Deltatherium: now a didelphid in the LRT

Shelley et al 2021 wrote,
“The dental and cranial anatomy of Deltatherium is a chimera, with morphological  similarities to both ‘condylarth’ and ‘cimolestan’ taxa. As such, the phylogenetic relationships of this taxon have remained elusive since its discovery, and it has variably been associated with Arctocyonidae, Pantodonta and Tillodontia.”

Wikipedia reports, “its relatives are far from clear.”
That usually means taxon exclusion is the culprit. The LRT minimizes taxon exclusion due to its large number of included taxa.

With new data entered into the LRT
on the dorsal skull and the number of possible premaxillary teeth, Deltatherium shifted its nesting spot.

In the process I learned the previous data, an otherwise precise illustration of a skull, restored missing premaxilla parts with imprecision.

In the LRT Deltatherium now nests closer
to Didelphis virginiana and Didelphis marsupialis, rather than the three clades listed above. So, Deltatherium is a big ‘possum’, a little closer to D marsupialis sharing 3 molars.

Shelley et al and other workers did not mention Didelphis in their text, but considered Deltatherium a eutherian = placenta-bearing mammal.

This means another case of taxon exclusion hobbling an otherwise informative study.

A little background from Lucas and Kondrashov 2004:
“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. Matthew considered arctocyonids to be primitive carnivorans (Creodonta), but Arctocyonidae are now assigned to archaic ungulates (Prothero et al., 1988).

“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). Cifelli (1983) suggested that a Deltatherium-like mammal could be the ancestor of both pantodonts and tillodonts. This idea of the close relationships between Deltatherium
and Pantodonta was rejected by some authors (Zhou et al., 1977; Muizon and Marshall, 1992), but the latter suggested that tillodonts are the sister group of pantodonts. Lucas (1993) stressed the close resemblance between Deltatherium and primitive tillodonts (Esthonyx) and identified Deltatherium as the sister taxon of tillodonts. However, he argued
that tillodonts are not true ungulates because they are not condylarthran descendants. Williamson and Lucas (1993) and Williamson (1996) repeated this opinion, and they included Deltatherium in Tillodontia.”

Lucas and Kondrashove adopted (without testing) the latter hypothesis.
All the above alternatives (except Esthonyx) were tested in the LRT.

Eocene Esthonyx is only known from
teeth attached to jaws – including tusk-like, but not ever-growing incisors typical of tillodonts, distinct from the large, sharp canines and tiny incisors found in Deltatherium.

This new placement in the LRT
was enabled by the recent addition of the common opossum, Didelphis marsupialis as a new taxon. It is found south of Texas to Bolivia, distinct from D virginiana, which is found from Canada to Costa Rica.

D marsupialis has three molars and a relatively shorter, taller skull, as in Deltatherium, distinct from the four molars and longer skull found in Didelphis virginiana.

Similar and related taxa in the LRT include
Ptolemaia  and a much larger Prodinoceras

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

Sorry, still no relief
from the tech issues preventing uploading or access to my blog library of uploaded images. Housekeeping continues in the mammal subset of the LRT.

References
Cope ED 1881. The Condylarthra (Continued). American Naturalist 84;18: 892–906.
Cope ED 1881. Mammalia of the lower Eocene beds. American Naturalist 15:337-338.
Kerr R 1792. The Animal Kingdom, or Zoological System, of the celebrated Sir Charles Linnæus; being a translation of that part of the Systema Naturæ.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
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.
Shelley SL et al (3 co-authors) 2021. Petrosal anatomy of the Palaeocene eutherian mammal Deltatherium fundaminis (Cope, 1881). Journal of Mammal Evolution 28, 1161–1180 (2021). https://doi.org/10.1007/s10914-021-09568-3

wiki/Common_opossum
wiki/Virginia_opossum
wiki/Deltatherium