High carbon dioxide levels during the middle Miocene epoch
have been documented in other studies through multiple lines of evidence, including the number of microscopic pores on the surface of plant leaves and geochemical evidence from soils and marine organisms.
Thus, such noteworthy events as the closure of the connection between the Atlantic and the Mediterranean at the end of the Miocene Epoch
and its subsequent opening would have had an insignificant effect on the global cycle.
Ancestors of the river dolphins existed from the Late Oligocene and Early Miocene epochs
, about 20 to 25 million years ago; while marine dolphins are known to exist from the Late Miocene epoch
, about 10 to 15 million years ago.
The youngest formation (ignoring the Quaternary alluvium and residual deposits) which crops out in the area is composed of rhyolites, rhyolite tuffs, breccias, and ignimbrites of the "Upper Volcanic Series" of the Miocene epoch
. This rock series is the predominant formation of the "Sierra Madre Occidental" province and covers the topographically highest parts of the province.
At some point in the Miocene epoch
, between about 23 million and 5 million years ago, the various hominoid species emerged in all their diversity.
The thickness of this layer is 60 metres and it dates back to the Miocene epoch
, in addition to many gas layers dating back to the Pliocene epoch.
The hammerhead ancestor probably lived in the Miocene epoch
about 20 million years ago.
These sites date back six to eight million years to the Late Miocene epoch
when Abu Dhabi was greener, with rivers teeming with hippos, crocodiles, turtles and fish.
"Like many freshwater fish, pupfish invaded from salt water, arriving during the Miocene epoch
, then making their way to the large Pleistocene lakes, and eventually becoming isolated in these various systems as the water receded," says John Wullschleger, a fisheries biologist with the park service.