[_ Old Earth _] A History of Life on Earth

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Gondwana:

Near the end of the Precambrian, roughly 700 million years ago, all landmasses assembled into a supercontinent. The continental collisions resulted in environmental changes that had a profound influence on the evolution of life. No broad oceans or extreme differences in temperatures existed to prevent species from migrating to various parts of the world. Between 630 and 560 million years ago, the supercontinent rifted apart and four or five continents rapidly drifted away. Evidence for the breakup exists in a long belt of volcanism near the present Appalachians.

Most of the continents were near the equator, which explains the existence of warm Cambrian seas. The continental breakup caused sea levels to rise and flood large portions of the land at the beginning of the Cambrian. The extended shoreline might have spurred the explosion of new species, with twice as many phyla living during the Cambrian than before or since.

Many organisms were in existence, none of which have any modern counterparts. One example is helicoplacus, whose body parts were configured in a manner not found in any living organism. It was about two inches long and shaped like a spindle covered with a spiraling system of armored plates. It emerged during the transition from the Precambrian to the Cambrian, when more types of body plans arose than at any other time. Helicoplacus, like most species of the early Cambrian, was unsuccessful in the long run and became extinct about 510 million years ago, just 20 million years after it first appeared.

During the Cambrian, continental motions assembled the present continents of South America, Africa, India, Australia, and Antarctica into Gondwana, named for an ancient region of east-central India. Evidence for Gondwana exists in geologic provinces with similar rock types from the late Precambrian to the early Cambrian; these show matches between Brazil and west Africa; eastern South America, South Africa, west Antarctica, and east Australia; and east Africa, India, east Antarctica, and west Australia. A great mountain-building episode deformed areas between all pre-Gondwana continents, indicating their collision during this interval.

Much of Gondwana was in the south polar region from the Cambrian to the Silurian. The present continent of Australia was at the northern edge of Gondwana and located on the Antarctic Circle. A later collision between North America and Gondwana near the end of the Cambrian about 500 million years ago created an ancestral Appalachian range that continued into western South America long before the Andes formed. North America then broke away from Gondwana and linked with Greenland and Eurasia to form Laurasia about 400 million years ago. Eurasia, the largest modern continent, assembled with about a dozen individual continental plates that welded together at the end of the Proterozoic.

A preponderance of evidence for the existence of Gondwana includes fossilized finds of a mammal-like reptile called lystrosaurus in the Transantarctic Range of Antarctica, which indicates a link with southern Africa and India, the only other known sources of lystrosaurus fossils. A fossil of a South American marsupial in Antarctica, which acted as a land bridge between the southern tip of South America and Australia, lends additional support to the existence of Gondwana.

Further evidence for Gondwana includes fossils of a reptile called mesosaurus in eastern South America and South Africa. Fossils of the late Paleozoic fern glossopteris, from the Greek word meaning featherlike and whose leaf impressions actually look like feathers, exist in coal beds on the southern continents of India. However, the plant is suspiciously absent on the northern continents, suggesting the existence of two large continents, one located in the Southern Hemisphere and another in the Northern Hemisphere, separated by a large open sea.

 

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Chapter Five:
Ordovician Vertebrates:

The Ordovician period from 500 to 435 million years ago was named for the ancient Ordovices tribe of Wales, Great Britain. Ordovician marine sediments are recognized on all continents of the Northern Hemisphere, in the Andes Mountains of South America, and in Australia, but they are absent in Antarctica, Africa, and India. Ordovician terrestrial deposits are not easily recognized because the lack of fossilized land organisms.

The multitudes of species that exploded onto the scene in the early Cambrian advanced significantly in the warm Ordovician seas. Corals began building extensive carbonate reefs in the Ordovician. The first fish appeared in the ocean, and the existence of freshwater jawless fish suggests that unicellular plants, including red and green algae, were inhabiting lakes and streams on land.


The Jawless Fish:

Beginning about 500 million years ago, the first vertebrates appeared on the scene with an internal skeleton made of bone or cartilage, on of life's most significant advancements. The vertebrate skeleton was light, strong, and flexible, with efficient muscle attachments, and the skeletons grew along with the animal. These new skeletons enabled the wide dispersal of free-swimming species into a variety of environments.

Invertebrates, supported by external skeletons, were at a disadvantage in terms of mobility and growth. Many animals, like crustaceans, shed their shells as they grew, which often made them vulnerable to predators. One such predator was an extinct giant sea scorpion with immense pincers called eurypterid, which ranged from the Ordovician to the Permian and grew to 6 feet long, terrorizing shell-less creatures on the sea floor.

The earliest vertebrates were probably wormlike creatures with a prominent rod called a notochord down the back, a system of nerves along it, and rows of muscles attached to the backbone and arranged in a banded pattern. Rigid structures made of bone or cuticle acted as levers, and with flexible joints they efficiently translated muscle contractions into organized movements such as rapid lateral flicks of the body to propel an animal through water. Later, a tail and fins evolved for stabilization, and the body became more streamlined and torpedo-shaped for speed. With intense competition among the stationary and slow moving invertebrates, any advancements in mobility was advantageous to the vertebrates.

The oldest known vertebrates were primitive jawless fish called agnathans, which first appeared in the early Ordovician about 470 million years ago. Remarkably well-preserved remains of these fish were discovered in the mountains of Bolivia, much of which was inundated by the sea in the Ordovician. Originally, fossil evidence was scant and fragmentary, and little was known of their appearance or about their evolutionary history. Earlier descriptions dismissed them as a headless, tailless mass of scales and plates or confused the head with the tail, giving the agnathans the dubious title "backwards fossil."

The widespread distribution of primitive fish fossils throughout the world suggest a long vertebrate record prior to the Ordovician. The first fish were small mud-grubbers and sea squirts lacking jaws and teeth. These ancient fish were probably poor swimmers and avoided deep water. The jawless fish were generally small (about the size of a minnow) and heavily armored with bony plates that protected the rounded head. The rest of the body was covered with thin scales that ended near a narrow tail. Although well protected from invertebrate predators, the added weight required the fish to live mostly at the bottom, where they sifted mud for food particles and expelled the waste products through slits on both sides of the throat, which later became gills.

The jawless fish whose modern counterparts include lampreys and hagfish had a flexible rod similar to cartilage instead of a bony spine typical of most vertebrates. Gradually, the protofish acquired jaws and teeth, the bony plates gave way to scales, lateral fins developed on both sides of the lower body for stability, and air bladders maintained buoyancy. Some fish were surprisingly large, up to 18 inches long and 6 inches wide. Thus, for the first time, vertebrates skillfully propelled themselves through the sea, and the fishes soon became the masters of the deep.

 

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Fauna and Flora:

Corals are marine coelenterates attached to the ocean floor. They began constructing extensive limestone reefs in the Ordovician, building chains of islands and altering the shorelines of the continents. Bryozoans, often called moss animals, are similar to corals but much smaller. They consist of microscopic individuals living in small colonies up to several inches across, giving the ocean floor a mosslike appearance. The bryozoans are retractable creatures, encased in a calcareous vaselike structure. Ciliated tentacles surrounded by a polyp, forming a netlike structure around the mouth used for filtering microscopic food that floats by.

Fossil bryozoans are common in Paleozoic formations, especially those of the American Midwest. They resembled modern descendants of bryozoans, and some larger groups might have contributed to Paleozoic reef-building. They are most abundant in limestone and less so in shales and sandstones. Fossil bivalve shells (brachiopod and pelecypod) are often encrusted with a delicate outline of bryozoans. Because of their small size, bryozoans make ideal microfossils for dating oil well cuttings. Their abundance from the Ordovician to the present makes bryozoans highly useful for rock correlations.

Of particular importance to geologists are the ostracods, or mussel shrimp, whose fossils are useful for correlating rocks from the Ordovician onward. Starfish were also common and left fossils in the Ordovician rocks of the central and eastern United States. Their skeletons comprised tiny silicate or calcite plates that were not rigidly joined and therefor usually disintegrated when the animal died, making whole starfish fossils rare. The sea cucumbers have large tube feet modified into tentacles and skeletons comprised of isolated plates that are occasionally found as fossils.

Near the end of the Ordovician 450 million years ago, the concentration of atmospheric oxygen generated sufficient levels of ozone in the upper stratosphere to shield the Earth from the sun's deadly ultraviolet rays. Thus, for the first time, plants began to come ashore to populate the land. When the early plants first left the oceans and lakes for a home on dry land, they were greeted by a harsh environment, where ultraviolet radiation, desert conditions, and lack of nutrition made life difficult. First to greet the land plants were soil bacteria that churned sediments into lumpy brown mounds. Their presence helped speed weathering processes, without which hot bare rocks would have covered the landscape and land plants would have had little success gaining a root hold.

Cyanobacteria, incorrectly called blue-green algae, might have been preparing the soil for the land invasion as early as 3 billion years ago. Ancient cyanobacteria, which were resistant to high levels of ultraviolet radiation, first lived in shallow tide pools, from which they eventually colonized the continents. They might have improved the terrestrial climate for a life out of water by drawing down atmospheric carbon dioxide, thereby countering the greenhouse effect, which prevents thermal energy from escaping the planet. The soil bacteria helped resist erosion by binding the sediment grains together and soaking up rainwater. Bacteria also provided nutrients for the early land plants.

Plant fossils of Ordovician age appear to be almost entirely composed of algae similar to present-day algal mats found on seashores and at the bottoms of ponds. Some marine algae lived in the intertidal zones, and were able to withstand only short amounts of time out of the sea, due to the risk of dehydration. Even after developing protective measures to help the organisms survive out of water for longer periods, they still depended on the sea for reproduction.

Lichens, which are a symbiotic relationship between algae and fungi, in which one lives off the other, probably took the first tentative steps on land. Following the lichens were mosses and liverworts. Fungi also had a symbiotic relationship with the roots of plants when they first evolved, aiding vegetation with the uptake of nutrients and receiving carbohydrates in return.

Bacteria also played an important role in the fixation of nitrogen, an abundant soil gas and an important nutrient for plant life. In what is called the nitrogen cycle, denitrifying bacteria convert dissolved nitrate back into elemental nitrogen. Without this cycle, all nitrogen would in the atmosphere would have long ago disappeared and the planet would have only a fraction of its present atmospheric pressure.

 

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The Ordovician Ice Age:

Plants began to invade the land and extend to all parts of the world during the late Ordovician about 450 million years ago. The early land plants absorbed large quantities of atmospheric carbon dioxide, and rapid burial under anaerobic conditions deposited the organic carbon into the geologic column, where it became coal. Plants also aided the weathering process, which leached minerals from the rocks, and locked up massive amounts of carbon dioxide in carbonate rocks such as limestone deposited by shelly organisms from the Cambrian onward.

The withdrawal of substantial amounts of carbon dioxide from the atmosphere weakened the greenhouse effect. The resulting climate cooling the initiated in large part by the plant invasion spawned a major ice age at the end of the Ordovician about 440 million years ago. The glaciations if the late Ordovician and the glacial epochs of the middle and late Carboniferous about 330 million and 290 million years ago might have been influenced by a reduction of atmospheric carbon dioxide to about one-quarter of its present value.

Atmospheric scientists have amassed information on global geochemical cycles to ascertain the cause of such a radical change in the carbon dioxide content of the atmosphere. Data from deep-sea cores show that carbon dioxide variations preceded changes in the extent of the more recent ice ages, therefor earlier glacial epochs might have been similarly affected. The variations of carbon dioxide levels might not have been the sole cause of glaciation. But when combined with other processes, such variations in the Earth's orbital motions or a drop in solar radiation, they could become a strong influence.

Continental movements also might be responsible for the late Ordovician glaciation. Magnetic orientations in rocks from many parts of the world indicate the position of continents relative to the magnetic poles at various times in Earth history. Paleomagnetic studies in Africa revealed very curious findings, however, with the data placing North Africa directly over the South Pole during the Ordovician.

Additional evidence for such a widespread glaciation came from another surprising location--the middle of the Sahara Desert. Geologists exploring for petroleum in the region stumbled upon a series of giant grooves cut into the underlying strata by roving glaciers. Rocks embedded at the base of glaciers scoured the landscape as the ice sheets moved back and forth. Other collaborating evidence that thick sheets of ice blanketed the Sahara Desert include erratic boulders (placed by moving ice) and eskers, which are sinuous sand deposits from glacial outwash streams.

As the Ordovician drew to a close, a mass extinction eliminated some 100 families of marine animals. Glaciation reached its peak, with ice sheets radiating outward from a glacial center in North Africa, which then hovered directly over the South Pole. Most of the victims were tropical species sensitive to fluctuations in the environment. Among those that went extinct were many trilobite species. Prior to the extinction, trilobites accounted for about two-thirds of all species but only one-third thereafter. The graptolites, which were colonies of cupped organisms that resembled a conglomeration of floating stems and leaves, also became extinct at the end of the Ordovician.

 

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The Iapetus Sea:

During the late Precambrian and early Cambrian, a proto-Atlantic Ocean called the Iapetus opened, forming extensive Cambrian inland seas. The inundation submerged most of the ancient North American continent called Laurentia and the ancient European continent called Baltica. The Iapetus Sea was similar in size to the North Atlantic and occupied the same general location about 500 million years ago. A continuous coastline running from Georgia to Newfoundland between 570 and 480 million years ago suggests that this ancient east coast faced a wide, deep sea.

The Iapetus stretched at least 1,000 miles across from east to west and bordered a much larger body of water to the south. It was dotted with volcanic islands and resembled the present-day Pacific Ocean between Southeast Asia and Australia. The shallow waters of the near-shore environment of this ancient sea from the Cambrian to the mid-Ordovician, about 460 million years ago, contained abundant invertebrates, including trilobites, which accounted for 70 percent of all species. Eventually, the trilobites faded, while mollusks and other invertebrates expanded throughout the seas.

The closing of this ancient ocean basin 400 million years ago as Baltica approached Laurentia signaled the formation of Laurasia. The island arcs between the two landmasses were scooped up and plastered against continental edges as the continents collided. The oceanic crustal plate carrying the islands subducted under, or was driven beneath, Baltica. The subduction rafted the islands into collision with the continent and deposited the formerly submerged rocks on the present west coast of Norway. Slices of land called terranes in western Europe migrated into the Iapetus from ancient Africa. In the same manner, slivers of crust from Asia have traveled across the ancient Pacific Ocean called the Panthalassa, Greek for universal sea, to form much of western North America.

A large portion of the Alaskan panhandle, known as the Alexander terrane, began its existence as part of eastern Australia some 500 million years ago. About 375 million years ago, it broke free from Australia, traversed the proto-Pacific Ocean, stopped briefly at the coast of Peru, slid past California, and rammed into the upper North American continent around 100 million years ago. The entire state of Alaska is an agglomeration of terranes that were pieces of ancient oceanic crust. Basaltic seamounts that accreted to the margin of North America traveled halfway across the ocean that preceded the Pacific.

Terranes are fault-bounded blocks, ranging in size from small crustal fragments to subcontinents, with geologic histories that are markedly different from those of neighboring blocks and of adjoining continental masses. They are a billion or more years old and are dated by analyzing fossil radiolarians, which are marine protozoans that lived in deep water and were abundant from about 500 million to 160 million years ago. Different species also defined specific regions of the ocean where the terranes originated. Many terranes traveled great distances before finally adhering to a continental margin. Some North American terranes have a western Pacific origin and were displaced thousands of miles to the east.

North and South America apparently abutted one another at the beginning of the Ordovician. A limestone formation in Argentina contains a distinctive trilobite species typical of North America but not of South America. The fossil evidence suggests that the two continents collided about 500 million years ago, creating an ancestral Appalachian range along eastern North America and western South America long before the present Andes formed. Later, the continents rifted apart, transferring a slice of land containing trilobite fauna from North to South America.

 

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The Caledonian Orogeny:

The Iapetus Sea was closed off as Laurentia approached Baltica at the end of the Silurian about 400 million years ago, some 200 million years before the modern Atlantic began to open. When the continents collided, they crumpled the crust and forced up mountain ranges at the point of impact. The sutures joining the landmasses are preserved as eroded cores of ancient mountains called orogens, from the Greek word oros for mountain. Paleozoic continental collisions raised huge masses of rock into several folded mountain belts throughout the world. A major mountain building episode from the Cambrian to the middle Ordovician deformed areas between all continents comprising Gondwana, indicating their collision during this interval.

Matching geologic provinces exist among South America, Africa, Antarctica, Australia, and India. The Cape Mountains in South Africa have counterparts in the Sierra Mountains south of Buenos Aires in Argentina. Matches also exist between mountains in Canada, Scotland, and Norway. Much of Gondwana was in the south polar region, where glaciers expanded across the continent.

The closing of the Iapetus Sea from the middle Ordovician to the Devonian as Laurentia approached Baltica resulted in the great Caledonian orogeny, or mountain building episode. This orogenic activity formed a mountain belt that extended from southern Wales, spanning Scotland, and ran through Scandinavia and Greenland, possibly including today's extreme northwest Africa as well. In North America, this orogeny built a mountain belt that extended from Alabama through Newfoundland and reached as far west as Wisconsin and Iowa. Vermont still preserves the roots of these ancient mountains, which were shoved up between about 470 and 400 million years ago, but have since been planed off by erosion.

The middles Ordovician Taconian orogeny, named for the Taconic Range of eastern New York State, culminated in a chain of folded mountains that extended from Newfoundland through the Canadian Maritime Provinces and New England, reaching as far south as Alabama. During the Taconian disturbance, extensive volcanic activity occurred in Quebec and Newfoundland and from Alabama to New York, extending as far west as Wisconsin and Iowa.

An inland sea that flooded the continent in the middle Ordovician and reached a maximum in the late Ordovician, partially withdrew in response to a flood of sediments eroded from the Taconian mountain belts. One of these sedimentary deposits is the widespread Ordovician St. Peter Sandstone of the central United States, composed of well-sorted, nearly pure quartz beach sands, ideal for the manufacture of glass.

 

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Chapter Six:
Silurian Plants:

The Silurian period, which ran from 435 to 400 million years ago, was named for the Silures, an ancient Celtic tribe of Wales, Great Britain. Many of today's mountain ranges were uplifted by middle Paleozoic continental collisions. Laurentia collided with Baltica to create Laurasia and close off the Iapetus Sea during the Silurian. The collision formed mountain belts on the margins of continents surrounding an ancient sea, producing intensely folded rocks.

Evidence of widespread reef building by Silurian corals indicates the existence of warm, shallow seas with little temperature variation. Regardless of the excellent climate, the trilobites, which were extremely successful in the early Paleozoic, began to rapidly decline in the Silurian, finally succumbing to extinction at the end of the period. The higher land plants were firmly established on the previously barren continents, and eventually creatures crawled out of the sea to dine on them.


The Age of Seaweed:

The distinction between plants and animals is somewhat blurred in the distant geologic record; at one time they shared many common characteristics. From humble beginnings as simple algae during the Precambrian, single-celled plants probably colonized for the same reasons unicellular animals grouped together: structural support, division of labor, and protection. However, complex marine plants did not appear in the fossil record until the Cambrian, after which they evolved rapidly.

Although the Cambrian is often referred to as "the age of seaweed," the geologic record does not support this contention with strong fossil evidence. Well-preserved multicellular algae and a variety of fossil spores were discovered in the late Precambrian and Cambrian sediments, suggesting complex sea plants had evolved, but no other significant remains have been found. Even as late as the Ordovician, plant fossils appear to have been composed almost entirely of algae, which probably formed stromatolite mounds and algal mats similar to those on seashores today.

The early seaweeds were soft and nonresistant and generally did not fossilize well. A seaweedlike plant grew half submerged in estuaries and rivers. However, for plants to be truly shore-bound, they had to reproduce entirely out of water. The first land plants achieved this function with sacs of spores attached to the ends of simple branches. When the spores matured, they were cast into the air and carried by the wind to suitable sites where they grew into new plants.

The first complex plants lived just below the surace, in shallow waters, probably as protection against high levels of solar ultraviolet radiation. When the atmospheric oxygen content rose to near present-day levels, the upper stratospheric ozone layer began to screen out the deadly ultraviolet rays, enabling life to flourish on the Earth's surface. Once plants crept ashore, the land was soon sprawling with lush forrests.

Before the invasion of true plants, a slimy coating of photosynthesizing cyanobacteria, or blue-green algae, might have inhibited the land. A cover of algae accelerated the weathering of rock and the formation of soils and nutrients required for advancing plant life. Therefor, prior to the emergence of land plants, microbial soils were making the Earth more hospitable for life out of water. The microorganisms probably formed a dark, knobby soil, resembling lumpy mounds of brown sugar spread over the landscape. In this manner and for about half a billion years, simple plants paved the way for their more advanced relatives.

Prior to the arrival of the terrestrial microbes, the continents were much too hot to support complex life forms. The early organisms played an important role in cooling the land surface by drawing down the atmosphere's surplus carbon dioxide for use in photosynthesis. The loss of this potent greenhouse gas cooled the climate and allowed higher forms of life to populate the continents. The microbes also aided in the weathering of rock into soil, helped to prevent soil erosion, and provided the nutrients more advanced plants needed for survival.

The first land plants were probably algae and seaweedlike plants that lived in the intertidal zones, as well as primitive forms of lichen and moss that existed on exposed surfaces. They were followed by tiny fernlike plants, the predecessors of trees, which lacked root systems and leaves and fertilized with spores. The most complex land plants grew less than an inch tall and probably resembled an outdoor carpet covering the landscape.

By the late Silurian, all major plants phyla were in existence. Except for simple algae and bacteria, the early plants diverged into two major groups. One gave rise to the lycopods, or club mosses, and the other gave rise to the gymnosperms, ancestors to the great majority of modern land plants. The simplest plants, among the first to live on shore, were the psilophytes, or whisk ferns, which lacked roots and leaves. They lived semisubmerged and reproduced by casting spores into the sea for dispersal.

The next major evolutionary step was the development of a vascular stem that uses channels to conduct water from a swamp or from the moist ground nearby to the plant's extremities. The strengthening of the stems enabled vascular plants to grow tall. The early club mosses, ferns, and horsetails were the first plants to make use of this system. When roots evolved plants could survive entirely on dry land by drawing water into their stalks from moist soil.

The lycopods, which included club mosses and scale trees, were the first plants to develop true roots and leaves. The branches were arranged in a spiral shape, and leaves were generally small. Spores were attached to modified leaves that became primitive cones. The scale trees, so named because of the scars on their trunk resembled large fish scales, grew upwards of 100 feet or more high and eventually became the dominant trees of the Paleozoic forests.

During their first 50 million years on dry land, plants displayed increased diversity and complexity, including root systems, leaves, and reproductive organs employing seeds instead of spores. When the true leaves evolved, plants developed a variety of branching patterns to expose them to as much sunlight as possible to maximize photosynthesis. Competition for light was of primary importance to the evolution of land plants, and those that developed the most efficient branching patterns gathered the most light and were the most successful.

As plants grew larger, they progressed from random branching to tiers of branches to achieve greater efficiency with a minimum of self-shading, similar to present-day pines. This added weight increased the mechanical stress on the plant, requiring stronger branches to prevent limbs from breaking during storms. These innovations gave rise to most flora in existence today.

 

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The Reef Builders:

Silurian marine invertebrates were intermediate in evolution between Ordovician and Devonian lines. Coral reef formation during the Silurian was widespread, indicating the presence of warm shallow seas with little seasonal temperature variation. Corals began constructing reefs in the Ordovician, forming barrier islands and island chains. They also built atolls atop extinct submerged volcanoes. As the volcanoes subsided beneath the sea, the rate of coral growth matched the rate of ocean subsidence, which kept the corals at a constant shallow depth for photosynthesis.

The coral polyp is a soft-bodied animal crowned by tentacles surrounding a mouthlike opening and tipped with poisonous stingers to attack prey swimming nearby. The polyp lives in an individual skeletal cup or tube composed of calcium carbonate called a theca. Polyps extend their tentacles to feed at night and withdraw into their theca during the day or at low tide to keep from drying out in the sun. Because the algae within the polyp's body require sunlight for photosynthesis, corals are restricted to warm, shallow water generally less than 100 feet deep.

A large variety of corals are well represented in the fossil record and resemble many of their modern counterparts. The tabulate corals, which became extinct at the end of the Paleozoic, consisted of closely packed polygonal or rounded thecae, some with pores covering the walls of the theca. The rugose, or horn corals, named for their shape, were particularly abundant in the Silurian and became extinct in the early Triassic. The hexacorals, with theca separated by six sepia, or walls, range from the Triassic to the present and were the major reef builders of the Mesozoic and Cenozoic eras.

In the geologic past, massive coral structures turned into some of the greatest limestone deposits on Earth. Corals constructed barrier reefs and atolls and played a major role in changing the face of the planet. Coral reefs contain abundant organic material. Many ancient reefs are composed largely of a carbonate mud with the skeletal remains of a variety of other species literally "floating" in the fine sediment, producing some of the finest fossil specimens.

Tropical plant and animal communities thrived on the reefs, due to the coral's ability to build massive, wave resistant structures. Unfortunately, these were the same species that suffered several episodes of extinction due to their narrow range if living conditions. The extinction hit hardest those organisms anchored to the ocean floor or unable to migrate out of the region because of physical and biological barriers.

The echinoderms, with their five-fold and bilateral symmetries and exoskeletons composed of numerous calcite plates, were among the most prolific animals of the Silurian seas. The most successful of the Silurian echinoderms were the crinoids, commonly called "sea lilies" because they resemble flowers atop stalks anchored to the seabed. Some crinoids were also free-swimming types. They became the dominant echinoderms of the middle and upper Paleozoic, with many species still in existence.

The long stalks of the crinoids grew upwards of 10 feet or more in length. They were composed of perhaps a hundred or more calcite disks and were anchored to the sea floor by a rootlike appendage. A cup called a calyx perched on top of the stalk and housed the digestive and reproductive organs. The animal strained food particles from passing water currents with five feathery arms that extended from the calyx, giving the crinoid a flowerlike appearance. The extinct Paleozoic crinoids and their blastoid relatives, whose calyx resembled a rose bud, made excellent fossils, especially the stalks, which on weathered limestone outcrops often looked like long strings of beads.

The echinoids are a class of echinoderms that include sea urchins, heart urchins, and sand dollars, having exoskeletons composed of limey plates that are characteristically spiny, spherical, or radially symmetrical about a central point. Some more advanced forms were elongated and bilaterally symmetrical along a single axis. The sea urchins lived mostly among rocks encrusted with algae, upon which they fed. Unfortunately, such an environment was not conductive to fossilization. The familiar sand dollars that occasionally washed up on beaches are also rare in the fossil record.

 

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The Land Invasion:

The first invertebrates to crawl out of the sea and populate the land were probably crustaceans. These ancient arthropods emerged from the ocean soon after land plants began to colonize the continents. The oldest known land-adapted animals were centipedes and tiny spiderlike arachnids about the size of a flea, both found in Silurian rocks some 415 million years old. The arachnids are air-breathing crustaceans and include spiders, scorpions, daddy longlegs, ticks, and mites. The early terrestrial communities probably consisted of small plant-eating arthropods that served as prey for the arachnids, which were predatory animals.

The early crustaceans were segmented creatures, ancestors of today's millepedes, and walked on perhaps 100 pairs of legs. At first, they stayed near shore, eventually moving farther inland along with the mosses and lichens. Having the land to themselves with no competitors and an abundant food supply, some species evolved into the first terrestrial giants. The crustaceans were easy prey for the descendants of the eurypterids, giant sea scorpions, when they came ashore.

The advent of forests, where leaves and other edible parts grew beyond easy reach from the ground, posed new problems for the ancestors of the insects. Climbing up tall tree trunks to feed on stems and leaves was probably less dangerous than the journey back down. It would have been much easier simply to jump or glide through the air on primitive winglike structures. These appendages probably originated as a means to regulate the insects body temperature and by natural selection developed into flapping wings. They worked well for launching insects to the tree tops and escaping predators when the vertebrates came to shore.

Insects are by far the largest living group of arthropods. They have three pairs of legs and typically two pair of wings and the thorax, or midsection. In most cases, the insect body is covered with an exoskeleton made of chitin, which is similar to cellulose. To achieve flight, insects had to be lightweight. As a result, their delicate bodies did not fossilize well, unless trapped in tree sap, which became hard amber, allowing the bodies to withstand the rigors of time. In some groups, the exoskeleton was composed of calcite or calcium phosphate, which enhanced the insects chances of fossilization.

 

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Laurasia:

During the Silurian, all northern continents collided to form Laurasia. The ancestral North American called Laurentia was assembled from several microcontinents that collided beginning about 1.8 billion years ago. Most of the continent evolved in a relatively brief period of 150 million years and comprised the interior of North America, Greenland, and Northern Europe. Laurentia continued to grow by garnering bits and pieces of continents and chains of young volcanic islands. After the rapid continent building, the interior of Laurentia erupted with igneous activity that lasted from 1.6 to 1.3 billion years ago.

A broad belt of red granites and rhyolites, which are igneous rocks formed by molten magma solidifying below ground and on the surface, extended several thousand miles across the interior of the continent from southern California to Labrador. The Laurentian granites and rhyolites are unique due to their shear volume, which suggests that the continent stretched and thinned almost to the point of rupture, bringing mantle material near the surface. These rocks are presently exposed in Missouri, Oklahoma, and a few other localities, but in most of the center of the continent they are buried under sediments up to a mile thick.

These massive outpourings of igneous rocks in the interior of the continent suggests that Laurentia was part of a supercontinent that formed about 1.6 billion years ago and broke up around 1.3 billion years ago, coinciding with the igneous activity. The supercontinent acted like an insulating blanket over the upper mantle, allowing heat to collect underneath it. About 1.1 billion years ago, cast quantities of molten basalt spewed to the surface out of a huge tear in the crust running from southeast Nebraska to the Lake Superior region.

About 700 million years ago, Laurentia collided with another large continent on its southern and eastern boarders, creating a new supercontinent centered over the equator. A superocean located approximately in the location of the present Pacific Ocean surrounded the supercontinent. The collision thrust up a 3,000-mile-long mountain belt in eastern North America during an episode called the Grenville orogeny. A similar mountain belt occupied parts of western Europe as well.

The supercontinent rifted apart between 630 and 560 million years ago, and its constituent continental blocks drifted away from one another. As the continents dispersed and subsided, seawater flooded the interiors, creating large continental shelves where vast arrays of organisms evolved. The rapid evolution of species at this time was highly remarkable. Another exceptional episode of explosive evolution occurred when a supercontinent called Pangaea rifted apart about 400 million years later.

About 500 million years ago, the continents dispersed around the Iapetus Sea, which opened during the breakup of the late Precambrian supercontinent. When the continents reached their maximum dispersal roughly 480 million years ago, subduction of the ocean floor beneath the North American plate initiated a period of volcanic activity and mountain building. From about 420 million to 380 million years ago, Laurentia collided with Baltica, the ancient European landmass, and closed off the Iapetus. The collision fused the two continents into the megacontinent of Laurasia, named for the Laurentian province of Canada and the Eurasian continent.

When Laurentia united with Baltica, island arcs in a proto-Pacific Ocean called the Panthalassa began to collide with the western margin of what is now North America. The collisions led to the Antler orogeny, which intensely deformed rocks in the Great Basin region, from the California-Nevada border to Idaho.

Laurasia occupied the Northern Hemisphere, while its counterpart Gondwana, which included the present Africa, South America, Australia, Antarctica, and India, inhabited the Southern Hemisphere. A large body of water called the Tethys Sea, named for the mother of the seas in Greek mythology, separated the two megacontinents. Evidence for the existence of a wide seaway between the landmasses comes from a unique specimen of flora called glossopterus found in the southern lands, but absent in Laurasia.

The continents were lowered by erosion and shallow seas flowed inland, flooding more than half the land surface. The inland seas and wide continental margins, along with a stable environment, provided excellent growing conditions and enabled marine life to flourish and spread throughout the world.

 

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Chapter Seven:
Devonian Fishes:

The Devonian period from 400 to 345 million years ago was named for the marine rocks of Devon in southwest England. Rocks of the Devonian age exist on all continents and show widespread marine and terrestrial conditions. The period coincides with the megacontinents Laurasia and Gondwana approaching each other and pinching off the Tethys Sea between them. The wide distribution of deserts, evaporite deposits, coral reefs, and coal deposits as far north as the Canadian arctic indicates a warm climate over large parts of the world.

The warm Devonian seas spurred the evolutionary development of marine species, including the first appearance of the ammonoids, which were coiled-shaped cephalopods that became fantastically successful in the succeeding Mesozoic era. The vertebrates, the highest form of marine animal life, dominating all other species, left their homes in the sea to establish a permanent residence on land, which by then was fully forested. Toward the end of the Devonian, the climate cooled, possibly causing glaciation near the poles. The climate change brought down many tropical marine animals, paving the way for entirely new species especially adapted to the cold.

The Age of Fishes:

The Devonian has been popularly name the "age of fished." The fossil record reveals so many and varied kinds of fish that paleontologists have a difficult time classifying them all. Every major class of fish alive today had ancestors in the Devonian, but not all Devonian fishes made it to the present.

The rise of fishes in the Devonian seas contributed to the decline of their less mobile invertebrate competitors, culminating in an extinction that eliminated many tropical marine groups at the end of the period. When a mass extinction occurs, those individuals that evolve into a better adaptive form are selected for survival, which is why certain species survive one major extinction after another. This is particularly true for marine species like the sharks, which originated in the Devonian around 400 million years ago and have survived every mass extinction since.

Fish comprise over half the species of vertebrates, both living and extinct. They include the jawless fish (lampreys and hagfish), the cartilaginous fish (sharks, skates, rays, and ratfish), and the bony fish (salmon, swordfish, pickerel, and bass). The ray-finned fishes are by far the largest group of living fish species. Fish progressed from rough scales, asymmetrical tails, and cartilage in their skeletons to flexible scales, powerful advanced fins and tails, and all-bone skeletons, much like they are today.

The jawless fish, which first appeared in the Ordovician, are the earliest known vertebrates and have been in existence for 470 million years. Instead of a bony spine like most vertebrates, the jawless fish had a flexible rod similar to cartilage. They were probably poor swimmers and avoided deep water. Bony plates surrounded the head for protection from predation, and thin scales covered the rest of the body. Although well protected from invertebrate predators, the added weight kept the fish mostly on the seafloor, where they sifted bottom sediments for food particles.

The extinct placoderms, which reached 30 or more feet in length, were ferocious giants that preyed on smaller fishes. They had well-developed articulated jaws and thick armor plating around the head that extended over and behind the jaws. The coelacanths were thought to have gone extinct along with the dinosaurs 65 million years ago. However, in 1938, a fisherman caught a five-foot coelacanth in the deep, cold waters of the Indian Ocean off the Comoro Islands near Madagascar.

The fish looked ancient, a castaway from the distant past, with a fleshy tail, a large set of forward fins behind the gills, powerful, square, toothy jaws, and heavily armored scales. The most remarkable aspect about this fish was that it had not changed significantly from its primitive ancestors, which evolved in the Devonian seas some 400 million years earlier, giving the coelacanth the title of "living fossil."

The coelacanth came from the same evolutionary branch that led to the land-dwelling vertebrates. Stout fins enabled the fish to crawl along on the deep ocean floor and were coordinated in a manner common in four-legged terrestrial animals. The fins moved like the legs of a lizard, with the forward appendage on each side advancing in concert with the rear appendage on the opposite side. Such an adaptation would have eased the transition from sea to land.

The sharks were highly successful from the Devonian to the present. An ancient freshwater shark called Xenacanthus had a back fin that stretched from head to tail, and it slithered through the water like an aquatic snake. Closely related to the sharks are the rays, with flattened bodies, pectoral fins enlarged into wings up to 20 feet across, and a tail reduced to a thin, whiplike appendage. The rays literally fly through the sea as they scoop up plankton into their mouths.

Sharks breath by drawing water in through the mouth, passing it over the gills, and expelling it through distinctive slits behind the head. The body of the shark is heavier than water, requiring it to swim constantly or else sink to the bottom. Instead of skeletons composed of bone like most fish, shark skeletons are comprised of cartilage, a much more elastic and lighter material. However, it does not fossilize well, and about the only common remains of ancient sharks are teeth, found in marine rocks of Devonian age or younger.

 

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Marine Invertebrates:

The Devonian marine invertebrates were similar to those that evolved in the Ordovician and include prolific brachiopods, corals, crinoids, trilobites, and gastropods. The conodonts, which are bony appendages of a possible leechlike animal that has the appearance of jawbones, show their greatest diversity during the Devonian and are important for long-range rock correlations of this period. Advanced insects, spiders, and poisonous centipedes appeared in the Devonian. The mollusks were well represented, and the first appearance of freshwater clams suggests that aquatic invertebrates had already conquered the land since they only lived in estuaries and rivers.

The nautiloids and ammonoids first appeared in the early Devonian about 395 million years ago. They had external shells subdivided into air chambers, and the suture lines joining the segments presented a variety of patterns used for identifying various species. The air chambers provided buoyancy to counterbalance the weight of the growing shell. Most shells were coiled in a plane, some forms were coiled in a spiral, and others were essentially straight.

The now-extinct nautiloids grew upwards of 30 or more feet long, and with straight, streamlined shells they were among the swiftest and most spectacular creatures of the Devonian seas. Their neutral buoyancy (they neither sank to the bottom nor rose to the top) and ability to use jet propulsion for high-speed travel (by expelling water under pressure through a funnel-like appendage) contributed to the nautiloid's great success in the period from the Devonian to the Cretaceous.

The belemnoids, which probably originated from more primitive nautiloids, were abundant during the Jurassic and Cretaceous but became extinct by the Tertiary. They were related to the modern squid and octopus and possessed a long, bulletlike shell. The shell was straight in most species and loosely coiled in others. The chambered part of the shell was smaller than the ammonoid, and the outer walls thickened into a fat cigar shape.

The ammonoids were the most significant cephalopods, with a large variety of coiled shell forms, which makes them ideal for dating Paleozoic and Mesozoic rocks. Shell designs steadily improved, making ammonoids the swiftest creatures of the deep, successfully competing with the fishes for food and avoiding predators. They lived mainly at middle depths and might have shared many features with living squids and cuttlefish. Some ammonoids grew to tremendous size, with shells up to 7 feet wide. The nautilus, which is commonly referred to as a "living fossil" because it is the only living relative of the ammonoids, lives in the depths of the South Pacific and Indian oceans down to 2,000 feet.

During a major extinction event near the end of the Devonian about 365 million years ago, many tropical marine groups disappeared, possibly due to climatic cooling. The die out of species apparently occurred over a period of 7 million years and eliminated species of corals and many other bottom-dwelling marine organisms. Primitive corals and sponges, which were prolific limestone reef builders early in the period, suffered heavily during the extinction and never fully recovered.

While these animal groups vanished, the glass sponges, which tolerated cold conditions, diversified, only to dwindle when the crisis subsided and other groups recovered. Their prosperity during the late Devonian signifies that less fortunate species had succumbed to the effects of climatic cooling. Large numbers of brachiopod families also died out at the end of the period. In contrast, cold-adapted animals living in Arctic waters fared quite well. Much of Gondwana was in the Antarctic during the Devonian, and sea flooded broad areas of the continent. The Gondwanan fauna, which lacked reef builders and other warm-water species, survived the extinction with few losses.

The oldest species living in the world's oceans today thrive in cold waters. Many Arctic species, including certain brachiopods, starfishes, and bivalves, belong to biological orders whose origins extend hundreds of millions of years backward into the Paleozoic. In contrast, tropical faunas such as reef communities, battered by periodic mass extinctions, have come and gone quite rapidly on the geologic time scale. But not all animals that shared the same environments were identically affected by the extinction.

A possible cause for the end-Devonian extinction was the bombardment of the Earth by one or two large asteroids or comets. The meteorite impact theory is supported by the discovery of deposits containing glassy beads called microtektites in the Hunan province of China and Belgium. Microtektites form when a large meteorite impacts the Earth and hurls droplets of molten rock into the air that quickly cool into bits of glass. The deposits also include and unusually high iridium content, which strongly indicates an extraterrestrial source. The Siljan crater in Sweden, about the same age as the microtektites, might be the source of the impact deposits. The evidence supports the notion that meteorite bombardments have contributed to many mass extinctions throughout Earth history.

 

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Terrestrial Vertebrates:

Plants had been greening the Earth for nearly 100 million years before the vertebrates finally set foot on dry land. Previously, freshwater invertebrates and fishes inhabited lakes and streams. Freshwater fish living in Australia around 370 million years ago were almost identical to those living in China, suggesting that the two landmasses were close enough for the fish to travel between them.

By the middle Devonian, stiff competition in the sea encouraged crossopterygians to make short forays on shore to prey on abundant crustaceans and insects. The crossopterygians were lobe-finned fish with heavy enamel-like scales. Their fin bones were attached to the skeleton and arranged into primitive elements of a walking limb. The crossopterygians strengthened their lobe fins into legs by digging in the sand for food and shelter. They eventually ventured farther inland, though never to distant from accessible sources of water such as swamps or streams. Primitive Devonian fish, similar to today's lungfish, crawled on their bellies from one pool to another, pushing themselves along with their fins.

Modern lungfish live in African swamps that seasonally dry out, forcing the fish to hole up for long stretches until the rains return. They burrow into the moist sand, leaving an airhole to the surface, and live in suspended animation, breathing with primitive lungs. In this manner, they can survive out of water for several months or even a year or more if necessary. When the rainy season returns, the ponds fill again, and the fish come back to life, breathing normally with their gills.

In Florida, a walking catfish originating from Asia will leave its drying pond and travel by pushing itself along with its tail and fins, sometimes a considerable distance before finding another suitable home. They breathe with primitive nostrils and lungs as well as gills, placing them midway on the line of evolution from fish to land-living vertebrates. Air-breathing was also important for fish striving to survive in warm, shallow waters low on oxygen.

The descendants of the lobe-finned fish and lungfish were the first advanced animals to populate the land some 370 million years ago. By the late Devonian, the crossopterygians had evolved into the earliest amphibians. Their legacy is well documented in the fossil record, and at no other time in geologic history were so many varied and unusual creatures inhabiting the surface of the Earth.

One of the earliest known amphibians, named icthyostega, crawled around on primitive legs with seven toes on each hind foot. Another ancient amphibian, called acanthostega, had eight fingers on each forelimb, perhaps the most primitive of walking limbs. Amphibians sporting six and eight digits also existed, suggesting that the evolution of early land vertebrates followed a flexible pattern of development. These early amphibians living in the late Devonian, when vertebrates were first making a transition from water to land, spent most of their time in the water, which led to their eventual downfall when the great swamps dried up toward the end of the Paleozoic.

Animal tracks tell of the earliest land invasion. Tracks of the primitive Devonian fish that first ventured on dry land and gave rise to four-legged amphibians exist in formations of late Devonian age onward. The amphibian tracks are generally broad with a short stride, indicating the animal could barely hold its squat body off the ground. It walked with a clumsy gait, and running was completely out of the question.

Amphibian footprints become abundant in the Carboniferous beginning about 350 million years ago and to a lesser extent in the Permian, owing to the amphibian's preference for life in water, in addition to the rise of the reptiles. The fossil remains of the amphibians are largely fragmentary, because of the manner by which vertebrate skeletons are constructed, with a large number of bones that are easily scattered by surface erosion, leaving a scant record of their existence.

 

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The Old Red Sandstone:

Beginning in the late Silurian and continuing into the Devonian, from about 400 million to 350 million years ago, a collision between present eastern North America and northwest Europe raised the Acadian Mountains. The terrestrial redbeds, composed of sandstones and shales cemented by red iron oxide, of the Catskills in the Appalachian Mountains extending from southwestern New York State to Virginia are the main evidence of the Acadian orogeny in North America. Extensive igneous activity and metamorphism accompanied the mountain building at its climax. The Devonian Antler orogeny was another mountain-building episode, producing intensely deformed rocks in the Great Basin of Nevada. The Innuitian orogeny, which deformed the northern margin of present North America, resulted from a collision between the continent and another crustal plate.

The middle Devonian Old Red Sandstone, a thick sequence of chiefly nonmarine sediments in Great Britain and northwest Europe, is the main expression of this mountain building episode in Europe called the Caledonian orogeny. The formation comprises great masses of sand and mud that accumulated in the basins between the ranges of the Caledonian Mountains from Great Britain to Scandinavia. The sediments are poorly sorted and consist of red, green, and gray sandstones and gray shales that often contain fish fossils.

Erosion leveled the continents and shallow seas flowed inland, flooding more then half the landmass. The inland seas and wide continental margins, along with a stable environment, provided favorable conditions for marine life to flourish and proliferate throughout the world. Seas flooding North America during the Devonian produced abundant coral reefs that lithified (became rock) into widespread limestones.

The rising Acadian Mountains on the east side of the inland sea eroded, producing flat-lying, fossiliferous deposits of shale in western New York State, possibly the best Devonian section in the world. The vast Chattanooga Shale Formation, which covers virtually the whole continental interior, was laid down during the Devonian and Carboniferous. The seas also blanketed much of Eurasia in the late Devonian. Terrestrial clastics comprised of rock fragments eroded from the Caledonian Mountains overlay the western part of the continent.

Gondwana, to this point located in the Antarctic, now shifted its position. Its location can be shown by paleomagnetic data, which indicate the locations of continents relative to the magnetic poles by analyzing the magnetic orientations of ancient iron-rich lavas. The south magnetic pole drifted from its present South Africa in the Devonian, ran across Antarctica in the Carboniferous, and ended up in southern Australia in the Permian.

The location of the southern pole is also indicated by widespread glacial deposits and erosional features on the continents that comprised Gondwana during the Paleozoic. The mass extinctions of the middle Devonian 365 million years ago and the late Ordovician 440 million years ago coincided with glacial periods that followed long intervals of ice-free conditions.

Gondwana in the Southern Hemisphere and Laurasia in the Northern Hemisphere were separated by the Tethys Sea, and into this seaway flowed thick deposits of sediments washed off the surrounding continents. Their accumulated weight formed a long, deep depression in the ocean crust, called a geosyncline, which later uplifted into folded mountain belts.

A warm climate and desert conditions over large areas are indicated by widespread distribution of evaporite deposits in the Northern Hemisphere, coal deposits in the Canadian arctic, and carbonate reefs. Warm temperatures of the past are generally recognized by abundant marine limestones, dolostones, and calcareous shales. A coal belt, extending from northeastern Alaska across the Canadian archipelago to northernmost Russia, suggests that vast swamps were prevalent in these regions.

Evaporite deposits generally form under arid conditions between 30 degrees north and south of the equator. However, extensive evaporite deposits are not currently being formed, suggesting a comparatively cooler global climate. The existence of ancient evaporite deposits as far north as the arctic regions implies that either these areas were once closer to the equator or the global climate was considerably warmer in the geologic past.

The Devonian years was 400 days long and the lunar cycle was about 30.5 days as determined by the daily growth rings of fossil corals. Paleomagnetic studies indicate that the equator passed from California to Labrador and from Scotland to the Black Sea during the Devonian and Carboniferous. The ideal climate setting helped spur the rise of the amphibians that inhabited the great Carboniferous swamps.

 

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Chapter Eight:
Carboniferous Amphibians:

The Carboniferous period, which ran from 345 to 280 million years ago, is further divided into the Mississippian and Pennsylvanian periods in North America, and was named for the coal-bearing rocks of Wales, Great Britain. Flora that appeared in the Devonian was plentiful and varied during the Carboniferous. Great coral forests of seed ferns and true trees with seeds and woody trunks spread across Gondwana and Laurasia in the lower Carboniferous.

All forms of fauna that existed in the lower Paleozoic flourished in the Carboniferous except the brachiopods, which declined in number and type. The fusulinids, which appeared for the first time in the Carboniferous, were large, complex protozoans that resembled grains of wheat, ranging from microscopic size up to 3 inches in length. Primitive amphibians inhabited the swampy forests, which were abuzz with hundreds of different types of insects, including large cockroaches and giant dragonflies. When the climate grew colder and widespread glaciation enveloped the southern continents at the end of the period, the first reptiles emerged and displaced the amphibians as the dominant land vertebrates.


The Age of Amphibians:

The ancestors of the amphibians appear to have been the crossopterygians, the stem group from which all terrestrial vertebrates descended. They were lobe-finned, air-breathing fish that grew upwards of 10 feet long with large teeth and the predecessors of modern lungfish. An abundance of food swept up on the beaches during high tide might have enticed these fish to come ashore. Fierce competition in the ocean for a scarce food supply provided extraordinary evolutionary incentive for any animal that could find food on land.

Amphibious fish probably spent little time on shore because their primitive legs could not support their body weight for long periods, requiring them to return to the sea. Eventually, as their limbs strengthened, the amphibious fish wandered farther inland, where crustaceans and insects were abundant. By the middle Devonian, they began to dominate the land and were especially attracted to the great Carboniferous swamps.

By about 335 million years ago, the amphibious fish had evolved into the earliest amphibians. Some species had strong, toothy jaws and resembled giant salamanders, reaching 3 to 5 feet in length. Although the amphibians had well-developed legs for walking on dry land, they apparently spent most of their time in water and depended on nearby water sources to moisten their skins for respiration. They reproduced like fish, laying small eggs without protective membranes in water or moist places. After hatching, the juveniles lived an aquatic, fishlike life-style and breathed with gills. As they matured, the young amphibians metamorphosed into air breathing, four-limbed adults.

The early amphibians were slow and ungainly creatures, with weak legs that could hardly keep their squat bodies off the ground. Therefor, to succeed as hunters without the benefit of speed or agility, the amphibians developed a remarkable tongue that lashed out at insects and flicked them into their mouths. The adaptation was so successful, the amphibians rapidly populated the land.

The necessity of having to live a semiaquatic life-style led to the eventual downfall of the amphibians when the great swamps began to dry out toward the end of the Paleozoic. The void left by the amphibians was quickly filled by their cousins the reptiles, which were better suited for a life totally out of water and destined to become the greatest evolutionary success story the world has ever known.

 

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The Great Coal Forests:

During the second half of the Paleozoic, the continents rose and sea levels dropped, causing the departure of the inland seas, which were replaced with immense swamps. About 315 million years ago, extensive forests grew in the great swamps. These regions formed a vast tropical belt that ran through the supercontinent Pangaea, which straddled the equator.

The lycopods, which ruled the ancient swamps, towering as high as 130 feet, were the first trees to develop true roots and leaves that were generally small. Branches were arranged in a spiral and spores were attached to modified leaves that became primitive cones. Their trunks were composed largely of bark, and for the most part lacked branches along the length of the tree, so the trees looked much like a forest of telephone poles. Only near the end of their lives did the lycopods sprout a small crown of limbs as they prepared for reproduction. Making their living among these trees were giant insects and, huge millipedes, walking fish, and primitive amphibians.

For millions of years, the lycopods endured changes in sea level and climate that alternately drained and flooded the swamps. Then about 310 million years ago, the climate of the tropics became drier and most swamplands disappeared entirely. The climate change set off a wave of extinctions that wiped out virtually all lycopods. Today, they exist only as small grass-like plants in the tropics. Late in the Carboniferous, as the climate grew wetter and the swamps reemerged, weedy plants called tree ferns dominated the Paleozoic wetlands.

The second most diverse group of living plants were the true ferns. They range from the Devonian to the present but were particularly widespread in the Mesozoic and prospered well in the mild climates, whereas today they are restricted to the tropics. Some ancient ferns attained heights of present-day trees. The Permian seed fern glossopteris was especially significant. Its fossil leaves are prevalent on the continents that formed Gondwana but are lacking on the continents that comprised Laurasia, indicating that these two landmasses were in separate parts of the world divided by the Tethys Sea. This sea was wide in the east and narrow in the west, where land bridges aided the migration of plants and animals from one continent to the other.

Although terrestrial fossils are not nearly as abundant as marine fossils, primarily because land species do not fossilize well and fossil-bearing sediments are subjected to erosion, some environments like ancient swamps and marshes provide an abundance of plant and animal fossils. Well preserved, carbonized plant material is commonly found between easily-separated sediment layers. Animals were also buried in the great coal swamps, where their bones were preserved and fossilized.

 

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Fossil Fuels:

The Carboniferous and Permian had the highest organic burial rates of any period in Earth history. Extensive forests and swamps grew successively on top of each other and continued to add to thick deposits of peat, which were buried under layers of sediment. The weight of the overlaying strata and heat from the Earth's interior reduced the peat to about 5 percent of its original volume and metamorphosed it into lignite, as well as bituminous and anthracite coal.

The world's coal reserves far exceed all other fossil fuels combined and are sufficient to support large increases in consumption well into the next century. The amount of economically recoverable coal reserves is upwards of one trillion tons. The Unites States holds substantial reserves of coal, which remain practically untouched. Since coal is the cheapest and most abundant fossil fuel, it will be a favorable alternate source of energy to replace petroleum when reserves run low.

Paleozoic sediments hold a large portion of the world's oil reserves, indicating a high degree of marine organic productivity during this time. The formation of oil and gas requires special geologic conditions, including a sedimentary source of organic material, a porous rock to serve as a reservoir, and a confining structure to act as a trap. The source material is organic carbon in fine-grained, carbon-rich sediments. Porous and permeable sedimentary rocks such as sandstones and limestones serve as reservoir. Geologic structures created by folding or faulting of sedimentary layers trap or pool the oil and gas.

Most of the organic material that produces petroleum derives from microscopic organisms that originated primarily in the surface waters of the ocean and were concentrated in fine particulate matter on the seafloor. For organic material to become petroleum, either the rate of accumulation must be high or the oxygen level in the bottom water must be low so the material does not oxidize before burial under thick sedimentary layers. This is because oxidation causes decay, which destroys organic material. Therefore, areas with high rates of accumulation of sediments rich in organic material are the most favorable sites for the formation of oil-bearing rock.

After deep burial in a sedimentary basin, high temperatures and pressures generated in the Earth's interior chemically alter the organic material into hydrocarbons. Of the hydrocarbons are overcooked, natural gas results. Oil is often associated with thick beds of salt. Because salt is lighter than the overlying sediments, it rises toward the surface, creating salt domes that help trap the oil.

Hydrocarbon volatiles (fluids and gases) along with seawater locked up in the sediments migrated upward through permeable rock layers and accumulated in traps formed by sedimentary structures that provide a barrier to further migration. In the absence of a cap rock, the volatiles continue rising to the surface and escape. Much petroleum has been lost in this manner, as well as by the destruction of the reservoirs by uplift and erosion of the last confining structure. Several tens of millions to a few hundred million years are required to process organic material into oil, depending mainly on the temperature and pressure conditions within the sedimentary basin.

 

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Carboniferous Glaciation:

During the latter part of the Carboniferous around 290 million years ago, Gondwana was in the south polar regions, where glacial centers expanded across the continents. Rocks heavily grooved by the advancing glaciers show lines of ice flow away from the equator and toward the poles, which would not be possible if the continents were situated where they are today. Furthermore, the ice would have had to flow from the sea onto the land in many areas, which is highly unlikely. Instead, the southern continents drifted en masse over the South Pole, and massive ice sheets crossed the present continental boundaries.

During the early part of the glacial epoch, the maximum glacial effects were in South America and South Africa. Later, the chief glacial centers moved to Australia and Antarctica, clearly showing that the southern continents that comprised Gondwana hovered over the South Pole. Continents residing near the poles are often the cause of extended periods of glaciation because land at higher latitudes has a high albedo (reflective quality) and a low heat capacity, encouraging the accumulation of ice.

Ice sheets covered large portions of east-central South America, South Africa, India, Australia, and Antarctica. In Australia, marine sediments were interbedded with glacial deposits and tillites were separated by seams of coal, indicating that periods of glaciation were interspersed with warm interglacial spells, when extensive forests grew. In South Africa, the Karroo Series, comprising a sequence of late Paleozoic tillites and coal beds, extended over an area of several thousand square miles and reached a total thickness of 20,000 feet. Among the coal beds, the best in Africa, are fossil glossopteris leaves, whose existence on the southern continents in among the best evidence for the theory of continental drift.

One cause of the ice age was the loss of atmospheric carbon dioxide. The burial of carbon dioxide in the crust might have been the key to the onset of all major ice ages since life evolved on Earth. A substantial carbon dioxide repository during the latter part of the Paleozoic were the great forests that spread over the land. Plants began to invade the land and extended to all parts of the world beginning about 450 million years ago. Lush forests that grew during the Carboniferous stored large quantities of carbon in their woody tissues. Burial under layers of sediment compacted the vegetative matter and converted it into thick seams of coal. The reduction of the carbon dioxide content in the atmosphere severely weakened the greenhouse effect, causing the climate to cool.

The continental margins became less extensive and narrower, confining marine habitats to near-shore regions, which might have influenced the great extinction at the end of the Paleozoic. Land once covered with great coal swamps dried out as the climate grew colder. No major extinction event occurred during the widespread Carboniferous glaciation around 330 million years ago, however. The relatively low extinction rates were probably due to a limited number of extinction-prone species following the late Devonian extinction.

 

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Pangaea:

Between 360 and 270 million years ago, Gondwana and Laurasia converged into the supercontinent Pangaea, meaning all lands. The massive continent had a total area of about 80 million square miles or 40 percent of the Earth's total surface area. It straddled the equator, extending practically from pole to pole, with an almost equal amount of land in both hemispheres, whereas today two-thirds of the continents lie north of the equator. A single great ocean called Panthalassa stretched uninterrupted across the planet, with the continents huddling to one side. Over the ensuing periods, smaller parcels of land continued to collide with the supercontinent until it reached its peak size about 210 million years ago.

The continental collisions crumpled the crust and pushed up huge masses of rock into several mountain belts throughout many parts of the world. Volcanic eruptions were extensive due to frequent continental collisions. During times of highly active continental movements, volcanic activity increases, especially at midocean spreading ridges where new oceanic crust is created and at subduction zones where old oceanic crust is destroyed. The amount of volcanism affects the rate of mountain building and the composition of the atmosphere, which affects the climate.

When Gondwana linked with Laurasia to form Pangaea, the collision raised the Appalachian and Ouachita mountains. Simultaneously, Laurasia connected with Serbia, thrusting up the Ural Mountains. The continued clashing of island arcs with North America resulted in an episode of mountain building in Nevada called the Sonoma orogeny, which coincided with the complete assembly of Pangaea 250 million years ago.

The sediments in the Tethys Sea separating Gondwana and Laurasia buckled and uplifted into various mountain belts, including the ancestral Hercynian Mountains of southern Europe. As the continents rose higher and the ocean basins dropped lower, the land became dryer and the climate grew colder, especially in the southernmost lands, which were covered with glacial ice. All known episodes of glaciation occurred during times of lowered sea levels. The changes in the shapes of the ocean basins greatly influenced the course of ocean currents, which in turn had a pronounced affect on the climate.

The closing of the Tethys Sea eliminated a major barrier to the migration of species from one continent to another, and they dispersed to all parts of the world. When all continents combined into Pangaea, plant and animal life experienced a proliferation of species in the ocean as well as on land. The formation of Pangaea marked a major turning point in the evolution of life, during which the reptiles emerged as the dominant species, conquering land, sea, and sky.

A continuous shallow-water margin ran around the entire perimeter of Pangaea, and no major physical barriers hampered the dispersal of marine life. Furthermore, the seas were largely confined to the ocean basins, leaving the continental shelves mostly exposed. The continental margins were less extensive and narrower, confining marine habitats to the near-shore regions. Consequently, habitat area for shallow-water marine organisms was very limited, which accounts for the low species diversity. As a result, marine biotas were more widespread but contained comparatively fewer species.

In the northern latitudes, thick forests of primitive conifers, horsetails, and club mosses that grew as tall as 30 feet dominated the mountainous landscape. Much of the interior probably resembled a grassless version of the steppes of central Asia, where temperatures varied from very hot in summer to very cold in winter. Since grasses would not appear for well over 100 million years, the scrubby landscape was dotted with bamboolike horsetails and bushy clumps of now-extinct seed ferns that resembled present-day tree ferns.

Browsing on the seed ferns were herds of moschops, 16-foot reptiles with thick skulls adapted for butting during mating season, a tactic similar to the behavior of modern herd animals. They were probably preyed upon by packs of lycaenops, which were reptiles with doglike bodies and long canine teeth projecting from their mouths. Mammal-like reptiles called dicynodonts also had two caninelike tusks and fed on small animals along riverbanks. A 2-foot-long amphibian with armadillolike plates rooted in the soil for worms and snails. Small reptiles probably ate insects like modern lizards do.

The Pangaean climate was one of the extremes, with the northern and southern regions colder than Siberia and the interior deserts hotter than the Sahara, and almost devoid of life. The massing of continents together created an overall climate that was hotter, drier, and more seasonal than at any other time in geologic history. These conditions might have led to the mass extinction of primarily terrestrial animals some 30 million years after most of Pangaea was assembled. Pangaea remained intact for another 40 million years, after which if rifted apart into the present continents.

 

[M82A1]

Chapter Nine:
Permian Reptiles:

The Permian period from 280 to 250 million years ago was named for a well-exposed sequence of marine rocks and terrestrial redbeds on the western side of the Ural Mountains in the Russian district of Perm. Rocks of the Permian age are distinct in western North America, particularly in Nevada, Utah, and Texas. Important reserves of oil and natural gas reside in the Permian Basin of Texas and Oklahoma. Extensive coal deposits of Permian age exist in Asia, Africa, and Australia.

During the Permian, all major continents combined into the supercontinent Pangaea, where widespread mountain building and extensive volcanism were prevalent. The interior of Pangaea was largely desert, causing the decline of the amphibians and the rise of the reptiles. At the end of the Permian, perhaps the greatest extinction the Earth has ever known eliminated over 95 percent of all species, paving the way for the ascension of the dinosaurs.


The Age of Reptiles:

The age of reptiles, which began in the Permian and lasted 200 million years, witnessed the evolution of some 20 orders of reptilian families. Amphibians, which were prominent in the Carboniferous, declined considerably in the Permian because of a preference for life in water. When the Carboniferous swamps dried out and were largely replaced with deserts, the amphibians gave way to their cousins the reptiles, which were well adapted to drier climates. In the latter half of the Permian, the reptiles succeeded the amphibians and became the dominant land-dwelling animals of the Mesozoic era. The generally warm climate of the era was also advantageous to the reptiles and aided them in colonizing the land.

The increase in the number of reptilian fossil footprints in Carboniferous and Permian sediments shows the rise of the reptiles, at the expense of the amphibians. The success of the reptiles was largely due to their more efficient mode of locomotion. The reptiles were also better suited to a full-time life on dry land, whereas the amphibians were dependent on a local source of water for moistening their skins and reproduction.

The reptilian foot was a major improvement over that of the amphibian, with changes in the form of the digits, the addition of a thumblike fifth digit, and the appearance of claws. In some reptiles, the tracks narrowed and the stride lengthened. Others maintained a four-footed walking gait and ran reared up on their hind legs. Although most reptiles walked or ran on all fours, by the late Permian some smaller reptiles often stood on their hind legs when they required speed an agility. The body pivoted at the hips and a long tail counterbalanced the nearly erect trunk. This stance freed the forelimbs for attacking small prey and completing other useful tasks.

Reptiles have scales that retain the animal's bodily fluids, whereas amphibians have a permeable skin that must be moistened frequently. Another major advancement over the amphibians was the reptile's mode of reproduction. Like fish, amphibians laid their eggs in water, and after hatching, the young fended for themselves, often becoming prey for predators. The reptile's eggs have hard, watertight shells so they can be laid on dry land. Parents protected their young, which gave them a better chance of survival, contributing to the reptile's great success in populating the land.

Like fish and amphibians, reptiles are cold-blooded, a term that is misleading since they draw heat from the environment. Therefor, the blood of a reptile sunning on a rock can actually be warmer than that of a warm-blooded mammal. An ancient reptile called dimetrodon had a large sail along its back, apparently to regulate its body temperature by absorbing sunlight during cold weather and radiating excess body heat when the weather was hot. A high body temperature is as important to a reptile as it is to a mammal to achieve maximum metabolic efficiency. In cold mornings, reptiles are sluggish and vulnerable to predators. They bask in the sun until their bodies warm and their metabolism can operate at peak performance.

Reptiles require only about one-tenth the amount of food mammals need to survive because mammals use most of their calories to maintain a high body temperature. Consequently, reptiles can live in deserts and other desolate places and flourish on small quantities of vegetation that would quickly starve a mammal of the same size. The generally warm climate of the Mesozoic was very advantageous to the reptiles and aided them in colonizing the land, whereas the amphibians, which avoided direct sunlight and were relatively cold and slow-moving, were at a disadvantage.

Perhaps the strangest reptile that ever lived was tanystropheus, dubbed the "giraffe-neck saurian." The animals measured as much as 15 feet from head to tail and is famous for its absurdly long neck, which was more than twice the length of the trunk. As it matured, its neck grew at a much faster rate than the rest of its body. The reptile was most likely aquatic because it could not possibly have supported the weight of its neck while on land. Tanystropheus probably used its grossly elongated neck for scavenging bottom sediments for food.

The phytosaurs were large, heavily armored, predatory reptiles with sharp teeth. They resembled crocodiles with their elongated snouts, short legs, and long tails, but were not closely related to them. They evolved from the thecodonts, which also gave rise to the crocodiles and dinosaurs. They thrived in the late Triassic, evolving quite rapidly, but apparently did not survive beyond the end of the period.

Near the close of the Triassic, when reptiles were the leading form of animal life, occupying land, sea, and air, a remarkable reptilian group called the crocodilians appeared in the fossil record. This group had originated in Gondwana, and was composed of the alligators with a blunt head, the crocodiles with an elongated head, and the gavials with an extremely narrow head. Members of this group adapted to life on dry land, a semiaquatic life, or an entirely aquatic life with a sharklike tail, a streamlined head, and legs modified into swimming paddles. A fossil of a gavial-like monster from the lower Cretaceous in Niger, West Africa, measured about 35 feet long.

The crocodilians diversified considerably over the past 200 million years, spreading to all parts of the world and adapting to a wide variety of habitats. Crocodile fossils found in the high latitudes of North America indicate a warm climate during the Mesozoic. They belong to the subclass Archosauria, which literally means ruling reptiles, along with dinosaurs and pterosaurs and are the only surviving members to escape the "great dying" at the end of the Cretaceous.