The Cenozoic

The Cenozoic is the uppermost of the three major divisions of the Phanerozoic, the other two being the Mesozoic and Paleozoic (Fig. 1). The term Cenozoic (or Cainozoic) means “recent life”, implying that the fossils encountered in these layers are more similar to modern species. Geologists divide the Cenozoic into three major systems, being, from bottom to top: the Paleogene, the Neogene, and the Quaternary. The Paleogene, is further subdivided into Paleocene, Eocene, and Oligocene, the Neogene into Miocene and Pliocene, and the Quaternary into Pleistocene and Holocene.

geol column

Figure 1: The mayor subdivisions of the geologic column.

Lower Paleocene layers are characterized by low diversity of terrestrial fauna and flora and of marine organisms, but diversity increases upwards through the layers. Plant fossils look very similar to modern species of tropical, sub-tropical and deciduous plants, including cacti and palm trees. Most fossils of mammals are only known from teeth and partial skeletons of small insectivores, herbivores, and carnivores. Most notable are the marsupial mammals, which make up more than 50% of the mammal species in the Paleocene layers of the southern continents.

Eocene fossils are recognizable as having body plans similar to living species. Geologic data suggest that there were no ice caps covering Earth’s poles and that latitudinal differences in temperature were small. Warm climate predominated in many regions of the planet facilitating the growth of large forests on Earth from pole to pole, recorded now as extensive fossil deposits of tropical to subtropical plants even in Arctic regions (Fig. 2). Tropical rainforests grew even in northern latitudes of North America and Europe. Toward the end of the Eocene, the evergreen forests were replaced by grasslands, plains and deciduous trees in North America, Eurasia and the Arctic. Similar changes happened in Antarctica, which became covered with tundra.

Minolta DSC

Figure 2: A perfectly fossilized palm frond associated with fish, from the Eocene Green River Formation, North America. This type of flora is indicative of a more subtropical climate than at present.

The first fossil representative of most of the modern mammal orders appear in the lower layers of the Eocene. A remarkable feature about these fossil mammals is their very small size compared to similar faunas of contiguous Paleocene and Oligocene deposits. Some reptiles, however, were very large, including Titanoboa, a large snake found in South America, and other reptilian megafauna. Birds are also abundant in Eocene layers, as well as fossil insects preserved in amber. Eocene layers also preserve many vertebrate fossils that lived in the oceans, including large carcharinid sharks, Basilosaurus (a large marine mammal), and sirenians.

The Oligocene sedimentary layers preserve a record of decline in temperatures, expansion of ice sheets, and global sea level fall. Important mountain building activity took place during the Oligocene, including areas such as the European Alps and western United States. In general the fossil fauna of the Oligocene, both on land and in the ocean, resembles that of modern organisms, except in South America, where large-sized litopterns, notoungulates and toxodonts (extinct orders of hoofed mammals), and extinct marsupial types lived. Some of these groups are found fossilized even in overlying strata, up to the Pleistocene (Fig. 3).

Miocene deposits record further cooling of the Earth and the extension of ice caps on both hemispheres. The Alps in Europe, the Andes in South America, and the Himalayas in Asia continued to rise, forming some of the greatest mountain ranges in the planet. Extensive grasslands allowed grazers such as horses, rhinoceroses, hippos, ground sloths, and also browsers such as camels, to thrive. All or almost all of the modern bird groups, including marine birds, are present as fossils in Miocene rocks. Marine fossils are abundant, including many specimens of marine mammals and other vertebrates. The abundance of biogenic sediments in Eocene strata indicate that the oceans sustained highly productive communities of microscopic algae (diatoms and other phytoplankton), which formed thick accumulations of diatomaceous sediments containing rich and exceptionally preserved fossils of marine mammals, birds, and reptiles.

Fig 3

Figure 3: A Pleistocene fossil toxodont from Argentina.

Pliocene layers record frequent and significant sea level changes, linked to contraction and expansion of ice sheets. Fossil Pliocene vegetation indicates a reduction of tropical species worldwide. Much of the northern hemisphere was covered by deciduous forests, coniferous forests and tundra, with grasslands spreading on all continents but Antarctica. A significant feature about the Pliocene fauna is gigantism: many species of land habitats were of large size, including mastodons, rodents, ground sloths, armadillos, glyptodonts, etc.

The Pleistocene record is dominated by the effects of glaciations that shaped the landscape in ways still discernible in North America, Russia, and the Nordic countries, as well as in mountain ranges in Asia and Europe. At the last glacial maximum, sea level decreased considerably.

During the Holocene, the global temperature rose and much of the ice that covered the northern hemisphere melted causing rapid sea level rise.

Two trends of interest emerge from this simple review of the Cenozoic fossil record. The first is that the fossil fauna and flora do not differ markedly from what observed today in terms of structure and major higher taxonomic groups. In other words, the type of life documented in Cenozoic layers does not appear to be fundamentally different from what seen in the modern world. The second is the importance played by climate patterns in shaping the geologic record of the Cenozoic, with a major trend for climate deterioration and establishment of glaciation, only recently reversed. Both these macro-scale observations could fit well with a model of the Cenozoic as representing geologic, climatic, and biologic processes unfolding in the post-flood world.

 

By Raul Esperante, PhD

Geoscience Research Institute

Image | This entry was posted in Fossils, Geological Column, Geology and tagged , , , . Bookmark the permalink.

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