The Miocene boundaries is not set at an easily identified worldwide event but rather at regional boundaries between the warmer Oligocene the relatively cooler Miocene and the even cooler Pliocene.
The Miocene Faunal stages from youngest to oldest are:
Aquitanian Burdigalian Langhian Serravallian Tortonian Messinian
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. Climates remained moderately warm although slow global cooling that eventual led to the Pleistocene glaciations continued. Mountain building in Western North America and Europe. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied contential exposures occur in the [American Great Plains]? and in Argentina.
Both marine and continental fauna?s were fairly modern. Recognizable dogs, horses, beaver, deer?, camels, crow?s, duck?s, owl?s, whales, etc existed in the Miocene. Only in isolated South America and Australia did widely divergant faunas exist.