Oligocene
The Oligocene (ˈɒlᵻɡəsiːn,_-ɡoʊ- ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present (33.9 to 23.03Ma). As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from the Ancient Greek ὀλίγος (olígos, "few") and καινός (kainós, "new"), and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period. The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene. Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt. The start of the Oligocene is marked by a notable extinction event called the Grande Coupure; it featured the replacement of European fauna with Asian fauna, except for the endemic rodent and marsupial families. By contrast, the Oligocene–Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene. The lower boundary of the Oligocene (its Global Boundary Stratotype Section and Point or GSSP) is placed at the last appearance of the foraminiferan genus Hantkenina in a quarry at Massignano, Italy. However, this GSSP has been criticized as excluding the uppermost part of the type Eocene Priabonian Stage and because it is slightly earlier than important climate shifts that form natural markers for the boundary, such as the global oxygen isotope shift marking the expansion of Antarctic glaciation (the Oi1 event).