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The Cambrian Period

Time encompassed: 500 to 570 million years ago. During this period, mass extinctions of sponges and trilobites occurred on a global scale. Stromatolites flourished in shallow seas. The first radiolarians, stromatoporoids, gastropods, bivalves, cephalopods, ostracodes, brachiopods, cystoids, graptolites, and conodonts appeared in the fossil records. This period is generally accepted to be the explosive appearance of shells. Initially the Cambrian Period was defined as the appearance of fossils, but the discovery of microscopic fossils ruined that marker. Modern acceptance of the start of the Cambrian is therefore the arrival of shells.

The most important fossil marker in the Cambrian is the Trilobite. These were segmented, 3-lobed animals unique to the 3 main divisions of the Cambrian Period.

The Cambrian ended with a mass extinction of almost 75% of trilobite families, 50% of all sponge families, and many brachiopods and gastropods. The cause of this extinction is unknown. However, the Cambrian ? Ordovician boundary is characterized by terrestrial sediments as sea levels fell and land mass began to erode into the shallow seas.

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Ostrocod (Image taken from: http://esp.cr.usgs.gov/info/lacs/ostracode.htm )


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Radiolarian (Image taken from:
http://www.answers.com/topic/radiolarian?cat=techn ology )

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Brachiopod (Image taken from: http://paleo.cortland.edu/tutorial/Brachiopods/bra chiopoda.htm
)

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Cambrian Stromatoporoid (Image taken from : http://www.humboldt.edu/~natmus/lifeThroughTime/Ca mbrian.web/index.html )

The Paleozoic Era encompasses the time from 430 million years ago up to 225 million years ago. It is composed of five periods called the Silurian, Devonian, Mississippian, Pennsylvanian and Permian. It is the time of spore bearing plants, sponges, corals, trilobites, brachiopods, crinoids, and fishes.

The Silurian Period

Time encompassed: 395 ? 430 million years ago. During this period the rugose and tabulate corals, eurypterids, bryozoans, crinoids, and graptolites were abundant. The first land plants, blastoids, and jawed fishes arrive into the fossil records.

Trilobites and graptolites were beginning to decline, but still locally abundant. Eurypterids of the genus Pterygotus became the giants of the Silurian period. Vertebrates are still very rare.

This period is characterized by a great deal of tectonic activity as the northern continents were joined together into a super continent called Laurasia. Volcanism and ash coupled with erosion created thick sedimentary deposits in the sinking basins.

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Tabulate Coral

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Please note: All coral photos are used by permission. The source of this material is the Kansas Geological Survey website at http://www.kgs.ku.edu/. All Rights Reserved." (tabulate and rugose coral photos)

The Mississippian Period (Lower Carboniferous)

Time encompassed: 325 ? 345 million years ago. Rugose corals and trilobites declined. Foraminiferans, bryozoans, crinoids, and blastoids became abundant. The first seed ferns and belemnoids arrive into the fossil records.

This fossil record is almost entirely marine, and represents the last time major limestone deposits from shallow seas would be seen. The predominant fossils are Crinoids, and for that reason, the Mississippian Period is often referred to as the Age of Crinoids.

Crinoids flourished because they were filter feeders, and most of the particles in the clear oceans would have been bits of food. They also required warm water to produce their skeletons because warmer water can hold more dissolved calcium carbonate. Blastoids, which first appeared in the Silurian were also abundant.

The lacy fenestrate bryozoans are another group of very successful Mississippian animals. These animals cannot survive in turbid waters, so the seas had to have been fairly clear.

The Foraminiferans first appeared in the Ordovician, but in the Mississippian these tiny single celled animals became abundant enough to produce whole strata composed almost entirely of their skeletons. The genus Endothyra, about the size of a pinhead, was so common in parts of Illinois and Missouri that the Salem Limestone is mostly composed of its globular shells. Endothyra lived on the bottom of shallow seas and picked up bits of food with its psuedopods in the manner of its close relatives, the amoebas.

There are very few fossils of terrestrial plants or animals, unlike the next period. Either conditions in N. America were unfavorable for land life, or unsuitable for its preservation. Coal did not form during this time in N. America, but it did form in Europe.

The Permian Period

Time encompassed was 55 million years, from 225 to 280 million years ago. The super-continent existed throughout the Permian. The western shores were the only coastal area in N. America, and the interior grew hot and dry. Red bed, iron deposits formed in the basins and were filled in with sediments. Salt deposits formed in Kansas, Texas, and New Mexico as sea levels dropped. The West Coast experienced volcanism, and deep troughs were filled with sediment.

Marine fauna thrived and it was a time of specialization. Huge Foraminiferans, called fusulines, were so numerous in some areas that rocks were deposited that are formed mostly of their shells. Ammonoids, insects, bryozoans, productid brachiopods and reptiles grew in abundance.

Plant life on land remained much as it had during the Pennsylvanian. As the Permian climate grew drier though, many of the spore bearing plants died off and becoming extinct by the end of the Permian. Ginkgos and cycads appeared for the first time. Conifers became more abundant, probably because they could colonize dry land. Cycads are still living, but are rare and restricted to Australia, Southern Africa, and the American tropics.

Insects continued to flourish. Twenty orders of insects have been described in the deposit near Elmo, Kansas called Insect Hill.

Amphibians left a solid fossil record in the Permian, especially in the Red Beds of the Southwest.

Reptiles also left an abundant fossil record. Among the more important and spectacular reptiles of the Permian were the pelycosaurs. They developed high, sail-like extensions of their backs, supported by spines from the vertebrae. These probably acted as heat regulators.

Mass extinctions of the Permian ended with the most severe of all mass extinctions; as many as 96% of all species were lost. Many classes and orders, and families disappeared entirely. The effects were felt both on land and in the seas. Rugose and tabulate corals, nearly all brachiopods vanished, trilobites, eurypterids, and blastoids were eliminated from existence.

The two most abundant orders of Paleozoic bryozoans, the trepostomes and cryptostomes perished, including the lacy fenestrate bryozoans that had flourished throughout the Mississippian and Permian. Almost all brachiopods became extinct, and they have bordered on complete extinction ever since.

Many mollusk families disappeared, the ammonoids lost all but one family. Trilobites and eurypterids became extinct. All Blastoids and all but one family of crinoids vanished. The echinoids, which had been abundant were reduced to 1 genus. It was the small, unspecialized animals that survived.

The land suffered mass extinctions as well. Fully 75% of all amphibian families and 80% of all reptile families became extinct. In addition, many groups of land plants perished, including most of those that had composed the great forests of the Pennsylvanian.

The most widely accepted explanation of this mass extinction is that there was a sudden, drastic drop in sea level and the area of shallow seas was reduced to virtually nothing, but there are problems with this hypothesis. There were other sudden drops in sea level that did not result in this kind of mass extinction events in the Phanerozoic Eon. Other explanations have attempted to tie the extinctions to the positions of the continents. But, if continental positions were an important factor, then extinctions should have begun in the Cambrian and continued all through the Paleozoic, with few new extinctions in the Permian.

Mammal-like reptiles appeared in the fossil records. Mammals are in a direct line of descent from the pelycosaur. The therapsids were a varied group, mostly fairly small meat eaters. Their teeth were differentiated rather than all alike, and they have separate nasal passages that opened on the tops of their skulls so they could eat and breathe at the same time. They had longer limbs and knees drawn inward instead of bent outward. Therapsids dominated the vertebrate faunas of the later Permian. The therapsids became extinct in the Triassic, but not before giving rise to the mammals.

Triassic Period

Time encompassed 35 million years, from: 190 ? 225 million years ago. Mass extinctions of the Permian were so severe that when the Triassic began, life forms were very different. The Atlantic Ocean began to open as North America and Europe were driven apart by plate tectonic activities. During the Triassic, the equator passed through central Mexico, and much of N. America lay in what is called the Horse Latitudes, which is a climatic zone about 30 degrees north of the equator where dry, high pressure air masses tend to descend, bringing very little rainfall. Most of the world?s deserts are located in the Horse Latitudes.

Tabulate corals go extinct as marine life had very few survivors from the Permian extinction. Conodonts disappear, ammonoids, amphibians and reptiles are decimated. Approximately 25% of all animal families disappeared in this extinction.

Gastropods, bivalves, ammonoids grow abundant, with the ammonoids being the first to recover. They rapidly diverged into over 400 genera. The first scleractinian corals, or modern corals arrived. Turtles, lizards, dinosaurs, marine reptiles, mammals appear in the fossil records as small, mouse-like insectivores.

One of the first new groups of reptiles to appear in the early Triassic were the thecodonts. They are the earliest of the archosaurs, or ruling reptiles, which included the dinosaurs, phytosaurs, birds and crocodiles.

The first dinosaurs appeared in the late Triassic. These were the saurischians. They walked on two legs and had short forelimbs and stabbing teeth. But they did not become abundant until the Jurassic.

Phytosaurs show up in the fossil record in the Late Triassic. They looked like crocodiles, but were not closely related to them. These became extinct by the end of the Triassic.

Crocodiles, which are still living, appeared a little later, nearer to the Triassic-Jurassic boundary. Crocodiles and birds are the only archosaurs to have survived the end of the Cretaceous.

One of the most successful groups of reptiles was the ichthyosaurs. These were fully marine reptiles, shaped like sharks or dolphins, and they quickly became the largest animals, terrestrial or marine, in the Triassic. Ichthyosaur vertebrae from Nevada indicate that some of these creatures were 9 meters long (30 feet). They had elongated jaws full of uniform stabbing teeth.

Plesiosaurs were another group of marine reptiles. Their legs were modified and turned into flippers. They had small heads and long necks.

Therapsids continued to be common in the Triassic, but did become extinct in the early Jurassic.

Cretaceous Period

Time encompassed: 65 ? 136 million years ago. There are more rocks from this period than any other period. There are three reasons for this phenomenon. First, the Cretaceous is relatively recent and erosion has had less time to remove these rock layers. Secondly, the Cretaceous was one of the longest periods, lasting about 70 million years. Finally, the seas stood higher on the continents during the Cretaceous than ever before or since. Thick deposits were laid down. Chalk deposits are very characteristics of the Cretaceous.

The Rocky Mountains rose and severe compressional forces operated on the western region of N. America.

The dominant modern group of fishes, the teleosts appeared in the Jurassic, but in the Cretaceous they outnumbered the previously dominant holosteans in marine rocks.

Marine reptiles were scarce in the early Cretaceous, but by the middle of the period the plesiosaurs were replaced as the largest marine animal by the lizard-like mosasaurs. The largest of these were over 17 meters long (56 feet)

The flying reptiles became larger and more specialized than they had been in the Jurassic, and their fossilized bones are more common than bird bones. One genus had a wingspan of 7.6 meters (25 feet), but it?s body was no bigger than that of a goose. Its legs were so weak that it probably could not have walked on land and must have spent all its time at sea.

Dinosaurs maintained a diverse population through the Cretaceous right up until the abrupt extinction at the end of this period. Among the saurischians was Tyrannosaurus, the largest land predator ever to live. It was 14 meters (45 feet) long and stood 6 meters (20 feet) tall.

This period had mass extinctions of some creatures, such as ammonoids, belemnoids, flying reptiles, dinosaurs such as the stegosaurs, marine reptiles long before the end of the period.

The angiosperms, scleractinian corals, gastropods, bivalves, ammonoids, belemnoids, bryozoans, teleosts, and some dinosaurs became abundant. First angiosperms, primates arrive in the fossil records.

The close of the Cretaceous is also the end of the Mesozoic Era. It is marked by the second most severe of all mass extinctions. At this time, about half of all animal families disappeared. All of the dinosaurs became extinct. Marine reptiles and pterosaurs vanished as well. Many families of bivalves and gastropods disappeared. Two thirds of all corals became extinct, and this group has never recovered its former diversity. Bryozoans and echinoids lost 25% of their families. Crinoids suffered and the number of sponge families was reduced by half. Planktonic Foraminiferans were almost extinguished with only 3 species are known to have survived.

At present, the only explanation for the mass extinction that seems plausible is that a large meteorite hit the earth 65 million years ago. Certain elements, most notably iridium, are concentrated at the boundary between the Cretaceous and Tertiary periods. Iridium of this magnitude would have had to come from a meteoric source approximately 10 kilometers (6 miles) in diameter. The combination of reduced photosynthesis, cooling climate from the dust and debris, and lowering of the sea level might well have caused this extinction.

Cretaceous

There are more rocks from the Cretaceous than any other period, and erosion has had less time to remove these rocks than those from earlier periods. The Cretaceous was also one o f the longest periods, lasting about 70 million years. The seas stood higher on the continents during the Cretaceous than ever before or since, and thick deposits were laid down.

Cretaceous rocks have a lot of chalk, limestone that is mostly composed of the skeletal remains of plants and animals The White Cliffs at Dover are composed almost entirely of the tiny, round shells of calcareous algae.

The Rocky Mountains began to rise during the Cretaceous. Severe compressional forces operated through Idaho, Wyoming and Utah, causing great sheets of rocks to be moved horizontally and folded. Vast quantities of sediments eroded from these early Rocky Mountains and poured into the seaway, leaving deposits of conglomerate rock 10,000 feet (3100 meters) in some places.

Marine invertebrates flourished, many groups reached the height of their abundance and diversity, never recovering after the Cretaceous-Tertiary extinction.

Corals: The scleractinian corals were far more various in the Cretaceous than they are now.

Mollusks: The rudists, a group of bivalves that resembled the rugose corals of the Paleozoic built reefs that rivaled those of the corals. Rudists had 1 valve modified into a cone, with the other valve serving as a lid. Ammonoids continued to be diverse and abundant. The most advanced of the shelled marine gastropods, the neogastropods, appeared in the Cretaceous. The mesogastropods were also numerous in the Cretaceous, although they had first appeared in the Ordovician.
Crustaceans: Lobsters and shrimps were present, but the number of fossils is small. Crabs, important as predators on gastropods and bivalves had appeared in the Jurassic and now became more abundant.

Teleost fishes: Appeared in the Jurassic, but it is not until the Cretaceous that they outnumber the previously dominant holosteans in marine rocks. A teleosts differs from earlier fishes in that the internal skeleton is fully ossified, and there are bones not present in earlier fishes. The backbone does not extend into the tail, which is composed of fin-rays. Scales are thin and flexible

Marine reptiles: The record is scanty for the Early Cretaceous. Ichthyosaurs were present, but small. By the Middle Cretaceous, the plesiosaurs reappeared, but the genera are different. Soon the plesiosaurs were replaced as the largest marine animals by the lizard-like mosasaurs. Mosasaurs had flippers instead of legs, and long, lizard-like tails. There teeth were those of reptilian predators. The largest mosasaurs were over 56 feet (17 meters) long. Turtle skeletons have the giant Archelon, 12 feet, 3.7 meters across and 11 feet, 3.4 meters long.

Pterosaurs became larger and more specialized. Their fossilized bones are more common than bird bones, and are found in the Niobrara Chalk. One genus has a wingspan of 25 feet (7.6 meters) but a body no bigger than that of a goose. It?s legs were so weak that it probably could not have walked on land, and must have spent all its time at sea.

Birds: The only common bird fossil from the N. American Cretaceous is the aquatic Hesperornis, which could not fly. It was similar to Archaeopteryx in that unlike living birds, it had teeth.

Dinosaurs: Between the Jurassic and the Cretaceous, some dinosaurs disappeared and new types emerged. Among the saurischians was Tyrannosaurus, the largest land predator ever to live. It was 45 feet (14 meters) long and stood 20 feet, (6 meters) high. There were also many smaller predators, some no larger than a rooster or a dog. Among the plant eating ornithischians, the stegosaurs became extinct early in the Cretaceous and were replaced by ankylosaurs, which looked something like large armadillos with heavy armor and club-like tails. Another four legged group was the ceratopsians.. Most genera had three horns on their skulls, strictly for defense. Most common ornithischians fossils are the duck billed dinosaurs.

Mass extinction: The close of the Cretaceous is also the end of the Mesozoic Era and is marked by the second most severe of all mass extinctions. At this time, about half of all animal families disappeared. All of the dinosaurs became extinct. Marine reptiles and pterosaurs vanished as well. Among the invertebrates, the ammonoids and belemnoids were entirely wiped out. Many families of bivalves and gastropods disappeared. Two thirds of the corals became extinct, the group was never to recover its former diversity. Bryozoans lost about twenty five percent of their families as did the echinoids. Crinoids suffered and the number of sponge families was reduced by half. Planktonic Foraminiferans were almost extinguished, only three species are known to have survived. Land plants were largely unaffected as were fishes. The brachiopods and amphibians lost only a few families. Mammals show little effect also, but this may be because the fossil record of mammals in the Mesozoic is so poor.

At present, the only explanation for the mass extinction that seems plausible is that a large meteorite hit the earth 65 million years ago. Certain elements, most notably iridium, which are known to occur in meteorites but are very rare in the Earth?s crust, are concentrated at the boundary between the Cretaceous and Tertiary periods, there appears to be no way to explain the presence of these elements except as the debris from a very large meteorite. Perhaps 6 miles (10 km) across. The extinctions would have been caused by the deterioration of the climate following the ejection of much dust into the upper atmosphere. Estimates vary as to how dark the Earth might have become; and for how long, but most likely the effects would have appreciably reduced sunlight and the ability of plants to photosynthesize for years. With plant growth halted, most animal species would simply have starved to death.

Some geologists think that the impact from a large meteorite could cause increased volcanism and changes in the rate of movement and direction of movement of crustal plates. Volcanism would further reduce sunlight and add to the food crisis. The dust from volcanoes and the meteorite impact could cool the Earth to the point where glaciers would begin growing, causing the sudden drop in sea level that is often observed at the time of a mass extinction.

The era following the Mesozoic is the Cenozoic, or Time of Recent Life. The Cenozoic includes the Tertiary Period, which lasted about 63 million years and is divided into five epochs, as well as the Quaternary Period, which is broken down into two epochs, together lasting less than two million years. From earliest to latest, the five Tertiary epochs are called: Paleocene, Eocene, Oligocene, Miocene, and Pliocene.

Tertiary Period: Sea level fell gradually during the Tertiary. On the average, it was low, with only 3% of present land areas covered by seas. Temperatures also fell gradually in the Tertiary until the end of the Oligocene, the climate was humid and subtropical as far north as the Dakotas. During the latter part of the Tertiary, subtropical vegetation retreated southward. Today there are probably more kinds of plants and animals living on Earth than ever before.

Continents continued to move apart during the Tertiary. The Pacific plate continued to slide under the West Coast of N. America, and this area remained geologically active throughout the Cenozoic. In the early Tertiary, Australia and Antarctica started to separate; Alaska and Siberia moved closer together and began to close off the Arctic Ocean from the Pacific Ocean. In the late Tertiary, India collided with Asia, and the Himalayas began to rise, and they are still colliding and driving the Himalayas higher.

Fossils contained in Tertiary coastal beds are modern in appearance, most of the genera and many of the species are still living. Gastropods and bivalves are dominant in these beds. Sea urchin and bryozoan fossils are present, but these forms were more fragile and easily destroyed. Solitary corals are common, but reef building corals are now restricted to Florida and Mexico.

The Tertiary is also called the Age of Mammals. The first mammals to appear in the fossil record were very small, with correspondingly small brains. They had 5 toes on each foot and walked on their soles. They probably ate insects and their teeth were unspecialized. Some, like shrews and moles have remained small insect eaters, but most began a trend toward increased size, with specializations of the feet for running, for grasping prey, or for swinging in trees. Teeth became specialized for predation or adapted for grinding up coarse vegetation.

Quaternary Period:
Pleistocene Epoch

Time encompassed: 11 thousand years ? 1.8 million years ago. First homo sapiens arrived.

Recent Epoch

Time encompassed: Current day ? 11,000 years ago. Mass extinction of large terrestrial mammals.