Primeval 25 – Miocene Epoch

 

The Miocene is the first geological epoch of the Neogene period and extends from about 23.03 to 5.332 million years ago. The Miocene boundaries are not marked by a single distinct global event but consist rather of regional boundaries between the warmer Oligocene and the cooler Pliocene.

Earth 20 Million Years Ago

Earth 20 Million Years Ago

The apes arose and diversified during the Miocene epoch, becoming widespread in the Old World. By the end of this epoch, the ancestors of humans are believed to have split away from the ancestors of the chimpanzees to follow their own evolutionary path. As in the Oligocene before it, grasslands continued to expand and forests to dwindle in extent. In the Miocene seas, kelp forests made their first appearance and became one of Earth’s most productive ecosystems. The plants and animals of the Miocene were fairly modern. Mammals and birds were well-established. Whales, seals, and kelp spread through the oceans. The Miocene epoch is of particular interest to geologists and palaeoclimatologists as major phases of Himalayan uplift had occurred during the Miocene epoch affecting monsoonal patterns in Asia, which were interlinked with glaciations in the northern hemisphere.(1)An Zhisheng et al. (2001) “Evolution of Asian monsoons and phased uplift of the Himalaya Tibetan plateau since Late Miocene times” Nature, Volume 411, Number 6833, Pages 62–66

Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula. Mountain building took place in western North America, Europe, and East Asia. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines.

A Promerycochoerus, Menoceras,  Merycochoerus, Moropus, and a Dinohyus in the Miocene

A Promerycochoerus, Menoceras, Merycochoerus, Moropus, and a Dinohyus in the Miocene

Life during the Miocene Epoch was mostly supported by the two newly formed biomes: kelp forests and grasslands. This allows for more grazers, such as horses, rhinoceroses, and hippopotamuses. Ninety five percent of modern plants existed by the end of this epoch. Both marine and continental fauna were fairly modern, although marine mammals were less numerous. Only in isolated South America and Australia did significantly different animals exist. In the Early Miocene, several Oligocene groups were still diverse, including nimravids, entelodonts, and three-toed horses. During the later Miocene mammals were more modern, with easily recognizable dogs, bears, raccoons, horses, beaver, deer, camels, and whales.

Approximately 100 species of apes lived during this time, ranging throughout Africa, Asia and Europe and varying widely in size, diet, and anatomy. Due to minimal fossil evidence it is unclear which ape or apes contributed to the modern hominid clade, but molecular evidence indicates this ape lived between 7 and 8 million years ago.(2)Kevin E. Langergrabe et al. (2012) “Generation times in wild chimpanzees and gorillas suggest earlier divergence times in great ape and human evolution” Proceedings of the National Academy of Sciences, Volume 109, Number 39, Pages 15716-15721 The first hominins (upright walking apes) appeared in Africa at the very end of the Miocene, including Sahelanthropus, Orrorin, and an early form of Ardipithecus.(3)David R. Begun (2007) Fossil Record of Miocene Hominoids

Four Tusked Gomphotheres

Four Tusked Gomphotheres

Although a long-term cooling trend was well underway, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene. The Miocene warming began 21 million years ago and continued until 14 million years ago, when global temperatures took a sharp drop, the Middle Miocene Climate Transition (MMCT). By 8 million years ago, temperatures dropped sharply once again, and the Antarctic ice sheet was already approaching its present-day size and thickness. Greenland may have begun to have large glaciers as early as 7 to 8 million years ago, although the climate for the most part remained warm enough to support forests there well into the Pliocene.

In the middle of the Miocene was a wave of extinctions both on land and in the ocean called the Middle Miocene Disruption following the Miocene Climatic Optimum (18 to 16 million years ago), around 14.8 to 14.5 million years ago, during the Langhian stage of the mid-Miocene. A major and permanent cooling step occurred between 14.8 and 14.1 million years ago, associated with increased production of cold Antarctic deep waters and a major growth of the East Antarctic ice sheet. A Middle Miocene relative increase in the heavier isotope of oxygen has been noted in the Pacific, the Southern Ocean and the South Atlantic.(4)Kenneth G. Miller and Richard G. Fairbanks (1983) “Evidence for Oligocene−Middle Miocene abyssal circulation changes in the western North Atlantic” Nature, Volume 306, Number 5940, Pages 250–253 The cause of the Middle Miocene Disruption is unknown.

References   [ + ]

1. An Zhisheng et al. (2001) “Evolution of Asian monsoons and phased uplift of the Himalaya Tibetan plateau since Late Miocene times” Nature, Volume 411, Number 6833, Pages 62–66
2. Kevin E. Langergrabe et al. (2012) “Generation times in wild chimpanzees and gorillas suggest earlier divergence times in great ape and human evolution” Proceedings of the National Academy of Sciences, Volume 109, Number 39, Pages 15716-15721
3. David R. Begun (2007) Fossil Record of Miocene Hominoids
4. Kenneth G. Miller and Richard G. Fairbanks (1983) “Evidence for Oligocene−Middle Miocene abyssal circulation changes in the western North Atlantic” Nature, Volume 306, Number 5940, Pages 250–253