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The Cenozoic Era of the Phanerozoic Eon:
the last 65.5 million years
| | |
With the extinction of the and the end of the , the swiftly inherit the Earth. Archaic mammals co-existed with birds and modern reptiles and invertebrates. The current continents emerged, and the initial tropical conditions were replaced by a colder drier climate, possibly caused by the . The appearance of meant the rise of grazing mammals, and the cooler drier world allowed modern mammalian groups to evolve, along with other lineages now extinct and a few archaic hold-overs. Among the newcomers were the that culminated in the of Africa. Decreasing temperatures and a polar landmass of resulted in a new . Most recently, in the blink of an eye geologically speaking, this era saw the rise of (, and ) and use of stone tools and fire, the extinction of much , and civilization and human activities that have transformed the globe, but at a cost of great environmental destruction.
During the 65 million years of the Cenozoic Era (also spelled "Cainozoic"), or Age of Mammals, the world took on its modern form. Invertebrates [], fishes [], reptiles [] etc were essentially of modern types, but mammals [], birds [], [] and flowering plants still evolved and developed during this period.
Traditionally, the Cenozoic Era was divided into two very unequal periods, the (65 to 1.8 million years ago), and the (1.8 million years ago to last Thursday). A later, alternative, scheme divided it rather less unequally into the (65 to 23 million years ago) and (23 million years ago to last Thursday). Sometimes the Paleogene and Neogene were considered divisions of the Tertiary, the Neogene thus excluding the last couple million years.
In 2009, the , in its ongoing quest for standardization of stratigraphic nomenclature, decided on a scheme with the Paleogene, a truncated Neogene (23 to 2.6 million years ago) and an expanded Quaternary (2.6 million years ago to last Thursday) as the three of the Cenozoic. The Tertiary has no official existence anymore, but will doubtless see continued informal use.
This is all less interesting than it could be, as in practice the periods of the Cenozoic are less often refered to than their constituent - the , , and ( = Paleogene); the and ( = Neogene); and the and ( = Quaternary).
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During the Cenozoic, the fragmentation of continental landmasses continued as the Earth's surface took on its present form. The major geologic events of the Cenozoic can be thought of as two basic processes. First, four different large fragments of the [] supercontinent moved north and became, to varying degrees, attached to the [] landmass. This resulted in a number of spectacular mountain-building events which climaxed about the Early [] Second, the north-south Atlantic spreading zone continued to widen the Atlantic, contributing to geologic strains in East Africa and the western parts of the Americas, as these continents were pushed into contiguous plates by the growing Atlantic Ocean.
The defecting Gondwanan fragments were , , , and . South America has not pushed far enough north to cause a the geological equivalent of a high speed collision with North America. Instead, the impact was cushioned by a sort of air bag of small plates in what is now the Caribbean Sea. In particular, the approach of the American continents pinched off part of the Pacific crust, a region containing the sea bottom south and west of Cuba. The cushioning effect of these intervening plates delayed the formation of a land bridge between the Americas until the Middle Pliocene ( []), and has confined the effects of continental collision to relatively mild and sporadic vulcanism around the Caribbean and its southern and western margins. Various other representations can be found at on the Caribbean plate.
A similar, but less pneumatic, effect has softened the impact of Africa on Europe. The numerous microplates of the Mediterranean have been repeatedly rearranged and compressed as Africa approached from the south. Nevertheless, Africa's attempt to subduct under the European Plate has been a little like a hippopotamus trying to hide under a bed sheet - there have been some inevitable little lumps and wrinkles. Some of these, like today's Alps, are difficult to overlook. Other, older ranges which run generally east to west across Europe are also products of this process. In addition, the approach of Africa squeezed shut the old Tethys Seaway, which played such a large part in Early history, leaving only a few puddles, such as the Mediterranean Sea and the Black Sea.
The impact of India doesn't seem to have been mitigated at all. A land bridge between India and the Asian mainland was not established until the . However, the continental shelves of Asia and India had been in contact for some time before this, and elevation of the Himalayas has been ongoing throughout the Cenozoic. Initially, most of the impact was in the East, as India attempted to subduct under Asia to become the basement level of Tibet. In the , the force of the collision was distributed further west, forming the high plateaus of Afghanistan and Iran, with collateral consequences as far west as Eastern Europe. Perhaps the same fate awaits Australia, the last of the Gondwanan refugees. However, Australia has been dawdling along in the Pacific and has only recently begun to interact with the outlying portions of the Indonesian plates.
While the northern and southern continents have been getting progressively cozier, the Mid-Atlantic spreading ridge has been busy separating east from west. In the north, after splitting Greenland from North America, the rift abruptly changed course in the Paleogene and began to separate Northern Europe from Greenland. As a result, the last land bridge between North America and Europe was broken in the Eocene. The westward pressure on the Americas may well have been responsible for the Laramide Orogeny in the Western United States during the Paleogene, and the seamless merger of the subduction zones of North and South America later on. It is less clear that it has had any role in the more recent events which raised the current complex set of north-south mountain ranges in North America.
On the other side, in East Africa, the eastward pressure of the Mid-Atlantic ridge, combined with the opposite forces generated by the impact of India, created enormous stresses. As a result, the Arabian peninsula was rotated and torn off the East coast of Africa, and a series of deep faults have begun to fracture the African plate. Late in the Cenozoic, the main rift valley running through Ethiopia, Kenya, and points south, became the home of several species of large, noisy, and nearly hairless apes.
Image: The satellite image is from NASA. It shows the southern part of the rift system in East Africa with a few of the great lakes which have developed in the rift valleys. Part of Lake Tanganyika is in the upper left corner. The lake in the upper central portion is Lake Malawi.
ATW040924. All text public domain. No rights reserved.
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The following table gives the component periods and epochs that make up the Cenozoic:
| Period | Epoch | Age | Base (duration) | Geomagnetic Polarity Zone (base) | Approximate Central Paratethys Stage | European Neogene Mammal Zones (base) | South American Land Mammal Ages ("SALMA") | North American Land Mammal Ages ("NALMA") |
|---|---|---|---|---|---|---|---|---|
| Holocene | 0.0118 (0.0118) | |||||||
| 0.126 (0.1142) | ||||||||
| 0.126 (0.1142) | Lujanian (0.3) | |||||||
| 1.81 (1.029) | C1 (1.8) | Ensenadan (1.5) | Irvingtonian (1.8), Rancholabrean (1.02) | |||||
| 2.59 (0.78) | MN 17 (2.5), MmQ1 (2.0) | Uquian (2.5) | ||||||
| MN 16 (3.2) | Chapadmalalan (3.0) | |||||||
| 5.33 (1.73) | C2 (4.2) | Dacian | MN 15 (4.2), MN 14 (4.9) | Montehermosan (5.4) | Blancan (4.9) | |||
| 7.25 (1.92) | Pontian | MN 13 (7.1) | ||||||
| 11.6 (4.35) | C3 (7.4), C4 (9.7) | Pannonian | MN 12 (7.7), MN 11 (8.7), MN 10 (9.7), MN 9 (11.3) | Huayquerian (9.0), Chasicoan (10) | Hemphillian (10.3) | |||
| 13.7 (2.1) | Sarmatian, later Badenian | MN 7/8 (12.7), MN 6 (13.8) | Mayoian (12), Laventenian (13.8) | Clarendonian (13.6) | ||||
| 16.0 (2.3) | earlier Badenian | MN 5 (16.0) | Colloncurian (15.5), Friasian (16.3) | Barstovian (16.3) | ||||
| 20.4 (4.4) | C5 (19.1) | Karpatian, Ottnangian, Eggenburgian | MN 4 (16.8) | Santacrucian (17.5) | Hemingfordian (20.6) | |||
| 23.0 (2.6) | Egerian | Colhuehuapian (21) | ||||||
| 28.4 (5.4) | C9 (28.2), C8 (26.5), C7 (24.9), C6 (24.1) | MN 1 (23.9) | Harrisonian (24.8) | |||||
| 33.9 (5.5) | C11 (30.6), C10 (29.3) | Deseadan (29) | Geringean (30.8), Whitneyan (33.3), Orellan (33.9) | |||||
| 37.2 (3.3) | C17/16 (36.3), C12 (33.0) | Tinguirican (36) | ||||||
| 40.4 (3.2) | C19 (41.3), C18 (40.4), C17 (37.6) | Chadronian (38) | ||||||
| 48.6 (8.2) | C21 (48.6), C20 (45.1) | Divisaderan (42), Mustersan (48) | Duchesnean (42), Uintan (46.2) | |||||
| 55.8 (7.2) | C23 (52.6), C22 (50.6) | Casamayoran (54.0-51.0)* | Bridgerian (50.3), Wasatchian (55.4) | |||||
| 58.7 (2.9) | C24 (56.6) | Ríochican (57.0-55.5) | Clarkforkian (56.8) | |||||
| 61.7 (3.0) | C25 (58.4) | Itaboran (59.0-57.5) | Tiffanian (60.2) | |||||
| 65.5 (3.8) | C29 (65.5), C28 (64.6), C27 (63.4), C26 (61.7) | Peligrosan (62.5-61.0), Tiumpampan (64.5-63.0) | Torrejonian (63.3), Puercan (65), Lancian (69.7) |
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During the the climate worldwide was warm and tropical, much as it had been for most of the preceding . The saw a drastic cooling in the world's climate, possibly caused by the Himalayan uplift (Tibetan plateau) that was generated by the Indian subcontinent ramming into the rest of Asia (and is still going on now). During the , the continuing cooling climate resulted in an ice age, or rather a series of ice ages with interspersed warm periods.
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With the end Cretaceous extinction event and the extinction of the ammonites and most of the belemnites, teleost fishes dominated neritic (near shore) and pelagic faunas. Plankton recovered and basically belonged to modern groups. Coleoidea, Crustaceans, nudibranch mollusks and polychaete worms make up a large part of the larger zooplankton. The large marine reptiles of the Mesozoic were replaced by cetacean mammals (dolphins, whales and their kin) that first appeared during the Eocene. And while the protostegids (which included giants like Archelon) disappeared with the end-Cretaceous extinction, modern sea turtles survived quite happily.
| Diatryma |
The Paleogene saw the diversification of many mammalian and bird groups, flourishing in the tropical conditions. During the early Paleogene the continents were isolated by shallow seas, and different lineages of Mammals evolved on each one. Mammals included many giant yet small-brained rhinoceros-like types - the Asiamerican uintatheres, and brontotheres and the African arsinoitheres. There were huge flightless carnivorous birds - the Laurasian diatrymids (left) and the South American phorusrhacids - 2 meters tall with cruel curved beaks, that mimicked the great theropod dinosaurs of the Mesozoic. All these animals lived in tropical forests. The champsosaurs, crocodile-like "eosuchian" reptiles - living fossils of their time - survived the dinosaurs and the K-T extinction but died out later in the Paleogene. In the seas the first archaic toothed whales appeared. Giant marine protozoa, (foraminifers) the size of lentils evolved during the Eocene. Bivalve and Gastropod molluscs were basically the same type as today. The nautilids experienced their last mild evolutionary radiation. Transitional forms ancestral to modern coleoid cephalopods evolved. Echinoderms, corals, bryozoa and sponges were basically of modern type. On land insects were generally of modern type. Ants were even more numerous then they are today.
| Deinotherium |
During the Neogene modern mammals and flowering plants evolve, as well as many strange mammals that are no longer around. The most astonishing thing to happen during the early Neogene was the evolution of grass. This led to the evolution of long-legged running animals adapted to life on the savanna and prairie. The horse family - Equiidae - was an especial success story during the Neogene. Horses and other grazing mammals evolved high-crowned teeth to cope with a diet of abrasive grass. There were still many forest animals however. The Mastodons lived on every continent except Australia. Many strange mammals - litopterns, notoungulates, ground sloths, borhyeanas, etc - continued to evolve in isolation in South America before a land bridge formed and allowed a devastating invasion of forms from the north. Meanwhile during the late Neogene Hominids appeared in the Africa savannas, the Australopithicines. The oceans were inhabited by whales basically like modern forms, which had replaced the archaic toothed whales. They were the most intelligent animals of their time, but they never developed the use of tools or a memetic noosphere. In the north Pacific were the Desmostylids - a sort of cross between an elephant and a seal. Also in the seas were the largest carnivorous sharks ever to live - the Carcharodon megalodon, a predecessor of the modern White Pointer but much larger and heavier.
| Megaceros |
The Pleistocene period saw essentially modern flora and invertebrate species. However many mammalian types were of species and genera now extinct, and generally of large size - the various species of mammoth, the Irish "elk" (left), a large diversity of rhinos, the giant ground sloths, the diprotodonts of Australia, and many more. Man evolved as an ice-age mammal in Europe. A combination of human hunting ("stone age overkill") and climatic change served to kill off most worlds megafauna.
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- - a short introduction at the .
- -- Discovery Channel -- a non-technical look at the age of mammals, with photo-realistic computer reconstructions of a number of extinct species.
- - from .
- - a good summary.
- - annotated links page from Richard Cowen's History of Life site.
- - annotated links page from Richard Cowen's History of Life site.