The Chadronian is a

North American Land Mammal Age The North American land mammal ages (NALMA) establishes a geologic timescale for North American fauna beginning during the Late Cretaceous and continuing through to the present. These periods are referred to as ages or intervals (or stages when ref ...

typically set from around 37,000,000 to 33,700,000 years BP, a period of . The Chadronian is preceded by the Duchesnean and followed by the Orellan NALMA stages. Relative to global geological

chronology Chronology (from Latin , from Ancient Greek , , ; and , ''wikt:-logia, -logia'') is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence of events. It is also "the deter ...

(the

geologic time scale The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochro ...

), it is usually considered to fall within the later part (

Priabonian The Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene Epoch or Series. It spans the time between . The Priabonian is preceded by the Bartonian and is followed by the Rupelian, the lowest stage ...

stage) of the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

epoch, ending around the same time as the Eocene-

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

boundary. The Chadronian is named after the Chadron Formation, a widespread component of the White River Group in

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and

South Dakota South Dakota (; Sioux language, Sioux: , ) is a U.S. state, state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota people, Dakota Sioux ...

. The most well-studied examples of Chadronian strata in the White River Group/Formation are the Flagstaff Rim area of eastern

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

and the Toadstool Park area of northwest Nebraska. The beginning of the Chadronian is defined by the first appearance of '' Bathygenys'' (a merycoidodontid, or " oreodont"). Other mammals which first appear at the start of the stage include '' Merycoidodon dunagani'' (a merycoidodontid), '' Brachyrhynchocyon dodgei'', (a daphoenine amphicyonid, or "bear dog"), and '' Archaeotherium'' (an entelodont). The early horse '' Mesohippus'' appears only slightly before the start of the Chadronian.

Subdivisions

Four subdivisions of the Chadronian were informally proposed by Prothero and Emry (1996), later formalized by the same authors in 2004. Type assemblages (diagnostic fossil faunas used to define the subdivision) are bolded and underlined. From youngest to oldest: Part of the Cypress Hills Formation (Saskatchewan, Canada) is also considered to be Chadronian in age. In Colorado, Chadronian mammal faunas are found in the Florissant Formation, the Antero Formation, and the White River Formation in the Kings Canyon area. In New Mexico, the Conejos, Espinaso, Spears, Sanders Canyon, Bell Top, upper Rubio Peak, and upper Palm Park formations all overlap with the Chadronian NALMA. The Iniyoo local fauna (

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, Mexico) was initially reported to be Chadronian, though revised dating suggests that it is

Arikareean The Arikareean North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 30,600,000 to 20,800,000 years BP, a period of . It is usuall ...

(late Oligocene) instead.

References

Eocene life Eocene animals of North America {{geochronology-stub