Introduction to Geology

Introduction to Physical Geology

Chapter 16 - Earth History

This chapter is a brief summary of the evolution of life on Earth through time.

Historical geology
is the science that examines concepts of evolution and geologic time as preserved in the fossil record. Historical geology is relevant to all other sciences that involve studies of the physical environment!. This chapter is a very brief summary of the history of life and discussions about some major geologic events shaping planet Earth. Figure 16-1 highlights many of the key geological and biological events that occurred, impacting life, leading to the present.

Earth formed from the accumulation of dust, gases, asteroids, and small planetesimal in the stellar nebula (as discussed in Chapter 1). During this early period in 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 the oceans were rich in organic compounds, essential for the development of evolution and life. The oldest sedimentary rocks on Earth preserve evidence of 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's 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.

Click on thumbnail images for a larger view.
Geologic time scale with highlights in evolution and events in Earth History
Fig. 16-1. Geologic Time Scale
with highlights in evolution and events in Earth history.

Key Developments In Understanding the Origin Of Life On Earth

Carl Linnaeus was a Swedish botanist, physician, and zoologist (lived 1707-1778), who laid the foundations for the modern scheme of binomial nomenclature. Lineaus is considered a founder of modern taxonomy and ecology (Figure 16-2). For instance, humans are called Homo sapiens in binomial nomenclature.

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 (commonly referred to as the evolutionary tree of life). In the past three centuries, millions of species have been identified and classified, but the lineages of different species are constantly being revised as new information becomes available.

Charles Darwin (1809-1882), a scientist/explorer, is credited with presenting the first published work dedicated to natural selection in his book entitled Origin of Species (published in 1859) (Figure 16-3). Darwin's theory on natural selection is now considered among be the main processes that brings about biological evolution. Darwin's book is 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. 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 (1822-1884) was 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 theories (Figure 16-4).

Over the past two centuries, many scientific discoveries and technological innovations have advanced our knowledge of biochemistry, cell structure and processes, and genetic evolution. In 1951, James Watson and Francis Crick discovered and reported the double helical structure of the DNA molecule (Figure 16-5). 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 and medical research.
Carl Linnaeus
Fig. 16-2. Carl Linnaeus
(1707-1778) is considered founder of modern taxonomy and ecology.

Mendel genetic variation
Fig. 16-4.
Statistical genetic variation illustrated by Mendel's research (applied to cats).
Charles Darwin (Smithsonian Institution drawing)
Fig. 16-3.
Explorer, Charles Darwin (1809–1882) published his theory of natural selection in a book titled Origin of Species in 1859.

DNA illustrated
Fig. 16-5. 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 an essential component of the knowledge supporting the current geologic time scale.
• Evolution supports an old earth (~4.56 billion years).
• The different time periods represented on the geologic time scale have uniquely defined populations of fossil species representative of those ages.

Natural selection (Darwinism): The strongest and best adapted organisms survive and produce offspring.

Divergent Evolution

• Populations that are separated environmentally can develop different features based upon an adaptation to their environment.
• One group of organisms can radiate (or diversify) 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.

Examples of Convergent Evolution:
• Both fish and marine mammals developed streamline bodies to swim efficiently.
• Marine mammals developed fur/thick blubber to protect them from cold waters.
Modern marine mammals share many of the same physical traits and life habits that ancient marine reptiles had before their disappearance from a mass-extinction event at the end of the Cretaceous Period (about 66 million years ago; discussed below).
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
Fig. 16-6. Evolution
and classification of living things (illustrated) based of shared or identifying characteristics.

Cat classification Fig. 16-8. Classification (taxonomy) of a house cat.

Classification of humans
Fig. 16-7. Human taxonomy
illustrated within the hierarchical classification of living things (kingdom, phylum, class, order, family, genus, and species).

Fig. 16-9. 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 changing situations (and evolve), or they face die offs or extinction.
EvolutionFig. 16-10. 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 (see Figure 16-1).

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 sedimentary deposits. Few things living today will survive to become fossils (see table to right on how fossils form).

The term fossil record is 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.

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

After an organism dies, its remains must:

survive being eaten (at least partially eaten).

2. must 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 to be useful.

Sedimentary Sequences Preserve the Fossil Record

The history of the evolution of life is partly preserved in sedimentary rocks found around the world. The ancient history of a species is also preserved in the DNA of living organisms. Although the fossil record is extensive, there are many gaps in the fossil record where sediments of different ages have not been preserved in many regions, and much has been erased as ocean crust is destroyed in the processes involving plate tectonics (discussed in Chapter 5). Also, ancient sedimentary deposits on continental are destroyed by erosion. Despite these issues, sedimentary deposits representing all geologic ages are preserved and exposed in different places around the world. The fossil record is best preserved and represented by sedimentary deposits associated with ancient shallow marine and coastal environmental settings preserved and exposed in continental settings.

Transgressions and Regressions of Ancient Shallow Seas

Figure 16-13 shows how sea level rose and fell through the ages across North America. A transgression occurs when sea levels rise and shallow seas advance onto the margins of a continent. When sea level falls, the seas retreat and land is exposed—a process called a regression. For much of the last billion years shallow seaways transgressed onto the North American continent. Many minor transgressions and regressions also occurred, and shallow seas intermittently covered large portions of the continent. When sea level rose, sediments were deposited blanketing large regions of the continents. 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 sedimentary sequences that preserve the fossil record.

Each of the sequences rests on the eroded surface on top of a previous sequence represented by a major regional unconformity (also called a sequence boundary, as illustrated in Figure 16-13). Six major sequences (with their underlying unconformities) are recognized throughout North America, with equivalent sequences and sequence boundaries on other continents. Each sequence represents a major marine advance (a transgressions) of shallow seas , replacing coastal plains and terrestrial environment. The major unconformities represent periods of regression (when the seas withdrew and coastal and terrestrial environmental setting replaced shallow marine environments).

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 these deposits in many regions, but portions of each sequence are still still preserved and exposed in different parts of the continents. For example, part of four of the great sequences are exposed in the Grand Canyon (Figure 16-14). Sedimentary rocks bearing fossils from all geologic-time periods have been identified in locations scattered around the world.

Tectonic and sedimentatiion cycles of North America
Fig. 16-13.
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).
Sequences of sedimentary rocks and unconformities exposed in the Grand Canyon
Fig. 16-14.
Paleozoic-age sedimentary sequences exposed in the Grand Canyon, Arizona include portions of the Sauk, Tippecanoe, Kaskaskia, and Absaroka sequences (shown in Figure 16-13). Each sequence is bounded (above and below) by unconformities. The oldest sequence 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 exposed in the deepest parts of the Grand Canyon.

Ecological Succession: How Species and Ecosystem Populations Change Over Time

Studies of the fossil record show that extinctions in Earth's history vary from a disappearance of a species (an extinction), to the disappearance of entire lineages and populations within regional communities or globally (a mass extinction). Paleontologist have scoured outcrop areas and made extensive collection of fossils. Their investigations have have revealed information about the appearance, changes, and extinction of many species. In many cases, they have made detailed analysis of fossil population and distributions across a region where rock layers of a particular age are preserved—one example involves extensive in sedimentary rock formations like the Triassic-age Chinle Formation in Painted Desert region of Arizona that contain an abundance of well preserved fossils (Figure 16-11).

The changes in species structure of an ecological community over time is called ecological succession. Ecological succession takes place on time scales ranging from decades (such as what happens to forest community after a massive wildfire or catastrophic superstorm) or even millions of years during an ice age or a mass extinction event. Figure 16-12 shows an interpretation of the changes in the species populations in within an ancient ecosystem over time as revealed by fossils preserved is successive layers of sedimentary strata. Changes in ancient species populations and ecosystems can be inferred from the abundance the fossil preserved (or missing), the character of the fossils themselves, and sometimes information can be inferred from the sediments surrounding fossils or trace fossils in the sedimentary layers investigated in a study area. Studies show that species appear, populations grow, and then decline and vanish, sometimes returning, or are often replaced by other species that either have out-competed them, or simply replaced them when climate changes or other processes occurred that changed an ecosystem community setting over time.
Outcrop area of the Triassic-age Chinle Formation in the Painted Desert, Arizona
Fig. 16-11. Outcrop area of the Triassic-age Chinle Formation in the Painted Desert, Arizona is an example of an ideal study area that has an abundance of fossils preserved in many layers of strata over a large region.
Population changes in a local ecosystem over time (select species and total population of all species).
Fig. 16-12. Population changes in a local ecosystem over time (select species and total population of all species). Interpretations like this may be made from exhaustive studies of fossil collections from an area like in Figure 16-11.

Geologic History and Biological Evolution

The following sections of this chapter is a review of major geologic events, biological evolution, 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

Precambrian is the general name for the geologic time period between when the Earth formed in the Solar System (in Hadean Time about 4.56 billion years ago) and the beginning of Phanerozoic Eon (about 540 million years ago). The oldest rock on Earth are Precambrian age. The Precambrian is subdivided into three Eons:
Hadean Eon (before about 4 billion years ago)
Archean Eon (between about 4.0 and 2.5 billion years ago)
Proterozoic Eon (between about 2.5 billion and 541 million years ago).

The Precambrian encompassed all of early Earth history and rocks from that time preserve evidence of the evolution of life forms on a microbial level. In biology, cell theory states that a cell is the fundamental structural and functional unit of living matter, and that the an organism a multicellular body composed of autonomous cells with its properties being the sum of those of its cells.

Multicellular organisms (animals and plants) do not appear in the fossil record until late in Precambrian (Late Proterozoic) time.

The Phanerozoic Eon began after the end of the Proterozoic Eon about 540 million years ago, and marks the change when fossil remains of multicellular organisms began to appear in great abundance in the fossil record (discussed below).
Geologic Time Highlights of Biological Evolution Formation of the moon in the early Solar System
Fig. 16-15.
Current thought is that the Moon formed from the debris created by the collision of a small planet-sized object with the ancestral Earth (or Proto Earth) early in the history of the Solar System about 4.5 billion years ago.


About 4.56 billion years ago Formation of Earth and Moon within the Solar System nebula (Figure 16-15). (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) (discussed below). 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. Without an ozone layer, intense solar UV probably would have killed life in the shallow ocean 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 through geologic time.

Cambrian Period
Beginning about 541 million years ago
The beginning of the Cambrian Period started a radiation of species preserved in the fossil record. This is, in-part, because many organisms began to develop the first hard skeletal material as part of defensive and functional body plans. The diversity of species preserved in Cambrian sediments is partly because soft-bodied organisms were not preserved in Precambrian-age sediments.

Significant changes happened in the global physical environment in Cambrian time. Formation of the ozone layer created hospitable habitats and new space for organisms to move up and utilize shallow, warm sea environments that followed a major transgression onto the continents. Organisms were finally able to adapt to this new environment by allowing them to utilized calcite (CaCO3) for hard body parts are (shells and exoskeletons). Organisms with calcareous body parts were selectively or preferentially preserved in Cambrian and younger sedimentary rocks. The selective preservation of calcareous body parts has therefore made it easier to find evidence of life forms today preserved as fossils. Sediments composed of the skeletal remains of organisms (with shells and exoskeletons rich in CaCO3) is called lime, which turns into a sometimes quite fossiliferous rock, limestone.

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) that formed about 3.0 to 1.8 billion years ago (Figure 16-16). Theory suggests BIFs are associated with the capture of free 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 experiment 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 dissolved in the ancient seawater was consumed, oxygen could gradually accumulate in the atmosphere. Once enough oxygen was free in the atmosphere, an ozone layer could form.

BIF deposits of Precambrian age are preserved in many locations around the world, occurring as massive and widespread deposits, hundreds to thousands of feet thick. The BIFs we see today are only remnants of what were probably every greater and more extensive deposits. During Precambrian time, BIF deposits probably extensively covered large parts of the ancient global ocean basins. Today, BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions.
Banded iron formation
Fig. 16-16.
A sample of Precambrian banded-iron formation (BIF) from Fremont County, Wyoming.

Cell Theory in Evolution

Cell Theory dictates that all known living things are made up of one or more cells (the fundamental structure and functioning living unit in all living things. All living cells arise from pre-existing cells by processes involving cell division.

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 16-17).

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 16-17).

The oldest known prokaryote fossils are about 3.5 billion years old.
The oldest known eukaryote fossils are about 1.5 billion years old.

The same basic molecular processes are involved in the lives of both prokaryotes and eukaryotes, suggesting that all present-day cells are descendant 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. Viruses likely have influenced evolution on a cellular level in Precambrian time, just as they impact species evolution today.

A stromatolite is a mound of calcareous sediment built up of layers of lime-secreting cyanobacteria (blue-green bacteria, algae and other more primitive eukaryotic life forms) that trap sediment, creating layers accumulations (Figure 16-18). 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 16-19).

Life in Late Precambrian Time (Late Proterozoic Eon)

Evidence of the first sexual reproduction appear in the fossil record about 1.2 billion years ago. Many eukaryotic organisms including protista (both unicellular and colonial forms), fungi, and multicellular organisms (including plants and animals) reproduce sexually.

Metazoans are multicellular animals that have cells that differentiate into tissues and organs and usually have a digestive cavity and nervous system. Metazoans appeared on Earth in Late Precambrian time (Late Proterozoic Eon) consisting of cells that with growth would differentiated into unique tissue or organs used for special purposes, such a locomotion, feeding, reproduction, respiration, tissues able to sense 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 share similar characteristics of some families or classes of organisms still found on Earth today (including segmented worms, jellyfish, chordates, and other invertebrates).

Prokaryotes and Eucaryotes
Fig. 16-17.
Cell structures of Prokaryotes and Eukaryotes

Fig. 16-18.
Stromatolites, fossils of cyanobacterial algae mats, occur in rocks dating back to early Precambrian time, but can still be found living in some aquatic environments today.

Shark Bay, Australia
Fig. 16-19.
Stromatolite 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 252 million years ago. Paleozoic means ancient life (even though evidence of microbial 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 is used to describe the age of 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 Early Paleozoic Era

Evolution of early plant and animal life (dominated mostly marine invertebrates) is revealed in the fossil record of the early part Paleozoic Era. Primitive land plants, insects, and the first vertebrates also appear.

Cambrian Period (541-485 million years)

The Cambrian Period is the oldest of the named geological periods 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 much of East Coast, Appalachian region, and most of the Midwest. The shallow 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 16-20 and 16-21, also see Figure 16-13). The base of the Sauk Sequence rests on an eroded surface of ancient Precambrian-age (mostly metamorphic and igneous rocks of the core of more ancient mountain systems). This sequence boundary 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 within the canyons Inner Gorge (see Figure 16-14). The Great Unconformity can be traced across most of North America wherever the base of the Cambrian-age Sauk Sequence is exposed.

Calcareous skeletal shell remains first appear in the Cambrian Period.

The term Cambrian explosion refers to evidence in the fossil record which shows that all major phyla were established in the transition from latest Precambrian to the Early Cambrian Period (about 700 to 541 million years ago) (Figure 16-22). 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 unique habitats (niches) which allowed species to evolve separately from common ancestors. In Cambrian time, escalation of predator-prey relationships and increased competition appears to have driven rapid evolution of new species (along with extinctions). In Cambrian time, shelled organisms first appear in abundance in sedimentary deposits preserved from that time period. The fossil record from Cambrian time show that organisms with chitonous and calcareous shells and exoskeletons appeared and diversified. Many Cambrian-age organisms have eyes, legs (or pods), spinal chord-like features, segmented body plans, and other unique body parts and characteristics. Representatives of all phyla from the Cambrian Explosion still exist in the world today (Figure 16-22). Sedimentary rocks from Cambrian Period are typically rich in evidence of life activity. They preserve an abundance of bioturbation features (also called trace fossils) even if the life forms that created them are not preserved (an example of bioturbation is shown in Figure 16-23).

dominate the fossil record in the early Paleozoic Era. An invertebrate is an animal lacking a backbone (spinal column or spinal chord), such as an arthropod, mollusk, annelid worm, coelenterate, echinoderm, and many others. The classification of invertebrates constitute a division of the animal kingdom, comprising about 95 percent of all 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 16-24 and 16-25). 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.
Great Unconformity between Precambrian and Cambrian rocks in Wyoming
Fig. 16-20.
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.

Cambrian Explosion illustrated Fig. 16-22. The Cambrian explosion refers to the diversification of life forms that began near the end of the Precambrian Eon.

Fig. 16-24. Trilobites are common shelled fossils in sedimentary rocks from the Cambrian Period.
Bright Angel Shale is a rock formation within the Sauk Sequence exposed in the Grand Canyon
Fig. 16-21.
The fossiliferous Bright Angel Shale of Cambrian age is one of the rock formations of the Sauk Sequence exposed throughout the Grand Canyon region.

Tracks and trails in Cambrian sediments, Grand Canyon
Fig. 16-23. Invertebrate tracks and trails appear in abundance in Cambrian-age sediments (Tapeates Sandstone) in the lower Grand Canyon in Arizona.

Cambrian fossils
Fig. 16-25. Cambrian fossils: trilobites, brachiopods, gastropods, and other invertebrates


Ordovician Period (485-444 million years)

Shallow 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 (see Figure 16-13). However, when sea level rose again (millions of years later) and shallow seas returned to cover large portions of the continents, communities of life forms in the oceans had significantly changed.

Trilobites no longer dominated the fossil record, but other life forms began to proliferate in warm, shallow marine environments. Communities similar to some modern reef-like settings appear in the fossil record. Corals (unrelated to modern varieties), crinoids, cephalopods, brachiopods, bryozoans and other fossil life forms with calcareous skeletons dominate the fossil record. Their abundance reflects their ability to live, proliferate, and upon death, survived burial and fossilization processes). Rare early examples of jaw-less, armored fish and land plants have been discovered in sediment deposits of Ordovician age. Sedimentary rocks of Ordovician age crop out in many locations around the country, but they are perhaps best known from the Cincinnati Arch region (of Ohio, Kentucky and Indiana) where a great abundance of well preserved fossils occur in strata preserved from that time period (Figures 16-26 to 16-28).
Ordovician-age strata exposed in the Cincinnati Arch region, this view is near Maysville, Kentucky
Fig. 16-26.
Fossil-rich sedimentary rocks of the Tippecanoe Sequence are perhaps most famous from the Cincinnati Arch region.

Brachiopods on a block of  limestone from the Cincinnati Arch region
Fig. 16-27.
Fossil brachiopods on a layer of Ordovician limestone from Brookville Indiana (on the Cincinnati Arch).

Ordovician fossils
Fig. 16-28. Common fossils of the Ordovician Period

Silurian Period (444-419 million years)

Few rocks of Silurian age are preserved in North America's fossil record (they are either not preserved or are not exposed at the surface). 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. Large fossil pinnacle reefs occur around the margins of an ancient sea basins that covered what is now the state of Michigan. 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 early insects,arachnids and centipedes, and scorpions)(Figure 16-29). The first jawed fishes and freshwater fishes appear in Silurian. Large marine, scorpion-like creatures called eurypterids grew up to nearly 7 feet long (much larger than anything like it that exists like it today). Early vascular plants evolved in the Silurian Period, setting the evolutionary stage for terrestrial ecosystems that followed.
Silurian fossils
Fig. 16-29. Common and unusual fossils of the Silurian Period.

Highlights of the Middle and Late Paleozoic Era

The Middle to Late Paleozoic Era is highlighted by the development of forest ecosystems and the development of vertebrate species on land, and rise of large fish in the oceans.

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 and flourish as revealed in the fossil record. The first coiled-shelled ammonoid mollusks appeared. Holdover families of marine invertebrates from earlier times persisted, including trilobites, brachiopods, cephalopods, and reef-forming tabulate and rugose corals flourished in shallow seas (Figure 16-30).

The current oil and gas boom in the United States is largely because of the fracking technologies used to extract petroleum from the tight (meaning low permeability), black shales associated with organic-rich muddy sediments deposited in inland seas of Devonian and Mississippian age. These black shales underlie large regions of the Appalachians, the Mid continent, and northern Great Plains regions in the United States. These deposits are part of the Kaskaskia Sequence (see Figure 16-13).
Devonian brachiopods
Fig. 16-30.
Devonian Period brachiopods and common fossils from Kentucky

Carboniferous Period (359 to 299 million years ago)

The Carboniferous Period got its name from the abundance of coal deposits in rocks of Late Paleozoic age in Europe. In the United States, the Carboniferous Period is subdivided into the Mississippian Period and Pennsylvanian Period. An abundance of coal deposits of these ages also exist in eastern and central United States. During the Carboniferous the world was very different than today. The Earth's atmosphere was much thicker, having as much as 40% more oxygen and a more uniform global environment than exists today by some estimates.

Common Mississippian invertebrate fossils
Fig. 16-31.
Mississippian Period marine invertebrate fossils from Pennsylvania.
Redwall Limestone of Mississippian Age in the Grand Canyon
Fig. 16-32.
The massive Redwall Limestone of Marble Canyon and the Grand Canyon formed in the Mississippian Period.

Mississippian Period (359 to 323 million years ago)

Sedimentary rocks of Mississippian age in North America are dominated by marine sediments preserved as limestones rock formations when shallow, warm seas covered much of North America. Massive fossiliferous limestone rock formations of Mississippian age exposed throughout the Midcontinent (Mississippi Valley), and throughout the Appalachian and Rocky Mountain regions (Figure 16-31). For example, the Redwall Limestone in the Grand Canyon region is about 800 feet thick (Figure 16-32). 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 (see Figure 13-16).

The southern Appalachian Mountains began to rise in Mississippian time, and terrestrial lowlands and coastal swamps began to replace shallow seas on the North American continent. Coastal swamps along the margins of rising mountain ranges rising above the shallow seas began to support forests. Amphibians became the dominant marginal-land vertebrates in Mississippian time (they still requires water to lay their eggs).

Pennsylvanian Period (323 to 299 million years ago)

The Pennsylvanian Period is named for the coal-bearing region in the Appalachian Plateau and Mountains region). Great coastal forests and swamplands covered large regions of North America and parts of Europe. Great coal deposits formed from extensive swamps that trapped organic sediments in locations around the world. Pennsylvanian rocks are perhaps best know for their coal-bearing basins in the Appalachians and Midwest regions (Figures 16-33 and 16-34).

Perhaps the greatest evolutionary innovation of the Carboniferous Period was the development of amniote egg which allowed lizard-like tetrapods to advance. Reptiles evolved and became the first totally terrestrial vertebrates, descendant from amphibian ancestors. With the abundance of vegetation on land, arthropods flourished, including species of insects that are much larger than any found on Earth today (Figure 16-31). In Pennsylvanian time, glaciation cycles in the Southern Hemisphere caused repetitious rise and fall in sea levels. The Appalachian and Ouachita Mountain systems also began to develop as ancient forms of the continents of Africa, South America, and North America began to collide with one-another.

It was during the Pennsylvanian Period that the world's continents assembled together to form the supercontinent of Pangaea (discussed in Chapter 5). The unconformable boundary between the Kaskaskia Sequence and the overlying Absaroka Sequence is the boundary between sedimentary rocks Mississippian and Pennsylvanian age (see Figure 16-13).The Absaroka Sequence includes sediments deposited during Pennsylvanian, Permian and Triassic Periods (see below).
Pennsylvanian coal basins in eastern North America
Fig. 16-33. Pennsylvanian age coal-bearing basins in the eastern United States are part of the Absaroka Sequence.

Pennsylvanian forest
Fig. 16-34. Reconstruction of a swamp forest of the Pennsylvanian Period.

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 (lizard-like, 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. Reptiles adapted and flourished in the more arid conditions.

During the Permian, the expansive fern forests that existed during the Carboniferous disappeared, and vast desert regions spread over the North American continental interior. Seed-bearing conifers (gymnosperms) first appear in the Permian fossil record.

In Permian time, seawater began to flood the great rift valleys associated with the opening of the Atlantic Ocean basin and the separation of North America and South America. One arm of the sea flooded westward into an inland sea basin located in the West Texas and New Mexico region (Figure 16-35). Great reef tracks developed in around this basin Figure 16-36). Eventually the Permian Basin (as it is called) completely filled in with massive accumulations of salts (gypsum and evaporite).

The end of the Permian Period (and Paleozoic Era) is marked by the greatest mass extinction in Earth history.

Capitan Reef map
Fig. 16-35.
Map of the Permian Reef complex in Texas and New Mexico

Guadelupe Peak Fig. 16-36. Permian reef track exposed in Guadalupe National Park, Texas
Fig. 16-37.
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 nearly simultaneously. 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. Many questions remain about the causes of the great mass extinctions (because they may shed light on what is happening or may happen to the world related to human activities impacting the modern environment).

Current estimates are that 90 percent of all species that have ever lived on Earth are now extinct. However, the rate of extinction has not been constant. Mass extinctions have occurred at least five times in the last 500 million years. With each mass extinction much as about 50 to 90 percent of previously existing species on Earth had disappeared in very short periods of geologic time (Figure 16-38).
Mass extinctions
Fig. 16-38.
Great mass extinction events in the fossil record (species diversity compared with the geologic time scale).

Asteroid impacting earth
Fig. 16-39.
An massive asteroid impact can ruin your day (and your species, and many others).

Gary Larson cartoon about the real reason dinosaurs when extinctFig. 16-40. A classic Far Side cartoon by Gary Larson.

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 of Permian and older age under all the world's ocean basins have be subducted back into the Earth's mantle or heavily altered by mountain-building processes. In addition, much of the sedimentary record associated with exposed land of that time were stripped away by erosion before sediments began to be deposited and preserved in 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 before in the Late Paleozoic Era. When this biodiversity returned, the world was host to completely different varieties of species and ecological 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 16-39, see Figure 16-58 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).

Are humans causing a sixth great mass extinction?

Many scientists believe evidence suggests that another mass extinction is currently under way. Global climate change, the growth of the human population, and the expansion of human activity into previously wild habitats 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 possibly 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 when the global climate was drastically changing. 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

The Mesozoic Era is the era between the Paleozoic and Cenozoic Eras, comprising the Triassic, Jurassic, and Cretaceous Periods. The Mesozoic Era is commonly referred to as the Age of Reptiles. Highlights of the Mesozoic Era include:

• Dominant large animals: Reptiles and dinosaurs; birds and mammals appear.
• Increased mountain building occurred in many regions around the globe, and with that, lots of sediments were generated from erosion.
• The ancient supercontinent, Pangaea, begins to breakup at about 200 million years ago (Pangaea is discussed in Chapter 5).
• With the breakup of Pangaea, continents began moving apart. This caused isolation of species and communities, and as a result, created more diversity in plant and animal species through divergent evolution.
• The ancestral Rocky Mountains and Cordilleran Ranges formed in western North America between about 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 through succession. 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, primitive mammals, and flying vertebrates (pterosaurs) but these families did not flourish until after 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 Gondwanaland 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 Red beds are oxidized, iron-rich sedimentary deposits that occur extensively throughout western North America that were deposited in coastal terrestrial and nearshore environments during the Triassic Period. Red beds of Triassic age are well exposed in west Texas, throughout the Colorado Plateau and Rocky Mountain region, and in the Newark and Connecticut Basins on the East Coast. These are associated with the Absaroka Sequence that accumulated while Pangaea was still assembled and hot and dry climate conditions prevailed across most of North America.
Fig. 16-43. Red beds of the Chugwater Group of formations of Triassic age exposed near Lander, Wyoming.
Tracks at Dinosaur State Park
Fig. 16-41.
Dinosaur tracks in Late Triassic sedimentary rocks, Dinosaur State Park, Connecticut.

Desmatosaur from West Texas
Fig. 16-44.
Desmatosuchus, an archosaur from the Triassic Period found in West Texas
petrified wood Fig. 16-42. Extensive coniferous forests covered coastal regions at illustrated by the massive deposit of fossil wood preserved in Triassic-age sedimentary rocks in and around Petrified Forest National Park, Arizona.
Fig. 16-45.
Placerias, a large mammal-like reptile from the Triassic Period from 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 associated with rapid and massive amounts of volcanism that 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. Large 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 (Figure 15-48).
Fig. 16-48. Massive sandstone cliffs of the Navajo Sandstone of Jurassic age are well exposed in Zion National Park, Utah
dinosaur tracks
Fig. 16-46.
Dinosaur tracks
in Jurassic-age rocks near Tuba City, Arizona.

Fig. 16-49. Reconstruction of Stegasaurus, a Jurassic-age vegetarian dinosaur with unusual spinal plates. Dinosaur National Monument, Utah.
Dino bones
Fig. 16-47.
Dinosaur bones preserved in ancient river bed sediments, Dinosaur National Monument, Utah.

Fig. 16-50. 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 White 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 more dominant reptile and dinosaur species that existed around them.

During Late Cretaceous time, a large mountain range and volcanic arc developed along the western margin of North America as the Atlantic Ocean basin began to rapidly expand. The rising mountains in the west forced an isostatic down warping of the central part of the North American continent, allowing the shallow Western Interior Seaway to flood across much of the region extending from Arctic Ocean in Alaska and Canada to the Texas Gulf Coast region (Figure 16-56).

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 merging to form the ancient 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. 16-54)
Fig. 16-53. Jurassic- and Cretaceous-age rock formations in Capitol Reef National Park in central Utah (sedimentary layers of the Zuni Sequence).

Ammonites of the Late Cretaceous
Fig. 16-51.
Late Cretaceous ammonites of the Western Interior Seaway - an ancient seaway that existed in the Great Plains and Rocky Mountain region during Cretaceous Time.

Fig. 16-54.
Parasaurolophus- a Late Cretaceous dinosaur with a crested skull.
Fig. 16-52. Triceratops
, a Late Cretaceous herbivore dinosaur. Chicago Field Museum.

Dinosaur Sue
Fig. 16-55. Dinosaur Sue
, a famous Tyrannosaurus rex fossil on display at the Chicago Field Museum. T. rex was a large carnivorous dinosaur of the Late Cretaceous Period.

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

The Cretaceous-Tertiary boundary is associated with one of the most investigated mass extinction events. The age of the K/T boundary is currently estimated to be about 66 million years based on absolute dating methods. 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 on land around the world and has been studied extensively in core samples from deep-sea drilling projects.

The K/T extinction 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 massive asteroid impact and following shock waves, monstrous tsunamis, firestorms, ash clouds, toxic gas clouds, and global winter-like condition that followed caused ecosystem collapse and failure of the food chains and webs in both the oceans and on land.

It is important to note that all species that exist today are descendant of the limited number of 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 also fell around the globe (see Figure 16-13). 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. 16-56. Western Interior Seaway and locations of plausible asteroid impact sites around North America.
Extinction curves
Fig. 16-57. 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. 16-58. A layer of highly disrupted sediments corresponds with the mass extinction horizon associated in marine sediments located along the Cretaceous-Tertiary Boundary exposed in and around 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. Mammals diversified, gradually replacing the niches held by dinosaurs wiped out by the K/T extinction.
• Mountain building continued, especially around the Pacific Ocean; the Himalayan Mountains, the Alps, and mountain ranges throughout southern Eurasia begin to form. The Rocky Mountains and Cordilleran Ranges in western North America continued 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 Period and Neogene Period.
The periods of the Cenozoic Era are also subdivided into time periods called epochs.

Paleogene Period (66 to 23 million years ago)

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. See more about the Paleocene: American Museum of Natural History 66 to 56 million
Eocene Epoch
Modern-like 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. See more about the Eocene: 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 broad-leaf forests became more restricted to the equatorial realm. See more about the Oligocene: 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. 16-59. Eocene lake deposits crop out as the Chadron Formation in Bryce Canyon National Park, Utah Fig. 16-60. Eocene-age sediments fill many of the basins throughout the Wyoming region. Fig. 16-61. Eocene through Miocene sedimentary rocks crop out in Badlands National Park, South Dakota. Fig. 16-62. Eocene and younger rock formations exposed at the Del Mar Dog Beach, California.

Neogene Period (23 to 2.6 million years ago)

Time Range
Miocene Epoch
Animals and plants of the Miocene Epoch are approaching modern life forms in diversity and appearance. 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. See more about the Miocene: 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. See more about the Pliocene: 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. 16-63. Miocene-age sedimentary rocks exposed along Chesapeake Bay at Calvert Cliffs, Maryland. Fig. 16-64. Pliocene-age sedimentary rocks exposed in sea cliffs near Santa Cruz at Wilder Ranch State Park Fig. 16-65. Pliocene-age sedimentary basin fill rocks exposed in Death Valley National Park, California Fig. 16-66. Neogene-age sedimentary rocks (Miocene to Pleistocene) crop out in Anza Borrego State Park, California

Quaternary Period (2.6 million years ago to Present)

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. Learn more about the Pleistocene of California preserved in the La Brea Tar Pits, Los Angeles (UC Berkeley Museum of Paleontology website). Ice ages of the Pleistocene Epoch are discussed in detail in Chapter 12.
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. Learn more about the Holocene: 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. 16-67. A thick sequence of coastal and nearshore deposits of Pleistocene age are exposed in the sea cliffs of Thornton State Beach south of San Francisco, California. Fig. 16-68. Big Bone Lick State Park, a source of Pleistocene-age megafauna fossils in northern Kentucky is the birthplace of North American vertebrate paleontology. Fig. 16-69. Glacial till and outwash exposed at Caumsett State Park, Long Island, New York. Long Island is underlain by unconsolidated Pleistocene-age glacial deposits. Fig. 16-70. 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

Some 15 to 20 different species of early human-like species (humanoids) are currently recognized. However, not all scientists studying human evolution agree how these species are related or how or why 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 or sub-species.

Humans are included in the family of primates (which include modern monkeys, apes, and humans). Primates are descendant from an earlier monkey-like group called prosimians that appear in the fossil record in Eocene to Oligocene time. Primate species appear in abundance in many locations around the world 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.

The famous Lucy skeleton (about 3 million years show species had ape-like proportions of face, brain case, 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. Evidence suggests fire was used 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 evidence suggest that they 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 (Figure 16-71). 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.

Climate change associated with the ice ages must have had significant impacts on the survival and extinction of human and human-like species. In addition, populations were impacted by massive volcanic episodes, such as the by the Toba Super Eruption in Sumatra that occurred about 75,000 years ago (discussed in Chapter 7).

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. The modern human population has benefited from advances in medicine, agriculture, and transportation. The world's population has doubled in the last 40 years, but the rate of population growth has declined by almost half in that time (but not enough to stop population growth)(Figure 16-73). However, this success is countered by the demands of land and resources that lead to war and conflicts between populations. Population growth is not evenly distributed around the world (Figure 16-74).

Human Evolution Time-line Interactive (Smithsonian Institution website)

Routes of human migration around the world
Fig. 16-71.
Routes of human evolution and migration around the world beginning in late Pleistocene time.

San Francisco Fig. 16-72. Within the past century, human activity has completely changed large regions of the planet's physical environment.

World population growth 1600 to 2017 and rate of population growth 1950 to 2017 from United Nations data. Fig. 16-73. World population growth 1600 to 2017 and rate of population growth 1950 to 2017 from United Nations data.

World population density map of the world for 2015.Fig. 16-74. World population density map of the world for 2015. Note that large populations have developed in regions of high agricultural productivity where water is abundant (and perhaps the most valuable resource to a region).

Refugia: How Life Goes On After Environmental Calamities

Even after any number of the great mass extinctions, life returned and flourished in abundance. Once the environmental calamity that caused the great mass extinction at the end of the Cretaceous Period ended, this allowed for the succession of living things from life forms that survived in place, or survived in refugia. A refugia is an area in which a populations of organisms can survived during an extended period of unfavorable conditions. Refugia are isolated or protected environmental setting that survive major climate changes—examples include:

• an unglaciated area on a south-facing mountain slope where plants and animals survive in isolation, surrounded by advancing continental glaciers.
• species surviving an isolated mountain peak cool and wet enough to allow some species to survive when surrounding lowlands change from forests to desert conditions.
• plants and animals that become isolated on islands when sea level rises, and relative species elsewhere are wiped out by disease and/or predation.
• a an isolated community surviving in a canyon with continuous water supply in a region of long-term extended drought.
• species living in an isolated bay far away from the annihilation caused by a massive asteroid impact elsewhere on the planet.

Many question remain why some species survive a mass extinction event. What was it about species turtles, snakes, crocodillians, birds, and mammals that allowed them to survive the K/T extinction event when all dinosaurs and other organisms did not?

Refugia In Our Modern Era

With the advance of human civilizations, we are witnessing unprecedented extinctions as cities and croplands replace forests and coastal plains. Some species are hunted to extinction, or environmentally sensitive species loose their refugia. Human activities, such as building interstate highways and expansion of urban corridors, are isolating populations that would otherwise be a part of a continuous breeding population across an area or region. For some species, surviving member of species now only exist in zoos or on isolated park lands and wildlife preserves. On the other hand, useful species, such as dogs, cats, goats, cows, chickens, etc., are protected, but are increasingly being genetically modified to suit the needs and interests of their human hosts.

The Anthropocene Epoch (1865 AD to present)?

The name Holocene Epoch has been applied to the time period extending from the end of the last ice age, encompassing the rise of human civilizations up to the present time. However, the name Anthropocene has been suggested to designate the current geological age, viewed as the period during which human activity has become the dominant influence on climate and the physical environment. Some question are: When did this happen? And, how will generations of consciously aware descendants of our times (human and otherwise) be able to recognize it from landforms and layers with sedimentary deposits? Many suggestions have been made, and deposits in one region may not completely match characteristics in another region. (This is an excellent discussion topic for examining other extinction boundaries in the geologic past!) Here are points to consider: when did the Anthropocene begin?

• Many scientists think the beginning of the Anthropocene began with the Industrial Revolution in the 1850s; the logical start starting point to the modern era. The start of the Industrial Revolution marks when major extraction of mineral resources began (coal, iron, and other metals), the spread transportation networks, the growth and expansion urban development.
• Durable pollen from eucalyptus trees imported from Australia and New Zealand to support expansion or the railroads start to appear in sediments throughout California sedimentary basin deposits starting in the 1850s.
• Mass production and distribution of durable glass, porcelain products, and lead bullets started in the 1850s, beginning the contribution to throw-away society materials that can be found in abundance wherever humans went. Durable man-made products began to accumulate as trash in the environment.

A later start to the Anthropocene Epoch is suggested for post World War II. Sediments from this period include:
• A universal boundary world-wide where radioactive isotopes and byproducts of the surface testing of nuclear weapons can now be identified as a boundary in sedimentary deposit around the world.
• Durable plastics, construction materials, porcelain tiles, composite materials, and other durable trash of the modern era released intentionally or accidentally (such as damaging effect caused by superstorm damage, tsunamis, floods, or other disasters) are now distributed throughout the environment.
• Construction of sprawling urban area, mining regions, transportation routes (such as interstate highways) , and agricultural activities have significantly modified the landscape in many regions that will have lasting effect on the landscape for many millennium into the future. Some estimates suggest that human activities are moving more materials than all the rivers, wind, ocean currents, and other natural geologic processes combined.
• Landfills will be a long-lasting time stamp on the landscape worldwide.
• Introduction of exotic species have completely changed the environment in many regions.

This discussion has many intriguing manifestations. Can humans organize and adjust to what might be considered sustainability? Or, perhaps without hope, are we destined to an apocalyptic fate as describe by Thomas Malthus (1766-1834), an English economist and demographer who proposed a theory that human population growth will always tend to outrun the food supply. Malthus suggested that the betterment of humankind is impossible without strict enforcement of limits on reproduction. So far in our modern era, it seems that some of the limitations on what might be considered sustainable have been addressed by advancing technology and changing social norms (globally). The question is, can we collectively achieve sustainability without enduring war, disease, and famine?
Proposed type section for the Holocene-Anthropocene Boundary on the Washington Monument in Washington D.C.
Fig. 16-75. The Washington Monument is a possibly a good choice for a type section for the Holocene/Anthropocene Boundary. The lower part of the monument was built (by slaves) before the Industrial Revolution began. The upper part of monument was completed in a second construction phase after the Civil War (by free men) after the Industrial Revolution was well in progress.

Interestingly, the H/A Boundary level depicted on the Washington Monument approximately marks the level that sea level will rise to if most of the ice on Greenland and Antarctica were to melt due to global warming (as has already occurred in the geologic past).
Human evolution cartoon Fig. 16-75. 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 and manage our populations in any way while avoiding catastrophe.

Concepts of evolution, refugia, and succession provide a valuable lesson about modern society.

In our life times we can witness the progress of evolution in many ways, and hopefully, learn. The advance of technology illustrates these concepts. Classic examples illustrate:
• cars and displacing or replacing trains and horse-drawn carts as primary means of transportation.
• cell phones replacing telephones, which replaced telegraphs and mail services as primary means of communication.
• cable television replacing radio/TV broadcasting.
• cities grow through succession following the changes in politics, industry, and development of infrastructure.

So, should calamity happen, and an area or region should loose electrical power or access to liquid fuels, what would survive? Populations would need to migrate, adapt, or face famine. Electric- and gas-power tools and equipment would be rendered useless, but hand-powered tools like hammers, water pumps, shovels, saws and axes would be increasingly valuable!

In the business world, evolution provides particularly important concepts. It is an interesting study to see how businesses and corporations survive economic calamities caused by wars and depressions, and the rise of competing new technologies. It is a jungle out there.

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 16-76 is a geologic map of the conterminous United States, and Figure 16-77 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)
Fig. 16-76.
Geologic map of the United States.
Geologic age units associated with the geologic map of the United States.
Fig. 16-77
. Geologic ages of bedrock on the geologic map.
Selected comprehensive websites for paleontology and evolution:

U.C. Berkeley Museum of Paleontology website

American Museum of Natural History

Smithsonian Institution, Department of Paleobiology
Chapter 16 Quiz Questions