Duchesnean

The Duchesnean North American Stage is a North American Land Mammal Age (NALMA), covering an age of around 41.4[1] to 37.0 million years ago. It corresponds to part of the Eocene epoch (specifically the Bartonian stage of the Middle Eocene and the earliest Priabonian or Late Eocene). The Duchesnean is preceded by the Uintan and followed by the Chadronian NALMA.[2]

Mammals which first appear in North America at the base of the Duchesnean include Hyaenodon (a large hyaenodont), Duchesneodus (a large brontothere), Duchesnehippus intermedius (an early horse), Amynodontopsis (an amynodont rhino), and Eotylopus (an oromerycid, an early relative of camels). Though there are major differences between early and late Duchesnean mammal faunas, the Duchesnean as a whole lacks formal subdivisions.[2]

The Middle Eocene Climatic Optimum (MECO) occured early in the Duchesnean, followed by a resumed cooling trend. This climate instability began to break up rainforests in some regions, such as California, where shrubbier plants became more dominant. North American primate diversity declined as their habitats became more open and seasonal, though predatory mammals diversified.[3] Hyaenodontines (large predators) and anthracotheriines (large herbivores related to hippos) immigrated into North America from Eurasia. Towards the end of the Duchesnean, mammal faunas are similar to the succeeding White River Fauna (Chadronian-Whitneyan). For example, merycoidodontids ("oreodonts", low-slung herbivores related to camels) begin to diversify around this time.[4]

Age

The oldest directly dated Duchesnean layer is the LaPoint ash in Utah, an ash bed which was dated to 39.74 ± 0.07 Ma in 1992.[5][2] This age was later recalibrated to 40.26 ± 0.08 Ma in 2012, and the actual start of the Duchesnean would be slightly older.[1] Based on magnetostratigraphy at early Duchesnean sites in the Sespe Formation of California, the Uintan-Duchesnean boundary may be within chron C19n (about 41.4 Ma).[1] In the Chambers Tuff of Texas, bracketing ash beds and magnetostratigraphy correlate the Duchesnean-Chadronian boundary to between 38-37 Ma, within chron C17n.[1][6]

Formations

The Duchesnean is named after the Duchesne River Formation in the Uinta Basin of Utah. The Duchesne River Formation's LaPoint Member is considered the type locality for the Duchesnean. Geological formations preserving Duchesnean fossils include:[2][1]

References

  1. 1 2 3 4 5 Kelly, T.S.; Murphey, P.C.; Walsh, S.L. (2012). "New records of small mammals from the middle Eocene Duchesne River Formation, Utah, and their implications for the Uintan–Duchesnean North American Land Mammal Age transition." Paludicola 8: 208–251.
  2. 1 2 3 4 Robinson, Peter; Gunnell, Gregg F.; Walsh, Stephen L.; Clyde, William C.; Storer, John E.; Stucky, Richard K.; Froehlich, David J.; Ferrusquia-Villafranca, Ismael; McKenna, Malcolm C. (2004), Woodburne, Michael O. (ed.), "4. Wasatchian Through Duchesnean Biochronology", Late Cretaceous and Cenozoic Mammals of North America, Columbia University Press, pp. 106–155, doi:10.7312/wood13040-006, ISBN 978-0-231-13040-0, JSTOR 10.7312/wood13040.10, retrieved 2026-08-29{{citation}}: CS1 maint: work parameter with ISBN (link)
  3. 1 2 Poust, Ashley W.; Barrett, Paul Z.; Tomiya, Susumu (2022). "An early nimravid from California and the rise of hypercarnivorous mammals after the middle Eocene climatic optimum". Biology Letters. 18 (10). doi:10.1098/rsbl.2022.0291. ISSN 1744-957X. PMC 9554728.
  4. ↑ Woodburne, Michael O. (2004), "8. Global Events and the North American Mammalian Biochronology", in Woodburne, Michael O. (ed.), Late Cretaceous and Cenozoic Mammals of North America, Columbia University Press, pp. 315–344, doi:10.7312/wood13040-005, ISBN 978-0-231-13040-0, JSTOR 10.7312/wood13040.14, retrieved 2026-06-06
  5. ↑ Prothero, Donald R.; Swisher III, Carl C. (1992), "2. Magnetostratigraphy and Geochronology of the Terrestrial Eocene-Oligocene Transition in North America", Eocene-Oligocene Climatic and Biotic Evolution, Princeton University Press, pp. 46–73, doi:10.1515/9781400862924.46, ISBN 978-1-4008-6292-4, retrieved 2026-08-29{{citation}}: CS1 maint: work parameter with ISBN (link)
  6. ↑ Nyborg, Torrey; Lander, E. Bruce (2022-03-14), "Vertebrate paleontology and Cenozoic depositional environments of Death Valley National Park, California, USA", Field Excursions from Las Vegas, Nevada: Guides to the 2022 GSA Cordilleran and Rocky Mountain Joint Section Meeting, Geological Society of America, pp. 1–22, doi:10.1130/2022.0063(01), ISBN 978-0-8137-0063-2, retrieved 2026-08-02



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