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 Cascade Mountain ranges formed, causing a non-seasonal and drier mid-continent climate. The increasing occurrences of drought and an overall decrease in absolute rainfall promoted drier climates. Additionally, grasslands began to spread, and this led to an evolutionary radiation of open habitat herbivores and carnivores. The first of the major periods of immigration via the Bering land connection between Siberia and Alaska occurred in the middle of the Miocene, and by the end of the Miocene the Panama isthmus had begun to form between Central and South America.
Plate tectonics also contributed to the rise of the Andes Mountains in South America, which led to the formation of a rain shadow effect in the southeastern part of the continent. The movement of the plates also facilitated trends favoring non-desert and highland environments.
In Australia, the climate saw an overall increase aridity as it continued to drift northwards, though the continent went through many wet and dry periods. The number of rainforests began to decrease, to be replace by dry forests and woodlands. The vegetation began to shift from closed broad-leaved forests to more open, drier forests as well as grasslands and deserts.
Eurasia also experienced increasing aridification during the Miocene. Extensive steppe vegetation began to appear, and the grasses became abundant. In southern Asia, grasslands expanded, generating a greater diversity of habitats. However, southern Asia was not the only area to experience an increase in habitat variability. Southern Europe also saw an increase in grasslands, but maintained its moist forests. Although most of Eurasia experienced increasing aridity, some places did not. The climate in some Eurasian regions, such as Syria and Iran, remained wet and cool.
During the Miocene, Eurasia underwent some significant tectonic rearrangements. The Tethys Sea connection between the Mediterranean and Indian Ocean was severed in the Mid-Miocene causing the increase in aridity in Southern Europe. The Paratethys barrier, w