Didymoconus colgatei (Matthew WD and Granger W 1924. Gingerich 1981, Eocene 52mya) is a former enigma taxon here nesting with Hyaenodon. Note the four relatively huge lateral teeth. Phylogenetic miniaturization is present in this tiny adult.

Hyaenodon leptorhynchus (Laizer and Parieu 1838; Scott 1895; Eocene to Miocene) was traditionally considered a credont and a placenta that hunted with its saberteeth. This was the type species.

H. gigas was the size of a bear (est. 1,100 lbs, 500 kgs, 3 m).
H. horridus was the size of a large dog (est. 88 lbs, 40 kgs).
H. microdon and H. mustelinus were tiny.
There were several mid-sized taxa, too.

Harpagolestes uintensis (Type H. macrocephalus Wortman 1901; Scott 1888, Szalay and Gould 1966; AMNH 1892, 1878; Eocene, 51 mya; 50 cm skull length) is widely considered to be a large, hyaena-like mesonychid, like Mesonyx, but with a deeper and wider skull. Here Harpagolestes is derived from a sister to Mesonyx and basal to Hippopotamus, all plant eaters. The teeth are often heavily to extremely worn, as is typical of herbivores. A diastema just develops around the anterior premolars. The jugal has the genesis of a postorbital bar to meet that of the frontal, which is complete in Hippopotamus. Stout limb bones indicate Harpagolestes was not able to pursure prey

Earlier mesonychids. like Yangtanglestes (= Lestes), extend to the early Paleocene, shortly after the asteroid impact.

Note: Achaenodon is not related to Harpagolestes, but represents a case of convergence.

Callistoe vincei (Babot et al. 2002; Argot and Babot 2011; Early Eocene, 35mya; PVL 4187) was originally considered a probohyaenid. Here it nests with Hyaenodon, sharing a descending jugal among other traits. Both are considered Sparassodonta (Ameghino 1894), but that appears to be a junior synonym of Creodonta (Cope 1875).

Traditional traits are focused on the teeth, such as the number of incisors and the open-rooted canines. The first molar (of four) is here reduced to three tiny roots.

The axial skeleton of Callistoe is characterized by the peculiar transverse processes of the cervical vertebrae emphasizing lateral instead of sagittal traction. There is no clavicle and eighteen thoracolumbar vertebrae, of which only five are lumbars. The pollex (= thumb) is not reduced and the ungual phalanges indicate very long claws, similar to those observed in some extant digging taxa. The pes is smaller than the manus. Pedal digits 1 and 5 are more slender. Callistoe was the largest mammalian predator of its time.

Borhyaena tuberata (Ameghino 1887; Early Miocene, 20mya; 50 cm at the shoulder; PU 015701) nests as a derived member of the Hyaenodon clade. It was similar to living hyenas. The lateral view skull image (upper left) had to be broken in order connect the glenoid to the jaw joint.

Argot C and Babot J 2011. Postcranial morphology, functional adaptations and palaeobiology of Callistoe vincei, a predaceous metatherian from the Eocene of Salta, North-Western Argentina. Palaeontology 54(2):447–480.

Babot MJ, Powell JE and Muizon C 2002. Callistoe vincei, a new Proborhyaenidae (Borhyaenoidea, Metatheria, Mammalia) from the Early Eocene of Argentina. Geobios 35:615-629.

Gingerich PD 1981. Radiation of Early Cenozoic Didymoconidae (Condylarthra, Mesonychia) in Asia, with a New Genus from the Early Eocene of Western North America. Journal of Mammalogy 62(3):526-538.

Laizier L and de Parieu J 1838. Description et determination d’une machoire fossile appartenant a un mammifere jusqu’a pressent inconnu, Hyaenodon leptorhynchus. Comptes-rendus hebdomadaires des séances de l’Académie des Sciences, Paris 7:442.

Matthew WD and Granger W 1924. New Carnivora from the Tertiary of Mongolia. American Museum Novitates (104).

Scott WM 1895. The osteology of Hyaenodon. Academy Natural Sciences Philadelphia Journal 9:499-536. online here.

Szalay FS and Gould SJ 1966. Asiatic mesonychidae (Mammalia, Condylarthra). Bulletin of the American Museum of Natural History 132(2):127–174.

Wang X et al 2001. Didymoconus (Mammalia: Didymoconidae) from Lanzhou Basin, China and its stratigraphic and ecological significance. Journal of Vertebrate Paleontology. 21 (3): 555–564.

Wortman JL 1901. Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum. The American Journal of Science, series 4 12(70):281-296.

wiki/Hyaenodon
wiki/Callistoe
wiki/Harpagolestes
wiki/Didymoconus