SVP 2022 abstracts – 18: on Deltatherium

Williamson T et al 2022 report,
“Deltatherium [Fig 1] is an early Paleocene (middle Torrejonian NALMA) mammal known only from the San Juan Basin of northwestern New Mexico. Deltatherium is a small, spaniel-sized mammal (~ 6 kg). The phylogenic placement of Deltatherium is contentious and previous workers have proposed close affinities with tillodonts, pantodonts, and chriacine arctocyonids.

In the large reptile tree (LRT, 2172 taxa) Deltatherium (Fig 1) nests with Arctocyon among the carnviorous marsupials, members of the Creodonta.

A chriacine arctocyonid would be related to Chriacus, which does not nest with Arctocyon in the LRT, but with the extant water oppossum, Chironectes.

This is why adding taxa is so important!

“New, relatively complete dental, cranial, and postcranial material allows for a renewed assessment of the functional morphology, phylogeny, life history, and evolution of Deltatherium. Postcranial fossils include portions of axial skeleton, bones of the fore- and hindlimb, tarsals, and phalanges. Histological examination of teeth indicates that Deltatherium was precocial.

Some pantodonts are bulky herbivores ancestral to Xenarthra (sloths and glyptodons) in the LRT. They are unlike little lethal Deltatherium. Other pantodonts (e.g. Pantolambda) are herbivores that nest closer to the transition to Placentalia. So let’s not use the polyphyletic traditional clade Pantodonta anymore.

Trogosus is a big, bulky tree shrew in the LRT, the only tillodont known from more than just skull fragments and teeth. It has rodent-like incisors nothing like Deltatherium. In the LRT Trogosus nests near the base of the gnawing clade, Glires.

Do not trust analyses
that recover enigma taxa in hopeful, nebulous suprageneric clades. In your own studies report that your genus nests with another genus. And then tell us which clade they both nest in or between. Unfortunately, Williamson and his four co-authors kept Deltatherium an enigma by not nesting it with another genus.

References
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.