A Jurassic digger, Docofossor, hints at an extinct paraplacental clade

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 arboreal Volaticotherium and the extant Monodelphis at the base of the Placentalia (Eutheria). Perhaps the reversion to a more primitive molar shape is a derived trait in this relatively newly formed clade. There is ample opportunity for Docofossor to nest outside of the Mammalia, but it does not do so in the large reptile tree (LRT, Fig. 4).

Figure 1. Original reconstruction of Docofossor and original reconstruction of its jaws and teeth along with DGS tracing of skeletal elements.

Figure 1. Original reconstruction of Docofossor and original reconstruction of its jaws and teeth along with DGS tracing of skeletal elements.

Broad, short-fingered hands,
larger than the feet, along with other traits mark Docofossor as a digging animal, similar to moles like Talpa and Chrysochloris. So this is at least the third time that mole-like placentals evolved and that does not count the marsupial mole or any of the burrowing lizards.

Figure 2. Docofossor partly reconstructed from DGS tracing in figure 1. Here a long tail is recovered along with a few other bones.

Figure 2. Docofossor partly reconstructed from DGS tracing in figure 1. Here a long tail is recovered along with a few other bones. At lower left are pelvis and femora tracings moved to in vivo positions.

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.

Well, if you look at a sister taxon
Monodelphis (Fig. 3) you find the same groove and no post dentary jaw bones. The image comes from a CT scan at Digimorph.org. In Monodelphis those posterior jaw bones are all tiny and hidden within the auditory bulla beneath the cranium where they serve as middle ear bones, as in humans. The groove remains unfilled by the splints of bones that were once located there in more ancient ancestors, like Castorocauda.

Figure 4. Mondelphis domestics with its posteromedial jaw groove highlighted in red. The ear bones are tiny and enclosed within the auditory bulla beneath the cranium.

Figure 3. Mondelphis domestics with its posteromedial jaw groove highlighted in red. The ear bones are tiny and enclosed within the auditory bulla beneath the cranium.B&W image from Digimorph. org and used with permission.

Figure 4. Basal mammals and the nesting of Docofossor with Volaticotherium and Monodelphis at the base of the Placentalia. Note (in yellow) how putative members of the Docodonta split up here.

Figure 4. Basal mammals and the nesting of Docofossor with Volaticotherium and Monodelphis at the base of the Placentalia. Note (in yellow) how putative members of the Docodonta split up here.

And here’s the subset of the LRT
(Fig. 4) showing where Docofossor and other putative docodonts nest within and outside the Mammalia.

Volaticotheria = Paraplacentalia
The fact that the only two good skeletons (Volaticotherium + Docofossor) in this new clade  come from a Jurassic climber and a Jurassic digger hint at the wide gamut of undiscovered taxa present in this clade of alternate and convergent placentals, the Paraplacentalia.

At the same time…
the several included members of the Docodonta are here (Fig. 4) split up into several clades, indicating that this putative clade is paraphyletic.

References
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.

wiki/Docofossor

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