Okay, I know this comes as no surprise…
a mouse-like marsupial at the origin of placental (eutherian) mammals. That’s old news.
But this EXTANT mouse-like marsupial
has traits found in basal eutherian mammals like Eomaia… not found in other tested marsupials… which is exactly the way it should be. We’re always looking for a gradual accumulation of traits in the large reptile tree (LRT, 764 taxa, subset Fig. 2).
So far among marsupials
Didelphis, the Virginia opossum nests basal to all other tested marsupials. Three marsupials nest together in a clade, Thylacinus, Dromiciops and Macropus. One marsupial, Monodelphis, nests between these taxa and the basal eutherian, Eomaia. So marsupials (Metatheria) are forming a grade, not a clade.
Monodelphis is approximately 50 times smaller in body size than Didelphis, and lacks a pouch which is found in the latter.
phylogenetic miniaturization strikes again, with a mini-opossum at the base of the Eutheria (if we delete the concept of time, of course).
By placing the appearance of each taxon on their time stratum, we can see that some basal mammals, like Ornithorhynchus, appear only in the present. The first radiation of basal carnivorous and insectivorous mammals was in the Jurassic. A second radiation of derived mammals occurred after the Cretaceous. There was no radiation of rodent-like pre-eutherian multituberculates in the Jurassic. Rather mutituberculates are rodent sisters, implying that rodents were around in the Jurassic, too. We just have not found them.
may modify the details, but the tree topology appears to be strong. This is a simplified topology without several pre-eutherian clades. It does not represent traditional hypotheses of interrelationships, but introduces a heretical new hypothesis that appears to indicate that many, but not all, mammal clades had deeper roots than previously thought.
Burnett GT 1830. Illustrations of the Quadrupeda, or Quadrupeds, being the arrangement of the true four-footed Beasts indicated in outline. Quarterly Journal of Science, Literature and Art, July to December, 1829, 336–353.
Macrini TE 2004. Monodelphis domestica. Mammalian Species 760:1-8.
Pine RH, Flores DA and Bauer K 2013. The second known specimen of Monodelphs unistriata (Wagner) (Mammalia: Didelphimorphia), with redescription of the species and phylogenetic analysis. Zootaxa3640 (3):425-441.