By the standards of "deep time," the Pliocene epoch was relatively recent, beginning only five million years or so before the modern historical record. During the Pliocene, prehistoric life around the globe continued to adapt to the prevailing climatic conditions, with some notable local extinctions and disappearances. The Pliocene is the second epoch of the Neogene period (23-2.6 million years ago), the first being the Miocene (23-5 million years ago); all of these periods and epochs were themselves part of the Cenozoic Era (65 million years ago to the present).
Climate and geography. During the Pliocene epoch, the earth continued its cooling trend, with tropical conditions holding at the equator (as they do today) and more pronounced seasonal changes at higher and lower latitudes; even still, average global temperatures were 7 or 8 degrees (Fahrenheit) higher than they are today. The major geographic developments were the reappearance of the Alaskan land bridge between Eurasia and North America, after millions of years of submersion, as well as the formation of an isthmus connecting North and South America. Not only did this latter development allow an interchange of fauna between the two continents, but it had a profound effect on the Atlantic ocean, which was cut off from the much warmer Pacific.
Terrestrial Life During the Pliocene Epoch
Mammals. During large chunks of the Pliocene epoch, Eurasia, North America and South America were all connected by narrow land bridges--and it wasn't all that difficult for animals to migrate between Africa and Eurasia, either. This wreaked havoc on mammalian ecosystems, which were invaded by migrating species, resulting in increased competition, displacement and even outright extinction. For example, camels (like the huge Titanotylopus) migrated from North America to Asia, while the fossils of giant prehistoric bears like Agriotherium have been found in Eurasia, North America and Africa. Apes and hominids were mostly restricted to Africa, though there were scattered communities in Eurasia and North America as well.
The most dramatic evolutionary event of the Pliocene epoch was the appearance of a land bridge between North and South America. Previously, South America had been a lot like modern Australia, a giant, isolated continent populated by a variety of strange mammals, including giant marsupials. (Confusingly, some animals had already succeeded in traversing these two continents before the Pliocene via the arduously slow process of "island hopping"; that's how Megalonyx, the Giant Ground Sloth, wound up in North America.) The winners in this contest were the mammals of North America, which either wiped out or greatly diminished their southern relatives.
The late Pliocene epoch was also when some familiar megafauna mammals appeared on the scene, including the Woolly Mammoth in Eurasia and North America, Smilodon (the Saber-Toothed Tiger) in North and South America, and Megatherium (the Giant Sloth) and Glyptodon (a gigantic, armored armadillo) in South America. These plus-sized beasts persisted into the ensuing Pleistocene epoch, when they went extinct due to climate change and competition with modern humans.
Birds. The Pliocene epoch marked the swan song of the phorusrhacids, or "terror birds," as well as the other large, flightless, predatory birds of South America. One of the last terror birds, the 300-pound Titanis, actually managed to traverse the pan-American isthmus and populate southeastern North America; however, this didn't save it from going extinct by the start of the Pleistocene epoch.
Reptiles. Crocodiles, snakes, lizards and turtles all occupied a backseat during the Pliocene epoch (as they did during much of the Cenozoic Era). The biggest developments were the disappearance of alligators and crocodiles from Europe (which was now much too cool to support their cold-blooded lifestyles), and the appearance of some truly gigantic turtles, such as the aptly named Stupendemys of South America.
Marine Life During the Pliocene Epoch
As during the preceding Miocene, the seas of the Pliocene epoch were dominated by the biggest shark that ever lived, Megalodon. Whales continued on their evolutionary way, approaching the forms seen in modern times, and pinnipeds (seals, walruses and sea otters) all flourished in various parts of the globe. (An interesting side note: the pliosaurs of the Mesozoic Era were once thought to date from the Pliocene epoch, hence their misleading name, "Pliocene lizards.")
Plant Life During the Pliocene Epoch
There weren't any wild bursts of innovation in Pliocene plant life; rather, this epoch continued the trend seen during the preceding Oligocene and Miocene epochs, the gradual confinement of jungles and rain forests to equatorial regions, while deciduous forests and grasslands dominated higher northern and southern latitudes.
Next: the Pleistocene Epoch