Updated July 18, 2018 with a new nesting for Docofossor with two other digging marsupials, Notoryctes and Anebodon.
Docofossor brachydactylus (Luo et al. 2015; Jurassic, 160 mya; BMNH 131735; 9 cm in precaudal length) was originally considered a member of the Docodontidae (Docodonta) along with Docodon and Haldanodon (and other taxa listed in figure 4), nesting outside of the Mammalia, chiefly based on their relatively ‘sophisticated(?)’ molar shape. Here Docofossor nests as a sister to the fossorial Early Cretaceous Anebodon and the extant fossorial (digging) Notoryctes (marsupial mole). There is ample opportunity for Docofossor to nest outside of the Mammalia, but it does not do so in the large reptile tree (LRT, 1251 taxa, Subset Fig. 4).
Broad, short-fingered hands,
larger than the feet, along with other traits mark Docofossor as a digging animal, similar to moles like the carnivore Talpa and and rodent-like Chrysochloris. So the Jurassic is the first time that mole-like mammals evolved. The two others followed later.
Perhaps the main reason
for not including docodonts within the Mammalia is their possession of a medial groove on the posterior dentary (mandible) that should contain tiny, splint-like posterior jaw elements, like the angular, surangular and articular. These splints are rarely if ever found, but are assumed to produce the jaw joint in lieu of or alongside the squamosal/dentary joint common to all mammals. Sometimes the grooves remain after the splints are gone.
And here’s the subset of the LRT
(Fig. 4) showing where Docofossor nests within the Metatatheria. Several included members of the Docodonta are split up into distinct clades, indicating that this putative clade is paraphyletic.
Luo Z-X, Meng QJ, Ji Q, Liu D, Zhang Y-G, Neande AI 2015.Evolutionary development in basal mammaliaforms as revealed by a docodontan. Science. 347 (6223): 760–764.