Diacodexis

Taxonomy

Class: Mammalia

Order: Artiodactyla

Family: DIACODEXEIDAE

Genus: Diacodexis

Species: Diacodexis antunesi, D. gracilis, D. ilicis, D. indicus, D. metsiacus, D. minutus, D. pakistanensis, D. secans

When and where Diacodexis lived: Diacodexis first appeared at the beginning of the Eocene 55.4 million years ago and lived until about 50 million years ago in North America, Europe, and Asia. It’s first appearance represents the beginning of the Eocene.

General Description

Diacodexis is the most primitive and earliest known artiodactyl (or even-toed ungulate) (Rose et al., 2005).   It had digitigrade feet with a hoof on each toe.  Diacodexis was an herbivore/omnivore about the size of a large rabbit and may have resembled the Asian mouse deer (Rose, 2001), with elongated hind limbs.  The modern group of artiodactyls includes animals such as camels, giraffes, deer, sheep, hippopotamuses and pigs.  It’s teeth, specifically the molars, are described as “bunodont”, being rounded with low crowns. 

Diacodexis lived in a paratropical to tropical rainforest which included evergreens, deciduous dicots, vines, palms, fern trees and ferns (Willis et al, 2002).  Diacodexis would have eaten fruit, seeds, and leafy vegetation and may have included insect protein in it’s diet (Stucky, 1998).  This and the lack of a ruminant digestive physiology may have played a part in it’s extinction when temperatures began to cool and the level of CO2 began to drop about 50 million years ago causing a change in the amount and quality of the vegetation (Janis, 2007).

Diacodexis was most likely prey for carnivores such as the cat-like oxyaenids Oxyaena and Malfelis or the wolf-like hyaenodonts, Tritemnodon and Gazinocyon.  With a double-pulley astralagus and long slender hind limbs, Diacodexis would have been a swift runner and an adept leaper (Rose, 1982).

Diacodexis or an animal closely related to it was likely the ancestor of all artiodactyls, having the most primitive dentition of all known artiodactyls.  Advanced species of Diacodexis were probably ancestral to the Eocene and Oligocene Leptochoeridae which lived in North America.  Diacodexis secans provides us with one of the most well documented examples of evolutionary change within a species over about 5 million years during the early Eocene (Krishtalka and Stucky, 1985).

Dentition:  “Metaconid moderately inflated; paraconid generally distinct but reduced especially on m2-3; hypoconulid on posterior ridge of talonid between entoconid and hypoconid and not on postcingulum; upper molars triangular with a central protocone, generally lacking hypocone and mesostyle; parconule, metaconule, and conular wings generally well developed; protocone, paracone, and metacone bulbous and distinct. Trend toward elongation of P3/3 relative to P4/4, bunodonty of molars and premolars, and inflation of metaconid.  Average length of M2: 3.7-5.3 mm.”  (Stucky, 1998)

Bibliography:

Janis, Christine M. 2007.  Artiodactyl Paleoecology and Evolutionary Trends; pp. 292-302 in Prothero, Donald R., and Scott E. Foss.  (eds.), The evolution of artiodactyls. Baltimore: Johns Hopkins University Press.

Krishtalka, Leonard, and Richard K. Stucky. 1985.  Revision of the Wind River Faunas, Early Eocene of Central Wyoming. Part 7.  Revision of Diacodexis (Mammalia, Artiodactyla).  Annals of Carnegie Museum 54:413-486.

Rose, Kenneth David. 1982.  Skeleton of Diacodexis, Oldest Known Artiodactyl. Science 216:621-623.
Rose, Kenneth David. 2001.  Wyoming’s Garden of Eden.  Natural History, April 55-59.

Rose, Kenneth David, and J. David Archibald. 2005. The rise of placental mammals: origins and relationships of the Major Extant  Clades. Baltimore: Johns Hopkins University Press.

Rose, Kenneth David.  2006. The beginning of the age of mammals.  Baltimore: Johns Hopkins University Press.

Sinclair, William John. 1914. A revision of the bunodont Artiodactyla of the Middle and Lower Eocene of North America. New York:  Published by order of the Trustees, American Museum of Natural History.

Stucky, Richard K. 1998.  Eocene bunodont and bunoselenodont Artiodactyla (“dichobunids”); pp. 358-374 in Janis, Christine M.,  Kathleen M. Scott, and Louis L. Jacobs (eds.), Evolution of Tertiary mammals of North America. Cambridge, UK:  Cambridge  University Press.

Willis, K. J., and J. C. McElwain. 2002. The evolution of plants.  New York: Oxford University Press.
 

Authored by Gloria Carbaugh

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