8 Earth History

Learning Objectives

By the end of this chapter, students should be able to:

  • Describe the turbulent beginning of Earth during the and .
  • Identify the transition to modern , , and evolution that occurred in the .
  • Describe the evolution and of invertebrates with hard parts, fish, amphibians, reptiles, tetrapods, and land plants; and and sedimentation associated with the .
  • Describe the evolution and of birds, dinosaurs, and mammmals; and and sedimentation associated with the breakup of .
  • Describe the evolution of mammals and birds, paleoclimate, and that shaped the modern world.
From oldest to newest time divisions: Hadean, archean, proterozoic, paleozoic, mesozoic, cenozoic. 4550 Ma: formation of the earth. 4527 Ma: formation of the moon.4000 Ma: end of the late heavy bombardment; first life. 3200 Ma: earliest start of photosynthesis. 2300 Ma: atmosphere becomes oxygen-rich; first snowball earth. 750-635 Ma: two snowball earths. 530 Ma: cambrian explosion. 380 Ma: first vertebrate land animals. 230-66 Ma: Non-avian dinosaurs. 2 Ma: first hominins.
Figure 8.1: Geologic time on Earth, represented circularly, to show the individual time divisions and important events. Ga=billion years ago, Ma=million years ago.

Entire courses and careers have been based on the wide-ranging topics covering Earth’s history. Throughout the long history of Earth, change has been the norm. Looking back in time, an untrained eye would see many unfamiliar life forms and terrains. The main topics studied in Earth history are paleogeography, paleontology, and paleoecology and paleoclimatology—respectively, past landscapes, past organisms, past ecosystems, and past environments. This chapter will cover briefly the origin of the universe and the 4.6 billion year history of Earth. This Earth history will on the major physical and biological events in each and .

8.1 Hadean Eon

Precambrian eon is made up of haydean, archean, and proterozoic eons. Phanerozoic eon is made up of paleozoic (permian, pennsylvanian, mississippian, devonian, silurian, ordovician, cambrian periods), mesozoic (cretaceous, jurassic, triassic periods), and cenozoic eras (quaternary and tertiary periods).
Figure 8.2: Geological time scale with ages shown.

Geoscientists use the geological time scale to assign relative age names to events and rocks, separating major events in Earth’s history based on significant changes as recorded in rocks and . This section summarizes the most notable events of each major time interval. For a breakdown on how these time intervals are chosen and organized, see chapter 7.

The , named after the Greek god and ruler of the underworld Hades, is the oldest and dates from 4.5–4.0 billion years ago.

Dark sky, fire and lighting everywhere. Volcanoes on the surface. Large sun in the background.
Figure 8.3: Artist’s impression of the Earth in the Hadean.

This time represents Earth’s earliest history, during which the planet was characterized by a partially molten surface, , and asteroid impacts. Several mechanisms made the newly forming Earth incredibly hot: gravitational , decay, and asteroid impacts. Most of this initial heat still exists inside the Earth. The was originally defined as the birth of the planet occurring 4.0 billion years ago and preceding the existence of many rocks and life forms. However, geologists have dated at 4.4 billion years, with evidence that liquid water was present. There is possibly even evidence of life existing over 4.0 billion years ago. However, the most reliable record for early life, the microfossil record, starts at 3.5 billion years ago.

8.1.1 Origin of Earth’s Crust

World map with the Moho depth color coded on the map: red is the deepest while blue is the shallowest. The Moho is deepest under central Asia and western South America, and the Moho is shallowest under the world ocean basins.
Figure 8.4: The global map of the depth of the moho.

As Earth cooled from its molten state, started to crystallize and settle resulting in a separation of based on density and the creation of the , , and . The earliest Earth was chiefly molten material and would have been rounded by gravitational forces so it resembled a ball of floating in space. As the outer part of the Earth slowly cooled, the high melting-point (see Bowen’s Reaction Series in chapter 4) formed solid slabs of early . These slabs were probably unstable and easily reabsorbed into the liquid until the Earth cooled enough to allow numerous larger fragments to form a thin primitive . Scientists generally assume this was and in , and littered with impacts, much like the Moon’s current . There is still some debate over when started, which would have led to the of and . Regardless of this, as Earth cooled and solidified, less dense floated to the surface of the Earth to form the , while the denser and materials sank to form the and the highest-density iron and nickel sank into the . This differentiated the Earth from a homogenous planet into a heterogeneous one with layers of , , , and iron and nickel .

8.1.2 Origin of the Moon

The far side of the Moon, gray in color and covered in numerous craters.
Figure 8.5: Far side of the Moon.

Several unique features of Earth’s Moon have prompted scientists to develop the current about its . The Earth and Moon are tidally locked, meaning that as the Moon orbits, one side always faces the Earth and the opposite side is not visible to us. Also and most importantly, the chemical compositions of the Earth and Moon show nearly identical ratios and volatile content. Apollo missions returned from the Moon with rocks that allowed scientists to conduct very precise comparisons between Moon and Earth rocks. Other bodies in the solar system and do not share the same degree of similarity and show much higher variability. If the Moon and Earth formed together, this would explain why they are so chemically similar.

The Earth and a Mars-sized object are colliding in a giant explosion.
Figure 8.6: Artist’s concept of the giant impact from a Mars-sized object that could have formed the moon.

Many ideas have been proposed for the origin of the Moon: The Moon could have been captured from another part of the solar system and formed in place together with the Earth, or the Moon could have been ripped out of the early Earth. None of proposed explanations can account for all the evidence. The currently prevailing is the giant-impact . It proposes a body about half of Earth’s size must have shared at least parts of Earth’s orbit and collided with it, resulting in a violent mixing and scattering of material from both objects. Both bodies would be composed of a combination of materials, with more of the lower density splatter coalescing into the Moon. This may explain why the Earth has a higher density and thicker than the Moon.

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Video 8.1: Evolution of the Moon.

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8.1.3 Origin of Earth’s Water

Gray dumbbell-shaped comet that has gaseous jets coming off of the comet's surface.
Figure 8.7: Water vapor leaves comet 67P/Churyumov–Gerasimenko.

Explanations for the origin of Earth’s water include outgassing, comets, and . The outgassing for the origin of Earth’s water is that it originated from inside the planet, and emerged via processes as vapor associated with eruptions. Since all eruptions contain some water vapor, at times more than 1% of the volume, these alone could have created Earth’s surface water. Another likely source of water was from space. Comets are a mixture of dust and ice, with some or most of that ice being frozen water. Seemingly dry meteors can contain small but measurable amounts of water, usually trapped in their structures. During heavy bombardment later in Earth’s history, its cooled surface was pummeled by comets and , which could be why so much water exists above ground. There isn’t a definitive answer for what process is the source of ocean water. Earth’s water isotopically matches water found in much better than that of comets. However, it is hard to know if Earth processes could have changed the water’s isotopic signature over the last 4-plus billion years. It is possible that all three sources contributed to the origin of Earth’s water.

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8.2 Archean Eon

Landscape shows clear water with numerous algal mats; in the distance is a large erupting volcano, meteors streaking through the sky, and the moon very large at the horizon.
Figure 8.8: Artist’s impression of the Archean.

The , which lasted from 4.0–2.5 billion years ago, is named after the Greek word for beginning. This represents the beginning of the rock record. Although there is current evidence that rocks and existed during the , the has a much more robust rock and record.

8.2.1 Late Heavy Bombardment

Planet covered in ice with numerous craters dotting the surface, with the exception of a smooth white plain covering a large portion of the lower right part of the planet.
Figure 8.9: 2015 image from NASA’s New Horizons probe of Pluto. The lack of impacts found on the Tombaugh Regio (the heart-shaped plain, lower right) has been inferred as being younger than the Late Heavy Bombardment and the surrounding surface due to its lack of impacts.

Objects were chaotically flying around at the start of the solar system, building the planets and moons. There is evidence that after the planets formed, about 4.1–3.8 billion years ago, a second large spike of asteroid and comet impacted the Earth and Moon in an event called . and comets in stable or semi-stable orbits became unstable and started impacting objects throughout the solar system. In addition, this event is called the lunar cataclysm because most of the Moons craters are from this event. During , the Earth, Moon, and all planets in the solar system were pummeled by material from the asteroid and Kuiper belts. Evidence of this bombardment was found within samples collected from the Moon.

Three diagrams showing a progressive simulation of the outer planets and the Kuiper belt with a horizontal scale of negative 50 to positive 50 labeled AU and a vertical scale of negative 50 to positive 50 labeled AU; the first diagram shows closely clustered white dots surrounding the drawn circular orbits of the four outer planets; the second diagram shows that the white dots have scattered outward but are still clustered in a ring and the outer four planets have changed their orbits; the third diagram shows that the white dots are completely scattered and the four outer planets' orbits are drawn as they occur today.
Figure 8.10: Simulation of before, during, and after the late heavy bombardment.

It is universally accepted that the solar system experienced extensive asteroid and comet bombardment at its start; however, some other process must have caused the second increase in impacts hundreds of millions of years later. A leading blames gravitational between Jupiter and Saturn for disturbing orbits within the asteroid and Kuiper belts based on a similar process observed in the Eta Corvi star .

8.2.2 Origin of the Continents

From core to surface: Inner core (solid), outer core (liquid), mantle (including asthenosphere), crust (including lithosphere)
Figure 8.11: The layers of the Earth. Physical layers include lithosphere and asthenosphere; chemical layers are crust, mantle, and core.

In order for to work as it does currently, it necessarily must have continents. However, the easiest way to create material is via and differentiation of existing continents (see chapter 4). This chicken-and-egg quandary over how continents were made in the first place is not easily answered because of the great age of material and how much evidence has been lost during and . While the timing and specific processes are still debated, action must have brought the first material to the Earth’s surface during the , 4.4 billion years ago. This model does not solve the problem of , since seems to need thicker . Nevertheless, the continents formed by some incremental process during the early history of Earth. The best idea is that density differences allowed lighter materials to float upward and heavier materials and iron to sink. These density differences led to the layering of the Earth, the layers that are now detected by studies. Early protocontinents accumulated materials as developing processes brought lighter material from the to the surface.

Block diagram showing an oceanic plate moving toward the right where it collides with another oceanic plate and subducts down beneath it. Above the contact between the two plates, there is an ocean trench. There is a volcanic island arc on top of the overriding oceanic plate, above where the oceanic crust has subducted beneath it.
Figure 8.12: Subduction of an oceanic plate beneath another oceanic plate, forming a trench and an island arc. Several island arcs might combine and eventually evolve into a continent.

The first solid evidence of modern is found at the end of the , indicating at least some must have been in place. This evidence does not necessarily mark the starting point of ; remnants of earlier activity could have been erased by the .

World map with geologic provinces color-coded: Shield are colored orange and are seen on northern North America, eastern South America, northwestern Europe, northern and southern Asia, northwestern Australia, and sub-Saharan and southern Africa. Platform are colored pink and are seen near the same locations as shield with the exception of large platforms covering most of northern Asia and Europe. Orogen are colored cyan and are seen along western North America, western South America, the northwestern edge of Africa, southern Europe, southeastern Australia, and southern, central, and northeastern Asia. Basin are colored blue and are seen as thin strips in central-west North America, central-west South America, southern and northeastern Asia, and small spots in central and northwestern Africa. Large igneous province is colored purple and are seen as small blobs in western North America, eastern South America, Iceland, eastern Africa, central India, and northern Asia. Extended Crust are colored yellow and are seen on the margins of all of the continents.
Figure 8.13: Geologic provinces with the Shield (orange) and Platform (pink) comprising the Craton, the stable interior of continents.

The stable interiors of the current continents are called and were mostly formed in the . A has two main parts: the , which is crystalline rock near the surface, and the made of sedimentary rocks covering the . Most have remained relatively unchanged with most activity having occurred around instead of within them. Whether they were created by or another process, continents gave rise to the continents that now dominate our planet.

Relief map centered on New Zealand with a pink outline extending north and southeast of New Zealand that represents the extent of the continent of New Zealandia.
Figure 8.14: The continent of Zealandia.

The general guideline as to what constitutes a and differentiates from is under some debate. At passive margins, grades into at passive margins, making a distinction difficult. Even island-arc and hot-spot material can seem more closely related to than . Continents usually have a in the middle with rocks. There is evidence that submerged masses like Zealandia, that includes present-day New Zealand, would be considered a . that does not contain a is called a fragment, such as the island of Madagascar off the east coast of Africa.

8.2.3 First Life on Earth

Rocks with a wrinkled texture, formed by microbial mats; the rocks are both gray and green in color and a rock hammer rests on the outcrop for scale.
Figure 8.15: Fossils of microbial mats from Sweden.

Life most likely started during the late or early . The earliest evidence of life are chemical signatures, microscopic filaments, and microbial mats. Carbon found in 4.1 billion year old grains have a chemical signature suggesting an organic origin. Other evidence of early life are 3.8–4.3 billion-year-old microscopic filaments from a deposit in Quebec, Canada. While the chemical and microscopic filaments evidence is not as robust as , there is significant evidence for life at 3.5 billion years ago. These first well-preserved are photosynthetic microbial mats, called , found in Australia.

Four diagrams showing various greenhouse gas molecules: water vapor consists of a red oxygen atom with two white hydrogen atoms attached on the bottom left and bottom right; nitrous oxide consists of two blue nitrogen atoms and a red oxygen atom connected in a straight line; methane consists of a black carbon atom at the center with four white hydrogen atoms connected to the carbon atom in a pyramidal shape; and carbon dioxide consists of a red oxygen atom, a black carbon atom, and another red oxygen atom connected in a straight line.
Figure 8.16: Greenhouse gases were more common in Earth’s early atmosphere.

Although the origin of life on Earth is unknown, include a chemical origin in the early and ocean, deep-sea vents, and delivery to Earth by comets or other objects. One is that life arose from the chemical environment of the Earth’s early and oceans, which was very different than today. The oxygen-free produced a reducing environment with abundant methane, carbon dioxide, sulfur, and nitrogen compounds. This is what the is like on other bodies in the solar system. In the famous Miller-Urey , researchers simulated early Earth’s and lightning within a sealed vessel. After igniting sparks within the vessel, they discovered the of amino acids, the fundamental building blocks of proteins. In 1977, when scientists discovered an isolated ecosystem around vents on a deep-sea (see chapter 4), it opened the door for another explanation of the origin of life. The vents have a unique ecosystem of critters with as the foundation of the food chain instead of photosynthesis. The ecosystem is deriving its energy from hot chemical-rich waters pouring out of underground towers. This suggests that life could have started on the deep and derived energy from the heat from the Earth’s interior via . Scientists have since expanded the search for life to more unconventional places, like Jupiter’s icy moon Europa.

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Video 8.2: Animation of the original Miller-Urey 1959 experiment that simulated the early atmosphere and created amino acids from simple elements and compounds.

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Another possibility is that life or its building blocks came to Earth from space, carried aboard comets or other objects. Amino acids, for example, have been found within comets and . This intriguing possibility also implies a high likelihood of life existing elsewhere in the cosmos.

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8.3 Proterozoic Eon

Water enters through the plants roots. Carbon dioxide enters through the plants leaves. Sunlight enters through the plants leaves. Oxygen leaves the plants leaves.
Figure 8.17: Diagram showing the main products and reactants in photosynthesis. The one product that is not shown is sugar, which is the chemical energy that goes into constructing the plant, and the energy that is stored in the plant which is used later by the plant or by animals that consume the plant.

The , meaning “earlier life,” comes after the and ranges from 2.5 billion to 541 million years old. During this time, most of the central parts of the continents had formed and processes had started. Photosynthesis by microbial organisms, such as single-celled cyanobacteria, had been slowly adding oxygen to the oceans. As cyanobacteria evolved into multicellular organisms, they completely transformed the oceans and later the by adding amounts of free oxygen gas (O2) and initiated what is called the (GOE). This drastic environmental change decimated the anaerobic bacteria, which could not survive in the presence of free oxygen. On the other hand, aerobic organisms could thrive in ways they could not earlier.

An oxygenated world also changed the chemistry of the planet in significant ways. For example, iron remained in in the non-oxygenated environment of the earlier . In chemistry, this is known as a reducing environment. Once the environment was oxygenated, iron combined with free oxygen to form solid precipitates of iron , such as the hematite or magnetite. These precipitates accumulated into large deposits with red known as -iron , which are dated at about 2 billion years.

Zoomed-in photo of a slice of rock, showing red and brown layers with glittering dots throughout; a scale bar at the lower left says 5.0 mm.
Figure 8.18: Alternating bands of iron-rich and silica-rich mud, formed as oxygen combined with dissolved iron.

The of iron and red (see figure 8.18) in the oceans lasted a long time and prevented oxygen levels from increasing significantly, since took the oxygen out of the water and deposited it into the rock . As oxygen continued to be produced and leveled off, oxygen gas eventually the oceans and started bubbling out into the . Oxygenation of the is the single biggest event that distinguishes the and environments. In addition to changing and ocean chemistry, the GOE is also tabbed as triggering Earth’s first event around 2.1 billion years ago, the Huron Glaciation. Free oxygen reacted with methane in the to produce carbon dioxide. Carbon dioxide and methane are called greenhouse gases because they heat within the Earth’s , like the insulated glass of a greenhouse. Methane is a more effective insulator than carbon dioxide, so as the proportion of carbon dioxide in the increased, the decreased, and the planet cooled.

8.3.1 Rodinia

Circular world map showing multiple gray cratons arranged together in the following way: just above the equator on the left is Madagascar, India, and East Antarctica; Laurentia spans across the equator with Australia, South China, and Siberia above it; Baltica is to the right of Laurentia, also across the equator; below Laurentia and Baltica are Rio Plato, Amazonia, and West Africa. There are two island cratons below the equator and toward the left: Congo-Sao Francisco and Kalahari. There are green strips across East Antarctica, Laurentia, and Congo-Sao Francisco that represent 1.1 Ga belts.
Figure 8.19: One possible reconstruction of Rodinia 1.1 billion years ago.

By the , lithospheric plates had formed and were moving according to forces that were similar to current times. As the moving collided, the ocean basins closed to form a called . The formed about 1 billion years ago and broke up about 750 to 600 million years ago, at the end of the . One of the resulting fragments was a mass called that would later become North America. Geologists have reconstructed by matching and aligning ancient mountain chains, assembling the pieces like a jigsaw puzzle, and using paleomagnetics to orient to magnetic north.

The disagreements over these complex reconstructions is exemplified by geologists proposing at least six different models for the breakup of to create Australia, Antarctica, parts of China, the Tarim north of the Himalaya, Siberia, or the Kalahari of eastern Africa. This breakup created lots of shallow-water, biologically favorable environments that fostered the evolutionary breakthroughs marking the start of the next , the .

8.3.2 Life Evolves

Brown, blobby stromatolites are slightly sticking out of the shallow water of the ocean.
Figure 8.20: Modern cyanobacteria (as stromatolites) in Shark Bay, Australia.

Early life in the and earlier is poorly documented in the record. Based on chemical evidence and evolutionary , scientists propose this life would have been single-celled photosynthetic organisms, such as the cyanobacteria that created . Cyanobacteria produced free oxygen in the through photosynthesis. Cyanobacteria, archaea, and bacteria are —primitive organisms made of single cells that lack cell nuclei and other organelles.

Concentric round structures in gray limestone.
Figure 8.21: Fossil stromatolites in Saratoga Springs, New York.

A large evolutionary step occurred during the with the appearance of around 2.1 to 1.6 billion years ago. cells are more complex, having nuclei and organelles. The nuclear DNA is capable of more complex replication and regulation than that of cells. The organelles include mitochondria for producing energy and chloroplasts for photosynthesis. The branch in the tree of life gave rise to fungi, plants, and animals.

Another important event in Earth’s biological history occurred about 1.2 billion years ago when invented sexual reproduction. Sharing genetic material from two reproducing individuals, male and female, greatly increased genetic variability in their offspring. This genetic mixing accelerated evolutionary change, contributing to more complexity among individual organisms and within ecosystems (see chapter 7).

land surfaces were barren of plants and animals and geologic processes actively shaped the environment differently because land surfaces were not protected by leafy and woody vegetation. For example, rain and would have caused at much higher rates on land surfaces devoid of plants. This resulted in thick accumulations of pure from the such as the extensive in the of the Uinta Mountains in Utah.

Oval fossil that resembles a leaf imprinted in tan sandstone.
Figure 8.22: Dickinsonia, a typical Ediacaran fossil.

Fauna during the , 635.5 to 541 million years ago are known as the , and offer a first glimpse at the diversity of ecosystems that evolved near the end of the . These soft-bodied organisms were among the first multicellular life forms and probably were similar to jellyfish or worm-like. did not have hard parts like shells and were not well preserved in the rock records. However, studies suggest they were widespread in the Earth’s oceans. Scientists still debate how many species were evolutionary dead-ends that became and how many were ancestors of modern groupings. The transition of soft-bodied life to life forms with hard body parts occurred at the end of the and beginning of the . This evolutionary explosion of biological diversity made a dramatic difference in scientists’ ability to understand the history of life on Earth.

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8.4 Phanerozoic Eon: Paleozoic Era

Top view of a black fossilized trilobite that has an overall oval shape with a segmented body covered in a pattern of fine ridges; a scale bar labeled 0.5 cm is in the lower right of the photo.
Figure 8.23: The trilobites had a hard exoskeleton, and is an early arthropod, the same group that includes modern insects, crustaceans, and arachnids.

The is the most recent, 541 million years ago to today, and means “visible life” because the rock record is marked by an abundance of . organisms had hard body parts like claws, scales, shells, and bones that were more easily preserved as . Rocks from the older time are less commonly found and rarely include because these organisms had soft body parts. rocks are younger, more common, and contain the majority of extant . The study of rocks from this yields much greater detail. The is subdivided into three , from oldest to youngest they are (“ancient life”), (“middle life”), and (“recent life”) and the remaining three chapter headings are on these three important .

Illustration of trilobites crawling on the seafloor.
Figure 8.24: Trilobites, by Heinrich Harder, 1916.

Life in the early was dominated by organisms but by the middle of the plants and animals evolved to live and reproduce on land. Fish evolved jaws and fins evolved into jointed limbs. The development of lungs allowed animals to emerge from the sea and become the first air-breathing tetrapods (four-legged animals) such as amphibians. From amphibians evolved reptiles with the amniotic egg. From reptiles evolved an early ancestor to birds and mammals and their scales became feathers and fur. Near the end of the , the had some of the most extensive forests in Earth’s history. Their fossilized remains became the that powered the industrial revolution

8.4.1 Paleozoic Tectonics and Paleogeography

Color-coded map of North America; covering the majority of the United States and Canada it is brown with the label North American Craton-continental crust that has remained relatively stable for the past 600 million years; there is a purple wedge in the American southwest that extends as a thin strip along the margins of the North American Craton labeled Deformed Craton; along the western edge of North America it is green with the label Accretionary Belt-welded to the North American continental margin within the last 600 million years; along the east coast and extending into the oceans it is blue with the label Coastal Plain.
Figure 8.25: Laurentia, which makes up the North American craton.

During the , sea-levels rose and fell four times. With each sea-level rise, the majority of North America was covered by a shallow tropical ocean. Evidence of these submersions are the abundant sedimentary rocks such as with corals and . Extensive sea-level are documented by widespread . Today, the midcontinent has extensive sedimentary rocks from the and western North America has thick layers of on block faulted mountain ranges such as Mt. Timpanogos near Provo, Utah

One supercontinent in the shape of a crescent. From top to bottom: Eurasia, North america, Africa, South america, India, Antarctica, Australia.
Figure 8.26: A reconstruction of Pangaea, showing approximate positions of modern continents.

The assembly of , sometimes spelled , was completed by the late . The name was originally coined by Alfred Wegener and means “all land.” is the when all of the major continents were grouped together as one by a series of events including island- accretion, and collisions, and ocean- closures. In North America, these events occurred on the east and are known as the Taconic, Acadian, Caledonian, and Alleghanian orogenies. The Appalachian Mountains are the erosional remnants of these mountain building events in North America. Surrounding was a global ocean known as the Panthalassa. Continued movement extended the ocean into , forming a large bay called the Tethys Sea that eventually divided the land mass into two smaller , Laurasia and Gondwana. Laurasia consisted of and Eurasia, and Gondwana consisted of the remaining continents of South America, Africa, India, Australia, and Antarctica.

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Video 8.3: Animation of plate movement the last 3.3 billion years. Pangea occurs at the 4:40 mark.

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While the east coast of North America was tectonically active during the , the west coast remained mostly inactive as a during the early . The western edge of North American was near the present-day Nevada-Utah border and was an expansive shallow near the paleoequator. However, by the , the Antler started on the west coast and lasted until the Pennsylvanian . The Antler was a that was accreted onto western North America with the direction away from North America. This created a mountain range on the west coast of North American called the Antler highlands and was the first part of building the land in the west that would eventually make most of California, Oregon, and Washington states. By the late , the Sonoma began on the west coast and was another of an . The Sonoma marks the change in direction to be toward North America with a along the entire west coast of North America by late to early .

By the end of the , the east coast of North America had a very high mountain range due to and the creation of . The west coast of North America had smaller and isolated highlands associated with accretion. During the , the size of the mountains on either side of North America would flip, with the west coast being a more tectonically active and the east coast changing into a after the breakup of .

8.4.2 Paleozoic Evolution

Elongate reddish brown animal with swimming flaps running along its body, large compound eyes, and a single pair of segmented frontal appendages.
Figure 8.27: Anomalocaris reconstruction by the MUSE science museum in Italy.

The beginning of the is marked by the first appearance of hard body parts like shells, spikes, teeth, and scales; and the appearance in the rock record of most animal phyla known today. That is, most basic animal body plans appeared in the rock record during the . This sudden appearance of biological diversity is called the . Scientists debate whether this sudden appearance is more from a rapid evolutionary diversification as a result of a warmer following the late environments, better preservation and fossilization of hard parts, or artifacts of a more complete and recent rock record. For example, fauna may have been diverse during the  , setting the state for the , but they lacked hard body parts and would have left few behind. Regardless, during the 541–485 million years ago marked the appearance of most animal phyla.

Elongate white fossil animal with a segmented trunk that has flaps along the sides and a fan-shaped tail.
Figure 8.28: Original plate from Walcott’s 1912 description of Opabinia, with labels: fp = frontal appendage, e = eye, ths = thoracic somites, i = intestine, ab = abdominal segment.

One of the best sites for the was discovered in 1909 by Charles Walcott (1850–1927) in the Burgess in western Canada. The Burgess is a , a site of exceptional preservation that includes impressions of soft body parts. This discovery allowed scientists to study animals in immense detail because soft body parts are not normally preserved and fossilized. Other sites of similar age in China and Utah have allowed scientist to form a detailed picture of biodiversity. The biggest mystery surrounds animals that do not fit existing lineages and are unique to that time. This includes many famous fossilized creatures: the first compound-eyed trilobites; Wiwaxia, a creature covered in spiny ; Hallucigenia, a walking worm with spikes; Opabinia, a five-eyed arthropod with a grappling claw; and Anomalocaris, the alpha predator of its time, complete with grasping appendages and circular with sharp . Most notably appearing during the is an important ancestor to humans. A segmented worm called Pikaia is thought to be the earliest ancestor of the phylum that includes , animals with backbones.

Colorful coral reef underwater in light blue, shallow water.
Figure 8.29: A modern coral reef.

By the end of the , mollusks, brachiopods, nautiloids, gastropods, graptolites, echinoderms, and trilobites covered the sea floor. Although most animal phyla appeared by the , the biodiversity at the family, genus, and species level was low until the . During the Great Ordovician Biodiversification Event, and invertebrates (animals without backbone) became more diverse and complex at family, genus, and species level. The cause of the rapid speciation event is still debated but some likely causes are a combination of warm temperatures, expansive shelves near the equator, and more along the . Some have shown evidence that an asteroid breakup event and consequent heavy impacts correlate with this diversification event. The additional added nutrients to ocean water helping support a robust ecosystem. Many life forms and ecosystems that would be recognizable in current times appeared at this time. Mollusks, corals, and arthropods in particular multiplied to dominate the oceans.

A mountain with steep cliffs rising from vegetation-covered slopes; the mountain has a blunt, nearly flat top.
Figure 8.30: Guadalupe National Park is made of a giant fossil reef.

One important evolutionary advancement during the was reef-building organisms, mostly colonial coral. Corals took advantage of the ocean chemistry, using to build large structures that resembled modern like the Great Barrier Reef off the coast of Australia. These reefs housed thriving ecosystems of organisms that swam around, hid in, and crawled over them. Reefs are important to paleontologists because of their preservation potential, size, and in-place ecosystems. Few other offer more diversity and complexity than assemblages.

According to evidence from deposits, a small caused sea-levels to drop and led to a major by the end of the . This is the earliest of five events documented in the record. During this , an unusually large number of species abruptly disappear in the record (see video below).

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Video 8.4: 3-minute video describing mass extinctions and how they are defined.

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Tan fish fossil that is covered with natural plate-like armor.
Figure 8.31: The placoderm Bothriolepis panderi from the Devonian of Russia.

Life bounced back during the . The ’s major evolutionary event was the development of jaws from the forward pair of gill arches in bony fishes and sharks. Hinged jaws allowed fish to exploit new food sources and ecological niches. This also included the start of armored fishes, known as the placoderms. In addition to fish and jaws, rocks provide the first evidence of or land-dwelling plants and animals. The first vascular plant, Cooksonia, had woody tissues, for gas exchange, and veins for water and food transport. Insects, spiders, scorpions, and crustaceans began to inhabit moist, freshwater environments.

Late Devonian lobe-finned fish and amphibious tetrapods. Land: Tiktaalik (375 million years ago), Ichthyostega (365 million years ago). Rivers, swamps, and shallows: Panderichthys (380 million years ago), acanthostega (365 million years ago). Sea: Eusthenopteron (385 million years ago), coelacanth (360 million years ago)
Figure 8.32: Several different types of fish and amphibians that led to walking on land.

The is called the Age of Fishes due to the rise in plated, jawed, and lobe-finned fishes. The lobe-finned fishes, which were related to the modern lungfish and coelacanth, are important for their eventual evolution into tetrapods, four-limbed animals that can walk on land. The first lobe-finned land-walking fish, named Tiktaalik, appeared about 385 million years ago and serves as a transition between fish and early tetrapods. Though Tiktaalik was clearly a fish, it had some tetrapod structures as well. Several from the are more tetrapod like than fish like but these weren’t fully . The first fully tetrapod arrived in the Mississippian (early ) . By the Mississippian (early ) , tetrapods had evolved into two main groups, amphibians and amniotes, from a common tetrapod ancestor. The amphibians were able to breathe air and live on land but still needed water to nurture their soft eggs. The first reptile (an amniote) could live and reproduce entirely on land with hard-shelled eggs that wouldn’t dry out.

Land plants had also evolved into the first trees and forests. Toward the end of the , another event occurred. This , while severe, is the least temporally defined, with wide variations in the timing of the event or events. building organisms were the hardest hit, leading to dramatic changes in ecosystems.

Top view of a 2-meter long brown millipede with segmented body; an inset diagram shows the millipede size compared with a human.
Figure 8.33: A reconstruction of the giant arthropod (insects and their relatives) Arthropleura.

The next time , called the (North American geologists have subdivided this into the Mississippian and Pennsylvanian ), saw the highest levels of oxygen ever known, with forests (e.g., ferns, club mosses) and swamps dominating the landscape. This helped cause the largest arthropods ever, like the millipede Arthropleura, at 2.5 meters (6.4 feet) long! It also saw the rise of a new group of animals, the reptiles. The evolutionary advantage that reptiles have over amphibians is the amniote egg (egg with a protective shell), which allows them to rely on non-aquatic environments for reproduction. This widened the reach of reptiles compared to amphibians. This booming life, especially plant life, created cooling temperatures as carbon dioxide was removed from the . By the middle , these cooler temperatures led to an (called the Karoo ) and less-productive forests. The reptiles fared much better than the amphibians, leading to their diversification. This event lasted into the early .

Four-legged brownish-black animal with a distinct tan sail-like structure on its back; it has a reptile-like body, covered in scaly skin, and its head features a long snout with sharp teeth. The sail on its back is composed of bony projections connected by a thin membrane.
Figure 8.34: Reconstruction of Dimetrodon.

By the , with assembled, the led to a dryer , and even more diversification and domination by the reptiles. The groups that developed in this warm eventually radiated into dinosaurs. Another group, known as the synapsids, eventually evolved into mammals. Synapsids, including the famous sail-backed Dimetrodon are commonly confused with dinosaurs. Pelycosaurs (of the Pennsylvanian to early like Dimetrodon) are the first group of synapsids that exhibit the beginnings of mammalian characteristics such as well-differentiated dentition: incisors, highly developed canines in lower and upper jaws and cheek teeth, premolars and molars. Starting in the late , a second group of synapsids, called the therapsids (or mammal-like reptiles) evolve, and become the ancestors to mammals.

Permian Mass Extinction

World map with geologic provinces color-coded: Shield are colored orange and are seen on northern North America, eastern South America, northwestern Europe, northern and southern Asia, northwestern Australia, and sub-Saharan and southern Africa. Platform are colored pink and are seen near the same locations as shield with the exception of large platforms covering most of northern Asia and Europe. Orogen are colored cyan and are seen along western North America, western South America, the northwestern edge of Africa, southern Europe, southeastern Australia, and southern, central, and northeastern Asia. Basin are colored blue and are seen as thin strips in central-west North America, central-west South America, southern and northeastern Asia, and small spots in central and northwestern Africa. Large igneous province is colored purple and are seen as small blobs in western North America, eastern South America, Iceland, eastern Africa, central India, and northern Asia. Extended Crust are colored yellow and are seen on the margins of all of the continents.
Figure 8.35: World map of flood basalts. Note the largest is the Siberian Traps.

The end of the is marked by the largest in earth history. The had two smaller , but these were not as large as the , also known as the . It is estimated that up to 96% of species and 70% of land-dwelling () went extinct. Many famous organisms, like sea scorpions and trilobites, were never seen again in the record. What caused such a widespread event? The exact cause is still debated, though the leading idea relates to extensive associated with the Siberian , which are one of the largest deposits of known on Earth, dating to the time of the event. The eruption size is estimated at over 3 million cubic kilometers that is approximately 4,000,000 times larger than the famous 1980 Mt. St. Helens eruption in Washington. The unusually large eruption would have contributed a large amount of toxic gases, aerosols, and greenhouse gasses into the . Further, some evidence suggests that the burned vast deposits releasing methane (a greenhouse gas) into the . As discussed in chapter 15, greenhouse gases cause the to warm. This extensive addition of greenhouse gases from the Siberian may have caused a runaway that rapidly changed the , acidified the oceans, disrupted food chains, disrupted carbon cycling, and caused the largest .

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8.5 Phanerozoic Eon: Mesozoic Era

Two well-preserved dinosaur fossils interlocked in a battle pose: one dinosaur is a Velociraptor, a smaller predator with sharp claws and teeth, while the other is a Protoceratops, an herbivorous dinosaur with a beak-like snout and a frilled neck.
Figure 8.36: Perhaps the greatest fossil ever found, a velociraptor attacked a protoceratops, and both were fossilized mid sequence.

Following the , the (“middle life”) was from 252 million years ago to 66 million years ago. As started to break apart, mammals, birds, and flowering plants developed. The is probably best known as the age of reptiles, most notably, the dinosaurs.

8.5.1 Mesozoic Tectonics and Paleogeography

GIF showing the progression of Pangea separating to become the continents in their current configuration.
Figure 8.37: Animation showing Pangea breaking up.

started breaking up (in a region that would become eastern Canada and United States) around 210 million years ago in the Late . Clear evidence for this includes the age of the in the Newark Supergroup basins and the Palisades of the eastern part of North America and the age of the Atlantic . Due to sea-floor spreading, the oldest rocks on the Atlantic’s floor are along the of northern Africa and the east coast of North America, while the youngest are along the .

Color-coded world map that shows the various ages of oceanic lithosphere. Continents are in gray. The color-coding and locations are as follows: the youngest oceanic lithosphere is 0 million years old and runs along the centers of the ocean basins where there are mid-ocean ridges, colored in red. Oceanic lithosphere ages get older away from the mid-ocean ridges, and the oldest oceanic lithosphere is 280 million years old near continental margins, colored purple.
Figure 8.38: Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along the coasts of the continents.

This age pattern shows how the Atlantic Ocean opened as the young Mid-Atlantic Ridge began to create the seafloor. This means the Atlantic ocean started opening and was first formed here. The southern Atlantic opened next, with South America separating from central and southern Africa. Last (happening after the ended) was the northernmost Atlantic, with Greenland and Scandinavia parting ways. The breaking points of each margin eventually turned into the passive boundaries of the east coast of the Americas today.

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Cross sectional sketch of an oceanic plate moving toward the right, subducting beneath continental crust; a volcanic arc forms on the continental crust above the subducting oceanic crust with a forearc in front of the volcanic arc; to the right of the volcanic arc is a series of mountain ridges thrusting upward labeled Sevier thrust belt; behind that is the foreland basin.
Figure 8.39: Sketch of the major features of the Sevier Orogeny.

In western North America, an active margin had started with , controlling most of the of that region in the . Another possible island- created the Sonoman in Nevada during the latest to the . In the , another island- caused the Nevadan , a large Andean-style and thrust belt. The Sevier Orogeny followed in the Cretaceous, which was mainly a volcanic arc to the west and a thin-skinned fold and thrust belt to the east, meaning stacks of shallow and built up the topography. Many of the structures in the Rocky Mountains today date from this .

Map of North America showing multiple seaways: the Western Interior Seaway spans north to south across the western part of Canada, the United States, and eastern Mexico where it connects to the Gulf of Mexico; the Hudson Seaway spans central Canada and connects to the Western Interior Seaway when it reaches the United States; and the Labrador Seaway spans east to west across the northernmost part of Canada; both the Western Interior Seaway and the Hudson Seaway connect to the Labrador Seaway at the northern ends.
Figure 8.40: The Cretaceous Interior Seaway in the mid-Cretaceous.

had an influence in one more important geographic feature in North America: the Western Interior Foreland , which flooded during high sea levels forming the . from the west was the Farallon Plate, an connected to the Pacific Plate (seen today as remnants such as the Juan de Fuca Plate, off the coast of the Pacific Northwest). was shallow at this time because a very young, hot and less dense portion of the Farallon was . This shallow caused a downwarping in the central part of North America. High sea levels due to shallow , and increasing rates of seafloor spreading and , high temperatures, and melted ice also contributed to the high sea levels. These factors allowed a shallow epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean to divide North America into two separate land masses, Laramidia to the west and Appalachia to the east, for 25 million years. Many of the deposits in Utah and Wyoming formed from swamps along the shores of this seaway. By the end of the , cooling temperatures caused the seaway to regress.

8.5.2 Mesozoic Evolution

Illustration of several dinosaurs walking on sandy beach with lush green forest in the foreground and background.
Figure 8.41: A Mesozoic scene from the late Jurassic.

The is dominated by reptiles, and more specifically, the dinosaurs. The saw devastated ecosystems that took over 30 million years to fully re-emerge after the . The first appearance of many modern groups of animals that would later flourish occurred at this time. This includes frogs (amphibians), turtles (reptiles), ichthyosaurs and plesiosaurs ( reptiles), mammals, and the archosaurs. The archosaurs (“ruling reptiles”) include ancestral groups that went at the end of the , as well as the flying pterosaurs, crocodilians, and the dinosaurs. Archosaurs, like the placental mammals after them, occupied all major environments: (dinosaurs), in the air (pterosaurs), aquatic (crocodilians) and even fully habitats ( crocodiles). The pterosaurs, the first group to take flight, like the dinosaurs and mammals, start small in the .

Illustration of two animals swimming underwater with long and streamlined bodies; their four limbs resemble paddle-like structures; their heads are elongated, featuring a pointed snout and sharp teeth; their bodies are covered in a smooth gray and tan skin texture.
Figure 8.42: A drawing of the early plesiosaur Augustasaurus from the Triassic of Nevada.

At the end of the , another event occurred, the fourth major in the geologic record. This was perhaps caused by the Central Atlantic Magmatic Province . The end- made certain lineages go extinct and helped spur the evolution of survivors like mammals, pterosaurs (flying reptiles), ichthyosaurs/plesiosaurs/mosasaurs ( reptiles), and dinosaurs.

A small shrew-like animal covered in white fur.
Figure 8.43: Reconstruction of the small (<5″) Megazostrodon, one of the first animals considered to be a true mammal.

Mammals, as previously mentioned, got their start from a reptilian synapsid ancestor possibly in the late . Mammals stayed small, in mainly nocturnal niches, with insects being their largest prey. The development of warm-blooded circulation and fur may have been a response to this lifestyle.

Drawing of the ilium bone; the pubis bone and ischium bone are each connected to the ilium bone; the bones of the pubis and ischium are close to each other.
Figure 8.44: Closed structure of a ornithischian hip, which is similar to a birds.

In the , species that were previously common, flourished due to a warmer and more tropical . The dinosaurs were relatively small animals in the of the , but became truly in the . Dinosaurs are split into two groups based on their hip structure, i.e. orientation of the pubis and ischium bones in relationship to each other. This is referred to as the “reptile hipped” saurischians and the “bird hipped” ornithischians. This has recently been brought into question by a new idea for dinosaur lineage.

Drawing of the ilium bone; the pubis bone and ischium bone are each connected to the ilium bone; the bones of the pubis and ischium are pointed away from each other.
Figure 8.45: Open structure of a saurischian hip, which is similar to a lizards.

Most of the dinosaurs of the were saurischians, but all of them were bipedal. The major adaptive advantage dinosaurs had was changes in the hip and ankle bones, tucking the legs under the body for improved locomotion as opposed to the semi-erect gait of crocodiles or the sprawling posture of reptiles. In the , limbs (or a lack thereof) were also important to another group of reptiles, leading to the evolution of Eophis, the oldest snake.

A four-legged animal in a bipedal stance with a long tail and slender body; its head is characterized by a long snout and sharp teeth; its body is covered in white feathers and it has long claws at the end of each leg.
Figure 8.46: Therizinosaurs, like Beipiaosaurus (shown in this restoration), are known for their enormous hand claws.

There is a paucity of dinosaur from the Early and Middle , but by the Late they were dominating the planet. The saurischians diversified into the giant herbivorous (plant-eating) long-necked sauropods weighing up to 100 tons and bipedal carnivorous theropods, with the possible exception of the Therizinosaurs. All of the ornithischians (e.g Stegosaurus, Iguanodon, Triceratops, Ankylosaurus, Pachycephhlosaurus) were herbivorous with a strong tendency to have a “turtle-like” beak at the tips of their mouths.

Archaeopteryx fossil embedded within flat face of tan sedimentary rock; the fossil shows features of reptiles, such as sharp teeth, fingers with claws, and a long bony tail, as well as features of birds, such as small, broad wings and the impression of feathers.
Figure 8.47: Archaeopteryx lithographica, specimen displayed at the Museum für Naturkunde in Berlin.

The pterosaurs grew and diversified in the , and another notable arial organism developed and thrived in the : birds. When Archeopteryx was found in the Solnhofen of Germany, a seeming dinosaur-bird hybrid, it started the conversation on the origin of birds. The idea that birds evolved from dinosaurs occurred very early in the history of research into evolution, only a few years after Darwin’s On the Origin of Species. This study used a remarkable of Archeopteryx from a transitional animal between dinosaurs and birds. Small meat-eating theropod dinosaurs were likely the branch that became birds due to their similar features. A significant debate still exists over how and when powered flight evolved. Some have stated a running-start model, while others have favored a tree-leaping gliding model or even a semi-combination: flapping to aid in climbing.

A massive four-legged dinosaur skeleton, with a long neck extending upwards and a massive body supported by sturdy legs; the head is small in proportion to its body.
Figure 8.48: Reconstructed skeleton of Argentinosaurus, from Naturmuseum Senckenberg in Germany.

The saw a further diversification, specialization, and domination of the dinosaurs and other fauna. One of the biggest changes on land was the transition to angiosperm-dominated flora. Angiosperms, which are plants with flowers and seeds, had originated in the , switching many plains to grasslands by the end of the . By the end of the , they had replaced gymnosperms (evergreen trees) and ferns as the dominant plant in the world’s forests. Haplodiploid eusocial insects (bees and ants) are descendants from wasp-like ancestors that co-evolved with the flowering plants during this time . The breakup of not only shaped our modern world’s geography, but biodiversity at the time as well. Throughout the , animals on the isolated, now separated island continents (formerly parts of ), took strange evolutionary turns. This includes giant titanosaurian sauropods (Argentinosaurus) and theropods (Giganotosaurus) from South America.

K-T Extinction

Bar graph with a horizontal axis going from the Cambrian to present toward the right and vertical axis going from 0 to 60 upward. There are multiple spikes with one spike at the end of the Cretaceous.
Figure 8.49: Graph of the rate of extinctions. Note the large spike at the end of the Cretaceous (labeled as K).

Similar to the end of the , the ended with the K-Pg (previously known as the ) 66 million years ago. This event was likely caused by a large (an extraterrestrial impactor such as an asteroid, , or comet) that collided with earth. Ninety percent of plankton species, 75% of plant species, and all the dinosaurs went at this time.

Massive rock on fire slamming into the earth.
Figure 8.50: Artist’s depiction of an impact event.

One of the strongest pieces of evidence comes from the iridium. Quite rare on Earth, and more common in , it has been found all over the world in higher concentrations at a particular layer of rock that formed at the time of the K-T boundary. Soon other scientists started to find evidence to back up the claim. Melted rock spheres, a special type of “shocked” called stishovite, that only is found at impact sites, was found in many places around the world. The huge impact created a strong thermal pulse that could be responsible for global forest fires, strong acid rains, a corresponding abundance of ferns, the first colonizing plants after a forest fire, enough debris thrown into the air to significantly cool temperatures afterward, and a 2-km high inferred from deposits found from Texas to Alabama.

Two satellite images of the Yucatan Peninsula; the second image has a dashed semicircle overlain on a faint semi-circular land imprint at the northern part of the peninsula labeled Trough, along with three black arrows pointing to small dot-shaped features labeled Cenotes (sinkholes).
Figure 8.51: The land expression of the Chicxulub crater. The other side of the crater is within the Gulf of México.

Still, with all this evidence, one large piece remained missing: the crater where the impacted. It was not until 1991 that the crater was confirmed using company geophysical data. Even though it is the third largest confirmed crater on Earth at roughly 180 km wide, the was hard to find due to being partially underwater and partially obscured by the dense forest canopy of the Yucatan Peninsula. Coring of the center of the impact called the peak ring contained , indicating the impact was so powerful that it lifted from the several miles toward the surface. In 2010, an international team of scientists reviewed 20 years of research and blamed the impact for the .

Map of south Asia with the geology of India color-coded: yellow Cenozoic rocks are found in northern India and in small slivers along the eastern coast; purple Deccan Volcanic rocks are found in west-central India; teal Mesozoic rocks are found in small slivers in northern India and central India; green Paleozoic rocks are found in small slivers in northern India; and pink Proterozoic-Archean rocks cover southern and eastern India with a large chunk in northern India as well.
Figure 8.52: Geology of India, showing purple as Deccan Traps-related rocks.

With all of this information, it seems like the case would be closed. However, there are other events at this time which could have partially aided the demise of so many organisms. For example, sea levels are known to be slowly decreasing at the time of the K-T event, which is tied to extinctions, though any study on gradual vs. sudden changes in the record is flawed due to the incomplete nature of the record. Another big event at this time was the Deccan Traps in India. At over 1.3 million cubic kilometers of material, it was certainly a large source of material hazardous to ecosystems at the time, and it has been suggested as at least partially responsible for the . Some have found the impact and eruptions too much of a coincidence, and have even linked the two together.

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8.6 Phanerozoic Eon: Cenozoic Era

A four-legged animal that resembles a rhinoceros with a long neck and a robust body supported by sturdy legs; the head is large and elongated, featuring a hornless snout; the skin texture is gray, thick, and wrinkled.
Figure 8.53: Paraceratherium, seen in this reconstruction, was a massive (15-20 ton, 15 foot tall) ancestor of rhinos.

The , meaning “new life,” is known as the age of mammals because it is in this that mammals came to be a dominant and large life form, including human ancestors. Birds, as well, flourished in the open niches left by the dinosaur’s demise. Most of the has been relatively warm, with the main exception being the that started about 2.558 million years ago and (despite recent warming) continues today. shifts in the west caused , but eventually changed the long-standing zone into a boundary.

8.6.1 Cenozoic Tectonics and Paleogeography

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Block diagram showing oceanic crust moving toward the right where it collides with continental crust and subducts down beneath it. Because the angle of subduction is shallow, the ocean crust travels inland before creating a volcanic arc on top of the continental plate, above where the oceanic crust has subducted beneath it--these are the Rocky Mountains.
Figure 8.54: Shallow subduction during the Laramide Orogeny.

In the , the of the Earth moved into more familiar places, with the biggest change being the closing of the Tethys Sea with such as the Alps, Zagros, and Himalaya, a that started about 57 million years ago, and continues today. Maybe the most significant feature that occurred in the of North America was the conversion of the west of California from a boundary zone to a boundary. off the of the western United States, which had occurred throughout the , had continued in the . After the Sevier in the late , a subsequent called the Laramide , occurred in the early . The Laramide was , different than the Sevier . It involved deeper crustal rocks, and produced bulges that would become mountain ranges like the Rockies, Black Hills, Wind River Range, Uinta Mountains, and the San Rafael Swell. Instead of descending directly into the , the shallowed out and moved eastward beneath the affecting the overlying hundreds of miles east of the margin and building high mountains. This occurred because the was so young and near the and the density of the was therefore low and was hindered. 

Map of western North America annotated with the location of the main San Andreas fault which runs from northwest of the Canadian coast, through western California, and southeast through Mexico. There are arrows on either side of the fault line showing relative movement: on the east side of the fault, movement is toward the lower right and on the west side of the fault, movement is toward the upper left.
Figure 8.55: Map of the San Andreas fault, showing relative motion.

As the itself started to subduct, the relative motion had changed. caused a relative convergence between the Farallon and the North American . On the other side of the from the Farallon was the Pacific , which was moving away from the North American . Thus, as the zone consumed the , the relative movement became instead of , which went on to become the San Andreas Fault System. As the San Andreas grew, it caused east-west directed forces to spread over the western United States, creating the province. The switched position over the last 18 million years, twisting the mountains around Los Angeles, and new in the southeastern California deserts may become a future San Andreas-style . During this switch from to , the nearly horizontal Farallon began to sink into the . This caused magmatism as the sank, allowing material to rise around it. This event is called the Oligocene ignimbrite flare-up, which was one of the most significant of ever, including the largest single confirmed eruption, the 5000 cubic kilometer Fish Canyon Tuff.

8.6.2 Cenozoic Evolution

Superfamily: hominoidea. Family: Hominidae and hylobatidae. Hominidae has 2 subfamilies: homininae and ponginae. Ponginae has genus pongo. homininae has 2 tribes: hominini (genuses homo and pan) and gorillini (genus gorilla)
Figure 8.56: Family tree of Hominids (Hominidae).

There are five groups of early mammals in the record, based primarily on teeth, the hardest bone in skeletons. For the purpose of this text, the most important group are the Eupantotheres, that diverge into the two main groups of mammals, the marsupials (like Sinodelphys) and placentals or eutherians (like Eomaia) in the and then diversified in the . The marsupials dominated on the isolated island continents of South America and Australia, and many went in South America with the introduction of placental mammals. Some well-known mammal groups have been highly studied with interesting evolutionary stories in the . For example, horses started small with four toes, ended up larger and having just one toe. Cetaceans ( mammals like whales and dolphins) started on land from small bear-like (mesonychids) creatures in the early and gradually took to water. However, no study of evolution has been more studied than human evolution. Hominids, the name for human-like primates, started in eastern Africa several million years ago.

Human-like skeleton in glass case. Skeleton is 1/2 fossil and 1/2 reconstructed skeleton.
Figure 8.57: Lucy skeleton, showing real fossil (brown) and reconstructed skeleton (white).

The first critical event in this story is an environmental change from jungle to more of a savanna, probably caused by changes in Indian Ocean circulation. While bipedalism is known to have evolved before this shift, it is generally believed that our bipedal ancestors (like Australopithecus) had an advantage by covering ground more easily in a more open environment compared to their non-bipedal evolutionary cousins. There is also a growing body of evidence, including the famous “Lucy” of an Australopithecine, that our early ancestors lived in trees. Arboreal animals usually demand a high intelligence to navigate through a three-dimensional world. It is from this lineage that humans evolved, using endurance running as a means to acquire more resources and possibly even hunt. This can explain many uniquely human features, from our long legs, strong achilles, lack of lower gut protection, and our wide range of running efficiencies.

Homo genus originated in southern africa and moved north through europe and asia, south through australia, and through greenland/alaska to north america and south america.
Figure 8.58: The hypothesized movement of the homo genus. Years are marked as to the best guess of the timing of movement.

Now that the hands are freed up, the next big step is a large brain. There have been arguments from a switch to more meat eating, cooking with fire, tool use, and even the construct of society itself to explain this increase in brain size. Regardless of how, it was this increased cognitive power that allowed humans to reign as their ancestors moved out of Africa and explored the world, ultimately entering the Americas through land bridges like the Bering Land Bridge. The details of this worldwide migration and the different branches of the hominid evolutionary tree are very complex, and best reserved for its own course.

Anthropocene and Extinction

Four graphs stacked on top of each other labeled Africa, Australia, North America, and Madagascar; each graph shows the large mammal population of the continent with an arrow to label when H. sapiens enters the continent; the horizontal axis is labeled Log (time) KYA and goes from over 100 on the left to below 0.1 on the right; the vertical axis is labeled Percent survival of Large Mammal species and goes from 0 on the bottom to 100 on the top. In each of the graphs, the large mammal population decreases after H. sapiens enters the continent.
Figure 8.59: Graph showing abundance of large mammals and the introduction of humans.

Humans have had an influence on the Earth, its ecosystems and . Yet, human activity can not explain all of the changes that have occurred in the recent past. The start of the , the last and current of the , is marked by the start of our current 2.58 million years ago. During this time , advanced and retreated, most likely due to (see chapter 15). Also at this time, various cold-adapted megafauna emerged (like giant sloths, saber-tooth cats, and woolly mammoths), and most of them went as the Earth warmed from the most recent maximum. A long-standing debate is over the cause of these and other extinctions. Is warming to blame, or were they caused by humans? Certainly, we know of recent human extinctions of animals like the dodo or passenger pigeon. Can we connect modern extinctions to extinctions in the recent past? If so, there are several ideas as to how this happened. Possibly the most widely accepted and oldest is the hunting/overkill . The idea behind this is that humans hunted large herbivores for food, then carnivores could not find food, and human arrival times in locations has been shown to be tied to increased rates in many cases.

Immense terraced dirt-lined open pit with snow-covered mountains visible around the back of the mine.
Figure 8.60: Bingham Canyon Mine, Utah. This open pit mine is the largest man-made removal of rock in the world.

Modern human impact on the environment and the Earth as a whole is unquestioned. In fact, many scientists are starting to suggest that the rise of human civilization ended and/or replaced the and defines a new geologic time interval: the . Evidence for this change includes extinctions, increased tritium (hydrogen with two neutrons) due to nuclear testing, rising pollutants like carbon dioxide, more than 200 never-before seen species that have occurred only in this , materials such as plastic and metals which will be long lasting “” in the geologic record, and large amounts of earthen material moved. The biggest scientific debate with this topic is the starting point. Some say that humans’ invention of agriculture would be recognized in geologic and that should be the starting point, around 12,000 years ago. Others link the start of the industrial revolution and the subsequent addition of vast amounts of carbon dioxide in the . Either way, the idea is that alien geologists visiting Earth in the distant future would easily recognize the impact of humans on the Earth as the beginning of a new geologic .

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Summary

The changes that have occurred since the inception of Earth are vast and significant. From the oxygenation of the , the progression of life forms, the assembly and deconstruction of several , to the of more life forms than exist today, having a general understanding of these changes can put present change into a more rounded perspective.

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URLs Linked Within This Chapter

Charles Walcott (1850–1927): http://www.nasonline.org/member-directory/deceased-members/20000936.html

Text References

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Figure References

Figure 8.1: Geologic time on Earth, represented circularly, to show the individual time divisions and important events. Woudloper; adapted by Hardwigg. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Geologic_Clock_with_events_and_periods.svg

Figure 8.2: Geological time scale with ages shown. USGS. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Geologic_time_scale.jpg

Figure 8.3: Artist’s impression of the Earth in the Hadean. Tim Bertelink. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Hadean.png

Figure 8.4: The global map of the depth of the moho. AllenMcC. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Mohomap.png

Figure 8.5: Dark side of the Moon. Apollo 16 astronauts via NASA. 1972. Public domain. https://en.wikipedia.org/wiki/File:Back_side_of_the_Moon_AS16-3021.jpg

Figure 8.6: Artist’s concept of the giant impact from a Mars-sized object that could have formed the moon. NASA/JPL-Caltech. 2017. Public domain. https://www.nasa.gov/multimedia/imagegallery/image_feature_1454.html

Figure 8.7: Water vapor leaves comet 67P/Churyumov–Gerasimenko. ESA/Rosetta/NAVCAM. 2015. CC BY-SA 3.0 IGO. https://commons.wikimedia.org/wiki/File:Comet_on_7_July_2015_NavCam.jpg

Figure 8.8: Artist’s impression of the Archean. Tim Bertelink. 2017. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Archean.png

Figure 8.9: 2015 image from NASA’s New Horizons probe of Pluto. NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute. 2015. Public domain. https://commons.wikimedia.org/wiki/File:Nh-pluto-in-true-color_2x_JPEG-edit-frame.jpg

Figure 8.10: Simulation of before, during, and after the late heavy bombardment. Kesäperuna. 2019. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Lhborbits.png

Figure 8.11: The layers of the Earth. Drlauraguertin. 2015. CC BY-SA 3.0. https://wiki.seg.org/wiki/File:Earthlayers.png

Figure 8.12: Subduction of an oceanic plate beneath another oceanic plate, forming a trench and an island arc. USGS. 1999. Public domain. https://commons.wikimedia.org/wiki/File:Oceanic-continental_convergence_Fig21oceancont.gif

Figure 8.13: Geologic provinces with the Shield (orange) and Platform (pink) comprising the Craton, the stable interior of continents. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:World_geologic_provinces.jpg

Figure 8.14: The continent of Zealandia. NOAA. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Zealandia_topography.jpg

Figure 8.15: Fossils of microbial mats from Sweden. Smith609. 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Runzelmarken.jpg

Figure 8.16: Greenhouse gases were more common in Earth’s early atmosphere. Kindred Grey. 2022. CC BY 4.0. Water molecule 3D by Dbc334, 2006 (Public domain, https://commons.wikimedia.org/wiki/File:Water_molecule_3D.svg). Nitrous-oxide-dimensions-3D-balls by Ben Mills, 2007 (Public domain, https://commons.wikimedia.org/wiki/File:Nitrous-oxide-dimensions-3D-balls.png). Methane-CRC-MW-3D-balls by Ben Mills, 2009 (Public domain, https://en.m.wikipedia.org/wiki/File:Methane-CRC-MW-3D-balls.png). Carbon dioxide 3D ball by Jynto, 2011 (Public domain, https://commons.wikimedia.org/wiki/File:Carbon_dioxide_3D_ball.png).

Figure 8.17: Diagram showing the main products and reactants in photosynthesis. At09kg. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Photosynthesis.gif

Figure 8.18: Alternating bands of iron-rich and silica-rich mud, formed as oxygen combined with dissolved iron. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:MichiganBIF.jpg

Figure 8.19: One possible reconstruction of Rodinia 1.1 billion years ago. John Goodge. 2011. Public domain. https://commons.wikimedia.org/wiki/File:Rodinia_reconstruction.jpg

Figure 8.20: Modern cyanobacteria (as stromatolites) in Shark Bay, Australia. Paul Harrison. 2005. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Stromatolites_in_Sharkbay.jpg

Figure 8.21: Fossil stromatolites in Saratoga Springs, New York. Rygel, M.C. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Stromatolites_hoyt_mcr1.JPG

Figure 8.22: Dickinsonia, a typical Ediacaran fossil. Verisimilus. 2007. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:DickinsoniaCostata.jpg

Figure 8.23: The trilobites had a hard exoskeleton, and is an early arthropod, the same group that includes modern insects, crustaceans, and arachnids. Wilson44691. 2010. Public domain. https://commons.wikimedia.org/wiki/File:ElrathiakingiUtahWheelerCambrian.jpg

Figure 8.24: Trilobites, by Heinrich Harder, 1916. Heinrich Harder. 1916. Public domain. https://commons.wikimedia.org/wiki/File:Trilobite_Heinrich_Harder.jpg

Figure 8.25: Laurentia, which makes up the North American craton. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:North_america_craton_nps.gif

Figure 8.26: A reconstruction of Pangaea, showing approximate positions of modern continents. Kieff. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Pangaea_continents.svg

Figure 8.27: Anomalocaris reconstruction by the MUSE science museum in Italy. Matteo De Stefano/MUSE. 2016. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Anomalocaris_canadensis_-_reconstruction_-_MUSE.jpg

Figure 8.28: Original plate from Walcott’s 1912 description of Opabinia, with labels: fp = frontal appendage, e = eye, ths = thoracic somites, i = intestine, ab = abdominal segment. Charles Doolittle Walcott. 1912. Public domain. https://commons.wikimedia.org/wiki/File:Opabinia_regalis_-_Walcott_Cambrian_Geology_and_Paleontology_II_plate_28_.jpg

Figure 8.29: A modern coral reef. Toby Hudson. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Coral_Outcrop_Flynn_Reef.jpg

Figure 8.30: Guadalupe National Park is made of a giant fossil reef. Zereshk. 2007. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Guadalupe_Nima2.JPG

Figure 8.31: The placoderm Bothriolepis panderi from the Devonian of Russia. Haplochromis. 2007. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Bothriolepis_panderi.jpg

Figure 8.32: Several different types of fish and amphibians that led to walking on land. Dave Souza; adapted by Pixelsquid. 2020. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Fishapods.svg

Figure 8.33: A reconstruction of the giant arthropod (insects and their relatives) Arthropleura. Tim Bertelink. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Arthropleura.png

Figure 8.34: Reconstruction of Dimetrodon. Max Bellomio. 2019. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Dimetrodon_grandis_3D_Model_Reconstruction.png

Figure 8.35: World map of flood basalts. Williamborg. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Flood_Basalt_Map.jpg

Figure 8.36: Perhaps the greatest fossil ever found, a velociraptor attacked a protoceratops, and both were fossilized mid sequence. Yuya Tamai. 2014. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Fighting_dinosaurs_(1).jpg

Figure 8.37: Animation showing Pangea breaking up. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Pangea_animation_03.gif

Figure 8.38: Age of oceanic lithosphere, in millions of years. Muller, R.D., M. Sdrolias, C. Gaina, and W.R. Roest (2008) Age, spreading rates and spreading symmetry of the world’s ocean crust, Geochem. Geophys. Geosyst., 9, Q04006, doi:10.1029/2007GC001743. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Age_of_oceanic_lithosphere.jpg

Figure 8.39: Sketch of the major features of the Sevier Orogeny. Pinkcorundum. 2011. Public domain. https://www.wikiwand.com/en/Sevier_orogeny#Media/File:Sevierorogeny.jpg

Figure 8.40: The Cretaceous Interior Seaway in the mid-Cretaceous. By William A. Cobban and Kevin C. McKinney, USGS. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Cretaceous_seaway.png

Figure 8.41: A Mesozoic scene from the late Jurassic. Gerhard Boeggemann. 2006. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Europasaurus_holgeri_Scene_2.jpg

Figure 8.42: A drawing of the early plesiosaur Agustasaurus from the Triassic of Nevada. Nobu Tamura. 2008. CC BY 3.0. https://commons.wikimedia.org/wiki/File:Augustasaurus_BW.jpg

Figure 8.43: Reconstruction of the small (<5″) Megazostrodon, one of the first animals considered to be a true mammal. Theklan. 2017. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Megazostrodon_sp._Natural_History_Museum_-_London.jpg

Figure 8.44: Closed structure of a ornithischian hip, which is similar to a birds. Fred the Oyster. 2014. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Ornithischia_pelvis_structure.svg

Figure 8.45: Open structure of a saurischian hip, which is similar to a lizards. Fred the Oyster. 2014. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Saurischia_pelvis_structure.svg

Figure 8.46: Therizinosaurs, like Beipiaosaurus (shown in this restoration), are known for their enormous hand claws. Matt Martyniuk. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Beipiao1mmartyniuk.png

Figure 8.47: Archaeopteryx lithographica, specimen displayed at the Museum für Naturkunde in Berlin. H. Raab. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Archaeopteryx_lithographica_(Berlin_specimen).jpg

Figure 8.48: Reconstructed skeleton of Argentinosaurus, from Naturmuseum Senckenberg in Germany. Eva K. 2010. CC BY-NC-ND 3.0. https://commons.wikimedia.org/wiki/File:Argentinosaurus_DSC_2943.jpg

Figure 8.49: Graph of the rate of extinctions. Smith609. 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Extinction_intensity.svg

Figure 8.50: Artist’s depiction of an impact event. Made by Fredrik. Cloud texture from public domain NASA image. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Impact_event.jpg

Figure 8.51: The land expression of the Chicxulub crater. NASA/JPL-Caltech. 2000. Public domain. https://commons.wikimedia.org/wiki/File:Yucatan_chix_crater.jpg

Figure 8.52: Geology of India, showing purple as Deccan Traps-related rocks. CamArchGrad. 2007. Public domain. https://en.wikipedia.org/wiki/File:India_Geology_Zones.jpg

Figure 8.53: Paraceratherium, seen in this reconstruction, was a massive (15-20 ton, 15 foot tall) ancestor of rhinos. Tim Bertelink. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Indricotherium.png

Figure 8.54: Shallow subduction during the Laramide Orogeny. Melanie Moreno, USGS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Shallow_subduction_Laramide_orogeny.png

Figure 8.55: Map of the San Andreas fault, showing relative motion. Kate Barton, David Howell, and Joe Vigil via USGS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Sanandreas.jpg

Figure 8.56: Family tree of Hominids (Hominidae). Fred the Oyster. 2014. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Hominidae_chart.svg

Figure 8.57: Lucy skeleton, showing real fossil (brown) and reconstructed skeleton (white). Andrew. 2007. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Lucy_Skeleton.jpg

Figure 8.58: The hypothesized movement of the homo genus. NordNordWest. 2014. Public domain. https://commons.wikimedia.org/wiki/File:Spreading_homo_sapiens_la.svg

Figure 8.59: Graph showing abundance of large mammals and the introduction of humans. ElinWhitneySmith. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Extinctions_Africa_Austrailia_NAmerica_Madagascar.gif

Figure 8.60: Bingham Canyon Mine, Utah. Doc Searls. 2016. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Bingham_Canyon_mine_2016.jpg

 

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