Placentalia 2 basal taxa and their marsupial ancestors to scale

These are allarboreal taxa.
Here in the large reptile tree (LRT, 2340 taxa) these taxa (Fig 1) are possum-like trending toward rodent-like, some going back to the early Jurassic, others extant.

Some later clade members became terrestrial = rabbits. Others were burrowers = gophers. Still others became aquatic = beavers and manatees. Some retarded the development of their inner ear bones from posterior jaw elelments = multituberculates.

This is the branch of the LRT 
where and when rabbit and mouse ancestors became placentals – by convergence with three other clades of placentals arising from separate marsupials. 

These are also the ancestors of the extant marine beavers = manatees.
Thus the largest rodent of all time was likely recently extinct Steller’s sea cow (Hydrodamalis gigas) some 8 to 9 meters long.

Figure 6. The aye-aye, Daubentonia in vivo. This is the closest living relative of multituberculates and is itself a plesiadapiform member of Glires, close to rodents, not primates. ” data-image-caption=”

Figure 6. The aye-aye, Daubentonia in vivo. This is the closest living relative of multituberculates and is itself a plesiadapiform member of Glires, close to rodents, not primates.

” data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/03/daubentonia_invivo588.jpg?w=584″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/03/daubentonia_invivo588.jpg?w=584″ alt=”Figure 6. The aye-aye, Daubentonia in vivo. This is the closest living relative of multituberculates and is itself a plesiadapiform member of Glires, close to rodents, not primates.” class=”wp-image-53650″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/03/daubentonia_invivo588.jpg?w=584 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/03/daubentonia_invivo588.jpg?w=150 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/03/daubentonia_invivo588.jpg?w=300 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/03/daubentonia_invivo588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Note the phylogenetic miniaturization 
demonstrated between Plesidapis (Fig 1) and tiny Middle Jurassic Megaconus (Fig 1), at present the last common ancestor of all extant placental 2 taxa. Given its phylogenetic position, Early Paleocene Plesiadapis can be considered a late-survivor from an earlier radiation. That goes double and triple for Petaurus (Fig 1) and its marsupial kin.

Plesidaapis cannot be considered a basal primate largely due to its rodent-like teeth. Editor’s note: It’s so strange that academics still have not recognized this long ticking academic time bomb still found in textbooks for what it is: an easily recognized untenable hypothesis of interrelationships.

The most primitive living placental 2 taxon
is the aye-aye, Daubentonia (Fig 2). That’s how we know it’s reproductive mode.

Daubentonia cannot be considered a basal primate largely due to its rodent-like teeth.
Editor’s note: It’s so strange that academics still have not recognized this long ticking academic time bomb still found in textbooks for what it is: an easily recognized untenable hypothesis of interrelationships.

Prior cladograms of the Mammalia
did not include enough taxa to recover 4 inventions of the placenta.

Genetic studies do not include fossils. 

The LRT currently needs testing with a similar taxon list. 

This appears to be a novel hypothesis of interrelationships. 
If not please provide a citation so I can promote it here. 

References
Kay RF, Thewissen JG and Yoder AD 1992. Cranial anatomy of Ignacius graybullianus and the affinities of the Plesiadapiforme”. American Journal of Physical Anthropology. 89 (4): 477–498. 

wiki/Plesiadapis
wiki/Placentalia
wiki/Aye-aye