Didymoconus: now a little Hyaenodon

Baby and juvenile mammals typically develop allometrically.
In other words, they tend to have short snouts and large eyes = cute features. Juvenile mammals also retain their milk teeth, replacing teeth only once during their lifetime.

By contrast, the following two mammals
(Figs 1, 2) do not have juvenile traits. They are similar to, but much smaller than their closest LRT relatives – and yet have adult teeth and adult proportions. Both small taxa chronologically preceded their larger relatives. So they could be ancestors.

Figure 1. Hyaenodon species + Didymoconus colgati and D berkeyi to scale. ” data-image-caption=”

Figure 1. Hyaenodon species + Didymoconus colgati and D berkeyi to scale.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/hyaenodon-faces588-1.jpg?w=143″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/hyaenodon-faces588-1.jpg?w=487″ class=”size-full wp-image-90902″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/hyaenodon-faces588-1.jpg” alt=”Figure 1. Hyaenodon species + Didymoconus colgati and D berkeyi to scale. ” width=”584″ height=”1229″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/hyaenodon-faces588-1.jpg?w=584&h=1229 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/hyaenodon-faces588-1.jpg?w=71&h=150 71w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/hyaenodon-faces588-1.jpg?w=143&h=300 143w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/hyaenodon-faces588-1.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Hyaenodon species + Didymoconus colgati and D berkeyi to scale. Note the longer skull of D  colgatei reflecting a longer mandible not shown in the original tracing. See figure 2.

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.

Figure 2. A new reconstruction of Didymoconus colgatei with the addition of the mandible. ” data-image-caption=”

Figure 2. A new reconstruction of Didymoconus colgatei with the addition of the mandible.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/didymoconus_colgatei_588.jpg?w=205″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/didymoconus_colgatei_588.jpg?w=584″ class=”size-full wp-image-90894″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/didymoconus_colgatei_588.jpg” alt=”Figure 2. A new reconstruction of Didymoconus colgatei with the addition of the mandible.” width=”584″ height=”856″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/didymoconus_colgatei_588.jpg?w=584&h=856 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/didymoconus_colgatei_588.jpg?w=102&h=150 102w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/didymoconus_colgatei_588.jpg?w=205&h=300 205w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/didymoconus_colgatei_588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 2. A new reconstruction of Didymoconus colgatei with the addition of the mandible.

Didymoconus berkeyi
(Matthew WD and Granger W 1924. Gingerich 1981, Wang X et al 2001; IVPP V11983, Eocene 52mya) is a former enigma taxon here nesting with Hyaenodon. The zygomatic arches are wider posteriorly and the orbits are dorsally open.

Hyaenodon leptorhynchus
(Laizer and Parieu 1838; Scott 1895; Eocene to Miocene) (Laizer and Parieu 1838; Scott 1895; Eocene to Miocene) was traditionally considered a credont and a placental that hunted with its saberteeth. It was derived from the Didymoconus specimens (Fig 1).

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. (I have not seen these yet).
There were several mid-sized taxa, too.

Earlier
a Didymoconus specimen entered the LRT with closely related Mesonyx. That minor move is rectified here. Earlier authors considered Didymoconus to be enigmatic, but considered Hyaenodon a good candidate.

References
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).
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.

wiki/Hyaenodon
wiki/Didymoconus