The Cenozoic

The Cenozoic Era of the Phanerozoic Eon: the last 65.5 million years

The Age of Mammals

During the 65 million years of the Cenozoic Era (also spelled "Cainozoic"), or Age of Mammals, the world took on its modern form. Invertebrates, fish, reptiles etc were essentially of modern types, but mammals, birds, protozoa and flowering plants still evolved and developed during this period.

Traditionally, the Cenozoic Era was divided into two very unequal periods, the Tertiary which made up the bulk of the Cenozoic), and the Quaternary, which is only the last one and a half million years or so. The Tertiary is in turn divided into Paleogene and Neogene. We do not adopt this use of the "Tertiary" as a formal stratigraphic division for the following reasons:

More than 95% of the Cenozoic era belongs to the Tertiary period, an unreasonable division which reflects the arbitrary manner in which the geological epochs were first named. From 1760 to 1770, Giovanni Arduino, inspector of mines in Tuscany and later professor of mineralogy at Padua, set forth the first classification of geological time, dividing the sequence of the Earth's rocks into Primitive, Secondary, and Tertiary. During the 18th century the names Primary, Secondary, and Tertiary were given to successive rock strata, the Primary being the oldest, the Tertiary the more recent. In 1829 a fourth division, the Quaternary, was added by P. G. Desnoyers. These terms were later abandoned, the Primitive or Primary becoming the Paleozoic Era, and the Secondary the Mesozoic. But Tertiary and Quaternary were retained for the two main stages of the Cenozoic. Admittedly, attempts to replace the obsolete "Tertiary" with a more reasonable division of Palaeogene (early Tertiary) and Neogene (later Tertiary and Quaternary) have not been completely successful, but most of the newer geological timelines have rejected the Tertiary.

Revised ATW040925

Geology of the Cenozoic

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 Gondwanan supercontinent moved north and became, to varying degrees, attached to the Laurasian landmass. This resulted in a number of spectacular mountain-building events which climaxed about the Early Miocene. 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 South America, Africa, India, and Australia. 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 (Piacenzian), and has confined the effects of continental collision to relatively mild and sporadic vulcanism around the Caribbean and its southern and western margins. See image from Kerr et al. (1999). Various other representations can be found at Prof. Manuel Iturralde's wonderful site on the Caribbean plate: Comparison of different opinions...

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 Mesozoic tetrapod 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 Eocene. 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 Miocene, 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.

Timescale

The following table gives the component periods and epochs that make up the Cenozoic. In addition to the Neogene and Paleogene Periods below, the helpful folks at the ICS have added a sub-era (or possibly period, subperiod, or supra-age), the Quaternary, which begins at the base of the Gelasian Age and extends through the present day. Obviously, this makes no sense at all, since the same body abolished the "Tertiary" just a year or two ago -- not to mention the fact that the rules on chronostratigraphic units don't seem to allow for a unit which begins in the middle of one epoch and ends in another. It would make much more sense to move the Gelasian Age (a unit which was only created in 1996) to the Pleistocene. Then the Pleistocene would become the age of continental ice sheets, which is precisely the reason it was thought necessary to recognize the "Quaternary." This sensible proposal was actually considered, but it was far too late. The ICS had already established a Subcommission on Quaternary Stratigraphy. Kings may die or abdicate. Nations may be conquered and governments dissolved. Even whole continents may fractured and dispersed by cataclysmic rifting. But committees do not vote themselves out of existence.

Climate

During the Paleogene the climate worldwide was warm and tropical, much as it had been for most of the preceding Mesozoic. The Neogene 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 Pleistocene, the continuing cooling climate resulted in an ice age, or rather a series of ice ages with interspersed warm periods

Life

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.

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.

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 - Equidae - 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, borhyaenids, 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 Australopithecines. 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.

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.

Links

University of California Museum of Paleontology -- Introduction to the Cenozoic - a short introduction at the University of California Museum of Paleontology site

Walking With Prehistoric Beasts -- Discovery Channel -- a non-technical look at the age of mammals, with photo-realistic computer reconstructions of a number of extinct species
Buy the DVD (a wonderful sequel to the Walking with Dinosaurs DVD - excellent computer special effects recreate the animals of the Cenozoic, from the early Eocene to the Pleistocene)

The Age of Mammals - from Biology 65: Biodiversity and Conservation

Cenozoic Era - a good summary

Cenozoic Mammals: Guilds and Trends - annotated links page from Richard Cowen's History of Life site

Geography and Evolution - annotated links page from Richard Cowen's History of Life site

DVD

Walking With Prehistoric Beasts - DVD also in VHS - by the team that brought you Walking With Dinosaurs