The Tertiary Period herald the beginning of the “Age of Mammals” and is considered to be the beginning of the “modern world” with the emergence of “higher” life forms including prehistoric man ("ape man"). During the Tertiary, the continents pretty much settled into their present form and shape; the climate changed from wet, tropical environments to cooler, drier ecosystems; and the mammals replaced the dinosaurs as the dominant form of life on Earth.
Tectonics and Paleoclimate
During the Tertiary, the Earth slowly formed into the continents we know today with each Tertiary Epoch brining on additive changes to the Earth’s geography.
During the Paleocene, sea-levels were high and the present-day continental land masses were largely separated into island continents. The climate was warm and tropical.
During the Eocene, the rifting of the North Atlantic cut off North America from Europe and South America lost links with Antarctica. India and Scotland experienced mountain-building events. The sea-level rose further invaded much of Africa, Australia, and Siberia. Climates were generally warm or mild worldwide with mean annual temperatures below those of present-day tropics and rainfall that exceeded present-day norms in the northern latitudes.
During the Oligocene, there was an increase in volcanic activity and plate tectonic movement as India collided with Asia. The last remnant of the super continent of Gondwanaland broke up into Australia and South America both of which are separated from Antarctica. The Oligocene marked the start of a generalized cooling of the Earth’s climate with glaciers forming in Antarctica for the first time during the Cenozoic. The increase in ice sheets led to a fall in sea level and the tropics diminished giving way to cooler woodlands and grasslands. The overall cooling trend continued throughout the Oligocene and the Neogene Periods culminating in the Ice Ages of the Quaternary Period (e.g., Pleistocene Epoch).
During the Miocene, the island continent of India slammed into Asia, pushing up the Himalayas and triggering a global cooling that was to culminate in the Quaternary (i.e., Pleistocene) ice ages. During this epoch, the Rockies and Andes mountain ranges rose. It was a time of warmer global climates than those in the preceding (i.e., Oligocene) and following (i.e., Pliocene) epochs. During this time modern patterns of atmospheric and ocean circulation formed. The isolation of Antarctica from Australia and South America meant the establishment of the circum-polar ocean circulation that significantly reduced the mixing or warmer tropical water with cold polar water which further led to the buildup of the Antarctic ice cap.
During the Pliocene, the Earth quickly approached that of modern day with the continents taking up their present-day positions. A shift in the Caribbean tectonic plate brought about the joining of North and South America thus creating a land bridge for mammals to migrate across. The Mediterranean Sea (the last remnant of the once mighty Tethy's ocean) dried out and was to remain dry plains and grassland for several million years. India collided with Asia and gave rise to the Himalaya Mountains. The Himalyan uplift accelerated the global cooling that had begun in the Miocene. Subtropical regions retreated toward the equator, large ice caps began to form especially in Antarctica which was not yet completely frozen, and the northern hemisphere began a broad scale cooling across land and ocean.
During the Paleocen,e dense forests extended to higher latitudes. The vegetation included sequoia trees with a dense undergrowth of shrubs (e.g., tea, laurel), ferns and horsetails.
During the Eocen,e dense forests thrived and included sequoia trees with a dense undergrowth of shrubs (e.g., tea, laurel), ferns and horsetails. Changes in vegetation were limited chiefly to the migration of types of plants in response to climatic changes.
During the Oligocen, the vegetation of the higher latitudes in the northern hemisphere changed from an essentially broad-leaved tropical evergreen forest to temperate deciduous woodlands of evergreen and broad-leaved trees. Grasses which originally appeared near water sources became more common in open habitats. In North America, the flora consisted of a mixture of subtropical life (e.g., cashews and lychee trees), temperate trees (e.g., roses, beech, pine), and leguminous plants of the pea and bean family and well as sedges, bulrushes, and a variety of ferns.
During the Miocene and Pliocene, two major ecosystems first appeared; kelp forests and grasslands. The drying of continental interiors and a global cooling resulted in the further reduction of both tropical and conifer forests which gave way to grasslands and savanna environments. Much of the flora was very much like that of modern-day.
During the Tertiary, the Earth’s fauna changed significantly from that of the proceeding Cretaceous Period. Mammals became the dominate life form replacing the reptiles as the apex occupants of the Earth.
During the Paleocene, the fauna of Laurasia seems to have developed in northern Asia and migrated from there to the rest of Asia, to Europe and to North America. The fauna of Gondwana is much less known as the land masses that comprised it (i.e., South America, Antarctica, Australia, New Zealand, India, Africa) were all isolated from each other and from other parts of the world. This isolation served as independent centers of evolution where unique types of placental mammals, marsupials, monotremes, birds, and reptiles developed in relative seclusion and safety. The seas were full of all sort of contemporary creatures including gastropods and bivalves which were very similar to modern forms, soft-bodied squid that displaced the hard-shelled ammonites as the leading mollusks, and new kinds of sea urchins and foraminifers which filled the ecological gaps caused by the kill off of their Mesozoic predecessors during the Cretaceous extinction. Sharks ruled the oceans.
On land, many new types of mammals appear none of which exceeded the size of a small modern bear. Most were short-legged and plantigrade (walking on the soles of their feet), had five toes on each foot, slim heads with narrow muzzles, and small brain cavities. The predominant mammals of the period were members of groups that are now extinct and included the flesh-eating Mesonychia (e.g., Dissacus or Hyaenodictis) and Creodontia as well as the mostly herbivorous Condylartha, Pantodonts, and Dinocerata. Other Paleocene groups included multituberculates (Cretaceous holdovers), marsupials, insectivores, Plesiadapiformes (a group of squirrel-like animals), and two unrelated groups of large clumsy herbivores, the Tillodonts and the Tainodonts. Giant flightless birds of prey evolve including the Diatryma and birds from similar orders such as the Diatrymiformes and the Phorusrhacids. Also living at this time, but still insignificant, were the first representatives of the rodents and the Miacidae (the ancestors of modern Carnivora).
During the Eocene, the mammals continue to diversify and flourish. Creodonts and amblypods including the spectacular Uintatheres thrive while the direct evolutionary ancestors of modern animals make their appearance. The ancient hoofed condylarths gave way to more modern ungulates and the modern hoofed mammals (i.e., perrisodactyls and artiodactyls) of Europe, Asia and North America make their appearance including proto-horses, tapirs, rhinoceroses, and camels, as well as extinct groups like the pig-like anthracotheres, horse-like chalicotheres, and rhinoceros-like Titanotheres. These animals began their evolution as domestic cat-sized creatures but some groups like the Titanotheres quickly grew huge in size. The rodents replaced the multituberculates, bats had evolved from primitive Insectivora, and primates, including forest-dwelling ancestors of today's lemurs and tarsiers, flourished in the trees. In Africa, the ancestors of elephants, hyrax, monkey, and the extinct rhinoceros-like Embrithotheres evolve. In South America, hoofed mammals, edentates, marsupials, and more giant flightless birds (Phorusrachids) appear. The first aquatic mammals, whales (extinct lineage called Archeocetes) and sea cows appeared in the oceans while the first modern birds, including eagles, pelicans, quail, and vultures as well as the great flightless Diatriyaformes, arrived on the scene.
During the Oligocen, the seas contain an abundance of Nummulitid life, the genus Lepidocyclina replaces Orthophragmin, and irregular echinoids (e.g., Scutella and Clypeaster) first appear. The terminal Eocene extinction eliminated the Dinocerata, Archeoceti, and most of the Titanotheres and creodonts thus providing the opportunity for the evolution of new kinds of mammals including the prehistoric ancestors of dogs, cats, rhinoceroses, and horses (e.g., Mesohippus). The most significant transition among terrestrial mammals was the replacement of the perrissodactyls by the artiodactyls (even-toed ungulates) which included the sheep-like oreodonts and giant omnivorous pig-like entelodonts (e.g., Archaeotherium). The largest land animals continued to be the perrisodactyls including the huge hornless rhino-like Indricotherium of central Asia. The ancestral elephant-like forms and the rhinoceros-like Arsinotheres grew to large size in Africa where the first anthropoid apes made their debut. In South America, the Edentates (e.g., sloths, armadillos), the Meridiungulates, and the giant flightless flesh-eating Phorusrhacids birds appear and/or expand their territories.
During the Miocene and Pliocene, Eurasia and North America witnessed the spread of grasslands which forced an evolutionary change in herbivorous mammals that resulted in the forest browsers giving way to the prairie grazers. Both the perissodactyls and artiodactyls underwent a period of rapid evolution especially in their digestive and dental features in order to adapt to their grassland existence. In addition, the Tertiary saw the continuation of bioregional evolution in some instances and great mammal migrations in other cases, as animals that had evolved on different continents during the Eocene and Oligocene spread via land-bridges.
The fauna of the Miocene and Pliocene is characterized by the appearance of all of the presently existing orders and families as well as many of the existing genera of mammals. Cattle, sheep, antelopes, gazelles, and other bovids reached their peak. North America was home to three-toed horses, sheep-like oreodonts, several types of rhinoceroses, pronghoms, camels, protoceratids, horse-like chalicothere, bear-dogs, saber-toothed cats, and the pig-like entelodonts. As the Tertiary headed toward the Quaternary, North American mammals included horses, camels, deer, pronghoms, peccaries, mastodonts, beavers, weasels, dogs, saber-toothed cats, and one-toed horses appear for the first time.
The Tertiary was a time of great migration, owing to the appearance of new land bridges. The North American three-toed Hipparion horse crossed the Bering Straits land bridge and entered Asia and Europe while mastodonts entered the Americas from Asia. The isthmus of Panama ended South America's isolation leading to a migration of many animals from South America to North America and vise versa. Heading north were armadillo, ground sloth, opossums, and Phoruscorid birds. Heading south were various dogs, cats, bears, horses, and mastodonts.
In Eurasia, the fauna included early deer and giraffes, indricatheres, and chalicotheres. Australia was a lush tropical rain forested land with an amazing abundance of marsupials, while South America was home to glyptodonts, armadillos, anteaters, New World monkeys, horse-like litopterns, a range of marsupial carnivores, giant carnivorous birds (i.e., Phorusrhacids), and strange crocodiles (e.g., sebecosuchids).
Africa was the home to early elephants, mastodons, apes, and Old World monkeys. The emerging savanna grasslands and retreating forests caused some apes to come down from the trees and take up life in the open where they co-existed with early elephants, antelopes, and other types of animals. An erect posture was necessary for these vulnerable creatures to watch for predators which also freed the hands for the use of makeshift tools (e.g., sticks). Thus the hominid lineage appeared in the rift valleys of north-east Africa during Early Pliocene. As with the bovids, the hominids underwent an evolutionary radiation with the robust Australopithecines inhabiting Ethiopia, Tanzania, and probably throughout most of Africa. The large-brained australopithecine Homo habilis emerged and continued on into the Early Pleistocene giving rise to Homo erectus, the hypothetical common ancestor of both Neanderthal and modern man.
In the seas, whales, dugongs, the extinct elephant-like desmostylida, and giant sharks such as Carcharodon megalodon flourished.
Meteorite Impacts on Earth
I included a list of meteorite impacts relevant to this time period as a point of reference since many of the explanations for mass extinctions throughout Earth’s history include meteorite impact(s) as a possible cause. The meteorite impact information below was obtained from the ‘Earth Impact Database’ maintained by the Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada ( www.passc.net/EarthImpactDatabase).