Overview of the Pliocene Epoch

Prehistoric Life 5.3-2.6 Million Years Ago

Glyptodon skeleten in glass display case

Fievet/Wikimedia Commons/CC BY-SA 3.0

By the standards of "deep time," the Pliocene epoch was relatively recent, commencing only five million years or so before the start of the modern historical record, 10,000 years ago. During the Pliocene, prehistoric life around the globe continued to adapt to the prevailing climatic cooling trend, with some notable local extinctions and disappearances. The Pliocene was 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 from previous epochs, with tropical conditions holding at the equator (as they do today) and more pronounced seasonal changes at higher and lower latitudes; 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, and the formation of the Central American Isthmus joining North and South America. Not only did these developments allow an interchange of fauna between three of the earth's continents, but they had a profound effect on ocean currents, as the relatively cool Atlantic ocean 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, ancestral camels (like the huge Titanotylopus) migrated from North America to Asia, while the fossils of giant prehistoric bears like Agriotherium have been discovered in Eurasia, North America, and Africa. Apes and hominids were mostly restricted to Africa (where they originated), though there were scattered communities in Eurasia and North America.

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 much 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 epoch, by the arduously slow process of accidental "island-hopping"; that's how Megalonyx, the Giant Ground Sloth, wound up in North America. The ultimate winners in this "Great American Interchange" 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 (combined with hunting by) 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, which resembled meat-eating dinosaurs that had gone extinct tens of millions of years earlier (and count as an example of "convergent evolution.") One of the last surviving terror birds, the 300-pound Titanis, actually managed to traverse the Central 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 an evolutionary backseat during the Pliocene epoch (as they did during much of the Cenozoic Era). The most important developments were the disappearance of alligators and crocodiles from Europe (which had now become much too cool to support these reptiles' 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, the 50-ton Megalodon. Whales continued their evolutionary progress, approximating the forms familiar in modern times, and pinnipeds (seals, walruses, and sea otters) flourished in various parts of the globe. An interesting side note: the marine reptiles of the Mesozoic Era known as pliosaurs were once thought to date from the Pliocene epoch, hence their misleading name, Greek for "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 trends seen during the preceding Oligocene and Miocene epochs: the gradual confinement of jungles and rain forests to equatorial regions, while vast deciduous forests and grasslands dominated higher northern latitudes, especially in North America and Eurasia.