doesn’t work on rock• Scientists use carbon-14 to date fossils and other objects

that are less than 50,000 years old, such as human artifacts.

• There is a LOT of Carbon-14 on the Earth. Meaning……?



Chapter 5

• Paleontology is the science involved with the study of past life.

• Scientists who study past life are called paleontologists.

• Paleontologists collect data by studying fossils, the remains of organisms preserved by geological processes.

Paleontology—The Study of Past Life


Chapter 5

• Vertebrate and invertebrate paleontologists study the remains of animals.

• Paleobotanists study fossils of plants.

• Other paleontologists reconstruct past ecosystems, study the traces that animals left behind, and piece together the conditions under which fossils formed.

Paleontology—The Study of Past Life, continued


Chapter 5

• Describe five ways in which different types of fossils form.

• List three types of fossils that are not part of organisms.

• Explain how fossils can be used to determine the history of changes in environments and organisms.

• Explain how index fossils can be used to date rock layers.

Objectives


Chapter 5

• The remains or physical evidence of an organism preserved by geologic processes is called a fossil.

• Rock in the shape of what was once alive.• Fossils in rocks can form when an organism dies and is

quickly covered by sediment.• The key is “quickly covered by sediment” not slow deposition

over time.• When the sediment becomes rock, hard parts of the

organism are preserved.• Soft parts have also been preserved through fossilization.

Fossilized Organisms


Chapter 5

• If an insect is caught in sticky tree sap, the sap covers its entire body and hardens quickly.

• Fossils in amber are entire organisms preserved inside hardened tree sap, called amber.

• Most species trapped in Amber are identical to current species.

• Some of the best insect fossils, as well as frogs and lizards, have been found in amber.

Fossilized Organisms, continued


Chapter 5

• Organisms can also be preserved by petrifaction.

• Petrifaction is a process in which minerals replace the organism’s tissues.

• Also called mineral replacement – where a living organism becomes an inorganic shape in rock.

• Permineralization and replacement are forms of petrifaction.



Chapter 5

• In the process of permineralization, pore space in an organism’s hard tissue (like bone or wood) is filled up with mineral.

• In the process of replacement, minerals completely replace the tissues of the organism.

• Some samples of petrified wood are composed completely of minerals.



Chapter 5

• In some places, asphalt wells up and forms thick, sticky pools at Earth’s surface.

• These asphalt pools can trap and preserve many organisms.

• The La Brea asphalt deposits in Los Angeles, California have preserved organisms for at least 38,000 years.



Chapter 5

• Frozen Fossils In 1999, scientists removed a 20,000-year-old woolly mammoth that was frozen in the Siberian tundra.

• These mammoths became extinct about 10,000 years ago.

• This is highly arguable!

• Because cold temperatures slow down decay, the mammoth was almost perfectly preserved.



Chapter 5

• Trace fossils are any naturally preserved evidence of animal activity.

• Tracks are an example of a trace fossil. They form when animal footprints fill with sediment.

• Tracks can reveal size and speed of an animal, and whether it traveled in groups.

Other Types of Fossils


Chapter 5

Huh!?

Chapter 5

• Burrows are another trace fossil.

• Burrows are shelters made by animals that bury themselves in sediment, such as clams.

• Another type of trace fossil is coprolite, or preserved animal dung.

• Dino-doo doo

Other Types of Fossils, continued


Chapter 5

• Molds and casts are two more examples of fossils.

• A cavity in rock where a plant or animal was buried is called a mold.

• A cast is an object that is created when sediment fills a mold and becomes rock.

Other Types of Fossils, continued


Chapter 5

• The Information in the Fossil Record The fossil record gives only a rough sketch of the history of life on Earth.

• Argumentative point on the “history of life on Earth”.

• Most organisms never become fossils.• But there are more than we ever thought could be fossils.

• Many fossils have yet to be discovered.

Using Fossils to Interpret the Past


Chapter 5

• Organisms with hard body parts have left more fossils than those with soft body parts.

• Organisms that lived in areas that favored fossilization have also left more fossils.

Using Fossils to Interpret the Past, continued


Chapter 5

• But fossils can show a history of environmental change.

• For example, the presence of marine fossils on mountaintops in Canada means that these mountains formed at the bottom of the ocean.

• Marine fossils can also help scientists reconstruct ancient coastlines and detect the presence of ancient seas.



Chapter 5

• Scientists can use fossils of plants and land animals to reconstruct past climates.

• By examining fossils, scientists can tell whether the climate of an area was cooler or wetter than that climate is now.



Chapter 5

• History of Changing Organisms Scientists study the relationships between fossils to interpret how life has changed over time.

• Since the fossil record is incomplete, paleontologists look for similarities between fossils over time to try to track change.

• This, of course, is requiring a HUGE assumption to be made again. In organic material cannot be linked together due to the lack of DNA to prove a relationship.



Chapter 5

• Scientists have found that particular types of fossils appear only in certain layers of rock.

• More assumptions to accept & be made.• By dating rock layers above and below these

fossils, scientists can determine the time span in which the organism lived.

• More assumptions to accept & be made.• If the organism lived for a relatively short period of

time, its fossils would show up in limited layers.• More assumptions to accept & be made.

Using Fossils to Date Rocks


Chapter 5

• Index fossils are fossils of organisms that lived for a relatively short, well-defined geologic time span.

• Well defined? By what definition? Defined by assumption?

• To be index fossils, these fossils must be found worldwide.

Using Fossils to Date Rocks, continued


Chapter 5

• Ammonites of the genus Tropites are index fossils.



Hmm look very similar don’t they?

Chapter 5

• These ammonites were marine mollusks similar to modern squids.

• Or nautilus

• Tropites lived between 230 million and 208 million years ago.

• How can we be so sure?

• Fossils of these ammonites are index fossils for that time period.




Chapter 5

• Trilobites of the genus Phacops are also index fossils.

• Trilobites are extinct. Their closest living relative is the horseshoe crab.



Chapter 5

• Phacops lived about 400 million years ago.• How can we be so sure?

• When scientists find fossils of trilobites anywhere on Earth, they assume the rock layers are also approximately 400 million years old.




Chapter 5 The Fossil Record Preview Section 1 Geologic HistoryGeologic History Section 2 Looking at FossilsLooking at Fossils Mr. Hall’s thoughts are in.

Documents