2.59 to 1.81 million years ago
The Gelasian is an age in the international geologic timescale or a stage in chronostratigraphy, being the earliest or lowest subdivision of the Quaternary period/system and Pleistocene epoch/series. It spans the time between 2.588 ± 0.005 Ma and 1.806 ± 0.005 Ma.. It follows the Piacenzian stage (part of the Pliocene) and is followed by the Calabrian stage.
During the Gelasian the Red Crag of Butley and Newbourn and the Norwich and Weybourn Crags, all from East Anglia (England) were deposited. The Gelasian is an equivalent of the Praetiglian and Tiglian stages as defined in the Netherlands, which are commonly used in northwestern Europe.
The Gelasian was introduced in the geologic timescale in 1998. It is named after the Italian city of Gela in southern Sicily. In 2009 it was moved from the Pliocene to the Pleistocene in order that the geologic time scale be more consistent with the key changes in Earth's climate, oceans, and biota that occurred 2.588 million years ago.
The base of the Gelasian is defined magnetostratigraphically as the base of the Matuyama (C2r) chronozone (at the Gauss-Matuyama magnetostratigraphic boundary), isotopic stage 103. Above this point there are notable extinctions of the calcareous nannofossils: Discoaster pentaradiatus and Discoaster surculus. The GSSP for the Gelasian is located at the Monte Sant Nicola near Gela.
The top of the Gelasian is defined magnetostratigraphically as the end of the Olduvai (C2n) chronozone, and faunally as the extinction level of the calcareous nannofossil Discoaster brouweri (base of biozone CN13). Above the Gelasian as the first occurrences of the calcareous nannofossil Gephyrocapsa sp. and the extinction level of the planktonic foraminifer Globigerinoides extremus.
During the Gelasian the ice sheets in the Northern Hemisphere began to grow, which is seen as the beginning of the Quaternary ice age. In part this may be related to the fact that the Isthmus of Panama was now closed for good. This caused a number of serious changes. One change pertains to pattern of oceanic currents. The connection between the Pacific and the Atlantic through the warm waters of the Caribbean was severed and the only connections were in the polar regions. This made the current around Antarctica more prominent and this aided in engulfing the entire continent in ice, pretty much extinguishing its native fauna.
Another consequence was that the gene flow between oceanic biota at either side of the isthmus was severed and they started to evolve into different species.
On land, the species of North and South America had started to penetrate in each others territory, a process known as the Great American Exchange. Modern carnivores moved from north to south. The led to the demise of the many species of marsupial carnivores of the south.
1. GeoWhen Database - Gelasian December 2007, from Internet archive
2. The Gelasian was first proposed by Rio et al. (1998)
3. Gradstein et al. (2005), p. 28; Rio et al. (1998)
4. Gradstein et al. (2005); Rio et al. (1998)
- Gradstein, F.M.; Ogg, J.G. & Smith, A.G. (eds.) (2005) A Geologic Time Scale 2004 Cambridge University Press, Cambridge, UK, ISBN 0-521-78142-6.
- Rio, D., R. Sprovieri, D. Castradori, and E. Di Stefano, 1998. The Gelasian Stage (Upper Pliocene): A new unit of the global standard chronostratigraphic scale. Episodes, 21(2): 82-87.