Life of the Past

The Cenozoic Era

The Cenozoic Era includes the last 70 million years of earth history. It is far more familiar than previous eras, for although some animals and plants died out other kinds have survived without drastic change and are alive today. Slow but striking changes in climate took place during this time. Polar regions cooled and the general warm temperate climate gave way to a wider climatic range.

The continents were similar to those of today, although there was mountain building, continental warping and volcanic activity. Cenozoic strata in the Gulf Coast, California, the Middle East and the East Indies are now important petroleum producers. Cenozoic life (the Age of Mammals) is dominated by the mammals and flowering plants. Mammals replaced the ruling reptiles of the Mesozoic in every environment. The flowering plants became broadly similar to living forms. Amphibia and reptiles became relatively inconspicuous. Birds continued to expand in numbers and variety. The bony fish (teleosts) outnumbered all other fish by twenty to one.

Marine invertebrates took on a modern look. Gastropods and pelecypods became the most abundant, and cephalopods and brachiopods were greatly reduced. This then is the last great era in the long history of life—the prelude to the present.

THE LOWER TERTIARY PERIOD

The Lower Tertiary period (Paleocene, Eocene, and Oligocene) in North America is represented by great continental, badland deposits which contain many mammalian fossils. Thick marine deposits formed in south-eastern U.S. and along the Pacific Coast where volcanoes also erupted.

Lower Tertiary fossils are strikingly different from those of the Cretaceous. Mammals increased explosively, spreading into all environments and becoming adapted to many ways of life on land, water and in the air. Mammalian history varied. In isolated Tertiary South America a great diversity of marsupial mammals developed. Other marsupials still survive in the isolation of Australia.

Early Tertiary mammals included primitive rodent-sized forms, insectivores and marsupials, that had persisted from the Cretaceous. New species included two main groups, hoofed mammals and carnivores.

Creodonts, forerunners of the carnivores, and condylarths, forerunners of hoofed mammals, were somewhat similar, differing mainly in details of teeth and feet. Both were squat, heavy-limbed, about the size of a collie, with dog-like heads and five bluntly clawed toes on each limb. Their brains were small and primitive. Other new arrivals were ancestral rodents and primates, represented by lemur-like animals.

Amblypods were heavy, clumsy, hoofed mammals, with broad feet. The early ones were sheep-sized, but the later kinds (uintatheres) were seven feet high, as big as a large rhinoceros, with three pairs of blunt horns. The males had large down-curved tusks.

During Eocene times more advanced mammals displaced many older forms in North America and Europe. These new arrivals included true rodents, mainly squirrel- like species, and a variety of rhinoceroses (one Oligocene giant, Baluchitherium, measured 18 feet high at the shoulders). Ancestral tapirs, titanotheres and the first even- toed hoofed mammals also appeared. The early titanotheres were small, hornless browsers with clumsy bodies and tiny brains. Condylarths, creodonts and uintatheres persisted for a time, but lost out to later arrivals such as early horses (the three-toed, collie-sized Mesohippus), giant pigs, ancestral camels, oreodonts, mastodons and large saber-toothed cats.

Oreodonts were sheep-sized, long-tailed herbivores that survived until the Pliocene. Titanotheres were huge, grotesque horned beasts. The cat family first appeared in the form of the earliest saber-tooth, Hoplophoneus, about the size of a mountain lion.

Lower Tertiary birds were modern in appparance, but included numbers of large, ground-living genera, one of which, Dial ryma, was 7 feet high. Fish, too, were of modern aspect. The fine fresh-water sediments of the Green River beds of Wyoming have yielded thousands of beautiful, well-preserved fish fossils.

Marine invertebrates were very much like modern forms. Large foraminifera (“Nummuliles”) abounded in the shallow seas of the Mediterranean and Caribbean. Plants resembled living forms, but palms grew in Canada, and temperate oak and walnut forests in Alaska. There was mountain building and crustal disturbance in the Alps, Carpathians, Pyrenees, Apennines, and Himalayas. The Coast Range of western North America was the scene of mountain building too, with volcanic activity.

THE UPPER TERTIARY PERIOD

The Upper Tertiary Period (Miocene and Pliocene) lasted about 25 million years, ending about a million years ago. It is marked by the continued rise of modern mammals.

Changes involving brain, limbs and teeth and the accompanying expansion of mammals as a group were closely related to climatic changes. Over wide areas of North America continental uplift produced drier climates, and converted lush lowland forests into grassy prairies. The oldest common grasses come from the Miocene. Many mammalian changes were associated with the change from browsing to grazing habits.

Changes of this kind are particularly well illustrated in the horse family. Some of these changes were correlated with increase in overall size, but others were not. Thus horse teeth became larger and deeper, but they also became high-crowned, with a square, infolded, chewing surface. The limbs became longer and changed in relative proportions, but the number of ground-touching toes was reduced. This reflects a radical change from a flat-footed to a tip-toe, spring-hoofed posture. Feeding on tough prairie grasses demanded tougher teeth, and the advantage of speed on the hard, open prairies favored the new foot structure.

Evolution of the Horse


Widespread changes also took place in other groups of mammals. Ancestral elephants, camels, rhinoceroses, dogs and smaller carnivores abounded. Other forms have no living descendants. Moropv’s resembled a clumsily constructed horse with claws. One giant pig had a skull four feet long. There were giraffes, camels, and the pathetic antelope, Syndyoceras, with the strangest horns of any of its tribe. Saber-toothed cats continued, and apelike creatures (Dryopllhecus) spread across Europe and Africa. Many of the older type mammals became extinct towards the close of Pliocene times.

Most Upper Tertiary marine invertebrates and plants are barely distinguishable from modern species. Renewed earth movements in the Alps, Himalayas and along the Pacific Coast of North America complicated and extended existing mountain ranges.

Our hurried excursion through geologic time has taken us, with giant steps, over a period of more than half a billion years from the late Pre-Cambrian to the end of the Tertiary. The late Tertiary world is modern except for minor species development including the explosive dominance of man.

Though fossil clues are meager, the panorama of life is understandable because of the length of geologic time. The slow organic changes which led to extinction or survival are the building blocks of evolution and, for their operation, time is needed. Man’s development is a matter of only a few million years, but a billion years of preparation lie behind it.

THE QUATERNARY PERIOD

The Quaternary period includes the moment in which we now live, together with the Pleistocene Epoch, which began about one million years ago. Continental ice sheets, up to ten thousand feet thick, spread over much of the Northern Hemisphere in at least four glacial advances, the last of which retreated about 11,000 years ago in North America. Antarctica and the mountains of the Southern Hemisphere were also glaciated.

There is evidence of repeated floral and faunal migrations in response to climatic changes. During the colder episodes vast herds of wild pigs, camels, bison and elephants ranged across North America, Europe and Asia. There were four American species of elephants, including the Imperial Mammoth, 14 feet high at the shoulder, with curved tusks 13 feet long.

Majestic woolly mammoths, which roamed across the tundra of Europe, Asia and North America, are pictured in early cave paintings. Most of these large mammals became extinct near the end of the Pleistocene. Carnivores, including wolves, foxes, badgers, and the terrible saber-tooth Smilodon, are well known from the Pleistocene tar pools of California. Huge armadillo-like glyptodonts and giant (20 ft. high) ground sloths had evolved in South America and spread into North America when the land bridge between the two continents was re-established in late Pliocene times. In this arid and partly frozen world, man emerged.

THE EMERGENCE OF HUMANS

CRO-MAGNON

Cro-Magnon man was finely built, tall, muscular, with modern brain and facial features, replaced Neanderthal. He manufactured finely worked tools from stone, ivory and bone, practiced ceremonial burial and was probably advanced socially as well. Cro-Magnon cave paintings, drawings and sculpture are beautifully executed.

NEANDERTHAL

Neanderthal man lived over a wide area of Europe and North Africa during the last glacial advance. He was short, stocky, stooped, with heavy brows, retreating forehead and a prominent but chinless jaw. A cave dweller and skilled hunter, he practiced ceremonial burial. He became extinct after about 100,000 years. Not ancestral to modern man, but probably from same stock.

PITHECANTHROPUS

Pithecanthropus (sometimes referred to as the "ape man") is known from specimens about 500,000 years old from Java and china.

Typical height was about 5 ft., semi-erect posture, heavy brows, powerful jaws with manlike teeth and a brain capacity between that of the large apes and modern man, charred bones found with simple stone tools suggest the possibility for cannibalism.

AUSTRALOPITHECUS

Australopithecus of South Africa and his relative, Zinjanthropus, of Olduvai Gorge, E. Africa, are more than 1 million yrs. old.

A recent Olduvai discovery is an advanced fossil man, Homo habilis, a hunter who probably built simple shelters and made most of the tools found with the split bones of game he ate.

Other discoveries have pushed the age of man’s direct ancestors back to about 20 million years.



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