ORIGIN AND STRUCTURE OF THE UNIVERSE, SOLAR SYSTEM & EARTH
Scientists believe the universe began between 15 and 20 billion years ago after a huge explosion sent matter out in all directions. The cloud of dust and gas produced from this event eventually began to consolidate, creating the galaxies and stars.

Our solar system was formed when a cloud of interstellar gas and dust, called a nebula, collapsed. The nebula became hotter, denser and began to rotate. Slow cooling caused various metals and minerals to condense and the collisions of these materials formed larger objects called planetesimals. The largest planetesimals became spherical from their own gravitational force and formed the present day planets.

Asteroids, also called minor planets, are large fragments of rock that orbit the sun between Mars and Jupiter. Scientists believe most asteroids are "left over" planetesimals.

Meteoroids and Meteorites
Any particle of debris traveling through space is called a meteoroid. They can be as small as a grain of sand or as large as an asteroid. The parent bodies of most meteoroids are located in the asteroid belt. Gravitational tides, caused by Jupiter, occasionally eject some of these meteoroids, sending them into Earth-crossing orbits where they have the potential of landing on earth. When a meteoroid burns up in earth's atmosphere, it is called a meteor or shooting star. When on lands on earth, it is called a meteorite. Meteorites are classified according to the amount of silicate minerals and nickel-iron metal that they contain. They include stony meteorites, stony-iron meteorites and iron meteorites. Stony meteorites contain less than 10% nickel-iron metal. Stony-iron meteorites contain roughly equal amounts of silicates and nickel-iron metal. The iron meteorites contain more than 90% nickel-iron metal.

Primordial Material
The stony meteorites are further divided into two subgroups, the chondrites and the achondrites. Chondrites, thought to be left over planetesimals, contain small spherical inclusions called chondrules. Scientists believe chondrules are droplets that condensed from the original nebula. Achondrites, irons and stony-irons are all igneous meteorites formed from melted chondrite material.

The Interior of the Earth
Scientists theorize that the earth may have been formed from the accumulation of chondritic material. However, as the Earth grew larger during its formation, it eventually developed enough internal heat from the decay of radioactive elements to melt. The denser materials such as nickel and iron sank to the center and the lighter rocky materials floated to the surface. The end result was a differentiated structure consisting of shells of various compositions.

The inner core is a 1,216 kilometer (756 mile) thick layer composed of solid nickel and iron.

The outer core is a 2,270 kilometer (1,410 mile) thick layer composed of molten nickel and iron.

The mantle is a 2,885 kilometer (1,789 mile) thick layer composed of dense silicates such as olivine and pyroxene.

The crust includes the continents and the ocean floor and ranges in thickness from 5 kilometers (3 miles) along the ocean floor to 50 kilometers (32 miles) along the highest mountain ranges. The continents are composed mostly of granite and the ocean floors are mostly basalt. The crust also contains a thin layer of sedimentary rocks.

The lithosphere is the rigid outer layer of the earth that includes both the upper mantle and the crust of the earth. The lithosphere, composed of approximately 18 plates, lies just above the Asthenosphere.

Lying just below the lithosphere is an area of the mantle known as the asthenosphere. Due to higher temperatures and pressures, the rock within this layer behaves like silly putty (a solid that flows). Motion in the asthenosphere, probably due to slow moving convection currents, causes the lithospheric plates to move around on the surface of the Earth. Scientists call this process Plate Tectonics.

The lithospheric plates are continually created along mid-ocean ridges (or spreading centers) and continually destroyed along the trenches (or subduction zones). The rigid lithospheric plates move slowly on top of the plastic-like asthenosphere and are driven by the internal heat of the Earth. This movement generates volcanoes and earthquakes along plate boundaries and results in mountain building along the continental edges.

Convergent Plates
When plates collide, one is subducted (forced under) the other. The force of these collisions usually results in the folding and faulting of crustal rock, producing earthquakes and volcanoes.
A continent/ocean convergent boundary occurs when an oceanic plate is subducted below a continental land mass. This results in the formation of a chain of volcanic mountains. Mount St. Helens is part of a volcanic mountain chain in western North America.

An ocean/ocean convergent boundary occurs when one oceanic plate is subducted below another oceanic plate. This results in the formation of a chain of volcanic islands. The Aleutian Islands were formed in this manner.

A continent/continent convergent boundary creates large mountain chains such as the Himalayan Mountains.

Divergent plates
When plates move apart, rifts occur in the crust and magma flows into these openings in the crust and pushes the material on either side upward, forming ridges. Rifting continues to form the Atlantic mid-ocean ridges and the East African rift valley.

Lateral Plate Boundaries, also called transform faults, occur where plates slide past each other but are neither created nor destroyed. Stresses build up along the fault line. When the stresses become too much the plates slip and an earthquake is generated. The San Andreas fault is an active transform fault.

ROCKS AND MINERALS

Marty the Miner introduces this section and Rocky Road addresses some major questions along the wall. An astounding amount of information is found here.

What are Minerals?

• Minerals occur naturally
• They are inorganic (nonliving)
• They are solids that have an orderly internal crystalline structure
• They have unique chemical compositions and physical properties
• They are the building blocks of rocks
• Minerals are composed of elements

What are Rocks?

A rock is a naturally formed combination of minerals.

Eight elements make up over 98% of the earth's continental crust; they include Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, and Magnesium.

Scientists classify rocks into three main groups: Igneous, sedimentary, and metamorphic.

Igneous rocks are formed when magma (molten rock) is cooled to form a solid. There are two types of igneous rock: Intrusive and extrusive.

Extrusive igneous rocks are formed from volcanic ash or lava (surface magma) that solidifies after a volcanic eruption. This type of rock usually has a fine grained appearance and is termed aphanitic. Some extrusive rocks are tuff, pumice, scoria, obsidian, basalt, andesite and rhylolite.

Intrusive igneous rocks are rocks that intrude (cut into) other rock material inside the earth. Since intrusive magma cools more slowly than extrusive magma, these rocks tend to have a texture where larger crystals can be seen and are termed phaneritic.

Sedimentary rocks are formed through the accumulation and solidification of sediments (rock debris, minerals, or organic material). There are three basic types: Clastic, Chemical, and Biochemical.

Clastics (also know as detrital) are sedimentary rocks composed of solid particles of preexisting rocks (clay, silt, sand, and gravel). Examples include breccia, conglomerate, sandstone, siltstone, and shale.

Chemical sedimentary rocks are created from material that was taken into solution during the process of erosion and carried to a lake or the sea. If the water evaporates, a rock can be formed. These types of rocks are called evaporites. Rock salt and gypsum are examples.

Biochemical sedimentary rocks result from the accumulation and eventual recrystalization of material that was organic (living). These type of rocks can be either clastic (termed bioclastic) or organic evaporites. They include limestone, dolomite and coquina.

Metamorphic rocks are formed from preexisting rocks that have been subjected to the forces of heat, pressure, or chemical action. There are two basic textures that identify metamorphic rocks. They include foliated and nonfoliated.

Foliated rocks such as gneiss, slate and schist have a banded or layered texture. Nonfoliated rocks have no distinct banding and the minerals do not align themselves in any particular arrangement. They often have a granular texture. Marble and quartzite are nonfoliated.

The Rock Cycle
The earth's surface changes constantly. Nothing is lost or gained as forces inside the earth bring newly formed rocks to the surface.

Natural forces such as gravity, wind, running water, chemical breakdown, or freezing and thawing wear down the earths rocks.

Transported fragments are deposited in a stream, lake , ocean, or on land. In time, sediments are covered and compacted to form sedimentary rocks.

Extreme temperatures and pressures can change rocks (metamorphic rocks) and may even melt them (igneous rocks.)

Round and round and round it goes. Elements make minerals that make rocks that change in time through the ROCK CYCLE.

EARTH, A TIMELESS TALE

The timeline in the north gallery displays and summarizes accumulated knowledge about the changes that have occurred since the earth's formation. The information is given on several levels, from simple cartoons to scientific data. This was done to accommodate different age and educational levels of visitors and to stimulate further interest and research.

One level consists of the major time periods identified on the top valance. On the posts are globes of earth that show the position of land masses as each time period changed.

The exhibit begins with Father Time's introduction and continues with comments from Ms. Lotsa Time on the posts between periods. Since fossils rarely portray the complete plant or animal, cartoon-like characters of significant organisms have been added, with pertinent information about the organism(s) and/or time. Fossils of organisms from each period are displayed and modern species have been added for comparison.

The reading rail in this gallery includes specific material about Pennsylvania and Berks County as well as small fossils and hands-on materials.

Fathertime
"The earth has been here about 4.6 billion years. Check out THE WALL with my niece Lotsa to see how rocks and fossils tell the story. If you want to know more, take a ride on the "READING RAIL."

Geological Time Scale
Many years ago, geologists (scientists who study rocks and minerals) and paleontologists (scientists who study fossils) created a relative time scale for the age of the earth. The scale is based on the correlation of fossils found within layered sedimentary rocks and is divided into two eons, the Phanerozoic and Precambrian.

The Precambrian eon is a very long, LONG period of time from the beginning of earth to the time of abundant life on earth. It covered approximately 87% of our planets history.

Our story begins with the first period of the Paleozoic era. Follow the color bar around the room and find your favorite period in each of the main eras. As you travel through time, it is important to note that Dramatic environmental changes throughout time have caused corresponding changes to the plants and animals that lived during each division on the time scale.

FOSSILS

A fossil is the remains or evidence of prehistoric life preserved in the earth's crust. Most are samples of an organisms, hard parts such as bones, teeth, shells, roots, bark, and seeds, but sometimes, a fossil can be the whole organism. The conditions for fossilization usually include the rapid burial an organism in sediments such as sand, silt, clay, or volcanic ash. Some animals become frozen, dried out, or covered by plant resin.

Types of Fossilization

Permineralization occurs when minerals fill the porous areas of bones or the empty spaces of a shell after the animal decays. The bone or shell becomes very heavy and often, the actual shell or tooth enamel is left intact.

Petrification or replacement occurs when the entire cell and other solid materials of the organism are replaced by minerals dissolved in the surrounding ground water. Most of the original detail of the organism is preserved.

Actual preservation occurs when the decay of an organism is stopped abruptly. Freezing, bog preservation, and tar pits are instances where organisms have been found intact.

Amber is fossilized tree resin that frequently contains the preserved remains of insects.

Desiccation or mummification occurs when an organism drys out, leaving behind its preserved remains.

Molds are created when an organism decays leaving a cavity that is the same shape and size of the organism. If the mold is filled with sediment, the result would be a three-dimensional cast of the organism.

Distillation or carbonization occurs following the compression of the sediments surrounding an organism. The only part of the organism that remains is a carbon residue.

Occasionally, an organism may fall into soft mud leaving an impression of itself. Many fossil leaves are impressions of the trees they fell from.

Trace fossils are the indirect evidence of an organism such as preserved tracks, trails, burrows, coprolites (fossilized excreta), and gastroliths (dinosaur stomach stones).

Pseudo fossils are rocks that resemble fossils. Common pseudo fossils include concretions and dendritic rocks.

PRECAMBRIAN EON
By the end of the Precambrian Era a super continent called Laurentia broke up and land masses were dispersed near the equator.

*A Berks County Highlight - Jutting into the center of the county from the east and just south of the Great Valley is the Reading Prong Section of the New England province. The bedrock in this area dates to the Precambrian time. It is composed of metamorphic and igneous rocks including gneiss, granite, and marble.

AGE OF INVERTEBRATES

Cambrian Period
During the Cambrian period, two elongated seas covered the eastern and western parts of what is now the North American continent. Cambrian seas supported a variety of marine invertebrates including mollusks, brachiopods, sponges, and arthropods (animals with jointed legs). The first animals with hard parts developed in the warm waters along the shores. The most abundant animal was the trilobite. All life at this time lived in the sea. Toward the end of the Cambrian cold water mixed with the warm seas, causing some species to become extinct.

Ordivician Period
During the Ordovician period, a land mass known as Gondwana moved south, opening the seas and flooding more of the land including much of North America. Coral reefs formed, providing food and shelter for many invertebrates.

Trilobites continued to thrive and brachiopods were more abundant. The first fish, called ostracoderms, began to appear and many types of straight shelled cephalopods (head-footed animals) roamed the seas.

By the end of the Ordovician, mountains began to form on the eastern border of North America. A large depression, called the Appalachian basin, was formed to the West of these mountains. Eventually, the mountains began to erode, leaving large quantities of silt, clay, sand and gravel in the watery basin.

As Gondwana continued its southward movement, it crossed the south pole. Huge glaciers that formed on this continent initiated the first ice age and the first global mass extinction.

A Berks County Highlight - The Great Valley Section of the Valley and Ridge province lies south of the Blue Mountains and extends across the middle of the county. The bedrock of the Great Valley contains shales, limestone, dolomite and sandstone deposited during the Cambrian and Ordovician Periods. The erosion of these sedimentary rocks has produced the rolling hills in the valley.

AGE OF FISHES

During the Silurian and Devonian periods, Gondwana began to move north. North America was still near the equator.

Silurian Period
At the start of the Silurian period, North America was covered with shallow seas. Sediments from the mountains created during the Ordovician continued to fill the Appalachian basin. As the erosion process continued, the height of the mountains was greatly reduced.

By the end of the Silurian, the source of sediments shifted from clastic (sand and gravel) to carbonates such as limestone. Oceans eventually retreated and landlocked seas quickly evaporated resulting in large deposits of salts and gypsum in parts of Pennsylvania, Ohio, New York, and Michigan.

Life during the Silurian was mainly confined to the sea but a few plants began to move onto land. Many new animal species appeared including the eurypterids, a large scorpion like creatures that could move between the land and water. Corals, trilobites, brachiopods, and crinoids were also abundant.

A Berks County Highlight - The northern border of the county includes the Blue Mountains which are part of the Appalachian Mountain Section of the Valley and Ridge province. They are composed of very hard quartz conglomerate and sandstone deposited during the Silurian and are part of the Tuscarora Formation.

Devonian Period
At the beginning of the Devonian period, inland seas were sparse but eventually rose again to cover large parts of North America. These seas were home to a variety of animals including large cephalopods with coiled and straight shells.

A variety of fish developed during the Devonian including the first jawed fish (acanthodians) , plate-skinned fish (placodoerms), the cartilaginous fish (chondrichthyes) and the bony fish (osteichthyes).

Land plants such as the lycopods, sphenophytes (horsetail rushes), and ferns became wide spread and the first insects, mites, and spiders appeared. Even more notable was the development of early amphibians from the crossopterygian (lobe finned) fishes.

THE DEVONIAN SEA

During the Devonian period, the Deer Lake area was covered by a warm shallow sea. Many organisms lived in this sea including bryozoans, corals, brachiopods, sponges, trilobites, jellyfish, crinoids, sea stars, and mollusks, as shown in the scene. Most of them are now extinct.

By the end of the Devonian, the collision of two large land masses created a new mountain range called the Acadian Mountains east of Pennsylvania. As this new range began to erode, sediments were again deposited in the Appalachian basin. These sediments would eventually become the dark shales and red sandstone common to Pennsylvania.

Another mass extinction marked the end of the Devonian period.

CARBONIFEROUS TIME - AGE OF AMPHIBIANS

The Mississippian and Pennsylvanian periods are commonly known as the Carboniferous time. During this time large quantities of the coal and oil we find today, were started. It was also the last time marine waters covered any part of Pennsylvania.

Mississippian Period
During the Mississippian period, shallow seas flooded much of the central and western portions of the North American continent including the southwestern and northeastern areas of Pennsylvania.

Corals, brachiopods, crinoids, bryozoans, and foraminifera were widespread and abundant. Fish continued to evolve including many varieties of sharks. Some scientists believe the sharks of the Mississippian contributed to the decline of trilobites.

Pennsylvanian Period
During this period, Pennsylvania was located just south of the equator. The tropical environment and periodic flooding of shallow seas provided habitats for a variety of plants and animals. Large cockroaches and huge dragonflies were abundant and the first reptiles appeared.

Great swamps with seed ferns (now extinct), calamites, and huge trees called lycopods (scale trees) covered much of our state. Over time, the repeated growth, death, and compaction of these plants resulted in the coal deposits found throughout Pennsylvania.

COAL

Geologists identify coal, oil and natural gas as "fossil fuels". A fossil is any remains, impression, cast, or trace of an animal or plant from the past, especially the parts petrified or converted to stone.

Coal is the only fossil fuel containing identifiable remains. Coal forms when dead plants accumulate under oxygen-poor conditions that prevent total decay. Bacteria in swamp water release oxygen and hydrogen from the plants but are destroyed by the acids formed in the process.

The soft brown Peat that forms from the plants contains easily recognized plant structures.

With shallow burial, peat slowly compacts into the soft brown coal, Lignite.

As deeper burial increases temperature and pressure, chemical reactions release water and organic gases (swamp gas). This forms Bituminous or soft coal, which is one tenth the thickness of the original peat. Lignite and Bituminous are sedimentary rocks.

Folding and deformation cause the release of more gases and water to form the metamorphic rock, Anthracite or hard coal. Anthracite is very hard and clean burning. It is almost pure carbon.

Permian Period
During the Permian period, the North American continent collided with Africa resulting in the creation of the Appalachian mountains. All the continents of the world eventually joined to form a supercontinent called Pangea.

The formation of Pangea during the Permian period affected all life on earth. The shallow seas that spread through Pennsylvania during the carboniferous time were cut off by the newly formed Appalachian Mountains. As a result, all the great swamps dried up and the plant life began to change. The new forests contained cycads, ginkgos and conifers.

Many marine organisms including trilobites became extinct. Reptiles were rapidly developing on land and amphibians were still abundant. New insects such as mayflies, beetles and grasshoppers began to appear.

Another mass extinction marked the end of the Paleozoic Era.

MESOZOIC ERA - AGE OF REPTILES

DINOSAURS
Dinosaurs or "terrible lizards" dominated the planet during the Mesozoic Era, over 160 million years ago. Most people are familiar with the larger species, but we now know that dinosaurs ranged in size from small as a mouse to bigger than a house. Recent fossil discoveries throughout the world have provided a more complete picture of dinosaur life.
Scientists now place most dinosaurs into two major groups based on their hip structure: Saurischian and Ornithischian.

Saurischian Dinosaurs have hip bones resembling other reptiles and include two groups, the carnivorous Theropods and the herbivorous Sauropodomorphs.

The Theropods are the Saurischian carnivores, from Tyranosaurus to the small, toothless Oviraptor. They characteristically had long necks, forefeet with a longer second finger and off-set thumb, and large, sharp, pointed teeth. They walked on two feet.

The Sauropodomorphs are four-footed Saurischian herbivores. Smaller ones appeared in the late Triassic, but this group also includes the giants of the Mesozoic; Diplodicus and Apatosaurus.

The Ornithiscian Dinosaurs have hip structure much like modern birds and a horn-like beak on the lower jaw but modern birds are not descended from these dinosaurs. There are five major groups of these herbivores.

The Ornithopods ran on their hind legs and varied greatly in size and appearance. They included the Iguanodon and duck-billed herd dinosaurs, the Hadraosaurs. Forms of Ornithopods lived throughout the Mesozoic and Cretaceous Periods.

The Ceratopians were frilled and horned, with narrow beaks. They were varied and abundant through the second half of the Cretaceous Period.

The Pachycephalosaurs have left few remains. They were the "boneheads" of the Mesozoic; their domed foreheads were solid bone.

The Stegosaurs (Stegosaurus) were the plated dinosaurs of the Jurassic. Their spines and plates suggest to some researchers that they were warm-blooded.

The Ankylosaurs had short legs and wore body armor of thick, bony plates. They first appeared in the Jurassic, but were not abundant until the late Cretaceous.

Triassic Period
The Triassic period marked the beginning of the Mesozoic era.

North America moved north placing Pennsylvania in the northern hemisphere.
Sometime during the early part of the Triassic period, Pangea began to separate into North America and Africa and the Atlantic Ocean was born. The separation of Africa and North America created large freshwater lakes in rift basins. These basins became populated with many new types of plants and animals.

On the lake shore and in the mud flats, many varieties of land reptiles, including dinosaurs, left their footprints in the mud and sand. The southeastern part of Berks County is one of the best areas in the world to find these fossil footprints.

Tracking Dinosaurs
Slab 96-61-1 , found in Exeter Township south of Jacksonwald, PA., dates from the late Triassic Period (approximately 200 million years ago). The slab contains two distinct types of fossil footprints: three-toed tracks known as Grallator and five-toed tracks known as Brachychirotherium.

The Grallator tracts were made by small to medium-sized carnivorous dinosaurs, walking on two feet. The lengths of the tracks tells researchers they were made by dinosaurs three to 15 feet in length.

Since many species had similar footprints, it is impossible to be certain which dinosaurs made these prints. Most researchers, however, believe dinosaurs known as coelurosaurs probably made the tracks. Coelurosaurs include the Coelophysis in the exhibit.

The Brachychirotherium were made by animals walking on four feet. Paleontologists think these prints were made by plant eating reptiles known as Aetosaurs, not by dinosaurs. The meat-eating group known as Rawsuchids, could also have made these tracks.

This discovery of Brachychirotherium near the Triassic-Jurassic boundary marks their extent in geological time. For paleontologists, this sheds light on the timing and duration of the large mass extinction at the end of the Triassic Period.

As the Triassic period advanced, reptiles continued to flourish and evolve. Many dinosaurs and mammal like reptiles appeared including marine reptiles such as ichthyosaurs and plesiosaurs. Other marine organisms such as ammonites became extinct. Gymnosperms became the dominant land plants.

A Berks County Highlight - The southern tip of the County lies in the Gettysburg-Newark Lowland Section of the Piedmont province. This area of the county is recognized by its rolling hills, steep ridges and open meadow areas. The bedrock consists of diabase, conglomerates, sandstone and shale deposited during both the Triassic and Jurassic Periods.

As the Triassic ended and the Jurassic began there was another unexplained destruction of species, especially marine organisms.

Jurassic Period
Africa and America continued to move apart, enlarging the Atlantic Ocean. During the Jurassic, marine waters invaded the western shores of North America.

Large reptiles and other land animals such as mammals survived extinction. Dinosaurs became dominant and the first birds arrived. Gymnosperms continued to dominate the land plants, and by the end of the period marine organisms again flourished.

TALE OF A TAIL

Following news of a fossil find in northeastern Utah, several museums and universities sent naturalists to investigate. In 1909 the Carnegie Museum of Natural History sent Earl Douglas. His excavation uncovered one of the largest deposits of dinosaur fossils ever found. There were so many specimens, the excavation continues today. The site is now the Dinosaur National Monument.

Many of the early excavated specimens were distributed to museums around the state. The Reading Public Museum received a portion of vertebrae in 1957. For years, these vertebrae, were identified as part of a Diplodocus from field #60 on the Earl Douglas quarry map.

In 1980, John McIntosh from the Carnegie Museum of Natural History reidentified the fossil as the caudal vertebrae of an Apatosaurus. This ended years of confusion about the identity of #60.

Land changes continued. As the Atlantic widened, pressure on the western side of America formed long chains of mountains.

Cretaceous Period
During the Cretaceous period rifting continued, broadening inland seas. Animal life flourished, especially marine invertebrates, fish, insects and dinosaurs.

As plants diversified, the first flowering plants (angiosperms) developed. Within ten million years, they replaced conifers and their relatives (gymnosperms) in most habitats.

The Cretaceous ended with another mass extinction. Half the earth's species, including dinosaurs, were eradicated.

CENOZOIC ERA
By the Cenozoic, continents were moving toward their present location. Several climactic changes occurred leading to the increase in the variety of mammal species. Flowering plants became the dominate land plants and many still exist today.

Paleocene Epoch
The age of mammals began in the Paleocene epoch and continues today. Flightless giant birds appeared, along with the first monkey-like prosimians.

Eocene Epoch
The first horses, (Eohippus) , elephants, whales and anthropoids appeared during the Eocene epoch.

Diatryma was a flightless bird of the Eocene. The head and feet are similar to some dinosaurs, but Diatryma was definitely a bird. The large, almost hookless beak and puny clawless wings suggest a herbivorous (plant eating) nature, although it may have consumed insects and carrion also.

Diatryma nested on the ground and probably walked most of the time. The strong legs could have supported the heavy body for only a short running distance. While never abundant, Diatryma was found across North America and Western Europe, in grasslands, open woodlands, swamps and marshes.

This specimen is a cast of a northern Wyoming fossil located at The American Museum of Natural History in New York City.

Crabs
During the Eocene, the fossil crab lived in what is now Florida. It is similar to modern crabs.
The smaller fossil shows the typical underside of a crab, indicating the segmentation typical of arthropods.

Oligocene Epoch
During the Oligocene epoch, grasslands spread, and primates (monkeys) appeared in the Americas, along with toothed whales and mesohippus (middle horses).

Oreodont
The sheep sized, long-tailed oreodont was a grazing mammal of the early Oligocene. It was limited to North America and became extinct in the Pliocene.

Miocene Epoch
Mammalian diversity reached its peak in the Miocene epoch, with primates (monkeys and apes) appearing on other continents.

American Mastodon, Mastodon americanus, was an elephant-like mammal belonging to the mammal order Proboscidea. It was common in North America from the Miocene until 10 thousand years ago.

The teeth, consisting of a series of paired conical cusps, were adapted to striping and grinding soft vegetation from spruce and hemlock trees. Some mastodons were more than 9 feet tall and the males had tusks up to 10 feet long.

Pliocene Epoch
As climates cooled in the Pliocene epoch, the first hominids (australopithecines) appeared and grasslands dominated.

Pleistocene Epoch
The Pleistocene epoch was an "Ice Age", when glaciers advanced over the North American continent several times. As this occurred, large mammals evolved and disappeared.

The fossil alligator, Alligator mississippiiensis, lived in the Pleistocene epoch. The same species is found today from Texas to the Atlantic coast. Alligators are reptiles of the Crocodilian class.

Hominoid Skull, Australopithicus a. robustus (cast) found by Robert Broom, 1948 Swarkrans, S. Africa
This hominoid lived 2 to 2.5 million years ago. Although it stood upright, was omnivorous, and had a complex brain, it is not on the direct line of descent of Homosapiens. Homo habilis, who is believed to be an ancestor of Homo sapiens, lived at the same time.

Lucy
Australopithicus afarensis
About 2.5 million years ago hominids began to make stone tools. There were several varieties of hominids, but most had three features that separated them from other primates.

1. They walked upright on two feet, which freed their hands to use tools.
2. They were omnivorous, thus able to survive if food sources changed.
3. Their brains were larger and more elaborate than hominoids, enabling individuals to learn and teach new patterns of behavior.

ICE AGE

An ice age is a period of time when the temperature of the earth drops, polar ice caps expand, the land cools and ocean water levels decrease, then the process reverses. Organisms which are unable to adapt to changing conditions migrate or die out (become extinct). Some scientists believe we are still recovering from the last ice age which began in the Pliocene epoch. The cause of ice ages is still a mystery, although there are several theories.

The last ice advance began about 24,000 years ago. The ice began retreating about 14,000 to 20,000 years ago and humans have become the dominant species.

Favorable climate changes, in a post ice age world, have allowed many organisms to thrive and multiply. The latest animal to make the geological time scene shares the earth with the greatest variety of organisms that ever existed.

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