Orellan

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.¹ 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.¹

The beginning of the Orellan is defined by the first appearance of Hypertragulus calcaratus (a hypertragulid ruminant). Other mammal species 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.¹

Orellan North America was cooler and drier than previous ages, though the middle of the continent was still a stable environment populated by the White River fauna. The Great Plains and Rocky Mountain regions were covered by temperate deciduous and conifer woodlands rather than the humid subtropical floras which characterized the Eocene.¹

Subdivisions

The Orellan is considered to contain the following four subdivisions, from oldest to youngest:¹

Orellan 1: Starting at the first appearance of Hypertragulus calcaratus and ending at the first appearance of Miniochoerus affinis.
Orellan 2: Starting at the first appearance of Miniochoerus affinis and ending at the first appearance of Miniochoerus gracilis.
Orellan 3: Starting at the first appearance of Miniochoerus gracilis and ending at the first appearance of Merycoidodon bullatus.
Orellan 4: Starting at the first appearance of Merycoidodon bullatus and ending at the first appearance of Leptauchenia major.

Fossil localities

The Orellan is named after the Orella Member of the Brule Formation, a widespread subunit of the White River Group.²¹

Colorado:
- White River Formation (Cedar Creek Member)
Mississippi:
- Byram Formation
Montana:
- Clarkston Basin, Three Forks Basin, South Townsend Basin, Jefferson Basin: Dunbar Creek Formation (= Renova Formation Dunbar Creek Member) (Easter Lily local fauna)³
- Sage Creek Basin: Cook Ranch Formation (= Renova Formation Cook Ranch Member) (Cook Ranch, Matador Ranch local faunas)³
Nebraska:
- White River Group: Brule Formation (Orella Member B-D)
North Dakota:
- White River Group: Brule Formation
Saskatchewan:
- Cypress Hills Formation (Fossil Bush local fauna)⁴
South Dakota:
- White River Group: Brule Formation (Scenic Member)
Wyoming:
- White River Group: Brule Formation (Orella Member)
- White River Formation
- Tepee Trail Formation (Hendry Ranch Member) (Cedar Ridge local fauna)⁵

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. JSTOR 10.7312/wood13040.11.
Prothero, Donald R.; Whittlesey, Karen E. (1998), "Magnetic stratigraphy and biostratigraphy of the Orellan and Whitneyan land-mammal "ages" in the White River Group", Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River and Arikaree Groups (Late Eocene to Early Miocene, North America), Geological Society of America, doi:10.1130/0-8137-2325-6.39, ISBN 978-0-8137-2325-9, retrieved 2026-06-08{{citation}}: CS1 maint: work parameter with ISBN (link)
Tabrum, Alan R.; Prothero, Donald R.; Garcia, Daniel (1996-06-13), Prothero, Donald R.; Emry, Robert J. (eds.), "Magnetostratigraphy and biostratigraphy of the Eocene-Oligocene transition, southwestern Montana", The Terrestrial Eocene-Oligocene Transition in North America (1 ed.), Cambridge University Press, pp. 278–311, doi:10.1017/cbo9780511665431.015, ISBN 978-0-521-43387-7, retrieved 2026-06-08{{citation}}: CS1 maint: work parameter with ISBN (link)
Storer, John E. (1996-06-13), Prothero, Donald R.; Emry, Robert J. (eds.), "Eocene-Oligocene faunas of the Cypress Hills Formation, Saskatchewan", The Terrestrial Eocene-Oligocene Transition in North America (1 ed.), Cambridge University Press, pp. 240–261, doi:10.1017/cbo9780511665431.013, ISBN 978-0-521-43387-7, retrieved 2026-06-08{{citation}}: CS1 maint: work parameter with ISBN (link)
Korth, William W. (2018). "Review of the marsupials (Mammalia: Metatheria) from the late Paleogene (Chadronian–Arikareean: late Eocene–late Oligocene) of North America". PalZ. 92 (3): 499–523. doi:10.1007/s12542-017-0396-y. ISSN 0031-0220.

[:0-1] 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. JSTOR 10.7312/wood13040.11.

[2] Prothero, Donald R.; Whittlesey, Karen E. (1998), "Magnetic stratigraphy and biostratigraphy of the Orellan and Whitneyan land-mammal "ages" in the White River Group", Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River and Arikaree Groups (Late Eocene to Early Miocene, North America), Geological Society of America, doi:10.1130/0-8137-2325-6.39, ISBN 978-0-8137-2325-9, retrieved 2026-06-08{{citation}}: CS1 maint: work parameter with ISBN (link)

[:1-3] Tabrum, Alan R.; Prothero, Donald R.; Garcia, Daniel (1996-06-13), Prothero, Donald R.; Emry, Robert J. (eds.), "Magnetostratigraphy and biostratigraphy of the Eocene-Oligocene transition, southwestern Montana", The Terrestrial Eocene-Oligocene Transition in North America (1 ed.), Cambridge University Press, pp. 278–311, doi:10.1017/cbo9780511665431.015, ISBN 978-0-521-43387-7, retrieved 2026-06-08{{citation}}: CS1 maint: work parameter with ISBN (link)

[4] Storer, John E. (1996-06-13), Prothero, Donald R.; Emry, Robert J. (eds.), "Eocene-Oligocene faunas of the Cypress Hills Formation, Saskatchewan", The Terrestrial Eocene-Oligocene Transition in North America (1 ed.), Cambridge University Press, pp. 240–261, doi:10.1017/cbo9780511665431.013, ISBN 978-0-521-43387-7, retrieved 2026-06-08{{citation}}: CS1 maint: work parameter with ISBN (link)

[5] Korth, William W. (2018). "Review of the marsupials (Mammalia: Metatheria) from the late Paleogene (Chadronian–Arikareean: late Eocene–late Oligocene) of North America". PalZ. 92 (3): 499–523. doi:10.1007/s12542-017-0396-y. ISSN 0031-0220.

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