Stratigraphy within the Miocene, as with much of the Cenozoic, is often defined on a highly regional basis. Terrestrial faunas are recognized in ages that vary from continent to continent, primarily because the animals themselves varied from place to place. These ages are usually defined on the basis of the land mammals, so that North America, Europe, Australia, etc. each have their own Land Mammal Ages.
For marine stratigraphy, diatoms and foraminifera are the primary groups used to recognize ages. By this time, both groups were abundant and diversified globally, so much so that diatomite is a common marine sediment of the Miocene. Because the diatoms are abundant, and are the substance of which many marine deposits are formed, they are particularly useful for identifying the relative ages of fossil deposits.
Life in the Miocene
Many of the fossils of species of this time have close relatives alive today, which can be both good and bad. It is good in that we have living representatives to infer past biology, but bad in that we may be tempted to infer too much from this.
The overall pattern of biological change for the Miocene is one of expanding open vegetation systems (such as deserts, tundra, and grasslands) at the expense of diminishing closed vegetation (such as forests). This led to a re-diversification of temperate ecosystems and many morphological changes in animals. Mammals and birds in particular developed new forms, whether as fast-running herbivores, large predatory mammals and birds, or small quick birds and rodents.
Figure 2 shows a Chalicothere, a Miocene mammal from Kazakhstan. Chalicothere was an odd-toed hoofed mammal, or Perissodactyl. Modern day ancestors of this organism include horses, zebras, rhinos and tapirs. Both the Perissodactyls and Artiodactyls (cloven-hooved mammals, modern day representatives include sheep, goats, camels, pigs, cows, deer, giraffes and antelopes) underwent a period of rapid evolution during the Miocene.
Plant studies of the Miocene have focused primarily on spores and pollen. Such studies show that by the end of the Miocene 95% of modern seed plant families existed, and that no such families have gone extinct since the middle of the Miocene. A mid-Miocene warming, followed by a cooling is considered responsible for the retreat of tropical ecosystems, the expansion of northern coniferous forests, and increased seasonality. With this environmental change came the diversification of modern graminoids, especially grasses and sedges.
Because of plate movements, the African-Arabian plate joined with Asia during the Miocene closing the seaway which had previously separated these two landmasses. This new land connection allowed for previously separated faunas and floras to come into contact with each other and resulted in a number of organism migrations.
In addition to changes on land, important new ecosystems in the sea led to new life-forms there. Kelp forests appeared for the first time, as did sea otters and other organisms unique to those environments. At the same time, such ocean-going mammals as the Desmostylia went extinct (Figure 3).
Paleoclimate and Geology
The Miocene was a time of warmer global climates than those in the adjacent Oligocene and Pliocene epochs. During the Miocene two major ecosystems first appear: kelp forests and grasslands. The expansion of grasslands during this epoch is correlated to a drying of continental interiors as the global climate first warmed and then cooled.
The Miocene saw a change in global circulation patterns due to slight position changes of the continents and globally warmer climates. Conditions on each continent changed somewhat because of these positional changes, however it was an overall increase in aridity through mountain building that favored the expansion of grasslands. Because the positions of continents in the Miocene world were similar to where they lie today, it is easiest to describe the plate movements and resulting changes in the paleoclimate by discussing individual continents.
In North America, the Sierra Nevada and 