Aclistomycter

Aclistomycter; Temporal range: 38–37 Ma PreꞒ Ꞓ O S D C P T J K Pg N Pal. Eocene Oligo. Miocene P P Late Eocene (late Duchesnean – earliest Chadronian)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Family:	†Merycoidodontidae
Subfamily:	†Aclistomycterinae; Lander, 1998
Genus:	†Aclistomycter; Wilson, 1971
Type species
	Aclistomycter middletoni; Wilson, 1971
Other species
	Aclistomycter? dunagani (Wilson, 1971);

Aclistomycter is an extinct genus of oreodont from the late Eocene of Texas and Mexico. It is the oldest known member of Merycoidodontidae (the family containing most oreodonts), and its distinctive qualities allow it to occupy its own monotypic subfamily, Aclistomycterinae. Aclistomycter was a small herbivorous artiodactyl with a short face and small, tusk-like canine teeth. Though small by the standards of Oligocene-Miocene oreodonts, it was somewhat larger than other late Eocene oreodonts.

The type species Aclistomycter middletoni was first reported from the Porvenir local fauna in the Chambers Tuff Formation, near Adobe Springs in Presidio County, Texas. This site is late Duchesnean in age (about 38 Ma). It has also been reported from the Rancho Gaitan site in Chihuahua, Mexico. A second potential species, Aclistomycter? dunagani, occurs in the nearby Little Egypt local fauna, which is slightly younger (earliest Chadronian, about 37 Ma).

Diagnosis

The type specimen consists of a skull and jaws.

Generic characters assigned by Wilson (1971):

Medium-small merycoidodontid with a very large and deep, probably perforated, antorbital fossae (a depression in front of the eyes).
Brachycephalic (with a short, broad snout)
Posterior part of skull extended.
Molar teeth with thick enamel and deep fossettes.
Posterior base of zygomatic processes wide, making the skull broad posteriorly.
Bullae thought to be inflated (though the bullae are not exposed in the figures of the type specimen illustrated by Wilson).

References

1 2 Lander, B. (1998). "Oreodontoidea". In Janis, C.M.; Scott, K.M.; Jacobs, L.L. (eds.). Evolution of Tertiary mammals of North America. Cambridge: Cambridge University Press. pp. 402–425. ISBN 0-521-35519-2.
1 2 Stevens, Margaret Skeels; Stevens, James Bowie (2007). "12. Family Merycoidodontidae". In Prothero, Donald R.; Foss, Scott E. (eds.). The Evolution of Artiodactyls. Baltimore: The Johns Hopkins University Press. pp. 157–168. doi:10.56021/9780801887352. ISBN 9780801887352.
1 2 Wilson, John A. (1971). "Early Tertiary Vertebrate Faunas, Vieja Group, Trans-Pecos Texas, Agriochoeridae and Merycoidodontidae". Bulletin of the Texas Memorial Museum, University of Texas at Austin. 18: 1–83.
↑ 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.
↑ 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

[Lander98-1] 1 2 Lander, B. (1998). "Oreodontoidea". In Janis, C.M.; Scott, K.M.; Jacobs, L.L. (eds.). Evolution of Tertiary mammals of North America. Cambridge: Cambridge University Press. pp. 402–425. ISBN 0-521-35519-2.

[:0-2] 1 2 Stevens, Margaret Skeels; Stevens, James Bowie (2007). "12. Family Merycoidodontidae". In Prothero, Donald R.; Foss, Scott E. (eds.). The Evolution of Artiodactyls. Baltimore: The Johns Hopkins University Press. pp. 157–168. doi:10.56021/9780801887352. ISBN 9780801887352.

[:1-3] 1 2 Wilson, John A. (1971). "Early Tertiary Vertebrate Faunas, Vieja Group, Trans-Pecos Texas, Agriochoeridae and Merycoidodontidae". Bulletin of the Texas Memorial Museum, University of Texas at Austin. 18: 1–83.

[:02-4] 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.

[:12-5] 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