Piacenzian Stage

What happened during this time?

Geophysical

• Started somewhat wet and warm period in North America

• Ocean circulation patterns similar to present

• Run-up to the first ice age

  • Ice sheets accumulate

  • High latitude temperatures dropped

• This drying trend was even more pronounced in East Africa, where the global aridification was augmented by regional uplifts (caused by the cracks which appeared in the African craton) casting rain shadows over parts of the area (Partridge et al. 1995).

Biological

Fauna

• Age of "Lucy," Australopithecus afarensis, and the radiation of A. robustus and A. gracile hominids which led to the genus Homo

  • Homo habilis appears 2.8 Ma

• In Europe, mammoth, tapirs, and the little bear, Ursus minimus, died out, which were adapted to temperate or subtropical climates and open woodlands.

• Piacanzian species replaced in the Gelasian by species more suited for colder and more open environments

• "Elephant-Equus Event," named for the conspicuous change from Piacenzian elephants to Gelasian horses as the dominant open-country ungulate (Azzaroli 1995)

  • This event is reflected by rapid changes in pollen, foraminifera, and mollusk indicators from the same time

• Faunal changes in Americas were marked by the Great American Interchange related to the formation of the interamerican land bridge. The formation of the Isthmus of Panama allowed the long-isolated North and South American vertebrates to mix. The land bridge allowed the armadillo, porcupine, opossum, and ground sloths to colonize the north, as well as the unique South American notoungulates. The sloths and, to a lesser extent, the notoungulates, prospered, but are now extinct. The others were initially restricted to the most southerly parts of North America but are now expanding their ranges. The Northern immigrants to South America included the rodents, Carnivora (bears, cats, dogs, etc.), llamas and horses, bovids (particularly deer), and the tapirs and elephants.

• Our understanding of the Interchange has changed greatly in recent years. It was originally viewed as a conquest of the South by the North. It now appears to have been a more even-handed and less catastrophic event. Many of the extinctions originally attributed to the Interchange, particularly the unique endemic South American ungulates, occurred either earlier, in the Late Miocene, or later, in the Pleistocene.