Placentalia 1 basal taxa and their marsupial ancestors to scale

These are all, more or less, opossum-like taxa.
Nevertheless in the large reptile tree (LRT, 2340 taxa) these taxa (Fig 1, sans Arctocyon) are deep human ancestors, elephant ancestors and mysticete ancestors.

Arctocyon is a dog, cat, bear and seal ancestor.

All these taxa are generalized, = arboreal and terrestrial = more or less like the opossum (Didelphis).

Among mammals, these clades retained true = plesiomorphic canines.
Other mammal clades either created a diastema or substituted a premolar. or both.

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

Note the phylogenetic miniaturization after Didelphis in Palaeosinopa (Figs 1, 2) the current last common ancestor of placental 1 taxa.

Figure 6. Palaeosinopa and Sinopa fossil and skeleton both have a long torso, long tail and relatively short legs. ” data-image-caption=”

Figure 6. Palaeosinopa and Sinopa fossil and skeleton both have a long torso, long tail and relatively short legs.

” data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/12/palaeosinopa-sinopa2scale588.jpg?w=584″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/12/palaeosinopa-sinopa2scale588.jpg?w=584″ alt=”Figure 6. Palaeosinopa and Sinopa fossil and skeleton both have a long torso, long tail and relatively short legs.” class=”wp-image-90612″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/12/palaeosinopa-sinopa2scale588.jpg?w=584 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/12/palaeosinopa-sinopa2scale588.jpg?w=150 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/12/palaeosinopa-sinopa2scale588.jpg?w=300 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/12/palaeosinopa-sinopa2scale588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Let’s talk about Palaeosinopa.
According to Wikipedia: “Palaeosinopa was a medium-sized, semi-aquatic mammal structurally convergent with modern otters. Based on an exceptionally complete, articulated skeleton discovered in the Eocene-aged Green River Formation of Wyoming, Palaeosinopa measured approximately 1 meter (3.3 ft) in total length from the snout to the tip of the tail and is estimated to have weighed roughly 5 kilograms (11 lbs).”

Palaeosinopa veterrima 
(Matthew 1901, Gingerich 1980; Rose and von Koenigswald 2005; Late Paleocene, 56-48 mya; YPM VP 027216) was originally considered a pantolestid insectivore. It has been known as an amphibious piscivore and perhaps mollusc eater as the molars of older specimens tend to be worn flat. Palaeosinopa has a long, robust tail with chevrons. Manual digit 2 is longer than the others. Pedal digit 3 is longer. The femur is short and robust. Fish were in the belly. Swimming style was similar to otters by convergence.

Considering its phylogenetic position in the LRT, late Palaeocene Palaeosinopa was specialized enough (swimming mollusk eater) and primitive enough (the rest of the skeleton) to have co-existed with dinosaurs and survive the asteroid. IMHO Palaeosinopa appeared in the Jurassic, because it was not far from Didelphis.

Prior cladograms of the Mammalia
did not include enough taxa to recover 4 inventions of the placenta, let alone split Carnivoramorpha from Pantodonta.

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
Gingerich PD 1980. A New Species of Palaeosinopa (Insectivora: Pantolestidae) from the Late Paleocene of Western North America. Journal of Mammalogy 61 (3): 449.
Matthew WD 1901. Additional observations on the Creodonta: Bulletin of the American Museum of Natural History, v. 14, p. 1–38. 
Patterson B 1958. Affinities of the Patagonian fossil mammal Necrolestes. Breviora Mus Comp Zool 94:1–14. 
Rose KD and von Koenigswald W 2005. An exceptionally complete skeleton of Palaeosinopa (Mammalia, Cimolesta, Pantolestidae) from the Green River formation, and other postcranial elements of the Pantolestidae from the Eocene of Wyoming (USA). 
Palaeontographica, Abteilung A: Palaozoologie – Stratigraphie  55–96 E. Schweizerbart’sche Verlagsbuchhandlung  10.1127/pala/273/2005/55.

wiki/Palaeosinopa
wiki/Pantolestidae

Source: https://pterosaurheresies.wordpress.com/2026/08/19/placentalia-1-basal-taxa-and-their-marsupial-ancestors-to-scale/