Yes, they all look alike (Fig. 1)
AND, the very subtle differences between them (aka microevolution) took the first steps that led to the major differences (aka macroevolution) seen in later more diverse clade members.
AND size does matter.
Phylogenetic miniaturization was once again at work here. Starting with the extant Didelphis at the base of the Theria, phylogenetic miniaturization gave us the smaller Monodelphis domesticus and the even smaller M. sorex and M. kunsi.
These in turn gave rise to
the larger Nandinia close to the base of the Carnivora, Tupaia, close to the base of the expanded Glires, Ptilocercus close to the base of the Ptilocercia, and Maelestes at the base of the Pantenreccetacea — aka the rest of the mammals.
This heretical origin hypothesis account runs counter
to all prior cladistic analyses, but I hope you’ll see a certain logic and order here that appears to echo Mesozoic evolutionary events. That order will be reflected in the LRT shortly.
Wonder why I added two more species from the genus Monodelphis?
Actually I was looking for skulls with larger premaxillary teeth, fewer molars, smaller canines, nares that open only anteriorly and other traits found in basal members of the various clades pictured above. I only found fewer molars among the smaller species show above.
M. sorex has the relatively shorter, taller skull, like that of the Carnivoran, Nandinia. M. kunis has the smaller naris and angled rostrum seen in Maelestes and the tenrec + whale clade. So, the larger trends are just barely present here, not enough to nest the smaller Monodelphis species apart from the the larger M. domesticus.
These largely overlooked taxa
must be among the most successful and well-adapted placental mammals of all time. After all, when you think about it, they have lasted the longest without having to evolve another morphology or die out. Putting them in the limelight will hopefully lead to future studies.