Miocene Epoch

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