The smaller (hamster-sized) type specimen of Didymoconus enters the LRT

According to Meng, Suyin and Schiebout 1994,
“Didymoconid relationships have long been a problem, which is fully reflected by the fact that has been related to about a dozen mammalian groups.”

That list includes placental and marsupial clades.

According to Wang et al 2001,
“The family Didymoconodontidae is an Asian endemic group of archaic mammals of uncertain affinities in the Eocene through Oligocene.”

According to Wikipedia – Didymoconus:
“Didymoconus is an extinct genus of mammal that lived during the early Oligocene epoch. It was endemic to Asia, and its fossils have been found in Mongolia, China and Kazakhstan. It is the type genus of the Didymoconidae, a family of eutherian mammals with uncertain taxonomic affinities.”

Figure 1. Didymoconus colagati and Didymoconus berkeyi at full scale on 72 dpi monitors. ” data-image-caption=”

Figure 1. Didymoconus colagati and Didymoconus berkeyi at full scale on 72 dpi monitors.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/11/didymoconus_colgatei_588-1.jpg?w=116″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/11/didymoconus_colgatei_588-1.jpg?w=395″ class=”size-full wp-image-89649″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/11/didymoconus_colgatei_588-1.jpg” alt=”Figure 1. Didymoconus colagati and Didymoconus berkeyi at full scale on 72 dpi monitors.” width=”584″ height=”1513″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/11/didymoconus_colgatei_588-1.jpg?w=584&h=1513 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/11/didymoconus_colgatei_588-1.jpg?w=58&h=150 58w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/11/didymoconus_colgatei_588-1.jpg?w=116&h=300 116w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/11/didymoconus_colgatei_588-1.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Hamster-sized Didymoconus colagati and Didymoconus berkeyi at full scale on 72 dpi monitors. These are phylogenetically miniaturized descendants of hippo-like Mesonyx (see figure 2).

Despite their size and morphology differences
the two species assigned to Didymoconus (Fig 1) nest together alongside the much larger taxon, Mesonyx (Fig 2) in the large reptile tree (LRT, 2325 taxa).

These three taxa
(Figs 1, 2) have bounced back and forth in the LRT along with their enclosing clade. No longer nesting with hippos, in the LRT these mesonychds nest with other creodonts, the proximal outgroup for the Placentalia. No creodonts are extant, so they may also be placental in reproduction. This is a mystery that might continue forever.

Note the far posterior location of the jaw joint,
leaving little to no lateral view room for the auditory bulla. Nevertheless, the otic chambers are not reduced in size. They’re just tucked under the small braincase, Note the appearance of the complete straight narrow vomers (purple in figure 2) at the palatal mid-line between the maxillae and palatines. In most mammals the vomers are not visible in palatal view, other than their anterior and posterior ends, as they extend above the sutured or fused maxillae and palatines.

Unusually,
this case of phylogenetic miniaturization occurred at the very end of a lineage. All prior members were larger. Didymoconids were neotonous mesonychids, so they provide some clues as to how weaned juvenile mesonychids might have appeared.

References
Matthew WD and Granger W 1924. New Carnivora from the Tertiary of Mongolia. American Museum Novitates (104).
Meng J, Suuin T and Schiebout JA 1994. The Cranial Morphology of an Early Eocene Didymoconid (Mammalia, Insectivora). Journal of Vertebrate Paleontology, 14(4):534–551.
Wang X, Downs W, Xie J and Xie G 2001. Didymoconus (Mammalia: Didymoconidae) from Lanzhou Basin, China and its stratigraphic and ecological significance. Journal of Vertebrate Paleontology 21, 555-564.

wiki/Didymoconus