Didelphis compared to Deltatherium

Deltatherium has been troublesome to nest within Mammalia.
Now the large reptile tree (LRT, 2340 taxa) nests Deltatherium (Fig 1) with Didelphis (Fig 1) and Megistotherium. Evidently this is a novel hypothesis of interrelationships.

Close hypothetical interrelationships should be obvious in hindsight.

The identities of the teeth in Deltatherium were corrected here. 
In the upper jaws two premolars are now identified following the loss of premolar 1 (see Didelphis frame in Fig 1). That means 4 molars are present and molar 1 has been premolarized. That happens within Carnivora and other clades, according to the LRT.

Lucas and Kondrashov 2004 reported,
“Matthew (1937) provided a detailed description of the skull and teeth of D. fundaminis and assigned the genus to Arctocyonidae, allying it closely to Chriacus.”

“Van Valen (1978, 1988) thought Deltatherium to be the ancestor of Pantodonta, thus deriving pantodonts from a condylarthran stock. McKenna (1975) assigned it to Pantodonta without discussion, a point of view later supported by Sloan (1987).”

Lucas (1993) stressed the close resemblance between Deltatherium and primitive tillodonts (Esthonyx) and identified Deltatherium as the sister taxon of tillodonts.”

Tillodon is shown in figure 2. In the LRT it nests within Glires, within Placentalia2 taxa.
Esthonyx is known from a complete palate + partial mandible and may be tested in the future, but so much is missing.

Williamson et al 2022 reported,
“Loss of the lower molar entoconids may be due to an increasing emphasis on vertical shearing function of the cheek teeth. Features of humerus and ulna indicate relatively unrestricted pronation and supination of the forearm. The tarsus indicates that the ankle was highly flexible. Morphology of the postcrania is consistent with an arboreal or scansorial adaptation.

“A consensus tree places Deltatherium as a basal member of a pantodont plus tillodont clade within crown Placentalia.”

Since this was an abstract, no cladogram was published. The authors were not aware of the several origins of the placenta revealed by the LRT, following an earlier hypothesis by Wortman 1902. Since the authors nested Deltatherium within the traditional Placentalia, it is unlikely that Didelphis (Fig 1) was on the tested taxon list.

Didelphis is a late survivor of a Late Jurassic radiation,
largely unchanged today. Despite the ideal adaptation to its niche, phylogenetic splinters were inevitable. Now Deltatherium appears to be one of many others previously identified.

References
Cope ED 1881. Mammalia of the lower Eocene beds. American Naturalist 15:337-338.
Lucas SG and Kondrashov PE 2004. A new species of Deltatherium (Mammalia, Tillodntia) from the Paleocene of New Mexico. In Lucas SG, Zeigler KE. and Kondrashov PE, eds., 2004, Paleogene Mammals, New Mexico Museum of Natural History and Science Bulletin No. 26.
Matthew WD 1937. Paleocene faunas of the San Juan Basin, New Mexico: Transactions of the American Philosophical Society, new series 30: 1-510.
Williamson T et al (4 co-authors) 2022. Triangular beast: new fossils shed light on Deltatherium, an enigmatic early Paleocene mammal from New Mexico. SVP abstracts p366.

SVP 2022 abstracts – 18: on Deltatherium

Source: https://pterosaurheresies.wordpress.com/2025/08/06/didelphis-compared-to-deltatherium/