Cenozoic Era


Cenozoic Era Paleobiology

"The Age of Mammals"

Back to Mesozoic Era Paleobiology

Also see:
Tertiary Fossils
Badlands Fossils
Messel Oil Shale Fossil Site
Fossils of the Carpathian Mountains
Green River Formation Lagerstatte

Cenozoic Era (65 million years ago to present)

Eocene Ginkgo biloba Fossil LeafThe KT Event set the stage for the Cenozoic Era Cenozoic Era that began 65 million years ago. As the dinosaurs perished at the end of the Cretaceous, the mammals took center stage. Even as mammals increased in numbers and diversity, so too did the birds, reptiles, fish, insects, trees, grasses, and other forms of life. Species changed as the epochs of the Cenozoic Era rolled by, with the mammals eventually becoming the largest land animals of the Era, as the dinosaurs had been during the Mesozoic. Flowering plants strongly influenced the evolution of both birds and herbivors throughout the Cenozoic era by providing a rich abundance of food. Those that could adapt to the changes in the environment survived; those that could not were doomed to extinction. A rich fossil record in rocks relatively undisturbed by geological forces reveals the history of both. The Cenozoic is when the continents moved to their current positions. Australia-New Guinea that split from Gondwana during the early Cretaceous drifted north eventually collidedine Asia. Antarctica moved into its current position over the South Pole. The Atlantic Ocean widened and South America became attached to North America.

Paleogene Period (65 to 23 million years ago); Also see: Tertiary Fossils

The Paleogene Period contains the Paleocene, Eocene, and Oligocene epochs. Invertebrates, fish and reptiles were similar to those of modern types, but mammals, birds, protozoa and flowering plants would undergo considerable evolutionary change.


The Paleocene Period began after the extinction of the dinosaurs. Mainly nocturnal mammals that had cowered in the shadows of dinosaurs for millions of years eventually evolved into a vast number of different forms to fill the newly vacant environmental niches. At the beginning of the Paleocene, most mammals were tiny and rodent-like. With time, mammals grew in size, number, and diversity. Many early mammal designs of this time would soon become extinct, but others would survive and then evolve into other forms. The diversity of birds, other animals, and plants increased, and species became more specialized. Although dinosaurs were gone, their reptile cousins lived on in the form of turtles, crocodiles, lizards, and snakes.


The first grasses appeared in the Eocene Epoch (from about 54 to 37 million years ago) with growth near the root Eocene Priscacara serrata from Green River Formation in Wyomingas opposed to the tip, providing a vastly expanded and renewable food resource for the herbovores; this allowed adaptation to life on the savanna and prairie and the evolution of running animals such as the Equiidae (the horse family). The grazing mammals evolved the teeth enabling a diet of harsh grass. The Eocene Epoch was a period when flowering plants continued a massive radiation that began in the Paleocene Epoch. Plants thrived, and with that many animals as new environmental niches were filled. The first grasses also provided a refuge for many animals. Small mammals radiate. Many new species of shrubs, trees and small plants appeared. A variety of trees thrived in a warm Eocene climate, including beech, elm, chestnut, magnolia, redwood, birch, and cedar, and more. The evolution of plants was providing a powerful selective pressure across the entire animal Kingdom, and many new symbiotic systems appeared.

Oligocene Oreodont Skull from Oligocene of South Dakota


The Oligocene Epoch extends from about 34 million to 23 million years years ago. The name Oligocene comes from the Greek oligos (meaning few) and ceno (meaning new) and is in reference to the paucity of new mammalian animals after their radiation during the preceding Eocene Epic. The Oligocene is often considered as an important window of environmental transition from the tropical Eocene and the cooler Miocene. The start of the Oligocene is marked by a major extinction event that might have been caused by a meteor impact in Siberia or near the Chesapeake Bay. Angiosperms continued their expansion throughout the world, as did grasses. Temperate deciduous woodlands mostly replaced tropical and sub-tropical forests, while plains and deserts became more commonplace. Among the animals, mammals diversified markedly, and marine fauna evolved to forms closely resembling those extant today. Ancestors of modern elephants and rhinoceros grew to large size in Africa, where the first apes primate belonging to suborder Anthropoidea that includes monkeys,Miocene Metailurus Saber Toothed Cat
apes, and humans, also appeared.

Neogene Period (23 to 2.58 million years ago); Also see: Tertiary Fossils

The Neogene Period contains the Miocene and Pliocene epochs. During this long period, while some mammal groups evolved markedly, others changed little. Importantly, the earliest hominids arose on the continent of Africa.


The Miocene Epoch extends from about 23 to 5 million years ago. The name comes from the Greek words meion (less) and ceno (new) because of the smaller proportion of modern sea invertebrates than the subsequent Pliocene Epoch. The Miocene is thus a very long 18 million years, and generally marks the transition from the far prehistoric world to a pseudo-modern world. A major expansion of grasslands occurred as forests declined in the cooler and dryer climate, driving selection and radiation of large herbivores, including the ruminants which are ancestors of modern cattle and deer. Mammals such as wolves, horses and deer as well as birds also generally evolved to closely resemble forms extant today.


The Pliocene Epoch extends from 5.3 million to 1.8 million years before present. The name comes from the Greek words pleion (more) and ceno (new) and roughly means the continuation of the recent in reference to the fact that mammals were essentially modern in form. The Pliocene climate was also relative cool and dry as in modern Pliocene Fossil Amber with Rare Isopod Crustaceatimes. These modern climates reduced tropical vegetation and shrank tropical forest to a band near the equator. Concurrently, deciduous and coniferous forests, tundra, grasslands, dry savannahs and deserts filled the space.

Continental drift would play a major role is how animals, and particularly terrestrial mammals were to distribute. South America linked to North America through the Isthmus of Panama, to the detriment of South American marsupials, and precipitating a drop in Atlantic Ocean temperatures. The collision of Africa and Europe formed the Mediterranean Sea, and disconnected part of the former Tethys Ocean. Receding sea levels formed a land bridge between Alaska and Asia.

Both marine and terrestrial life was for the most part modern, though discernibly more primitive. Herbivores grew in size, as did their predators. The first recognizable human ancestors, the australopithecines, appeared in the Pliocene. Mammalian life evolved in continent-dependent ways, and some migration occurred between continents. In North America, rodents, mastodonts, elephant-like gomphotheres, and opossums were notably prolific, while hoofed animals generally declined. Africa’s hoofed animals and primates were notably successful, and the australopithecines (some of the first hominids) appeared late in the Pliocene The Pliocene seas were thrived with mammals such as seals and sea lions.

Quaternary Period (2.6 million years ago to present)Pleistocene Coyote Fossil Skull

The Quaternary Period that began some 2.6 million years ago marked the origin of the close human ancestors as well as the modern forms of the animals we see today. The period includes two geologic epochs: the Pleistocene and Holocene, and contained a series of glaciations. Huge glaciers advanced and retreated over much of North America and Europe, parts of South America and Asia, and over all of Antarctica. The Great Lakes formed and giant mammals thrived in parts of North America and Eurasia not covered in ice. These mammals became extinct when the last Ice Age ended some 11,700 years ago. Modern humans evolved about 190,000 years ago. During the Quaternary period, mammals, flowering plants, and insects dominated the land.


The Pleistocene is the geological epoch which lasted from about 2.6 million years ago to about 11,700 years ago, comprising the most period of repeated glaciations. Both marine and land faunas were essentially like today, albeit mammals were generally larger than their modern descendants. The dramatic climatic changes during the ice age impacted fauna and flora in majoe ways. When glaciers advanced south, so did plants and animals reduced living space and food availability. A major extinction event of large mammals (megafauna) that included mammoths, mastodons, saber-toothed cats, glyptodons, ground sloths, Irish elk, and cave bears began late in the Pleistocene and continued into the Holocene. Neanderthals also became extinct during this period. By the end of the last ice age, smaller and swifter mammals and migratory birds, had displaced the megafauna and then returned northward.


Animal and plant life have not evolved much during the relatively short Holocene, and are essentially as they are today. A number of large animals including mammoths and mastodons, saber-toothed cats and giant sloths disappeared in the late Pleistocene and early Holocene, especially in North America, where animals that survived elsewhere (including horses and camels) became extinct.