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Chapter 4 - Evolution of Earth & Life Through Time

The concepts of evolution and geologic time are essential components of oceanography (and all other sciences as well!). This chapter is a very brief summary of the history of life and major geologic events shaping planet Earth.

Earth formed from the accumulation of dust, gases, asteroids, and small planetesimal in the stellar nebula. During this period in early Earth history conditions on the surface of the planet were probably too hot for oceans to exist. However, over time the surface cooled enough for oceans to form and persist. However, the oceans and atmosphere were chemically very different than what exists today. The Early Earth had no significant free oxygen in the air or oceans, and they were rich in organic compounds, essential for the development of evolution and life. Where preserved, the oldest rocks in the sedimentary "geologic record" preserve evidence or biological activity, but only on a primitive microbial level. Early evolution was taking place on the molecular, intercellular, and microbial scales for the first 3 billion years of Earth history. Eventually primitive life forms began to use photosynthesis as a source of energy, and gradually (over a billion years) the atmosphere and oceans became an oxygen-rich environment allowing more complex life forms to evolve.

The theory of "natural selection" was first advanced by Charles Darwin and is now believed to be the main process that brings about evolution. Natural selection is the processes whereby organisms that are better adapted to their environment tend to survive and produce more offspring.
Click on thumbnail images for a larger view.
Geologic time scale with highlights in evolution and events in Earth History
Fig. 4-1. Geologic Time Scale
with highlight in evolution and events in Earth history.

Key developments in understanding the origin of life on Earth

Carl Linnaeus—a Swedish botanist, physician, and zoologist (lived 1707-1778), who laid the foundations for the modern scheme of binomial nomenclature and considered a founder of modern taxonomy and ecology (Figure 4-2).

* Linnaeus's system of classification grouped organisms based on shared characteristics. Modern taxonomy attempts to connect taxonomy to the evolutionary framework of shared common ancestors ("the evolutionary tree of life").

The theory of "natural selection" was first advanced by Charles Darwin and is now believed to be the main process that brings about evolution. Charles Darwin, a scientist/explorer, is credited with presenting the first published work dedicated to "natural selection" (fundamental to evolution theory) in his book "Origin of Species" (1859)—a compilation of his observations and thoughts about plants, animals, and fossils initially gathered during a five-year voyage around the world studying nature onboard the Royal Navy ship, the HMS Beagle (Figure 4-3). Natural selection is the processes whereby organisms that are better adapted to their environment tend to survive and produce more offspring. Note: Darwin did not release his research for nearly two decades after the expedition largely out of fear of repression, but his work arguably became one of the world's greatest scientific works of modern times.

Gregor Johann Mendel—an Austrian geneticist/researcher (and monk) who conducted experimental research on creating hybrids of garden peas. In 1865 and 1866, he published his research on how hereditary characteristics are passed from parent organisms to their offspring. Mendelian theory is fundamental to much of what is known about modern genetics theory (Figure 4-4).

* Over the past two centuries, many scientific discoveries about the nature of biochemistry, cell structure and processes, and development of analytical methods equipment (such as microscopes and chromatography) have contributed to the modern knowledge base fundamental to understanding genetic evolution. In 1951, James Watson and Francis Crick discovered and reported the double helical structure of the DNA molecule (Figure 4-5). Today, the entire genetic structure of human DNA has been mapped and reported via the Human Genome Project (2001). Genome mapping is now central to many kinds of biological research.
Carl Linnaeus
Fig. 4-2.
Carl Linnaeus is considered founder of modern taxonomy and ecology.
Charles Darwin (Smithsonian Institution drawing)
Fig. 4-3.
Explorer, Charles Darwin (1809–1882) published "Origin of Species" in 1859.
Mendel genetic variation
Fig. 4-4.
Statistical genetic variation illustrated by Mendel's research (applied to cats).
DNA illustrated
Fig. 4-5.
DNA occurs within chromosomes within a cell nucleus (illustrated).


Evolution means (in general usage) the gradual development of something, especially from simple to more complex forms. In biological sciences, evolution involves the processes by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the Earth.

Biological evolution also involves changes in heritable genetic traits within biological populations over successive generations (first described by Gregor Johann Mendel in 1865). Evolution occurs at many scales including the molecular level, cell level, organism level, species level, and ecosystem community level.

Evolutionary Theory Highlights

Evolutionary theory is a basic building block for the current geologic time scale.
• Evolution supports an old earth (~4.55 billion years).

Natural selection (Darwinism): The strongest and best adapted organisms survive and produce offspring. (For example, compare tuna to other fish!)

Divergent evolution

• Populations that are separated environmentally can develop different features based upon an adaptation to their environment.
• One group of organisms radiates (diversifies) into many different groups and species.
• Divergence leads to different and distinct populations and communities of organisms

Convergent evolution
Populations can develop similar features based upon a utilizing a similar environment and living habits. (The term niche is used in biology to define an organism's role in an ecosystem.)
• Both fish and marine mammals developed streamline bodies to swim efficiently.
• Marine mammals developed fur/thick blubber to protect them from cold waters.

Birds, bats, flying squirrels, insects, and flying fish all independently developed means in order to take flight.
Marsupial mammals in Australia adapted similar characteristics of mammal elsewhere (see table to right).

Evolution and classification Classification of humans
Fig. 4-6. Evolution and classification of living things (illustrated) Fig. 4-7. Human taxonomy within the scientific classification of life
Cat classification dog
Fig. 4-8. Classification (taxonomy) of a house cat Fig. 4-9. Classification (taxonomy) of a dog.

Populations that evolve in separate settings
may develop similar traits

Marsupial mammals
in Australia
Birthing manner
Tasmanian wolves

How Evolution Works

The life mission of individuals in any species is to eat, survive, and reproduce.

While living, individuals must deal with competition (within a population of their own species, or with other species).

Individuals must also adapt to "environmental changes" (changes in living space, availability of food resources, climate changes, catastrophes, etc.).

As time passes, species with either adapt to their situation (and evolve), or they face die offs or extinction.
Fig. 4-10.
How evolution works.

Essential Concepts of Historical Geology & Evolution

The geologic time scale is a systematic and chronological organization of time related to the history of the Earth and universe used by scientists (geologists, paleontologists, astronomers) to describe the timing and relationships between events that have occurred during the history of the Earth (and the universe) (see Figure 4-1).

Paleontology is the scientific study of life forms existing in former geologic periods, as represented by their fossils; the science involves reconstructing the physical characteristics of organisms, life habits, and the environments where they lived (paleoecology).

A fossil is a remnant or trace of an organism of a some earlier geologic age, such as a skeleton or leaf imprint, embedded and preserved in the earth's crust. Few things living today will survive to become fossils (see table to right).

The "fossil record" is a term used by geologists and paleontologists (scientists who study paleontology) to refer to the total number of fossils that have been discovered, as well as to the information derived from them. Many species that we see today do not get a chance to be preserved as fossils, but we can still learn about them by comparing them to fossils that have been found and properly recorded.

is the processes that turn plant or animal remains eventually to stone.

A trace fossil is a fossil impression of a footprint, trail, burrow, or other trace of an animal rather than of the animal itself.

An "everything" website for paleontology and evolution: U.C. Berkeley Museum of Paleontology website

Review of how fossils form
(or how they survived destruction)

After an organism dies it must:

survive being eaten (at least partially eaten).

2. survive transport to site of preservation.

3. survive burial in sediments or volcanic materials.

4. survive bioturbation (being chewed up underground by burrowing organisms).

5. survive microbial decay.

6. survive compaction with burial.

7. survive chemical changes associated with lithification.

8. survive uplift, weathering and erosional exposure.

9. be discovered and identified.

10. be researched, reported, and curated.

Sedimentary Sequences Preserve the "Fossil Record"

The history of the evolution of life is partly preserved in sedimentary rocks found around the world. The history of life is also recorded in the DNA of living organisms. The fossil record is extensive, but there are many "gaps" where sediments of different ages have not been preserved, and much has been erased as ocean crust is destroyed in the process of subduction and continental deposits are destroyed by erosion. Despite these issues, sedimentary deposits representing all geologic ages are preserved and exposed around the world. Figure 4-11 shows how the rise and fall of sea level through the ages resulted in the advance (transgression) and retreat (regression) of shallow seaways onto the North American continent. When sea level rose, sediments were deposited blanketing large regions of the continent. These deposits are preserved as sedimentary rock formations that accumulated in terrestrial, coastal, and marine depositional environments. Groups of these rock formations are parts of "sequences" that preserve the fossil record. Each of the sequences rests on the eroded surface of a previous sequence representing a major unconformity (or "sequence boundary"). Six major sequences (with their underlying unconformities) are recognized throughout North America, with equivalent features on other continents. Each of the sequences preserve fossils and evidence of biological activity that occurred when the sediments were deposited and preserved. Erosion through time has stripped away most of these deposits, but portions of each sequence are still still preserved and exposed in different regions of the continent. For example, four of the great sequences are exposed in the Grand Canyon (Figure 4-12).
Tectonic and sedimentatiion cycles of North America Sequences of sedimentary rocks and unconformities exposed in the Grand Canyon
Fig. 4-11. Major sedimentary sequences of North America preserve some of the evidence of the "fossil record" (after Sloss, 1931). The Sauk Sequence is the oldest containing shell fossils of the Cambrian Period. Each sequence represents a major advance (transgression) of shallow seas and coastal environments. Major unconformities represent periods of regression (dominated by erosion when the seas withdrew). Fig. 4-12. Paleozoic sedimentary sequences exposed in the Grand Canyon, Arizona include the "Sauk, Tippecanoe, Kaskaskia, and Absaroka" sequences. Each sequence is bounded by unconformities. The oldest bearing an abundance of fossils is the Sauk Sequence that rests on top of the Great Unconformity, an erosional boundary between rocks of Precambrian and Cambrian age.

Geologic History and Biological Evolution

The following sections of this chapter is a review of major geologic events, evolution of life through time, and selected important concepts related to Earth history, starting with the most ancient events and appearance of life forms in the fossil record and leading to the Present.

Precambrian Eon

The Precambrian Eon is the period of geologic time between when the Earth formed in the Solar System (in Hadean Time about 5 billion years ago) and the beginning of Phanerozoic Eon (about 540 million years ago). The oldest rock on Earth are Precambrian age.

Early Earth history and cell theory:

Geologic Time Highlights of Biological Evolution
Formation of the moon in the early Solar System
Fig. 4-13. Current thought is that the Moon formed from the debris created by the collision of a small planet-sized object with the ancestral Earth early in the history of the Solar System.
About 4.5 billion years ago Formation of Earth and Moon within the Solar System nebula (Figure 4-13). (This is discussed in detail in Chapter 1).
About 4 billion years ago Evidence of earliest cell-based life of Earth (prokaryotes)
About 3 billion years ago Evidence of photosynthesis and first eukaryotic cells capable of oxygen-based respiration.
About 3.0 to 1.8 billion years ago World-wide deposition of banded-iron formations fundamental to the gradual conversion of Earth atmosphere rich in carbon dioxide (CO2) to oxygen (O2). This conversion took nearly a billion years. Once there was enough free oxygen in the atmosphere, this allowed the development of an ozone layer to protect Earth from deadly solar ultraviolet radiation (UV). UV destroys many organic organic compounds. Intense solar UV probably would have killed life in the shallow waters.
About 1.8 billion years ago Sexual reproduction fully established in eukaryotes. Sexual reproduction increased the rate of mutation in species, leading to increased biodiversity.
About 1 billion years ago Earliest evidence of multicellular organisms (metazoans). Early multicellular organism were very "primitive" but diversified very quickly in geologic time.
Beginning about 541 million years ago
Beginning of the Cambrian Period started "radiation of species" - in part, because many organisms began to develop the first hard skeletal material as part of defensive and functional body plans. Warm, shallow ocean water (protected by an ozone layer) allowed new "chemistry" for organisms to adapt to, starting with calcite (CaCO3). Hard body parts are (shells and exoskeletons) were selectively preserved (and therefore easier to find as fossils) in Cambrian and younger sedimentary rocks. Sediments composed of the skeletal remains of organisms with shells and exoskeletons rich in CaCO3 is called "lime."


Early Evidence of Life on a Global Scale

Banded-iron formations (BIFs) are sedimentary mineral deposits consisting of alternating beds of iron-rich minerals (mostly hematite) and silica-rich layers (chert or quartz) formed about 3.0 to 1.8 billion years ago (Figure 4-14). Theory suggests BIFs are associated with the capture of oxygen released by photosynthetic processes by iron dissolved in ancient ocean water. The ancient oceans were enriched in CO2 (just like the atmosphere). Iron easily dissolves in CO2-rich water — this is easy to illustrate: drop and iron nail in a bottle of soda and it will dissolve completely in a few days! The early oceans must have been rich in iron (similar to salt is today)! Once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form. BIF deposits are extensive in many locations, occurring as deposits, hundreds to thousands of feet thick. During Precambrian time, BIF deposits probably extensively covered large parts of the global ocean basins. The BIFs we see today are only remnants of what were probably every extensive deposits. BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions.

Cells are divided into two main classes: prokaryotic cells and eukaryotic cells

Prokaryotic cells
include (bacteria and related organisms). Prokaryotes lack a "nucleus" (or nuclear envelope) and are generally smaller, structurally simpler, and less complex genomes (genetic material) than eukaryotic cells (Figure 4-15).

Eukaryotic cells
contain cytoplasmic organelles or a cytoskeleton, and contain a nucleus in which the genetic material is separated from the cytoplasm. Eukaryotes include fungi, plants, animals, and some unicellular organisms. Eukaryotic cells are capable of sexual reproduction (Figure 4-15).

*The same basic molecular processes are involved in the lives of both prokaryotes and eukaryotes, suggesting that all present-day cells are descendent from a single primordial ancestor.

is a theory that suggests organelles evolved in eukaryotic cells and occurred when one type of cell became incorporated into another type of cell, creating a symbiotic relationship to the benefit of both (such as chloroplasts in plants, and mitochondria in animals).

are "non-living" organic structures capable of genetic self replication that are not classified as cells and are neither unicellular nor multicellular organisms; viruses lack a metabolic system and are dependent on the host cells that they infect to reproduce.

A stromatolite is a mound of calcareous sediment built up of layers of lime-secreting cyanobacteria (blue-green bacteria, algae and other "simple" eukaryotic life forms) that trap sediment, creating layers accumulations (Figure 4-16). Stromatolites are found in Precambrian rocks and represent some of the earliest known fossils. Stromatolites are known from all geologic time periods and are still occurring today, with exceptional examples resembling ancient life forms still being formed today in places like Shark Bay, Australia (Figure 4-17).

Metazoans are multicellular animals that have cells that differentiated into tissues and organs and usually have a digestive cavity and nervous system. Metazoans appeared on earth in Late Precambrian time (late Proterozoic Era) consisting of cells in that with growth would differentiated into unique tissue or organs used for special purposes, such a locomotion, feeding, reproduction, respiration, sensing the environment, etc.

* Late Precambrian life forms have been discovered, but fossils from this period are scarce and poorly preserved because they did not contain "hard parts" (skeletons, teeth, etc.). Impression is sediments are dominantly "trace fossils" (tracks, trails, resting and feeding traces) and rare body impressions have been found.

A group of ancient fossil organisms called the Ediacaran fauna is one of the earliest known occurrence of multicellular animals is the fossil record. They were named for the Ediacaran Hills of South Australia where they were first discovered. Traces of Ediacaran fauna has been found worldwide in sedimentary rocks of about 635 to 541 million years (very late Precambrian age) and consisted of frond- and tube-shaped, soft-body organisms, mostly sessile life forms (sessile meaning attached to the seabed). Many of the fossils from this time period match "families"or "classes" of organisms still found on Earth today (including segmented worms, jellyfish, chordates, and other invertebrates).
Banded iron formation
Fig. 4-14.
A sample of Precambrian banded-iron formation (BIF) from Fremont County, Wyoming
Prokaryotes and Eucaryotes
Fig. 4-15.
Prokaryotes and Eukaryotes
Fig. 4-16.
Stromatolites, fossils of cyanobacteria "algae" mats, occur in rocks dating back to early Precambrian time.
Shark Bay, Australia
Fig. 4-17.
Stomatolites of Shark Bay, Australia, are modern living examples of stromatolites that resemble fossils from the Precambrian Eon.

The Paleozoic Era

The Paleozoic is the era of geologic time spanning about 541 to 248 million years ago. Paleozoic means "ancient life" (even though evidence of life extends well back in time to some of the earliest sedimentary rocks still preserved and discovered on Earth). The Paleozoic Era follows the Precambrian Eon and precedes the Mesozoic Era. The term "Paleozoic" also applies to rocks that formed and accumulated in that time period. Highlights include:

• Dominant large animals: Invertebrates dominate early; fish and amphibians appear in the middle Paleozoic, and reptiles appear even later.
• Continents were mostly clustered together throughout the Paleozoic Era.
• Large, warm, clear, shallow seas covered large portions of continents.
• Similar animal and plant species existed on each continent.
• Continents were mostly low with little relief. Few large mountain ranges existed on and around most continental landmasses (compared with today).
• The combined Appalachians and Atlas Mountains formed 350 to 400 MYA (between what was North America and Africa before the opening of the Atlantic Ocean basin).

Highlights of the Paleozoic Era

Evolution of plant and animal life as revealed in the Paleozoic fossil record.

Cambrian Period (540-485 million years): The Cambrian Period is the first geological period of the Paleozoic Era. At the beginning of the Cambrian Period the combination of tectonic forces and erosion of the landscape allowed shallow seas to gradually cover much of North America. Shallow seas covered most of what is now the Great Basin, Rocky Mountains, and Great Plains in the west, and the East Coast, Appalachian region and most of the Midwest. The seas withdrew at the end of Cambrian time, but what was left behind was a blanket of Cambrian sedimentary rocks, collectively called the "Sauk Sequence" (see Figures 4-18 and 4-19). The base of the Sauk Sequence rests on an eroded surface of ancient Precambrian-age (mostly metamorphic and igneous rocks) and is called the Great Unconformity. The Great Unconformity is exposed in many places throughout the western United States, and is particularly well known from exposures along the base sedimentary rocks of Cambrian age exposed in the Grand Canyon above the canyons Inner Gorge (see Figure 4-12). The Great Unconformity can be traced across most of North America wherever the base of the Cambrian-age Sauk Sequence is exposed.

Skeletal shell remains first appear in the Cambrian Period

The "Cambrian Explosion" refers to evidence in the fossil record which shows that all major phyla were established in the transition from latest Precambrian to to the Early Cambrian Period (about 700 to 541 million years ago) (Figure 4-20). The cause of this radiation from earlier metazoan life forms is uncertain, but it may have been driven by global climate changes (hot to cold cycles) and the establishment of micro habitats (niches) which allowed species to evolve separately from common ancestors. In this time, chitonous and calcareous shells and exoskeletons appear. Many Cambrian-age organisms have eyes, legs (pods), spinal chords, segmented body plans, and other unique body parts and characteristics. Representatives of all phyla from the Cambrian Explosion still exist in the world today. Sedimentary rocks from Cambrian Period are typically rich in evidence of life activity. They preserve an abundance of bioturbation features (trace fossils) even if the life forms that created them are not preserved (an example of bioturbation is shown in Figure 4-21).

dominate the fossil record in the early Paleozoic Era. An invertebrate is an animal lacking a "backbone," such as an arthropod, mollusk, annelid worm, coelenterate, echinoderm, and many others. The classification of invertebrates constitute a "fabricated" division of the animal kingdom, comprising about 95 percent of animal species and about 30 different "known" phyla.

By the end of the Cambrian Period several groups of invertebrates were well established in shallow marine environments, perhaps most notably were trilobites, brachiopods, crinoids, bryozoans, sponges, and gastropods (snails) are locally common fossils preserved in Cambrian sedimentary rocks (Figures 4-22 and 4-23). At the end of the Cambrian Period, sea level fell and a long period of exposure and erosion occurred throughout North America and the other continents worldwide.

Ordovician Period (485-444 million years): The seas once again flooded across much of North America through much of the Ordovician Period. Deposition of sediments during this marine transgression resulted in the Tippecanoe Sequence which rests unconformably on top of the Sauk Sequence. However, when the seas returned (millions of years later) life in the oceans had significantly changed. Trilobites no longer dominated the fossil record, but other life forms began to proliferate in the shallow marine environment. Corals (unrelated to modern varieties), crinoids, cephalopods, brachiopods, bryozoans and other fossils with calcareous skeletons dominate the fossil record (because of their ability to live, proliferate, and upon death, survived burial and fossilization processes). Rare early examples of fish and land plants have been discovered in Ordovician age sedimentary rocks. Ordovician age sedimentary rocks crop out in many locations around the country, but they are perhaps best known from the Cincinnati Arch region (Ohio, Kentucky and Indiana) where they have a great abundance of well preserved fossils of that time period (Figures 4-24 to 4-26).

Silurian Period (444-419 million years):
Few rocks of Silurian age are preserved in North America's fossil record (they are not preserved or are not exposed). Some sedimentary rocks of Silurian age are preserved in upstate New York, around the Cincinnati Arch, and around the margins of the Michigan Basin are notable exceptions). The fossil record shows that the Silurian world was dominated by marine invertebrates, but the first fish-like chordates appear. Simple and primitive forms of land plants began to flourish and diversify during Silurian time. Plants on land became a food source allowing the first animals to emerge onto dry land (including insects and arthropods, including scorpions)(Figure 4-27).
Great Unconformity between Precambrian and Cambrian rocks in Wyoming Bright Angel Shale is a rock formation within the Sauk Sequence exposed in the Grand Canyon
Fig. 4-18. The Great Unconformity is an erosional boundary at the base of the Sauk Sequence throughout much North America. This view is in Wind River Canyon, Wyoming. Fig. 4-19. The fossiliferous Bright Angel Shale of Cambrian age is one of the rock formations of the Sauk Sequence exposed throughout the Grand Canyon region.
Cambrian Explosion illustrated Tracks and trails in Cambrian sediments, Grand Canyon
Fig. 4-20. The "Cambrian Explosion" refers to the diversification of life forms that began near the end of the Precambrian Eon. Fig. 4-21. Invertebrate tracks and trails appear in abundance in Cambrian-age sediments (Tapeates Sandstone) in the lower Grand Canyon in Arizona.
Trilobite Cambrian fossils
Fig. 4-22. Trilobites are common shelled fossils in sedimentary rocks from the Cambrian Period. Fig. 4-23. Cambrian fossils: trilobites, brachiopods, gastropods, and other invertebrates
Ordovician-age strata exposed in the Cincinnati Arch region, this view is near Maysville, Kentucky Brachiopods on a block of  limestone from the Cincinnati Arch region
Fig. 4-24. Fossil-rich sedimentary rocks of the Tippecanoe Sequence are perhaps most famous from the Cincinnati Arch region. Fig. 4-25. Fossil brachiopods on a layer of Ordovician limestone from Brookville Indiana (on the Cincinnati Arch).
Ordovician fossils Silurian fossils
Fig. 4-26. Common fossils of the Ordovician Period Fig. 4-27. Common and unusual fossils of the Silurian Period.
Devonian Period (419 to 359 million years): On land, free-sporing vascular plants adapted and spread across the landscape, allowing the first forests to cover the continents. By the middle of the Devonian several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Terrestrial arthropods began to flourish. In the marine world, early ray-finned and lobe-finned-bony fish, and sharks appear in the fossil record. The first ammonoid mollusks appeared. Holdover families of marine invertebrates from earlier times persisted, including trilobites, brachiopods, cephalopods, and reef-forming corals remained common (Figure 4-28).

The current oil and gas boom in the United States is largely because of the "fracking technologies" used to extract petroleum from the "tight" black shales associated with organic-rich sediments deposited in inland seas of Devonian and Mississippian age. These black shales underlie large regions of the Appalachians, Midcontinent, and northern Rocky Mountains regions in the United States. These deposits are part of the Kaskaskia Sequence (see Figure 4-11).

Mississippian and Pennsylvanian Periods: These periods are also collectively called the Carboniferous Period (359 to 299 million years ago) because great quantities of coal are preserved in rocks of these ages around the world. Great coastal forests and swamplands covered large regions of North America and parts of Europe. Amphibians became the dominant land vertebrates in Mississippian time. Descendent from amphibian ancestors, reptiles evolved and became the first terrestrial vertebrates. With the abundance of vegetation on land, arthropods flourish, including species of insects that are much larger than any found on Earth today (Figure 4-29). Toward the end of the Carboniferous Period, glaciation cycles caused repetitious rise and fall in sea levels. The Appalachian and Ouachita Mountain systems also began to develop as ancient forms of the the continents of Africa, South America, and North America began to collide with one-another.

Sedimentary rocks of Mississippian age in North America are dominated by marine limestones exposed throughout the mid continent, and Appalachian and Rocky Mountain regions. Mississippian rocks throughout these regions are host to many cavern systems (such as Mammoth Cave in Kentucky). Mississippian rocks are part of the Kaskaskia Sequence. The Redwall Limestone in the Grand Canyon region is part of the Kaskaskia Sequence (Figure 4-30).

The unconformable boundary between the Kaskaskia Sequence and the overlying Absaroka Sequence is the boundary between sedimentary rocks Mississippian and Pennsylvanian age (see Figure 4-11).The Absaroka Sequence includes sediments deposited during Pennsylvanian, Permian and Triassic Periods (see below). It was during the Pennsylvanian Period that the world's continents assembled together to form the supercontinent of Pangaea (discussed in Chapter 5). Pennsylvanian rocks are perhaps best know for their coal-bearing basin in the Appalachian and Mid continent region (Figures 4-31 and 4-32).

Permian Period (299 to 252 million years):
The last period of the Paleozoic Era was a time of colossal changes. All the continents of the world had combined to form the supercontinent of Pangaea. In the fossil record, a group of tetrapods (four legged animals with backbones or spinal columns) called amniotes appeared, capable of living on dry land and producing terrestrially adapted eggs. All modern land species are descendant from a common ancestral group of amniotes. During the Permian, the expansive forests that existed during the Carboniferous disappeared, and vast desert regions spread over the North American continental interior. Reptiles adapted and flourished in the more arid conditions. Great reef tracks developed in the Texas and New Mexico region (Figures 4-33 and 4-34). The end of the Permian Period (and Paleozoic Era) is marked by the greatest mass extinction in Earth history.
Devonian brachiopods Common Mississippian invertebrate fossils
Fig. 4-28. Devonian brachiopods and common fossils from Kentucky Fig. 4-29. Mississippian invertebrate fossils from Pennsylvania
Redwall Limestone of Mississippian Age in the Grand Canyon Pennsylvanian coal basins in eastern North America
Fig. 4-30. The massive Redwall Limestone of Marble Canyon and the Grand Canyon is part of the Kaskaskia sequence. Fig. 4-31. Pennsylvanian coal-bearing basins in the eastern United States are part of the Absaroka Sequence.
Pennsylvanian forest Capitan Reef map
Fig. 4-32. Reconstruction of a swamp forest of the Pennsylvanian Period Fig. 4-33. Map of the Permian Reef complex in Texas and New Mexico
Guadelupe Peak Dimetrodon
Fig. 4-34. Permian reef track exposed in Guadalupe National Park, Texas Fig. 4-35. Dimetrodon, a mammal-like reptile from the Permian Period on display at the Chicago Field Museum

Evidence of large mass extinctions preserved in the fossil record

Extinction is the state or process of a species, family, or larger group being or becoming extinct (ceasing to exist).

* Extensive studies of microfossils in deep well cores extracted from around the world show that the appearance and disappearance (extinction) of species has happened continuously through geologic time, but the rate was not constant.

*As climates and landscapes changed, new species evolved to fit ever changing ecological niches; older species fade away.

A mass extinction is an episode or event in earth history where large numbers of species vanish from the fossil record. The causes of mass extinctions are debated, but some are linked to possible global climate changes associated with asteroid impacts, massive volcanism episodes, onset of ice ages, or a combination of effects that affected environments globally.

*Current estimates are about 90 percent of all species that have ever lived on Earth are now extinct. However, the rate of extinction has not been constant. At least five times in the last 500 million years, as much as about 50 to 90 percent of all species on Earth have disappeared in very short periods of geologic time (Figure 4-36).

The Permian/Triassic (P/T) Boundary Extinctionthe greatest of all mass extinctions

* The greatest mass extinction event occur at the end of the Permian Period (about 252 million years ago). Most families of organisms that existed in the Paleozoic Era vanished at the end of the Permian Period. A 2008 report published by the Royal Society of London provided estimates that as much as 96 percent of marine species and about 70 percent of terrestrial vertebrates that existed in Late Permian time vanished during the end of the Permian extinction event. This occurred during the assembly and breakup of the supercontinent Pangaea. Great amounts of volcanism are known from that period associated with the rifting and opening of the Atlantic Ocean basin. However, other causes, such as glaciation, ocean circulation collapse, or possibly asteroid and comet impacts, extraterrestrial radiation events, and others have been pondered. The problem with studying mass extinctions like the one associated with the Permian-Triassic boundary is that the world has significantly changed since that time. Bedrock under all the world's ocean basin have be subducted back into the Earth's mantle or heavily altered by mountain-building processes, and much of the sedimentary record associated with exposed land were stripped away by erosion before sediments began to be deposited and preserved in the fossil record for the Triassic Period. Whatever the cause, it took many millions of years after the P/T extinction event (or events) for the biodiversity of the planet to return to levels that existed in the Late Paleozoic Era, and when this biodiversity returned, the world was host to completely different host of species and communities, many replacing or occupying the same life habits (niches) and environments occupied by organisms that disappeared before the P/T extinction.

*Great extinction events" created opportunity for new life-forms to emerge. For instance, dinosaurs and many other life forms appeared only after the mass extinction at the end of the Permian Period (about 252 million years ago). The same is true for when mammals replaced dinosaurs when they went extinct at the end of the Cretaceous Period.

* Perhaps the most studied extinction event has been the Cretaceous-Tertiary Boundary where strong evidence suggests at least one asteroid collided with earth in the vicinity of the Yucatan Peninsula in Mexico (about 66 million years ago)(Figure 4-37, see Figure 4-56 below). This extinction killed off the dinosaurs and many other families of organism that lived in the oceans and on land. However, the catastrophe made room for mammals and other groups of organisms to rapidly diversify and evolve. Unlike the P/T extinction which has limited exposure around the world from 252 million years ago, there are many locations world wide and on all continents and within sediments extracted from the sea floor that reveal information about what happened at the end of the Cretaceous Period about 66 to 65 million years ago (discussed below).
Mass extinctions
Fig. 4-36.
Great mass extinction events in the fossil record
Asteroid impacting earth
Fig. 4-37.
An massive asteroid impact can ruin your day (and your species, and many others).
Gary Larson cartoon about the real reason dinosaurs when extinctFig. 4-38. A classic Far Side cartoon by Gary Larson.

Are Humans causing the Sixth Great Mass Extinction?

Many scientists believe evidence suggests that a sixth mass extinction is currently under way. Global climate change and the expansion of human activity are largely to blame. Some estimates suggest that human activities such as land clearing (for agriculture), pollution, mining, urban development, and over fishing may drive more than half of the world's marine and land species to extinction within the next century. This extinction event perhaps began during the end of the last ice age when humans spread around the globe and their populations expanded. Many species of large land animals and birds have vanished in the past 10,000 years, but the rate of changes has drastically increased in the past 100 years with the tremendous expansion in the global human population.

Mesozoic Era

Mesozoic Era—the era between the Paleozoic and Cenozoic eras, comprising the Triassic, Jurassic, and Cretaceous periods; commonly referred to as the "Age of Reptiles."
• Dominant large animals: Reptiles and dinosaurs; birds and mammals appear
• Increased mountain building and with that, lots of sediment from erosion.
• The ancient supercontinent, Pangaea, begins to breakup at about 200 million years ago (Pangaea is discussed with the chapter on Plate Tectonics).
• The continents began pulling apart. The isolation of species and communities created more diversity in plant and animal species through divergent evolution.
• The "ancestral" Rocky Mountains formed between 120 to 66 million years ago.
Triassic Period (252 to 201 million years): Following the great extinction event at the end of the Permian Period, life on Earth gradually reestablished itself both on land and in the oceans. Scleractinians (modern corals) replaced earlier forms as dominant reef-forming organisms. On land, reptilian therapsids (an order related to the distant ancestors of mammals) and archosaurs (ancestors of dinosaurs and modern crocodillians) became the dominant vertebrates. New groups evolved in the middle to late Triassic Period including the first dinosaurs, mammals, and flying vertebrates (pterosaurs) but these families did not flourish until another global extinction event at the close of Triassic time. Current thought is that ancestral forms of both mammals and dinosaurs first appear in the fossil record in Late Triassic time, about 200 million years ago.

During the middle Triassic, the supercontinent of Pangaea began to rift apart into separate landmasses, Laurasia to the north and Gondwana to the south. With the breakup of Pangaea, terrestrial climates gradually changed from being mostly hot and dry to more humid condition. Another mass extinction in the fossil record marks the end of the Triassic Period.

The Chugwater Formation in Wyoming is a classic example of the Zuni Sequence Triassic "red beds" are "oxidized iron-rich" sedimentary deposits that occur extensively throughout western North America with notable occurrences in Texas, through the Colorado Plateau, Rocky Mountain region and the Newark and Connecticut Basins on the East Coast. These are associated with the Absaroka Sequence that accumulated while Pangaea was still assembled and dry climate conditions prevailed across most of North America.
Fig. 15-41. Red beds of the Chugwater Group of formations exposed near Lander, Wyoming.
Tracks at Dinosaur State Park
Fig. 4-39.
Dinosaur track in Late Triassic sedimentary rocks, Dinosaur State Park, Connecticut
petrified wood
Fig. 4-40.
Fossil wood in Triassic sedimentary rocks.
Petrified Forest National Park, Arizona
Desmatosaur from West Texas
Fig. 4-42.
Desmatosuchus, an archosaur from the Triassic Period found in West Texas
Fig. 4-43.
Placerias, a large mammal-like reptile from the Triassic Period. Petrified Forest National Park, Arizona
Jurassic Period (201 to 145 million years): The cause of the mass extinction at the end of Triassic is still unclear, but evidence shows that it was rapid and massive amounts of volcanism was taking place with the breakup of Pangaea (created by the opening of the Atlantic Ocean basin as North and South America gradually split away from the African and European continents).

With other life forms out of the way, dinosaurs adapted and diversified into a wide variety of groups. Although pterosaurs were the dominant flying vertebrates during the Jurassic Periods, the first birds appeared having evolved from a branch of theropod dinosaurs. Rare small mammals occur in the fossil record during the Jurassic Period, but remained insignificant compared to the dinosaurs that dominated the landscape. Marine reptiles including ichthyosaurs and plesiosaurs dominated the oceans.

Waterpocket Monocline in Capitol Reef National Park, Utah Sedimentary rocks of the Zuni Sequence are well preserved and throughout the Colorado Plateau region. During the late Jurassic Period a great coastal sand desert covered much of the western part of the continental United States. This ancient sand desert would rival the large deserts of the Sahara or Arabian Peninsula that exist today. Through time, the desert conditions gave way to more humid coastal conditions with river systems and coastal swamplands (home to a variety of dinosaurs of the Jurassic and following Cretaceous Periods). The massive white cliffs of Zion National Park preserve evidence of this great sand desert in the western United States.
Fig. 15-46. Massive sandstone cliffs of the Jurassic Navajo Sandstone are well exposed in Zion National Park, Utah
dinosaur tracks Dino bones
Fig. 4-44. Dinosaur tracks
in Jurassic-age rocks near Tuba City, Arizona
Fig. 4-45. Dinosaur bones preserved in ancient river bed sediments, Dinosaur National Monument, Utah
stegasaurus Allosaurus
Fig. 4-47. Reconstruction of Stegasaurus, a Jurassic-age vegetarian dinosaur with unusual spinal plates. Dinosaur National Monument, Utah Fig. 4-48. Skeleton of Allosaurus, a massive carnivorous dinosaur from the Jurassic Period. Dinosaur National Monument, Utah
Cretaceous Period (145 to 66 million years): During the Cretaceous Period the Earth was relatively warm compared to the world today. There were no glaciers on the planet and sea level was as much as 200 feet higher that today. Fossils of warm-water organisms are found in rocks that are arctic regions today. The dinosaurs that survived into the Cretaceous Period diversified and evolved into many unusual forms. Large marine reptiles called Mosasaurs were the dominant organism in the ocean. Sediments deposited in shallow sea flooding onto the continents had an abundance of ammonites—squid-like organisms that had calcareous shells similar to modern nautilus species. Cretaceous gets its name for "Creta"—latin for the word chalk. The shallow warm seas of the Cretaceous Period were locations where the calcareous skeletal remains of planktonic organisms called coccoliths accumulated, forming great accumulations of chalk, such as exposed in the Great Cliffs of Dover, England. In many places in the equatorial realm oyster-like organism called rudists formed great reefs. Flowering plants also first appear in the fossil record, birds existed in Cretaceous time but were insignificant compared to flying non-avian pterosaurs. Small mammals first appear in abundance in the Cretaceous Period, but they were still generally insignificant compared with reptiles and dinosaurs that existed around them.
Waterpocket Monocline in Capitol Reef National Park, Utah Sedimentary rocks of the Zuni Sequence are well preserved and throughout the Colorado Plateau region. During the late Jurassic Period a great coastal sand desert covered much of the western part of the continental United States. As Pangaea broke apart, a great volcanic arc system began to form along the western margin of North America (called the Cordilleran Range). At the same time shallow seaways began to expand across the central North America Seaways eventually merged forming the Western Interior Seaway. This ancient seaway extended from Texas to Alaska by Cretaceous time and covered what are now the Great Plains and Rocky Mountain regions of the United States and Canada (Fig. 4-54)
Fig. 4-51. Jurassic- and Cretaceous-age rock formations in Capitol Reef National Park, Utah (encompassing the Zuni Sequence).
Ammonites of the Late Cretaceous
Fig. 4-49.
Late Cretaceous ammonites of the Western Interior Seaway (South Dakota)
Fig. 4-50.
Triceratops, a Late Cretaceous herbivore dinosaur. Chicago Field Museum
Fig. 4-52.
Parasaurolophus- a Late Cretaceous dinosaur with a crested skull.
Dinosaur Sue
Fig. 4-53.
Dinosaur "Sue"
Tyrannosaurus rex
, a large carnivorous dinosaur of the Late Cretaceous Period, Chicago Field Museum

The Cretaceous-Tertiary Boundary (or K/T Boundary) Extinction

The Cretaceous-Tertiary boundary is one of the most investigated mass extinction events. It is has been well investigated partly because it is the youngest of the large extinctions that totally changed the nature of life on Earth. It is also well exposed in many locations around the world and has been studied extensively in core samples from deep-sea drilling projects.

* The "K/T event" is believe to have been caused by a massive asteroid impact in the Yucatan region of Mexico, although other possible sites of large impacts are being considered. What is known is that all species of dinosaurs on land, and marine reptiles and ammonites in the marine realm vanished.

* The asteroid impact and following shock waves, monstrous tsunamis, firestorms, ash and gas clouds, and following global winter-like condition caused ecosystem collapse in both the food web of the oceans and on land.

* All species that exist today are descendent of the few species that survived the global catastrophe... small mammals, birds, invertebrates, reptiles, amphibians, fish and other surviving groups had evolutionary advantages that allowed them to survive. With the dinosaurs, pterosaurs, large swimming reptiles and other large animals of the Cretaceous Period out of the way, the surviving species proliferated and moved into empty and new niches that allowed them to prosper and diversify.

The K-T boundary occurred near the end of the Zuni Sequence Cycle when sea level fell around the globe (see Figure 4-11). In the following Cenozoic Era many changes continued to occur including the uplift of the Rocky Mountain region and the withdrawal and disappearance of shallow inland seas and great lakes that previously flooded the Western Interior region.
Western Interior Seaway
Fig. 4-54. Western Interior Seaway and locations of asteroid impact sites around North America
Extinction curves
Fig. 4-55. Appearance, expansion, decline and extinction of Late Cretaceous ammonite genus Baculites. Diagram shows changes in sea level and abundance of marine species.
K/T Boundary in Badlands National Park The person is pointing toward a zone of disrupted bedding that corresponds to the zone where many terrestrial and marine species vanished from the fossil record at the end of the Cretaceous Period.
Fig. 4-56. Disrupted zone of sediments along the mass extinction horizon associated in marine sediments located along the Cretaceous-Tertiary Boundary in Badlands National Park, South Dakota.

Cenozoic Era

The Cenozoic is commonly referred to as the "Age of Mammals." The Cenozoic Era began with the mass extinction event associated with the K/T Boundary (discussed above). Highlights of the Cenozoic Era include:
• Dominant large animals: Mammals (gradually replacing the niches held by dinosaurs wiped out by the K/T extinction).
• Mountain building continues, especially around the Pacific Ocean; Himalayas begin to form 45 million years ago. The Rocky Mountains and Cordilleran Ranges in western North America continue to form.
• Lots of erosion of existing mountains fed sediments to coastal plains and ocean margin basins.
• The youngest Tejas Sequence began to accumulated in the early Cenozoic Era and continues to the present day, forming the Atlantic and Gulf Coast regions.

The Cenozoic Era is generally divided into two (or three) periods:
Time Range


Tertiary Paleogene 66 million to 23 million years ago
Neogene 23 million to 2.6 million years ago
Quaternary 2.6 million years ago to the present

The older name "Tertiary Period" in now subdivided into two periods: Paleogene and Neogene.
The Cenozoic's periods are also subdivided into epochs.
Time Range
Paleocene Epoch
The mass extinction at the end of the Cretaceous Period left many of the niches filled by dinosaurs and large swimming reptiles empty. Mammals with placental-type live birth appear. Shallow seas of the Cretaceous period withdrew or were gradually replaced by lakes. In North America, the Rocky Mountains began to rise. American Museum of Natural History 66 to 56 million
Eocene Epoch
"Modern" forms of mammals appear and diversify in the fossil record during the Eocene Epoch. The Eocene was a warm period with an expanded tropical realm. The end of the Eocene period is marked by a mass extinction that may have involved asteroid collisions in Siberia and in the vicinity of Chesapeake Bay. American Museum of Natural History 56 to 33.9 million
Oligocene Epoch
The Oligocene was a time of transition when older life forms were replace with life forms that dominate the world today. The warmer, more tropical environments of the Eocene Epoch gave way to dryer landscapes dominated by grasslands, whereas broadleaf forests became more restricted to the equatorial realm. American Museum of Natural History 33.9 to 23.0 million

Bryce Canyon National Park Eocene sedimentary rocks in the Wind River Basin, Wyoming Pinnacles at Badlands National Park Cliffs at the Delmar Dog Beach, California
Fig. 4-57. Eocene lake deposits crop out as the Chadron Formation in Bryce Canyon National Park, Utah Fig. 4-58. Eocene-age sediments fill many of the basins throughout the Wyoming region. Fig. 4-59. Eocene through Miocene sedimentary rocks crop out in Badlands National Park, South Dakota. Fig. 4-60. Eocene and younger rock formations exposed at the Del Mar Dog Beach

Time Range
Miocene Epoch
Animals and plants of the Miocene Epoch are approaching modern life forms in diversity. Earth was warmer with expanded tropical realms compared to the modern world. The Himalayan Mountains begin to rise as the Indian continental landmass began to collide with Asia. American Museum of Natural History 23 to 5.3 million
Pliocene Epoch
Global climates cooled and became dryer with the onset of glaciation cycles. Most families of animals and plants found in the world had ancestral forms during the Pliocene including humans. Greenland's ice sheet starts to form. South America and North America became linked at the Isthmus of Panama, allowing the cross migration of many species between continents; but also shutting off the migration of species from the Atlantic to the Pacific oceans. The same kind of interactions took place when Africa collided with Europe. American Museum of Natural History 5.3 to 2.6

Calvert Cliffs, Maryland Sea cliffs at Wilder Ranch Zabriskie Point in Death Valley Anza Borrego State Park, California
Fig. 4-61. Miocene-age sedimentary rocks exposed along Chesapeake Bay at Calvert Cliffs, Maryland. Fig. 4-62. Pliocene-age sedimentary rocks exposed in sea cliffs near Santa Cruz at Wilder Ranch State Park Fig. 4-63. Pliocene-age sedimentary basin fill rocks exposed in Death Valley National Park, California Fig. 4-64. Neogene-age sedimentary rocks (Miocene to Pleistocene) crop out in Anza Borrego State Park, California

Time Range
Pleistocene Epoch
Time period of major ice ages where continental glaciation advance and retreated; glaciers covering much of northern North America and Europe during cold periods. Modern human species appears in the fossil record. Many species of large land mammals went extinct at the end of the Pleistocene Epoch. La Brea Tar Pits, Los Angeles
(UC Berkeley Museum of Paleontology website)
2.6 million to 11,000 years
Holocene Epoch
End of the Wisconsinian ice age to the present. Includes a 400 foot-rise in sea level and the rise of human civilizations. Humans rise to become the dominant species on Earth. American Museum of Natural History
11,500 years
to present

Thornton Beach State Park, California Big Bone Lick State Park, Kentucky Glacial till and outwash Glacial moraine
Fig. 4-65. A thick sequence of coastal and nearshore deposits of Pleistocene age are exposed in the seacliffs of Thornton State Beach south of San Francisco, California. Fig. 4-66. Big Bone Lick State Park, a source of Pleistocene-age megafauna fossils in northern Kentucky is the "birthplace" of North American vertebrate paleontology. Fig. 4-67. Glacial till and outwash exposed at Caumsett State Park, Long Island, New York. Long Island is underlain by unconsolidated Pleistocene-age glacial deposits. Fig. 4-68. Glacial moraine at Montauk Point on Long Island, New York is part of the southern terminal moraine of the Wisconsin glaciation at the end of the Pleistocene Epoch.


Evolution of humans and the rise of modern civilization

Human Evolution Timeline Interactive (Smithsonian Institution website)

Some 15 to 20 different species of "early humans" (humanoids) are currently recognized. However, not all scientists studying human evolution agree how these species are related or how they died out. The majority of early human species left no living descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.

Humans are included in the family of primates (which include modern monkeys, apes, and humans). Primates are descendent from an earlier monkey-like group called prosimians that appear in the fossil record in Eocene to Oligocene time. Primate species appear in local abundance during the Miocene Epoch (between 23 to 5.7 million years ago).

Fossils of earliest recorded human-like ancestors come from sediments of 6-7 million years ago in western Africa; the species had chimpanzee-sized brains and were able to walk upright on two legs.

Fossils of 6 to 3 million years recovered in eastern Africa (Ethiopia) show species with ape-like features that walked upright and lived in forested environments.

By 4 million years ago, early human species lived in near open areas in forested environments; bone structures show they were able to walk upright (bipedal) and still climb trees.

Famous "Lucy" skeleton (about 3 million years show species had ape-like proportions of face, braincase, strong arms [for climbing], but walked upright on arched feet.

The oldest stone tools have been found in sediments deposited 2.6 million years ago. Homo habilis (2.4-1.4 million years ago) species thought to represent the first stone toolmaker.

Multiple species of the genus Homo have been discovered from the time period of about 2 to 1 million years ago; some sharing the same environments.

Human use of fire began about 800,000 years ago-for warmth, cooking, socializing, and safety from predators.

Homo erectus is known from ages about 1.89 million to 143,000 years ago, and fossils have been recovered from places as distant as eastern to southern Africa; western Asia (Republic of Georgia), China and Indonesia. The species used fire and ate meat, and took care of old and weak members of their clans.

A rapid increase in human brain size took place from 800,000 to 200,000 years ago, giving humans better survival skills the ability to adapt to changing environmental conditions (such as the onset of ice ages and interglacial warm and dry periods).

Our species, Homo sapiens, first appear in the fossil record about 200,000 years ago in Africa, but spread out into Europe and Asia by at 100,000 years ago. We now inhabit land everywhere on the planet and we are the sole surviving species of a once diverse group of ancestral family of human-like species. As human populations spread around the world, populations became isolated and developed characteristics associated with major races of humans that exist throughout the world today.

Although new discoveries are constantly being made, current though is that humans first came to Australia within the past 60,000 years and to the Americas within the past 30,000 years. Use of agriculture methods and the rise of the first civilizations developed within the past 12,000 years. As the "human species" has expanded, diversified, adapted, and populated. In contrast, many other species have already gone extinct due to human predation, isolation, and habitat destruction. Learn more about the ongoing Holocene Extinction (Wikipedia).
Routes of human migration around the world San Francisco Human evolution cartoon
Fig. 4-69. Routes of human evolution and migration around the world beginning in late Pleistocene time. Fig. 4-70. Within the past century, human activity has completely changed large regions of the planet's physical environment. Fig. 4-71. A famous cartoon depicting human evolution. Many people agree that humans are greatly altering our global environment with potentially catastrophic consequences without drastic changes in how we use our planet's limited resources. We need to learn how to manage and sustain our world's natural resources.

Where are rocks of different ages exposed in the United States?

Rocks of all geologic ages are exposed in different parts of the United States. Figure 4-72 is a geologic map of the conterminous United States, and Figure 4-73 is the geologic map legend that shows colors associated with regions where rocks of different ages are exposed at the surface. Earth scientists use geologic maps like these to locate areas where they may go study the fossil record where rocks of different ages (and the fossils they contain) occur. Each region of the country has unique fossil record. The best place to start an investigation is to visit museums, universities, and government organizations that host fossil and rock collections in the vicinity where rocks are exposed. Learn more about the regional geology and natural resources of the United States on this link: Regional Geology of the United States.
Geologic Map of the United States (48 states) Geologic age units associated with the geologic map of the United States.
Fig. 4-72. Geologic map of the United States. Fig. 4-73. Geologic ages of bedrock on the geologic map.
Chapter 4 quiz questions 7/17/2017