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

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Mammal-Like Reptiles:

The pelycosaurs were the first animals to depart from the basic reptilian stock some 300 million years ago. They were distinguished from other reptiles by their large size and varied diet, and the earliest predators were capable of killing relatively large prey, including other reptiles. A pelycosaur called dimetrodon grew to a length of about 11 feet. It had a large dorsal sail composed of webs of membrane well supplied with blood, stretching across bony protruding spines and probably used for temperature control. When the animal was cold, it turned its body broadside to the sun to absorb more sunlight. When the animal was hot, it sought out a shaded area or exposed itself to the wind. This appendage might have been a crude forerunner of the temperature control system in mammals.

As the climate warmed, the pelycosaurs lost their sails and perhaps gained some degree of internal thermal control. They thrived for about 50 million years and then gave way to their descendants, the mammal-like reptiles called the therapsids. The first therapsids retained many characteristics of the pelycosaurs, but their legs were well adapted for much higher running speeds. They ranged in size from as small as a mouse to as large as a hippopotamus.

The early members invaded the southern continents at the beginning of the Permian when those lands were recovering from the Carboniferous glaciation, suggesting the animals were warm-blooded enough to withstand the cold. They probably had undergone some physiological adaptations to enable them to feed and travel through the snows of the cold winters. They were apparently too large to hibernate, as shown by the lack of growth rings in their bones, an indicator similar to the tree rings that mark alternating seasons of growth. The development of fur appeared in the more advanced therapsids, as they migrated into colder climates. Therapsids might also have operated at lower body temperatures than most living mammals to conserve energy. The family of mammal-like reptiles clearly shows a transition from reptile to mammal. Mammals evolved from the mammal-like reptiles over a period of more than 100 million years, during which time the animals adapted so as to function better in a terrestrial environment. Teeth evolved from simple cones that were replaced repeatedly during the animal's lifetime to more complex shapes that were replaced only once. However, the mammalian jaw and other parts of the skull still shared many similarities with reptiles.

The advantages of being warm-blooded are tremendous, and a stable body temperature finely tuned to operate within a narrow thermal range provides a high rate of metabolism independent of the outside temperature. Therefor, the work output of leg muscles, heart, and lungs increases enormously, giving mammals the ability to outperform and outendure reptiles. The principle of heat loss, by which a large body radiates more thermal energy than a small one, applies to large reptiles as well as mammals. In addition, mammals have a coat of insulation, including an outer layer of fat and fur, to prevent the escape of body heat during cold weather.

The therapsids appear to have reproduced like reptiles, by laying eggs. They might have protected and incubated the eggs and fed on their young. This in turn might have resulted in longer egg retention in the female and given rise to live births. The therapsids dominated animal life fore more than 40 million years until the middle Triassic, and then for unknown reasons they lost out to the dinosaurs. From then on, primitive mammals were relegated to the role of a shrewlike nocturnal hunter of insects until the dinosaurs finally became extinct.

 

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The Appalachian Orogeny:

Perhaps the most impressive landforms on Earth are the mountain ranges, created by the forces of uplift and erosion. Paleozoic continental collisions crumpled the crust, pushing up huge masses of rock into several mountain chains throughout many parts of the world.

The Appalachian Mountains, extending some 2,000 miles from central Alabama to Newfoundland, were upraised during continental collisions between North America, Eurasia, and Africa in the late Paleozoic, from about 350 million to 250 million years ago during the construction of Pangaea. The southern Appalachians are underlain with more than 10 miles of flat-lying sedimentary and metamorphic rocks, whereas the surface rocks were highly deformed by the collision.

This type of formation suggests that these mountains were the product of thrust faulting, involving crustal material carried horizontally for great distances. The sedimentary strata rode westward on top of Precambrian metamorphic rocks and folded over, buckling the crust into a series of ridges and valleys. The existence of sedimentary layers beneath the core of the Appalachians suggests that thrusting involving basement rocks is responsible for the formation of all mountain belts, possibly since the process of plate tectonics began. The shoving and stacking of thrust sheets during continental collision also might have been a major mechanism in the continued growth of the continents.

The Mauritanide mountain range in Western Africa is the counterpart, the "other side," of the Appalachians. It is characterized by a series of belts running east to west that are similar in many respects to the Appalachian belts. The eastern parts of the range comprise sedimentary strata partially covered with metamorphic rocks that have overridden the sediments from the west along thrust faults. Older metamorphic rocks resembling those of the southern Appalachians lie westward of this region, while a coastal plain of younger horizontal rocks covers the rest. Furthermore, a period of metamorphism and thrusting similar to the formation of the Appalachians occurred prior to the opening of the Atlantic. In this respect, the two mountain ranges are practically mirror images of each other.

This episode of mountain building also uplifted the Hercynian Mountains in Europe, which extended from England to Ireland and continued through France and Germany. The folding and faulting was accompanied by large-scale igneous activity in England and on the European continent. The Ural Mountains were similarly formed by a collision between the Siberian and Russian continental shields. The Transantarctic Range, comprising great belts of folded rocks, formed when two plates came together to create the continent of Antarctica. Prior to the end of the Permian, the younger parts of West Antarctica had not yet formed, and only East Antarctica was present.

 

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Late Paleozoic Glaciation:

The continents of Africa, South America, Australia, India, and Antarctica were glaciated in the late Paleozoic, around 270 million years ago, as evidence by glacial deposits and straitions in ancient rocks. The lines of ice flow were away from the equator and toward the poles. Therefor, the continents must have joined in such a manner that ice sheets moved across a single landmass, radiating outward from a glacial center over the South Pole.

The Late Paleozoic was a period of extensive mountain building, which raised massive chunks of crust to higher elevations, where glaciers are nurtured in the cold, thin air. Glaciers also might have formed at lower latitudes when lands were elevated during continental collisions. When Gondwana and Laurasia combined into Pangaea, the continental collisions crumpled the crust and pushed up huge blocks into several mountain chains throughout many parts of the world. Generally speaking, with high mountains come low temperatures and increased precipitation, a situation that maintains glaciers in the high altitudes.

Besides folded mountain belts, volcanoes were prevalent, and usually long periods of volcanic activity blocked out the sun with clouds of volcanic dust and gases, thereby significantly lowering surface temperatures. As the continents rose higher, the ocean basins dropped lower. The change in shape of the ocean basins greatly affected the course of ocean currents, which in turn had a profound effect on the climate. All known episodes of glaciation occurred at times when sea levels should have been low, although not all mass extinctions were associated with lowered sea levels.

The continental margins became less extensive and narrower, confining marine habitats to near-shore areas. Such an occurrence might have had a major influence on the great extinction at the end of the Paleozoic era. During this time, land once covered with great coal swamps completely dried out as the climate grew colder, culminating in the deaths of multitudes of species.

 

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Mass Extinction:

Throughout the Earth's history, massive numbers of species have vanished in several short periods. During geologically brief intervals of perhaps a few million years, mass extinctions in the oceans have eliminated half or more of the existing families of plants and animals. Devastations of this magnitude are generally due to radical global changes in the environment. Drastic changes in environmental limiting factors, including temperature and living space on the ocean floor, determined the distribution and abundance of species in the sea.

Many episodes of extinction coincided with periods of glaciation, and global cooling had a major effect on life. The living space of warmth-loving species was restricted to the tropics. Species trapped in confined waterways, unable to move to warmer seas, were particularly hard hit. Furthermore, the accumulation of glacial ice in the polar regions lowered sea levels, thereby reducing shallow water shelf areas, which limited the amount of habitat and consequently the number of species.

Ocean temperatures is by far the most important factor limiting the geographic distribution of marine species, and climatic cooling is the primary culprit behind most extinctions in the sea. Species unable to migrate to warmer regions or adapt to colder conditions are usually the most adversely affected. This is especially true for tropical faunas that can tolerate only a narrow range of temperatures and have nowhere to migrate. Since lowered temperatures also slow down the rate of chemical reactions, biological activity during a major glacial event should function at a lower energy state, which could affect the rate of evolution and species diversity.

The greatest extinction event took place when the Permian ended 250 million years ago. The extinction was particularly devastating to Permian marine fauna. Half the families of marine organisms, including more than 95 percent of all known species, abruptly disappeared. In effect, the extinction left the world almost devoid of life forms at the end of the Paleozoic as at the beginning. The extinction followed on the heels of a late Permian glaciation, when thick ice sheets blanketed much of the Earth, significantly lowering ocean temperatures.

Corals, which require warm, shallow water, were affected the worst, as evidenced by the lack of coral reefs in the early Triassic. Another group of animals that disappeared at this time were the fusulinids, which were formanifers shaped like a grain of wheat and populated the shallow seas of the world for about 80 million years, during which their shells accumulated into vast deposits of limestone.

The crinoids and brachiopods, which had their heyday in the Paleozoic, were relegated to minor roles during the succeeding Mesozoic. The spiny brachiopods that were plentiful in the late Paleozoic seas vanished without leaving any descendants. The trilobites, which were extremely successful during the Paleozoic, completely died out at the end of the era. A variety of other crustaceans, including shrimps, crabs, crayfish, and lobsters, occupied the habitats vacated by the trilobites. On land, 75 percent of the amphibian families and over 80 percent of the reptilian families disappeared.

Whatever the agents of biological stress were -- climatic changes, shifts in ocean currents, shallowing seas, or disruptions in food chains -- the ability of the biosphere to resist them varied in different parts of the world. But one very consistent pattern in mass extinctions was that, although each event typically affected different suites of organisms, tropical biotas, which contain the highest number of species, were almost always the hardest hit.

When all continents had converged into Pangaea by the end of the Permian around 250 million years ago, the change in geography spurred a great proliferation of plant and animal life on land and in the sea. The formation of Pangaea marked a major turning point in the evolution of life, during which the reptiles emerged as the dominant species.

The Pangaean climate appears to have been equable and warm throughout most of the year. However, much of the interior of Pangaea was desert, where temperatures fluctuated wildly from season to season, with scorching hot summers and freezing cold winters. These climate conditions might have contributed to the widespread extinction of land-based species during the late Paleozoic. It also explains why the reptiles, which adapt readily to hot, dry climates, replaced the amphibians as the dominant land animals.

The sea level lowered during the formation of Pangaea and drained the interiors of the continents. The drop in sea level caused the inland seas to retreat, producing a continuous, narrow continental margin around the supercontinent. This in turn reduced the amount of shoreline, which radically limited the marine habitat area. Moreover, unstable near-shore conditions resulted in an unreliable food supply. Many species unable to cope with the limited living space and food supply died out in tragically large numbers, paving the way for the ascension of entirely new species.

 

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Chapter Ten:
Triassic Dinosaurs:

The Triassic period, which marks the start of the Mesozoic era, ran from 250 to 210 million years ago and was named for a sequence of redbed and limestone strata in central Germany. In North America, continental sediments and redbeds add to the rugged beauty of Utah, Wyoming, and Colorado. In Arizona's Petrified Forest lie the fossilized remains of primitive Triassic-age conifers that once flourished in the upland regions. At the end of the period, the supercontinent Pangaea started rifting apart into the present continents, and massive amounts of basalt spilled onto the landscape.

During the late Triassic, large families of terrestrial animals died off in record numbers. The mass extinction spanned a period of less than a million years but was responsible for killing nearly half the reptile families. The dying-out of species forever changed the character of life on Earth and initiated the rise of the dinosaurs, one of biology's great success stories.


The Age of Dinosaurs:

At the beginning of the Mesozoic era, the continents consolidated into a supercontinent, at the era's midpoint they began to rift apart, and at its end they were well on their way to their present locations. The breakup of Pangaea created three new bodies of water that included the Atlantic, Arctic, and Indian oceans. The climate was exceptionally mild for an unusually long time. The reptiles especially prospered during these extraordinary conditions. Besides conquering the land, some species went to sea and others took to the air, occupying nearly every corner of the globe.

Early in the Triassic, the Earth was recovering from a major ice age and an extinction that took the lives of over 95 percent of all species. The early Mesozoic marked the rebirth of life, and 450 new families of plants and animals came into existence. But instead of inventing entirely new body plans as during the Cambrian explosion at the beginning of the Paleozoic, species developed new variations on already established themes. Therefore, fewer experimental organisms arose, and many lines of today's species evolved. Several major groups of terrestrial vertebrates made their debut, including the ancestors of dinosaurs, modern reptiles, mammals, and the predecessors of birds. True birds did not appear in the fossil record for another 50 million years.

Dinosaurs arose to become the dominant terrestrial species for the next 150 million years. They suppressed the rise of other creatures, including the mammals, which were then tiny and inconsequential. The amphibians continued to decline during the Mesozoic, with all large, flat-headed species becoming extinct early in the Triassic. The group thereafter was represented by the more familiar toads, frogs, and salamanders. Although the amphibians did not achieve complete dominion over the land, their descendants the reptiles became the undisputed rulers of the world.

The oldest dinosaurs originated on the southern continents of Gondwana when the last glaciers of the great Permian ice age were departing and the region was still recovering from the cold conditions. About 230 million years ago, when mammal-like reptiles dominated the land, dinosaurs represented only a minor percentage of all animals. Several reptile species living at the same time of the early dinosaurs still far outweighed them. However, in just 10 million years, dinosaurs became the dominant species, evolving from moderate-sized animals less than 20 feet long to become the giants for which they are famous.

The dinosaurs descended from the thecodonts, the apparent common ancestors of crocodiles and birds. The earliest thecodonts were small to medium-size predators that lived during the Permian-Triassic transition. One group of thecodonts took to the water and became large fish eaters. They included the phytosaurs, which died out in the Triassic, and the crocodilians, which remain successful today. Pterosaurs were also descendants of the Triassic thecodonts. The appearence of featherlike scales ostensively used for insulation suggests that thecodonts were also the ancestors of birds. By the end of the Triassic, the dinosaurs replaced the thecodonts as the dominant terrestrial vertebrates.

Dinosaurs are classified as sauropods, which were long-necked herbivores, or as carnosaurs, which were bipedal carnivores that possibly hunted in packs. The camptosaur, ancestor to many later dinosaur species, was a herbivore up to 25 feet long. Not all dinosaurs were giants, however, and many were no larger than most modern mammals. Protoceratops and ankylosaurs were very common and ranged over wide areas like modern-day sheep. The smallest known dinosaur foorprints are about the size of a penny. The smaller dinosaurs had hollow bones similar to those of birds. Some had long, slender hind legs, long delicate forelimbs, and a long neck. And, if not for a lengthy tail, their skeletons would closely resemble those of modern ostriches.

Many early small dinosaurs reared up on their hind legs and were the first animals to establish a successful permanent two-legged stance. Bipedalism increased speed and agility and freed the forelimbs for foraging and other tasks. The back legs and hip supported the entire weight of the animal, while a large tail counterbalanced the upper portions of the body, and the dinosaur walked much like birds. Therefor, dinosaurs are classified as ornithischians with a birdlike pelvis or as suarischians with a lizard-like pelvis. The ornithischians probably arose from the same group of thecodont reptiles that gave rise to crocodiles and birds. Indeed, birds are the only living relatives of the dinosaurs, and the skeletons of many small dinosaurs resemble those of birds.

Some bipedal dinosaurs later reverted to a four-footed stance probably because of their increased weight. They eventually evolved into gigantic long-tailed, long-necked sauropods such as the apathosaurus, formerly called brontosaurus. Others, like tyrannosaurus rex, possibly the greatest land carnivore that ever lived, maintained a two-legged stance with powerful hind legs, a muscular tail for counterbalance, and arms shortened to almost useless appendages.

Dinosaur tracks are the most impressive of all fossil footprints because the great weight of many species caused deep indentations to be left in the ground. Their footprints exist in relative abundance in terrestrial sediments of the Mesozoic age throughout the world. The study of dinosaur tracks suggests that some species were highly gregarious, gathering in groups. Large carnivores like tyrannosaurus rex were swift, agile predators that could sprint up to 45 miles per hour.

The giant herbivores might have traveled in great herds, with the largest adults in the lead and the juveniles placed in the center for protection. Duck-billed hadrasaurs, among the most successful of all dinosaur groups, were up to 15 feet tall and lived in Arctic regions, where they either adapted to the cold and dark or migrated in large herds over long distances to warmer climates. Triceratops, whose vast herds roamed the entire globe toward the end of the Cretaceous and were among the last to go during the dinosaur extinction, might have contributed to the decline of other species of dinosaurs possibly due to extensive habitat destruction or the spread of diseases.

Females of some dinosaur species might have given live births. Many nurtured and fiercely protected their offspring until they could fend for themselves, allowing larger numbers to mature into adulthood, thus ensuring the continuation of the species. The parents might have brought food to their young and regurgitated seeds and berries as modern birds do. This parental care for infants suggests strong social bonds and might explain why dinosaurs were so successful for so long.

Some dinosaurs might have developed complex mating rituals. Besides regulating its body temperature, the large sail on dimetrodon's back might have been used to attract females. Other dinosaurs might have sported elaborate head gear for much the same purpose. The carnivores were cunning and agressive creatures that charged at their prey with speed and agility. The cranial capacity of some carnivores indicates they possessed relatively large brains and were fairly intelligent, able to react to a variety of environmental pressures. The velociraptors with their sharp claws and powerful jaws were among the most vicious hunters, whose varacious appetites suggest they were warm-blooded.

Other dinosaur species might have aquired a certain degree of temperature control like mammals and birds. Dinosaurs descended from the thecodonts, the same common ancestors of warm-blooded birds, the distant living relatives of the dinosaurs. An arguement in favor of warm-blooded dinosaurs contends that the skeletons of smaller, lighter species bear many resemblances to those of birds. Evidence for rapid juvenile growth, which is common among mammals, also exist in the bones of some dinosaur species, possibly providing another sign of warm-bloodedness.

When the dinosaur age began, the climates of southern Africa and the tip of South America where the early dinosaurs roamed experienced cold winters, during which large cold-blooded animals could not have survived without migrated to warmer regions. The stamina needed for such long-distance migration would have required sustained energy levels that only warm-blooded bodies could provide. Warm-blooded animals mature more rapidly than cold-blooded animals, which continue growing steadily until death. A comparison among the bones of dinosaurs, crocodiles, and birds, all of which had a common ancestor, shows a similarity between bird and dinosaur bones -- another sign of possible warm-bloodedness. Furthermore, dinosaur bones had a higher blood vessel density than those of living mammals. Some dinosaur skulls show signs of sinus membranes, which only exist in warm-blooded animals.

Several dinosaur species appear to have been swift and agile, requiring a high rate of metabolism that only a warm-blooded body can provide. The complex social behavior of dinosaurs appears to be an evolutionary advancement that results from being warm-blooded. Even the females of some species might have produced live births like mammals. Yet at the end of the Cretaceous, when the climate supposedly grew colder, the warm-blooded mammals survived while the dinosaurs did not.

The period between the end of the Triassic and the beginning of the Jurassic was one of the most exciting times in the history of land vertebrates. In the late Triassic, all the continents were joined only at their western ends with Laurasia in the north and Gondwana in the south. Nonetheless, animal life on Laurasia was becoming distinct from that on Gondwana. A land bridge connecting Laurasia to the Indochina microcontinent might have been the last link, enabling the migration of animals when the two blocks collided at the close of the Triassic.

At the end of the Triassic, about 210 million years ago, a huge meteorite slammed into the Earth, creating a 60-mile-wide Manicouagan impact structure in Quebec, Canada. The gigantic explosion appears to have coincided with a mass extinction over a period of less than a million years that killed off 20 percent or more of all families of animals, including nearly half the reptile families. In the ocean, ammonoids and bivalves were decimated and the conodonts, fossil skeletons of a leechlike animal, completely disappeared. The extinction forever changed the character of life on Earth and paved the way for the rise of the dinosaurs.

Almost all animal groups, including amphibians, reptiles, and mammals, made their debut on the evolutionary stage at this time. This was also the time when the dinosaurs achieved dominance over the Earth -- and held their ground for the next 150 million years. After this, theoretically, another large meteorite struck the Earth with the explosive force of 100 trillion tons of dynamite, turning the planet into an inhospitable world. In this manner, the dinoaurs might have been both created and destroyed by meteorites.

 

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The Tethyan Fauna:

At the beginning of the Triassic, the Tethys Sea was a huge bay separating the northern and southern arms of Pangaea, which took the shape of a gigantic letter C that straddled the equator. Between the late Paleozoic and middle Cenozoic, the Tethys was a broad tropical seaway that extended from western Europe to southeast Asia and harbored diverse and abundant shallow-water marine life. A great circumglobal ocean current that distributed heat to all parts of the world maintained warm climatic conditions. The energetic climate eroded the high mountain ranges of North America and Europe down to the level of the prevailing plain.

Early in the Triassic, ocean temperatures probably remained cool after the late Permian ice age. Marine invertebrates that managed to escape extinction lived in a narrow margin near the equator. Corals, which require warm, shallow water for survival, were particularly hard hit, as evidenced by the lack of coral reefs at the beginning of the Triassic. When the great glaciers melted and the seas began to warm, reef-building became intense in the Tethys Sea, with thick deposits of limestone and dolomite laid down by prolific lime-secreting organisms.

The mollusks appear to have weathered the hard times of the late Permian extinction quite well and continued on to become the most important shelled invertebrates of the Mesozoic seas, with some 60,000 distinct species living today. The warm climate of the Mesozoic influenced the growth of giant animals in the ocean as well as on land. Giant clams grew to 3 feet wide, giant squids were upwards of 65 feet long and weighed over a ton, and crinoids reached 60 feet in length.

The cephalopods were extremely spectacular and diversified to become the most successful marine invertebrates of the Mesozoic seas. The coiled-shell ammonoids, which evolved in the early Devonian period 395 million years ago, grew to as much as 7 feet across. They traveled by jet propulsion, using neutral buoyancy to maintain depth, which contributed to their great success. However, of the 25 families of widely ranging ammonoids in the late Triassic, all but one or two became extinct at the end of the period. Those species that escaped extinction eventually evolved into scores of ammonite families in the Jurassic and Cretaceous.

Among the marine vertebrates, fish progressed into more modern forms. The sharks regained ground lost from the great Permian extinction and continued to become the successful predators they are today. The sea serpentlike plesiosaurs, the seacowlike placodonts, and the dolphinlike ichthyosaurs were reptiles that returned to the sea, where they achieved great evolutionary success. The placodonts were a group of short, stout marine reptiles with large, flattened teeth, which they probably used to feed primarily on bivalves and other mollusks. Many other reptilian species, including lizards and turtles that were quite primitive in the Triassic, also went to sea. However, only the smallest turtles made it past the extinction at the end of the Cretaceous.

During the final stages of the Cretaceous, when the seas drained from the land as the level of the ocean dropped, the temperatures in the Tethys Sea began to fall. As the continents drifted poleward during the last 100 million years, the land accumulated snow and ice, which had an additional cooling effect on the climate.

Most warmth-loving species, especially those living in the Tethys Sea, disappeared when the Cretaceous ended. The most temperature sensitive Tethyan fauna suffered the heaviest extinction rates. Species that were so successful in the warm waters of the Tethys dramatically declined when ocean temperatures dropped. Afterward, marine species acquired a more modern appearance as ocean bottom temperatures continued to plummet.

 

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The New Red Sandstones:

The Triassic witnessed a complete retreat of marine waters from the land as the continents continued to rise. Abundant terrestrial redbeds and thick beds of gypsum and salt were deposited in the abandoned basins. Also, the amount of land covered with deserts was much greater in the Triassic than today, as indicated by a preponderance of red rocks composed of terrestrial sandstones and shales now exposed in the mountains and canyons in the western United States. Terrestrial redbeds covered a region from Nova Scotia to South Carolina and the Colorado Plateau.

Redbeds are also common in Europe, where in northwestern England they are particularly well developed. Over northern and western Europe the terrestrial redbeds are characterized by a nearly fossil-free sequence called the New Red Sandstone, named for a sedimentary formation in Scotland famous for its dinosaur footprints. The unit shows a continuous gradation from Permian to Triassic in the region, with no clear demarcation between the two periods.

The wide occurrences of red sediments probably resulted from massive accumulations of iron supplied by one of the most intense intervals of igneous activity the world has ever known. Air trapped in ancient tree sap suggests a greater abundance of atmospheric oxygen, which oxidized the iron to form the mineral hematite, named so because of its blood-red color.

The mountain belts of the Cordilleran of North America, the Andean of South America, and the Tethyan of Africa-Eurasia contain thick marine deposits of Triassic age. The Cordilleran and Andean belts were created by the collision of east Pacific plates with the continental margins of the new American plates, formed when Pangaea rifted apart. The Tethyan belt formed when Africa collided with Eurasia, raising the Alps, which contain an abundantly fossil-bearing Triassic section. The Tethys Sea, located in the tropics during the Triassic, contained widespread coral reefs that uplifted to form the Dolomites and Alps during the collision of Africa and Eurasia in the Cenozoic.

Late in the Triassic, an inland sea began to flow into the west-central portions of North America. Accumulations of marine sediments eroded from the Cordilleran highlands to the west, often referred to as the ancestral Rockies, were deposited on the terrestrial redbeds of the Colorado Plateau. Important reserves of phosphate used for fertilizers were precipitated in the late Permian and early Triassic in Idaho and adjacent states. Huge sedimentary deposits of iron were also laid down. The ore-bearing rocks of the Clinton Iron Formation, the chief iron producer in the Appalachian region from Alabama to New York, were deposited during this time.

Evaporite accumulation peaked during the Triassic, when the supercontinent Pangaea was just beginning to rift apart. Evaporite deposits form when shallow brine pools generally replenished by seawater evaporite. Few evaporite deposits date beyond 800 million years ago, however, probably because most of the salt was buried or recycled into the sea. Ancient evaporite deposits exist as far north as the Arctic region, indicating that these areas were once closer to the equator or that the global climate was considerably warmer in the past.

 

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Triassic Basalts:

Over the past 250 million years, 11 episodes of massive flood basalt volcanism have occurred worldwide. They were relatively short-lived events, with major phases generally lasting less than 3 million years. These large eruptions created a series of overlapping lava flows, giving many exposures a terracelike appearance known as traps, from the Dutch word for "stairs."

Many flood basalts lie near continental margins, where great rifts began to separate the present continents from Pangaea near the end of the Triassic. These massive outpourings of basalt reflect one of the greatest crustal movements in the history of the Earth. The continents probably traveled much faster than they do today because of more vigorous plate motions, resulting in tremendous volcanic activity.

Triassic basalts common in eastern North America indicate the formation of a rift that separated the continent from Eurasia. The rift later breached and flooded with seawater, forming the infant North Atlantic Ocean. The Indian Ocean formed when a rift separated the Indian subcontinent from Gondwana. By the end of the Triassic, India drifted free and began its trek toward southern Asia. Meanwhile, Gondwana drifted northward, leaving Australia still attached to Antarctica, in the southern polar region.

Huge lava flows and granitic intrusions occurred in Siberia, and extensive lava flows covered South America, Africa, and Antarctica as well. Southern Brazil was paved with three-quarters of a million square miles of basalt, constituting the largest lava field in the world. Great floods of basalt, upwards of 2,000 feet or more thick, covered large parts of Brazil and Argentina when the South American plate overrode the Pacific plate, and the subduction fed magma chambers underlying active volcanoes. Basalt flows also blanketed a region from Alaska to California.

Near the end of the Triassic, North and South America began to move away from each other; India, nestled between Africa and Antarctica, began to separate from Gondwana; the Indochina block collided with China; and a great rift began to divide North America from Eurasia. The rifting of continents radically altered the climate and set the stage for the extraordinary warm periods that followed.

 

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Chapter Eleven:
Jurassic Birds:

The Jurassic period, from 210 to 135 million years ago, was names for the limestone and chalks of the Jura Mountains in northwest Switzerland. Early in the period, Pangaea began rifting apart into the beginnings of the present continents, forming the Atlantic, Indian, and Arctic oceans. Mountains formed by upheavals during previous periods were leveled by erosion, and inland seas invaded the continents, providing additional offshore habitats for a bewildering assortment of marine species. By now, terrestrial faunas had attained the basic composition they would keep until the dinosaurs became extinct.

The dinosaurs were highly diversified by this time and reached their maximum size, becoming the largest terrestrial animals to ever live. There was a warm, moist climate, as suggested by widespread plant growth and coal formation. The beneficial climate and magnificent growing conditions contributed to the giant size of many dinosaur species, many of which became extinct at the end of the period. Reptiles were extremely successful and occupied land, sea, and air. Mammals were small, rodentlike creatures, sparsely populated and scarcely noticed. The first flight-worthy birds appeared and shared the skies with flying reptiles called pterosaurs.


The Early Birds:

Birds first appeared in the Jurassic about 150 million years ago, although some accounts push their origin as far back as the late Triassic, 225 million years ago. They descended from the thecodonts, the same ancestors of dinosaurs and crocodilians, and consequently birds are often referred to as "glorified reptiles." They were warm-blooded to obtain the maximum metabolic efficiency needed for sustained flight but retained the reptilian mode of reproduction by laying eggs. This ability to maintain a warm body temperature has led to speculation that some dinosaur species with similar skeletons were warm-blooded as well.

Archaeopteryx, from the Greek word for "ancient wing," was the earliest known fossil bird. It was about the size of a modern pigeon and appeared to be a transitional species between reptiles and true birds. Archaeopteryx was first thought to be a small dinosaur until fossils showing impressions of feathers were found in a unique limestone formation in Bavaria, Germany in 1863. The discovery sparked a long-standing controversy. Prominent 19th-century geologists claimed Archaeopteryx was a hoax, and the feather impressions were simply etched into the rock containing the fossil. However, an Archaeopteryx fossil discovered in 1950 from the same Bavarian formation produced a well-preserved specimen that clearly showed feather impressions.

Although Archaeopteryx had many of the accouterments necessary for flight, it likely was a poor flyer and might have flown only short distances. It probably achieved flight by running along the ground with its wings outstretched and then glided for a brief moment or leapt from the ground while flapping its wings to catch an insect flying by. Archaeopteryx had teeth, claws, a long bony tail, and many of the skeletal features of a small dinosaur but lacked hollow bones for light weight. Its feathers were out-growths of scales and probably originally functioned as insulation. Many bird species retained their teeth until the end of the Cretaceous.

Upon mastering the skill of flight, birds quickly radiated into all environments, and their superior adaptability enabled them to compete successfully with the pterosaurs, possibly leading to that reptile's extinction. Giant flightless land birds appeared early in the avian fossil record. Their wide distribution is further evidence for the existence of Pangaea, since these birds would then have had to walk from one corner of the world to another.

After being driven into the air by carnivorous dinosaurs and kept there by hunting mammals, birds found life a lot easier on the ground once this threat was eliminated because they had to expend much energy to remain airborne. Some birds also successfully adapted to a life in the sea. Certain diving ducks are specially equipped for "flying" underwater to catch fish. Penguins, for example, are flightless birds that have taken to life in the water and are well adapted to survive in the Antarctic.

 

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The Pterosaurs:

Pterosaurs, including the ferocious-looking pterodactyl, were flying reptiles with wingspans up to 40 feet and more. They originated in the early Jurassic and appear to have been the largest animals that ever flew, dominating the skies for more than 120 million years. Their wings were constructed by elongating the fourth finger of each forelimb, which supported the front edge of a membrane that stretched from the flank of the body to the fingertip, leaving the other fingers free for such purposes as climbing trees.

By comparison, a bat's wing is constructed by lengthening and splaying all fingers and covering them with membrane. The wing membranes might have originally served as a cooling mechanism used to regulate body temperature by fanning the forelimbs. Why pterosaurs took to the air is still a mystery. Their ancestors might have grown skin flaps for jumping out of trees like flying squirrels.

The pterosaurs resembled both birds and bats in their overall structure and proportions, with the smallest species roughly the size of a sparrow. Like birds, they had hollow bones to conserve weight for flight. The larger pterosaurs were proportioned similar to a modern hang-glider and weighed about as much as the human pilot. Many pterosaurs had tall crests on their skulls, which possibly functioned as a forward rudder to steer them in flight.

Pterosaurs might have achieved flight by jumping off cliffs and riding the updrafts, by climbing trees and diving into the wind, or by gliding across the tops of wave crests like modern albatrosses. They could have trotted along the ground flapping their wings and taking off gooney bird-fashion or simply stood on their hind legs, caught a strong breeze, and with a single flap of their huge wings and a kick of their powerful legs became airborne. The pterosaur probably spent most of its time aloft riding air currents like present-day condors.

When landing, it simply stalled near the ground, gently touching down on its hind legs like a hang-glider does. While on the ground, pterosaurs might have been ungainly walkers, sprawling about on all fours like a bat. However, fossil pterosaur pelvises seem to indicate that the hind legs extended straight down from the body, enabling the reptiles to walk upright on two feet. They could then trot along for short bursts to gather speed for takeoff. That the animals could fly is not doubted, and they went on to become the greatest animals aviators the world has ever known.

 

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The Giant Dinosaurs:

The oldest dinosaurs originated on the southern continent Gondwana when the last glaciers from the great Permian ice age were departing. They ventured to all major continents, and their distribution throughout the world is strong evidence for the theory of continental drift. At the time the dinosaurs came into existence all continents were assembled into Pangaea. Early in the Jurassic, it began to rift apart, and the continents drifted toward their present locations. Except for a few temporary land bridges, the oceans that filled the rifts between the newly formed continents provided a barrier to further dinosaur migration. At this time, almost identical species lived in North America, Europe, and Africa.

The success of the dinosaurs is exemplified by their extensive range, wherein they occupied a wide variety of habitats and dominated all other forms of land-dwelling animals. Indeed, if the dinosaurs had not become extinct, mammals would never have achieved dominance over the Earth and humans would not have come into existence. The dinosaurs would have continued to suppress further advancement of the mammals, which would have remained small, nocturnal creatures, keeping out from underfoot of the dinosaurs.

Over 500 dinosaur genera have been discovered over the last 175 years. Among the largest dinosaur species were the brachiosaurs and the apathosaurs, formerly called brontosaurs. The were sauropods with long, slender tails and necks, and the front legs were longer than the hind legs. Their fossils are found in Colorado and Utah, southwestern Europe, and East Africa, some species probably having traveled to Africa by way of Europe.

The Jurassic Morrison Formation, a famous bed of sediments in the Colorado Plateau region, has yielded some of the largest dinosaur fossils. Many of the best specimens are displayed at Dinosaur National Monument near Vernal, Utah. Perhaps the tallest and heaviest dinosaur ever discovered is the 80-ton ultrasaurus, which could look down onto the roof of a five-story building. Seismosaurus, meaning "Earth-shaker," was the longest known dinosaur, possibly reaching a length of more than 140 feet from its head, which was supported by a long, slender neck, to the tip of its even longer, whiplike tail.

Dinosaurs attained their largest sizes and longest life spans during the Jurassic. Large reptiles possess the power of almost unlimited growth. Adults never cease growing entirely but continue to increase in size until disease or accident take their lives. The giant Komodo dragon lizards of southeast Asia, for example, grow to over 300 pounds and prey on monkeys, pigs, and deer. Reptiles with their continuous growth achieved a measure of eternal youth, whereas mammals grow rapidly to adulthood and then slowly degenerate and die.

A large body allows a cold-blooded reptile to retain its body temperature for long periods. A large body retards heat loss better than a small one because it has a better surface-area-to-volume ratio. Thus, the animals is less susceptible to short-term temperature variations such as cool nights or cloudy days. Conversely, a large reptile takes much longer to warm up from an extended cold period than a small one. Muscles also generate body heat, although for reptiles it is only about a quarter of that produced by mammals during exertion. A high steady body temperature maintains an efficient metabolism, and higher temperatures enhance the output of muscles. Therefore, the performance of some large dinosaurs probably could match that of large mammals.

The generally warm climate of the Mesozoic produced excellent growing conditions for lush vegetation, including ferns and cycads, to satisfy the diets of the plant-eating dinosaurs. The herbivorous dinosaurs developed a large stomach to digest the tough, fibrous fronds, requiring an enormous body to carry it around. The dinosaurs grew to such giants probably for the same reasons that large ungulates like the rhinoceros and elephant are so big. Most large dinosaurs were herbivores that consumed huge quantities of coarse cellulose that required much time to digest. This required a large stomach and therefor a large body to carry it about.

Some dinosaur species swallowed cobbles called gizzard stones, similar to the grit used by many modern birds, to pulp the coarse vegetation in their stomachs. The digestive juices further broke down the rough material, and this long fermentation process required a large storage capacity. The rounded polished stones were often left in a heap where the dinosaur died. Sometimes deposits of these stones lie atop exposed Mesozoic sediments.

The large size of the herbivores spurred the evolution of giant carnivorous dinosaurs to prey on them, such as tyrannosaurus rex, perhaps the fiercest carnivore of them all. The giant dinosaurs were prevented from growing any larger due to the force of gravity. When an animal doubles its size, the weight on its bones is four times greater. The only exceptions were dinosaurs living permanently in the sea. As with modern whales, some of which are even larger than the biggest dinosaurs, the buoyancy of seawater kept the weight off their bones. If an animal accidentally beached itself, as whales sometimes do, it suffocated because its bones were unable to support the weight of its body, crushing the lungs.

Many families of large dinosaurs, including apathosaurs, stegosaurs, and allosaurs, became extinct at the end of the Jurassic. Following the extinction, the population of small animals exploded, as species occupied niches vacated by the large dinosaurs. Most of the surviving species were aquatic, confined to freshwater lakes and marshes, and small land-dwelling animals. Many of the small non-dinosaur species were the same types that survived the dinosaurs extinction at the end of the Cretaceous, probably due to their large populations and ability to find places to hide.

 

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The Breakup of Pangaea:

Throughout the Earth's history, continents appear to have undergone cycles of collision and rifting. Smaller continental blocks collided and merged into larger continents. Millions of years later, the continents rifted apart, and the chasm filled with seawater to form new oceans. The regions presently bordering the Pacific Basin apparently have never collided with each other. The Pacific Ocean is a remnant of an ancient sea called the Panthalassa, which narrowed and widened in response to continental breakup, dispersal, and reconvergence in the area occupied by the present Atlantic Ocean.

Several oceans have repeatedly opened and closed in the vicinity of the Atlantic Basin, while a single ocean has existed continuously in the area of the Pacific Basin. The Pacific plate was hardly larger than the United States after the breakup of Pangaea in the early Jurassic about 180 million years ago. The rest of the ocean floor consisted of other unknown plates that disappeared as the Pacific plate grew, and consequently no oceanic crust is older than Jurassic in age.

Early in the Jurassic, North America separated from South America, and a rift divided the North American and Eurasian continents. India, nestled between Africa and Antarctica, drifted away from Gondwana, and Antarctica--still attached to Australia--swung away from Africa toward the southeast, forming the proto-Indian Ocean. The rift separating the continents breached and flooded with seawater, forming the infant North Atlantic Ocean. Many ridges of the Atlantic's spreading seafloor remained above sea level, creating a series of stepping-stones for the migration of animals between the Old and New Worlds.

When Pangaea began to separate into today's continents, a great rift developed in the present Caribbean. It sliced northward through the continental crust connecting North America, northwest Africa, and Eurasia and began to open the Atlantic Ocean. The process took several million years along a zone hundreds of miles wide. The breakup of North America and Eurasia might have resulted from upwelling basaltic magma that weakened the continental crust. Many flood basalts exist near continental margins, evidence of where rifts separated the present continents. The episodes of flood basalt volcanism were short-lived events, with major phases generally lasting less than 3 million years.

About 125 million years ago, the infant North Atlantic obtained a depth of about 2.5 miles and was bisected by an active midocean ridge system producing new ocean crust. At about the same time, the South Atlantic began to form, opening up like a zipper from south to north. The rift propagated northward several inches per year, comparable to the plate separation rate. The entire process of opening the South Atlantic was completed in only about 5 million years. By 80 million years ago, the North Atlantic had become a fully developed ocean. Some 20 million years later, the Mid-Atlantic rift progressed into the Arctic Basin, detaching Greenland from Europe.

After breakup, the continents traveled in spurts rather than drifting apart at a constant speed. The rate of seafloor spreading in the Atlantic was matched by plate subduction in the Pacific, where one plate drives under another, forming a deep trench. This is why the oceanic crust of the Pacific Basin dates back no further than the early Jurassic. A high degree of geologic activity around the Pacific rim produced practically all the mountain ranges facing the Pacific and the island arcs along its perimeter.

Much of western North America was assembled from island arcs and other crustal debris skimmed off the Pacific plate as the North American plate continued heading westward. Northern California is a jumble of crustal fragments assembled within about 200 million years ago. A nearly complete slice of ocean crust, the type that is shoved up on the continents by drifting plates, sits in the middle of Wyoming. The Nevadan orogeny produced the Sierra Nevada Range in California from the middle to late Jurassic.

The breakup of Pangaea compressed the ocean basins, causing a rise in sea level and a transgression of the seas onto the land. In addition, an increase in volcanism flooded the continental crust with vast amounts basalt. The rise in volcanic activity also increased the carbon dioxide content of the atmosphere, resulting in a strong greenhouse effect that led to the warm Mesozoic climate.

Continental breakup and dispersal might also have contributed to the extinction of many dinosaur species. The shifting continents changed global climate patterns and brought unstable weather conditions to many parts of the world. Massive lave flows from perhaps the most volcanically active period since the early days of the Earth might have dealt a major blow to the climatic and ecological stability of the planet.

 

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Marine Transgression:

Throughout most of the Earth's history, several crustal plates constantly in motion reshaped and rearranged continents and ocean basins. When continents broke up, they overrode ocean basins, which compressed the seas, thereby raising global sea levels several hundred feet. The rising seas inundated low-lying areas inland of the continents, dramatically increasing the shoreline and shallow-water marine habitat area, which in turn supported many more species.

The vast majority of marine species live on continental shelves, shallow-water portions of islands, and subsurface rises generally less than 600 feet deep. The richest shallow-water faunas live in the tropics, which contain large numbers of highly specialized organisms. Species diversity also depends on the shapes of the continents, the width of shallow continental margins, the extent of inland seas, and the presence of coastal mountains, all of which are effected by continental motions.

Extensive mountain building is also associated with the movements of crustal plates. The upward thrust of continental rocks alter patterns of river drainage and climate, which in turn affects terrestrial habitats. The raising of land to higher elevations, where the air is thin and cold, spurs the growth of glacial ice, especially in the higher latitudes. Furthermore, continents scattered in all parts of the world interfere with ocean currents, which distribute heat over the globe.

During the Jurassic and continuing into the Cretaceous, an interior sea flowed into the west-central portions of North America. Massive accumulations of marine sediments eroded from the Cordilleran highlands to the west and were deposited on the terrestrial redbeds of the Colorado Plateau, forming the Jurassic Morrison Formation, well-known for fossil bones of large dinosaurs. Eastern Mexico, southern Texas, and Louisiana were also flooded. Seas invaded South America, Africa, and Australia as well.

The continents were flatter, mountain ranges were lower, and sea levels were higher. Thick deposits of sediment that filled the seas flooding North America were uplifted and eroded, giving the western United States its impressive scenery. Reef building was intense in the Tethys Sea, and thick deposits of limestone and dolomite were laid down in the interior seas of Europe and Asia, later to be uplifted during one of geologic history's greatest mountain building episodes.

 

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Chapter Twelve:
Cretaceous Corals:

The Cretaceous, which ran from 135 to 65 million years ago, was named from the Latin word creta, meaning chalk, due to the vast deposits of carbonate rock laid down worldwide at this time. It was the warmest period of the Phanerozoic as evidenced by extensive coral reefs, which built massive limestone deposits. Coral reefs and other tropical biota, for which bright sunlight and warm seas are essential, ranged far into higher latitudes. They fringed the continents and covered the tops of extinct marine volcanoes.

The warm climate was particularly advantageous to the ammonites (coil-shaped cephalopods), which grew to tremendous size, becoming the predominant creatures of the Cretaceous seas. The dinosaurs did exceptionally well during the Cretaceous, but along with the ammonites and many other species, they mysteriously vanished at the end of the period. The extinction was apparently caused by a cataclysm that created intolerable living conditions for most species on Earth.


The Age of Ammonites:

Coral reefs were the most widespread during the Cretaceous, ranging a thousand miles farther away from the equator, whereas today they are restricted to the tropics. The corals began constructing reefs in the early Paleozoic. The hexacorals from the Triassic to the present were the major reef builders of the Mesozoic and Cenozoic seas. The corals constructed barrier reefs and atolls, which were massive structures composed of calcium carbonate lithified into limestone. The Great Barrier Reef, stretching more than 1,200 miles along the northeast coast of Australia, is the largest feature built by living organisms.

The cephalopods were the most spectacular, diversified, and successful marine invertebrates of the Mesozoic seas. The nautiloids grew to lengths of 30 feet or more, and with straight, streamlined shells they were among the swiftest creatures of the deep. The ammonites, the most significant cephalopods, had a variety of coiled-shell forms identified by their complex suture patterns, making them the most important guide fossils for dating Mesozoic rocks.

Unfortunately, after surviving the critical transition from the Permian to the Triassic and recovering from serious setbacks during the Mesozoic, the ammonites suffered final extinction at the end of the Cretaceous, when the recession of the seas reduced their shallow-water habitats worldwide. The ammonites declined over a period of about 2 million years, possibly becoming extinct 100,000 years prior to the end of the Cretaceous.

A fast-swimming, shell-crushing marine predator called ichthyosaur, Greek for "fish lizard," apparently preyed on ammonites by first puncturing the shell from the ammonite's blind side, causing it to fill with water and sink to the bottom, where the attack could then be made head on. These highly aggressive predators might have caused the extinction of most ammonite species before the Cretaceous was out.

All shelled cephalopods were absent in the Cenozoic seas except the nautilus, found exclusively in the deep waters of the Indian Ocean and the ammonite's only living relative, along with shell-less species, including cuttlefish, octopus, and squids. The squids competed directly with fish, which were little affected by the extinction. Other major marine groups that disappeared at the end of the Cretaceous include the rudists, which were huge coral-shaped clams, and other types of clams and oysters. The gastropods, including snails and slugs, increased in number and variety throughout the Cenozoic and presently are second only to insects in diversity.

 

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The Angiosperms:

The Mesozoic was a time of transition, especially for plants, which showed little resemblance at the beginning of the era to those at the end, when they more closely resembled present-day vegetation. The gymnosperms originated in the Permian and bore seeds without fruit coverings, including conifers, ginkgoes, and palmlike cycads. The true ferns prospered in the higher latitudes, whereas today they live only in the warm tropics.

The cycads, which resembled palm trees, were also highly successful and ranged across all major continents, possibly contributing to the diets of the plant-eating dinosaurs. The ginkgo, of which the maidenhair tree in eastern China is the only living relative, might have been the oldest genus of the seed plants. Also dominating the landscape were conifers up to 5 feet across and 100 feet long. Their petrified trunks are especially plentiful at Yellowstone National Park.

About 110 million years ago, vegetation in the early Cretaceous underwent a radical change with the introduction of the angiosperms, flowering plants that evolved alongside pollinating insects. They might have originally exploited the weedy rift valleys that formed as Pangaea split apart. The earliest angiosperms appear to have been large plants, growing as tall as magnolia trees. However, fossils discovered in Australia suggests that the first angiosperms there and perhaps elsewhere were small herb-like plants. Within a few million years after their introduction, the efficient flowering plants crowded out the once abundant ferns and gymnosperms.

The angiosperms were distributed worldwide by the end of the Cretaceous, and today they include about a quarter-million species of trees, shrubs, grasses, and herbs. The plants offered pollinators, such as honey bees and birds, brightly colored and scented flowers, and sweet nectar. The unwary intruder was dusted with pollen, which it transported to the next flower it visited for pollination. Many angiosperms also depended on animals to spread their seeds, which were encased in tasty fruit that passed through the body and dropped some distance away.

Near the end of the Cretaceous, forests extended into the polar regions far beyond the present tree line. The most remarkable example is a well preserved fossil forest on Alexander Island, Antarctica. To survive the harsh arctic conditions, trees had to develop a means of protection against the cold, since plants are more sensitive to the lack of heat than the absence of sunlight. They probably adapted mechanisms for intercepting the maximum amount of sunlight during a period when global temperatures were considerably warmer than today.

The cone-bearing plants prominent during the entire Mesozoic occupied only a secondary role during the Cenozoic. Tropical vegetation that was widespread during the Mesozoic withdrew to narrow regions around the equator in response to a colder, drier climate, a result of a general uplift of the continents and the draining of the interior seas. Forests of giant hardwood trees that grew as far north as Montana were replaced by scraggly conifers, a further indication of a cooler climate.

The rise of angiosperms near the end of the Cretaceous might even have contributed to the death of the dinosaurs and certain marine species. By absorbing large quantities of carbon dioxide from the atmosphere, they caused a drop in global temperatures. Also contributing to the dinosaur's downfall, forests of broadleaf trees and shrubs that were a favorite food of the dinosaurs apparently disappeared just prior to the ending of the Cretaceous.

 

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The Laramide Orogeny:

Beginning about 80 million years ago, a large part of western North America uplifted, and the entire Rocky Mountain region from northern Mexico into Canada rose nearly a mile above sea level. This mountain-building episode, called the Laramide Orogeny, resulted from the subduction of oceanic crust beneath the West Coast of North America, causing an increase in crustal buoyancy. The Canadian Rockies consist of slices of sedimentary rock that were successively detached from the underlying basement rock and thrust eastward on top of each other. A region between the Sierra Nevada and the southern Rockies experienced a spurt of uplift over the past 20 million years, raising the area over 3,000 feet.

During the late Cambrian, the future Rocky Mountain region was near sea level. Farther west, within about 400 miles of the coast, a mountain belt comparable to the present Andes formed above a subduction zone during the 80 million years prior to the Laramide. It might have been responsible for the Cretaceous Sevier orogeny that created the Overthrust Belt in Utah and Nevada. A region from eastern Utah to the Texas panhandle that deformed during the late Paleozoic, Ancestral Rockies orogeny was completely eroded by the time of the Laramide. The Rocky Mountain foreland region subsided as much as 2 miles between 85 million and 65 million years ago and then rose well above sea level, reaching its present elevation around 30 million years ago.

To the west of the Rockies, numerous parallel faults sliced through the Basin and Range Province between the Sierra Nevada of California and the Wasatch Mountains of Utah, resulting in a series of about 20 north-south-trending fault-block mountain ranges. The Basin and Range covers southern Oregon, Nevada, western Utah, southeastern California, and southern Arizona and New Mexico. The crust bounded by the faults is literally broken into hundreds of steeply tilted blocks and raised nearly a mile above the basin, forming nearly parallel mountain ranges up to 50 miles long.

Death Valley, which is presently 280 feet below sea level, the lowest place on the North American continent, sat several thousand feet higher during the Cretaceous. The region collapsed when the continental crust thinned from extensive block faulting, with one block of crust lying below another. The Great Basin area is a remnant of a broad belt of mountains and high plateaus that subsequently collapsed after the crust was pulled apart following the Laramide.

The rising Wasatch Range of north-central Utah and south Idaho is an excellent example of a north-trending series of faults, one below the other. The fault blocks extend for 80 miles, with a probable net slip along the west side of 18,000 feet. The Tetons of western Wyoming were upfaulted along the eastern flank and downfaulted to the west. The rest of the Rocky Mountains evolved by a process of upthrusting similar to the plate collision and subduction that raised the Andes Mountains of Central and South America. The Andes continue to rise due to an increase in crustal buoyancy caused by the subduction of the Nazca plate to the west beneath the South American plate.

 

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Cretaceous Warming:

During the Cretaceous, plants and animals were especially prolific and ranged practically from pole to pole. The deep ocean waters, which are now near freezing, were about 15 degrees Celsius (60 degrees Fahrenheit) during the Cretaceous. The average global surface temperature was 10 to 15 degrees warmer than at present. Temperatures were also much warmer in the polar regions, with a temperature difference between the poles and the equator of only 20 degrees Celsius, or about half that of today.

The drifting of continents into warmer equatorial waters might have accounted for much of the mild climate during the Cretaceous. By the time of the initial breakup of the continents about 180 million years ago, the climate had begun to warm dramatically. The continents were flatter, with lower mountains and higher sea levels. Although the geography during this time was important, it did not account for all the warming.

The movement of the continents was more rapid than today, with perhaps the most vigorous plate tectonics the world has ever known. About 120 million years ago, an extraordinary burst of submarine volcanism struck the Pacific Basin, releasing vast amounts of gas-laden lava onto the ocean floor. These volcanic spasms are evidenced by a collection of massive undersea lava plateaus that formed almost simultaneously, the largest of which, the Ontong Java, is about two-thirds the size of Australia. It contains at least 9 million cubic miles of basalt, enough to bury the United States under 3 miles of lava.

The surge of volcanism increased the production of oceanic crust as much as 50 percent. This rise in volcanic activity provided perhaps the greatest contribution to the warming of the Earth, producing 4 to 8 times the present amount of atmospheric carbon dioxide, and worldwide temperatures averaged 7.5 to 12.5 degrees Celsius higher than today.

For the next 40 million years, the Earth's geomagnetic field, which normally reverses polarity quite often on a geologic time scale, stabilized and assumed a constant orientation due to several mantle plumes that produced tremendous basaltic eruptions. This greater volcanic activity increased the carbon dioxide content of the atmosphere, producing the warmest global climate in 500 million years. Carbon dioxide also provided an abundant source of carbon for green vegetation and contributed to its prodigious growth, supplying substantial diet for herbivorous dinosaurs.

Polar forests extended into latitudes 85 degrees north and south of the equator, as indicated by fossilized remains of an ancient forest that thrived on the now frozen continent of Antarctica. Evidence of a warm climate that supported lush vegetation is provided by coal seams that run through the Transantarctic Mountains that are among the most extensive coal beds in the world. Alligators and crocodiles lived in the high latitudes as far north as Labrador, whereas today they are confined to warm tropical areas. The duck-billed hadrasaurs also lived in the Arctic regions of the Northern and Southern Hemispheres.

The positions of the continents might have contributed to the warming of the climate during most of the Mesozoic. Continents bunched together near the equator during the Cretaceous allowed warm ocean currents to carry heat poleward. High-latitude oceans are less reflective than land and absorb more heat, further moderating the climate.

 

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The Inland Seas:

In the late Cretaceous and early Tertiary, land areas were inundated by high seas levels that flooded continental margins and formed great inland seas, some of which split continents in two. Seas divided North America in the Rocky Mountains and high plains regions, South America was cut in two in the region that later became the Amazon basin, and Eurasia was split by the joining of the Tethys Sea and the newly formed Arctic Ocean.

The oceans of the Cretaceous were also interconnected in the equatorial regions by the Tethys and Central American seaways, providing a unique circumglobal oceanic current system that made the climate equable. Mountains were lower and sea levels higher, and the total land surface declined to perhaps half its present size. The Appalachians, which were an imposing mountain range at the beginning of the Triassic, were eroded down to stumps in the Cretaceous. Erosion toppled the once towering mountains ranges of Eurasia as well.

Great deposits of limestone and chalk were laid down in Europe and Asia, which is how the Cretaceous received its name. Seas invaded Asia, Africa, Australia, South America, and the interior of North America. About 80 million years ago, the Western Interior Cretaceous Seaway was a shallow body of water that divided the North American continent into the western highlands, comprising the newly forming Rocky Mountains and isolated volcanoes, and the eastern uplands, consisting of the Appalachian Mountains.

Eastward of the rising Rocky Mountains was a broad coastal plain composed of thick layers of sediments eroded from the mountainous regions and extended to the western shore of the interior seaway. These sediment layers were later lithified and upraised and today are exposed as impressive cliffs in the western United States. Along the coast and extending some distance inland were extensive wetlands, where dense vegetation grew in the subtropical climate. Inhabiting these areas were fishes, amphibians, aquatic turtles, crocodiles, and primitive mammals. The dinosaurs included herbivorous hadrasaurs and triceratops, and the carnosaurs that preyed on them.

Toward the end of the Cretaceous, North America and Europe were no longer in contact, except for a land bridge that spanned Greenland to the north. The straight between Alaska and Asia narrowed, creating the practically landlocked Arctic Ocean. The South Atlantic continued to widen, with South America and Africa separated by over 1,500 miles of ocean. Africa moved northward, leaving behind Antarctica, which was still joined to Australia, and began to close the Tethys Sea.

Meanwhile, the northward-drifting subcontinent of India, traveling about 2 inches per year, narrowed the gap between itself and southern Asia. During this journey after breakup with Gondwana, no known mammals appear to have existed in India until after the collision with Eurasia. Apparently, during the middle Cretaceous, Australia -- still attached to Antarctica -- Wandered near the Antarctic Circle and acquired a thick mantle of ice. As Antarctica and Australia continued to move eastward, a rift developed that eventually separated them. Australia moved into the lower latitudes, while Antarctica drifted into the south polar region, accumulating a massive ice sheet.

When the Cretaceous ended, the seas regressed from the land because of lowered sea levels, and the climate grew colder. The last stage of the Cretaceous, called the Maestrichtian, was the coldest interval of the period. The decreasing global temperatures and increasing seasonal variation in the weather made the world stormier, with powerful gusty winds that wreaked havoc over the Earth.

There is no clear evidence of significant glaciation during this time. However, most warmth-loving species, especially those living in the Tethys Sea, disappeared when the Cretaceous came to an end. The extinctions appear to have been gradual, occurring over a period of 1 to 2 million years. Moreover, those species already in decline, including the dinosaurs and pterosaurs, might have been dealt a fatal blow from above.

 

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An Asteroid Impact:

One theory that attempts to explain the extinction of the dinosaurs and over 70 percent of other species at the end of the Cretaceous suggests that one or more large asteroids or comets struck the Earth with an equivalent explosive force of 100 trillion tons of dynamite or about a million eruptions of Mount St. Helens. The impact would have sent 500 billion tons of debris into the atmosphere and plunged the planet into environmental chaos. Generally, no animal heavier than 50 pounds survived the extinction, and a large body size appears to have been a severe disadvantage among terrestrial mammals.

Following the impact, glowing bits of debris flying through the atmosphere set ablaze globe-wide forest fires, burning perhaps a quarter of all vegetation on the continents and turning a large part of the Earth into a smoldering cinder. A heavy blanket of dust and soot encircled the entire globe and lingered for months, cooling the planet and halting photosynthesis.

A catastrophe on this scale would have destroyed most terrestrial habitats and caused extinctions of tragic proportions. Species living in the tropics that relied on steady warmth and sunshine, like the coral reef communities, were especially hard hit. For example, the rudists, which built reef-like structures, completely died out, along with half of all bivalve genera.

A massive bombardment of meteorites also might have stripped away the upper atmosphere ozone layer, bathing the Earth in the sun's deadly ultraviolet rays. The increased radiation would have killed land plants and animals as well as primary producers in the surface waters of the ocean. The mammals, which were no larger than rodents, coexisted with the dinosaurs for more than 100 million years. But because they were mostly nocturnal and remained in their underground burrows in the daylight hours, only coming out at night to feed, the mammals would have been spared the onslaught of ultraviolet radiation during the daytime.

In the aftermath of the bombardment, the Earth would have succumbed to a year of darkness, under a thick brown smog of nitrogen oxide. Surface waters, poisoned by trace metals leached from the soil and the rock, and global rains as corrosive as battery acid would have destroyed terrestrial life forms. Plants that survived as seeds and roots would have been relatively unscathed. The high acidity levels would have dissolved the calcium carbonate shells of marine organisms, while those with silica shells would have survived as they have done during other crises. Land animals living in burrows would have been well protected, and creatures living in lakes buffered against the acid would have survived the meteorite impact quite well.

The impacts also could have caused the widespread extinctions of microscopic marine plants called calcareous nannoplankton, which produce a sulfur compound that aids in cloud formation. With the death of these creatures, cloud cover would have decreased dramatically, triggering a global heat wave extreme enough to kill off the dinosaurs and most marine species. This contention is supported by the fossil record, which shows that ocean temperatures rose 5 to 10 degrees Celsius for tens of thousands of years beyond the end of the Cretaceous. During this time, over a period of almost a half a million years, more than 90 percent of the calcareous nannoplankton disappeared along with most marine life in the upper portions of the ocean.

Sixty-five-million-year-old sediments found at the boundary between the Cretaceous and Tertiary periods throughout the world contain shocked quartz grains with distinctive lamellae, common soot from global forest fires, rare amino acids known to exist only on meteorites, the mineral stishovite, a dense form of silica found nowhere except at known impact sites, and unique concentrations of iridium, a rare isotope of platinum relatively abundant on meteorites and comets but practically nonexistent in the Earth's crust.

The geologic record holds other iridium anomalies thought to be associated with other giant meteorite impacts that also coincide with extinction episodes. However, they are not nearly as intense as the iridium concentrations at the end of the Cretaceous, which are as high as a thousand times normal background levels, suggesting that the end of Cretaceous extinction might have been a unique event in the history of life on Earth.

 

[M82A1]

Chapter Thirteen:
Tertiary Mammals:

The Tertiary period, running from 65 to 3 million years ago, is known as the "age of mammals," and because of their great diversity, many more plant and animal species are alive today than at any other time in geologic history. The appearance of the grasses early in the period spawned the evolution of hoofed animals as well as voracious carnivores to prey on them. The prosimians (pre-apes) were also on the scene and gave rise to the anthropoids, ancestors of the apes and humans.

Extremes in climate and topography created a greater variety of living conditions than existed during any other equivalent span of geologic time. The rigorous environments presented many challenging opportunities for plants and animals, and the extent to which they invaded diverse habitats was truly remarkable. The Tertiary was a time of constant change, and all species had to adapt to a wide range of living conditions. The changing climate patterns resulted from the movement of continents toward their present positions and from the intense mountain building that raised most ranges of the world.


The Age of Mammals:

Mammals originated in the late Triassic at roughly the same time as the dinosaurs, and the two groups coexisted for about 150 million years thereafter. When the dinosaurs left the stage at the end of the Cretaceous, the mammals were waiting in the wings, poised to conquer the Earth. Because the dinosaurs represented the largest group of animals, their departure left the world wide open to invasion by the mammals.

About 10 million years after the extinction of the dinosaurs, mammals began to radiate into dazzling arrays of new species. The small, nocturnal mammals eventually evolved into larger animals, some of which were evolutionary dead ends. Of the 30 or so orders of mammals that existed during the early Cenozoic, only half had lived in the preceding Cretaceous while almost two-thirds are still living today.

The evolution of the mammals following the dinosaur extinction was not gradual but progressed in fits and starts. The early Tertiary was characterized by an evolutionary lag, as though the world had not yet awakened from the great extinction. By the end of the Paleocene epoch, about 54 million years ago, mammals began to diversify rapidly. About 37 million years ago, a sharp extinction event took out many of the archaic mammal species, most of which were large, peculiar looking animals. Afterward, the truly modern mammals began to evolve.

The extinction coincided with changes in the deep-ocean circulation and eliminated many species of marine life on the European continent, which was flooded with shallow seas. The separation of Greenland from Europe might have allowed frigid Arctic waters to drain into the North Atlantic, significantly lowering its temperature and causing most types of foraminifera (marine protozoans) to disappear. The climate grew much colder, and the seas withdrew from the land as the ocean dropped 1,000 feet to perhaps its lowest level of the last several hundred million years.

Much of the drop in sea level might have resulted from the accumulation of massive ice sheets atop Antarctica, which had drifted over the South Pole. A large fall in sea level due to a major expansion of the Antarctic ice sheet led to another extinction about 11 million years ago. These cooling events removed the most vulnerable of species, so that those living today are more robust, having withstood extreme environmental swings over the last 3 million years, when glaciers spanned much of the Northern Hemisphere.

Marine species that survived the great Cretaceous extinction were similar to those that lived in the Mesozoic. Although the extinction in the oceans was severe and many species died out, few radical species appeared because habitats left vacant were simply taken over by the next of kin. Species inhabiting unstable environments such as those in the higher latitudes were especially successful.

Some 70 species of marine mammals known as cetaceans were among the most adaptable animals and included dolphins, porpoises, and whales, which evolved during the middle Cenozoic. The ancestors of the whales walked on land and swam in rivers and lakes about 50 million years ago. Today, their closest relatives are the artiodactyls, of hoofed animals with an even number of toes, such as cows, pigs, deer, camels, and giraffes. Ancestors of the blue whale, the largest animal on Earth, evolved from ancient toothed whales about 40 million years ago.

The drifting continents isolated many groups of mammals, and these evolved along independent lines. For the last 40 million years or so, Australia has been an island continent, without a land link to the other continents. The large island is home to many strange egg-laying mammals called monotremes that include the spiny anteater and the platypus, which should rightfully be classified as surviving mammal-like reptiles. Marsupials, which are primitive mammals that suckle their tiny infants in belly pouches, originated in North America around 100 million years ago, migrated to South America, crossed over to Antarctica when the two continents were still in contact, and landed in Australia before it broke away from Antarctica. Today, 13 of the world's 16 marsupial families reside only in Australia.

The Australian marsupials consist of kangaroos, wombats, and bandicoots, with opossums and related animals occupying other parts of the world. The largest marsupial fossil found is that of the diprotodon, which was about the size of a rhinoceros. The giant kangaroos disappeared soon after early humans invaded the continent some 60,000 years ago. Madagascar, which broke away from Africa about 125 million years ago, has none of the large mammals on the African continent except the hippopotamus, which mysteriously landed on the island after it had drifted some distance from the African mainland.

Camels, which originated about 25 million years ago, migrated out of North America to other parts of the world by connecting land bridges. Horses originated in western North America during the Eocene when they were only about the size of small dogs. As they became progressively larger, their faces and teeth grew longer as the animals switched from browsing to grazing, and their toes fused into hoofs. The giraffes shifted from grazing on grass to browsing on leaves, and their necks lengthened to reach the tall branches. Many types of hoofed animals called ungulates evolved in response to increased grassland all over the world.

All major groups of modern plants were represented in the early Tertiary. The angiosperms dominated the plant world, and all modern families appear to have evolved by about 25 million years ago. Grasses were the most important angiosperms, providing food for ungulates throughout the Cenozoic. The grazing habits of many large mammals probably evolved in response to the widespread availability of grasslands.

The first primates lived some 60 million years ago and were about the size of a mouse. Afterward, the primate family tree split into two branches, with the monkeys on one limb and the great apes, including our human-like ancestors, the hominoids, on the other. Beginning about 37 million years ago, the New World monkeys unexplainably migrated to South America from Africa when those continents had already drifted far apart. About 30 million years ago, the precursors of apes lived in the dense tropical rain forests of Egypt, which is now mostly desert. These ape-like ancestors migrated from Africa into Europe and Asia between about 25 and 10 million years ago.

Between 12 and 9 million years ago, the forests of Europe were home for a tree-living, fruit-eating ape called Dryopithecus, which probably evolved into Ramapithecus, an early Asian hominid with more advanced characteristics than earlier species. Between 9 and 4 million years ago, the fossil record jumps from the hominid-like but mainly ape form of Ramapithecus to the true hominids and the human line of evolution. During this time, much of Africa entered a period of cooler, drier climates and retreating forests, offering many evolutionary challenges to the ancestors of humans.