The Paleogene Period* is the first of three periods comprising the Cenozoic Era. The Cenozoic, sometimes known as the "Age of Mammals", as the Mesozoic was the "Age of Reptiles", is known by its Epochs. The Paleogene is composed of the first three of these Epochs, (Paleocene, Eocene, and Oligocene Epochs). Four additional Epochs comprise the Neogene and Quaternary Periods that are to follow. The Paleogene sees the rapid filling of the environment following the K/T extinction, though it took more than two million years for the Earth’s ecosystems to recover from this event. On land primitive mammals and birds began to spread rapidly.  In the seas planktonic foraminifera and nanofossils begin new evolutionary paths.  Most marine life resembles modern forms: the wonder of Cenozoic fossils is seeing recognizable organisms cast in stone, rather than the exotic, 'alien' life forms, such as sea lilies, ammonoids, and trilobites, of the Mesozoic and Paleozoic Eras. The three Epochs are discussed in more detail below. Additional information about the mammals of these epochs can be found in our Prehistoric Mammals of the Cenozoic exhibits

Paleocene Epoch (65.5–55.8 Ma)

Small mammals and birds diversify in dense forests as Earth recovers from the (K-T) extinction. The loss of the giant reptiles that dominated the Mesozoic Era left the world open for evolutionary experiments by mammals and birds as they filled Earth’s environments in turn.The diverse mammalian fauna remained small, the largest only the size of a small pony. Ferns were initially abundant following the K-T extinction, but flowering plants and conifers soon took over as they returned to abundance. Deciduous trees dominated swamp forests in North America from middle latitudes to the Arctic ocean. Grasses, an immensely important group in later epoch ecologies, originated early in the Paleogene. Insect herbivory finally recovered from the K-T extinction event in the late Paleogene, nine million years after the event.

In the oceans, most reptiles vanished, turtles and crocodilians being exceptions. Sharks and teleost fish become more common, and bony fishes dominate the seas as they will continue to do to the present day. Among invertebrates more modern forms of gastropods and bivalves, foraminiferans and echinoids appear. As a result of various geological events like the island continent of India colliding with Asia, there was a rapid worldwide rise in temperature at the end of the epoch.

Eocene Epoch (55.8–33.9 Ma)

The Eocene begins with extreme Global warming, the warmest five million years of the Cenozoic. This warming was probably due to a large methane release from the ocean floor. As a result of global warming trees grew even in Polar Regions, while subtropical or tropical angiosperm forests cover most of what is now the United States. Palm trees grew in Alaska and Spitzbergen island and crocodilians lived above the Arctic circle. Many new grasses evolve. However, grasslands had yet to develop and herbivorous mammals were browsers, feeding on leaves and herbs rather than grass. The first odd-toed mammals (perissodactyls, such as rhinos and horses) and even-toed mammals (artiodactyls such as camels) were present at the beginning of the epoch. The first marine mammals, including the first whales, appear in the seas, and the first primates appear on land. Large mammals make their first appearance on land, then die off by the end of the epoch. Carnivores include the first members of the dog, weasel, bear and cat families. Most modern orders of bird had appeared by the Eocene. Africa is now an island continent. The climate began the long cooling trend that would continue through the Cenozoic in the middle of the Eocene.

In the middle Eocene grasslands had not yet spread North America was dominated by forest and forest mammals as seen above. The odd-toed perrisodactyls, such as the Palaeosyops in the rear center had already appeared. In the trees next to them are the primitive rodent Ischyrotomys and the primate Smilodectes. A group of primitive tapirs (Helatetes) are in the center of the frame. Just in front of them, looking at a group of Orohippus (a perrisodactyl), is a predatory Patreofelis. Just below a saber-toothed Machaeroides defends its kill (a lizard, Echmatemys) from the fox-size carnivore, Sinopa. A couple of artiodactyls, the tiny Homacodon and Helohyus, are seen just above an edentate Metacheiromys leaving the frame above the saber-tooth and its kill.**

Oligocene Epoch (33.9–23.0 Ma)

The global cooling that eventually leads to later ice ages begins during this epoch. Forests begin to shrink and grasslands expand at the expense of forests. Anthropoid apes make their first appearance. The newly opened landscapes favor the evolution of fast running prey and predators. Oreodonts were very common in North America along with three toed horses and a variety of rhinoceros species. The giant Indricothere, a type of rhinoceros the size of a medium sauropod and the largest land mammal ever, lived in central Asia. The anthropoid apes appeared during this epoch. South America, isolated for a few million years, evolves a distinct fauna including giant carnivorous birds and predatory marsupials. Both cats and dogs were represented among carnivores, including saber tooth cats. In the ocean both seals and sea lions appear late in this epoch. The two suborders of whales alive today, the toothed and baleen whales evolved. Invertebrates continued to look more modern, while coral reefs spread in the southern hemisphere as far as New Zealand. South America breaks away from Antarctic, allowing an isolating circum-antarctic current and a permanent ice cap to form, lowering world temperatures.

Dominating this scene in the distance are giant Brontotherium, the largest land animals to ever live in North America. Just in front of them is one the the first true rhinoceros, Subhyracodon, and in front of it is a group of Merycoidodon herbivorous oreodonts. Next to them, in the center of the frame, is a Protapirus, the first true tapir, that in turn is above a group of Poëbrotherium, small ancestral camels. Two Hyaenodon fight over a Glyptosaurus lizard, while a saber-tooth cat, Hoplophoneus lies in the bushes in the center foreground. Moving across the front from left to right a series of small animals are seen: the primitive rabbit Paleolagus, the insectivore Ictops, three kinds of ruminants, Hypisodus, a couple of Leptomeryx, and on the far right lower corner, a group of Hypertragulus.**

*The term Paleogene is from Naumen’s “Paleogen Stufe” (1866) in which he combined the Eocene and the Oligocene.

** The names and information concerning the animals in these murals are taken from the original legends in the 1964 Time-Life book, The Land and Wildlife of North America, and may be outdated.