Miocene Epoch
Geologic Age
23.03–5.33 million years ago
Subdivisions
Messinian: 7.246–5.333 Ma
Tortonian: 11.63–7.246 Ma
Serravallian: 13.82–11.63 Ma
Langhian: 15.97–13.82 Ma
Burdigalian: 20.44–15.97 Ma
Aquitanian: 23.03–20.44 Ma
Eon / Era / Period
What happened during this time?
Biological
Flora
Kelp forests may appear at this time, although Looy (2014) indicates that it may have occurred during the Cretaceous
Forests declined in cooler and drier climate
Fossil evidence suggests that deciduous hardwood forests were once widespread throughout the Northern Hemisphere, when Asia, Europe and North America were still joined in the supercontinent called Laurasia.
As the climate of Laurasia began to cool, only the forest remnants in what are now China, Japan, Appalachia and small parts of Europe survived the periods of climate change and glaciation that followed, leaving nothing but fossils in the thousands of miles in between.
Laurasia’s deciduous forests started to die out, after which the continents were only intermittently connected by land bridges -- shallow parts of the ocean floor that were exposed when ice age glaciers tied up vast amounts of ocean water and sea levels were lower.
Analysis of oxygen isotopes of soil carbonates and clays shows that this shift from forests to open habitat co-occurs with a decline in winter precipitation leading to increased aridity across west-central North America 26–15 million years ago (Kukla et al. 2022)
Grasses diversified greatly, co-evolving with large herbivores and grazers
There was open grassland, dominated by C3 plants, between 25 and 17 Ma
This is ~7 Ma before supposed adaptations to grasslands in ungulates (hypsodonty) originate (Stromberg 2001)
C4 plants, which assimilate carbon dioxide more efficiently, begin to expand, and were locally abundant, contributing to heterogeneous habitats ranging from forests to wooded grasslands between 21 and 16 Ma (Peppe et al. 2023)
The expansion of grasslands and radiations among terrestrial herbivores, such as horses, can be linked to fluctuations in CO2
Grass is tough and richer in silica, causing a worldwide extinction of large herbivores
Herbivores, like horses, evolved very high-crowned teeth to cope with the wear.
Both the perissodactyls and artiodactyls underwent a period of rapid evolution
Fauna
In Eurasia and North America, the spread of grasslands forced an evolutionary change in herbivorous mammals, with the forest browsers giving way to the prairie grazers.
Mammal diversity reached its peak during the Miocene. Many were hoofed grazers or browsers. The epoch was marked by further evolution of horses, which became plains type animals as large as ponies, the chalicotheres, camels, rhinoceroses and anthropoid apes.
The appearance of the mastodons, raccoons, and weasels.
The first deer and giraffes also appear, along with the first hyenas.
The slow clumsy creodonts, well adapted to the jungle thickets, were replaced by the swift intelligent cat and dog type Carnivora as the dominant predators.
There were many eccentric browsing types as well - the chalicotheres - think of a horse crossed with a gorilla - were able to rear up on their hind legs and pull down the branches of trees.
The Asian indricotheres and the American entelodonts both flourished then died out during this epoch.
In the seas whales, dugongs and extinct elephant-like desmostylida flourished.
Giant sharks such as Carcharodon megalodon reached 13 to 15 metres in length and preyed on the early whales.
Geophysical
After about 40-50 million years of movement, the Indian and Eurasian Plate collide during the Miocene, forming the Himalayan Mountains.
This geologic process consumes large amount of carbon pushing into the Earth's mantle.
Panama Isthmus begins to form between 13-15 million years ago, cutting off the Atlantic from the Pacific Oceans (Montes et al. 2015)
Caused gulf stream to carry warmer waters to northern Europe.
Massive volcanic eruptions
CO2 levels continue to drop during this time
Earth becomes cooler and drier
Additional Resources
Discovery of a new species of fossil tree helps paint picture of ancient African forest (Phys.org 18Mar2025)
└Pan et al., (2025) Fossil Astropanax Seem. (Araliaceae) from the early Miocene (21.73 Mya) Mush Valley plant assemblages of Ethiopia
Current carbon dioxide levels last seen 14 million years ago (Science 8Dec2023)
└The Cenozoic Co2 Proxy Integration Project (CENCO2PIP) Consortium (2023) Toward a Cenozoic history of atmospheric CO2
Plants are Cool, Too! Episode 2: Fossil Forests