This one seemed pretty obvious from the first impression,
but failed to make the same impression on the original authors (Huttenlocker et al., 2018).
Usually mammal teeth are found without a skull.
Huttenlocker et al., 2018 found a skull largely without teeth (most don’t erupt beyond the rim of the very few alveoli), certainly a derived trait for mammals. And this is one more way tetrapods became toothless. They named their new taxon, Cifelliodon wahkarmoosuch (Fig. 1).
According to Wikipedia:
“Cifelliodon is an extinct genus of haramiyid mammal from the Lower Cretaceous of North America. It is a mammaliaform, and is one of the latest surviving haramiyids yet known, belonging to the family Hahnodontidae. Its discovery led to the proposal to remove hahnodontids from the larger well-known group, the multituberculates.”
As usual the LRT recovered a different nesting.
in the large reptile tree (LRT, 1233 taxa) Cifelliodon wahkarmoosuch from the Early Cretaceous of Utah nests strongly with Tachyglossus (Figs. 1, 4, 5), one of the extant egg-laying echidnas, currently restricted to Australia and surrounding islands. Tachyglossus was tested in the (Huttenlocker et al. analysis of basal mammal relationships, but the two taxa did not nest together.
Unfortunately Huttenlocker et al., 2018
experienced taxon exclusion problems that nested Cifelliodon with the distinctly different wombat Vintana and those two with the distinctly different multituberculates all more primitive than monotremes.
To their credit
Huttenlocker et al. linked this North American taxon with others from Gondwana which includes Australia, which broke off 99 mya, 40 million years after the appearance of Cifelliodon in Utah. In an interview for USC, Huttenlocker reported, “Most of the fossil record of early mammal relatives is based on teeth. Cifelliodon is unique in that it is one of the only near-complete skulls of a mammal relative from the basal Cretaceous of North America and is the only fossil of early mammal relatives from this time interval in Utah.”
“The fact that the skull looked so primitive compared to other known mammal groups from the Cretaceous made figuring out its relationships extremely difficult. It shows some unique dietary specializations that are seen in only a handful of groups that lived during the age of dinosaurs. Ultimately, the structure of the preserved molars showed clear similarities to some neglected fossil teeth from Northern Africa. So we think that Cifelliodon represents an archaic offshoot whose relatives may have dispersed into the southern continents and became fairly successful during the Cretaceous.”
The skull of Tachyglossus
retains only a tiny lateral temporal fenestra (because the jaws don’t move much in this anteater. Compared to Cifelliodon the braincase is greatly expanded, the lateral arches are expanded and the two elements fuse, unlike most mammals.
Huttenlocker AD, Grossnickle DM, Kirkland JI, Schultz JA and Luo Z-X 2018. Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature Letters Link to Nature