The Miocene epoch marks the interval when prehistoric life (with some notable exceptions in South America and Australia) substantially resembled the flora and fauna of historical times, due in part to the long-term cooling of the earth's climate. The Miocene was the first epoch of the Neogene period (23-2.5 million years ago), followed by the much shorter Pliocene epoch (5-2.6 million years ago); both the Neogene and Miocene are themselves subdivisions of the Cenozoic Era (65 million years ago to the present).
Climate and geography. As with the preceding Eocene and Oligocene epochs, the Miocene epoch witnessed a continuing cooling trend in the earth's climate, as global weather and temperature conditions approached their modern patterns. All of the continents had long since separated, though the Mediterranean Sea was dry for millions of years (effectively joining Africa and Eurasia) and South America was still completely cut off from North America. The most significant geographic event was the slow collision of the Indian subcontinent with the underside of Eurasia, causing the gradual formation of the Himalayan mountain range.
Terrestrial Life During the Miocene Epoch
Mammals. There were a few notable trends in mammalian evolution during the Miocene epoch. The prehistoric horses of North America took advantage of the spread of open grasslands and began to evolve toward their modern form; common genera included Hypohippus, Merychippus and Hipparion (oddly enough, Miohippus, the "Miocene horse," actually dated to the Oligocene epoch!) At the same time, various animal groups--including prehistoric dogs, camels and deer--became well-established, to the point that a time traveler to the Miocene epoch, encountering a proto-canine like Tomarctus, would immediately recognize what type of mammal she was dealing with.
Perhaps most significantly, from the perspective of modern humans, the Miocene epoch was the golden age of apes and hominids. These prehistoric primates mostly lived in Africa and Eurasia, and included such important transitional genera as Gigantopithecus, Dryopithecus and Sivapithecus. Unfortunately, apes and hominids (which walked with a more upright posture) were so thick on the ground during the Miocene epoch that paleontologists have yet to sort out their exact evolutionary relationships, both to each other and to modern Homo sapiens.
Birds. Some truly enormous flying birds lived during the Miocene epoch, including the South American Argentavis (which had a wingspan of 25 feet and may have weighed as much as 200 pounds); the slightly smaller (only 75 pounds!) Pelagornis, which lived all over the world; and the 50-pound, sea-going Osteodontornis of North America and Eurasia. All of the other modern bird families were pretty much in place, if occasionally a bit bigger than usual (penguins being the most notable examples).
Reptiles. Although snakes, turtles and lizards continued to diversify, the Miocene epoch was most notable for its gigantic crocodiles, which were nearly as impressive as the plus-sized species of the Cretaceous period. Among the most important genera were Purussaurus, a South American caiman, Quinkana, an Australian crocodile, and the Indian Rhamphosuchus, which may have weighed as much as two or three tons.
Marine Life During the Miocene Epoch
Pinnipeds (the mammalian family that includes seals and walruses) first came into prominence at the end of the Oligocene epoch, and prehistoric genera like Potamotherium and Enaliarctos went on to colonize Miocene rivers. Whales--including the gigantic, carnivorous sperm whale ancestor Leviathan and the sleek, gray whale ancestor Cetotherium--could be found in oceans worldwide, alongside enormous sharks like Megalodon. The Miocene seas were also home to one of the first forebears of modern dolphins, Eurhinodelphis.
Plant Life During the Miocene Epoch
As mentioned above, grasses continued to run wild during the Miocene epoch, clearing the way for the evolution of fleet-footed horses and deer, as well as more stolid, cud-chewing ruminants. The appearance of new, tougher grasses toward the later Miocene may have been responsible for the sudden disappearance of many megafauna mammals, which were unable to extract sufficient nutrition from their favorite food source.
Next: the Pliocene Epoch