The Orellan is a North American Land Mammal Age typically set from around 33,700,000 to 32,000,000 years BP, a period of 1.7 million years. The Orellan is preceded by the Chadronian and followed by the Whitneyan NALMA stages. Relative to global geological chronology (the geologic time scale), it is usually considered to fall within the earliest part (early Rupelian stage) of the Oligocene epoch, beginning around the same time as the Eocene-Oligocene boundary.
The Orellan is named after the Orella Member of the Brule Formation, a widespread subunit of the White River Group. The Orella Member crops out most extensively in northwest Nebraska and eastern Wyoming. In the White River badlands of southwest South Dakota, an equally extensive Orellan component of the Brule Formation is known as the Scenic Member. Orellan strata of the Brule Formation can be found in more restricted exposures across northeast Colorado (Cedar Creek Member) and North Dakota. Certain sites in Montana preserve Orellan mammals: the Dunbar Creek Formation and the Cook Ranch and Matador Ranch local faunas of the Cook Ranch Formation. The Byram Formation of Mississippi also appears to be Orellan.
The beginning of the Orellan is defined by the first appearance of Hypertragulus calcaratus (a hypertragulid ruminant). Other mammals which first appear at the start of the stage include Leptomeryx evansi (a leptomerycid ruminant) and Palaeolagus intermedius (an early lagomorph). Poebrotherium eximium (an early camelid) and Miohippus grandis (an early horse) last appear around the base of the Orellan, and brontotheres go extinct only shortly above the base.
References
- ^ Prothero, Donald R.; Emry, Robert J. (2004). "5. The Chadronian, Orellan, and Whitneyan North American Land Mammal Ages". In Woodburne, Michael O. (ed.). Late Cretaceous and Cenozoic Mammals of North America: Biostratigraphy and Geochronology. Columbia University Press. pp. 156–168. doi:10.7312/wood13040-007. ISBN 978-0-231-13040-0.
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