Kolponomos ,”the Miocene beach bear,” moves to the desmostylians in the LRT

Updated February 2, 2023
with new scores moving Kolponomos (Figs 1, 2) close to the desmostylian, Cornwallius (Fig 2).

Knee-high to a human,
early Miocene Kolponomos (Stirton 1960, Tedford, Barnes and Ray 1994, Fig 1) moves in the large reptile (LRT, 2189 taxa then, 2212 taxa now) between Paleoparadoxia and Cornwallius. The latter is from the same formation in Vancourver.

Figure 2. The pre-desmostylian Cornwallius. Here the tympanic bulla (bright green) was considered “a mass” in the text and otherwise was not labeled.

Traditionally
Kolponomus was considered a bear relative, “a beach bear” perhaps close to pinnipeds. A few postcranial bones are known indicating this taxon was amphibious, but not a strong swimmer.

According to Wikipedia,
“The discovery of more fossils including a nearly complete cranium (Fig 1) from the original locality of K. clallamensis which helped identify it as part of the group from which pinnipeds evolved.”

The LRT indicates this a matter of convergence with desmostylians.

“In life, species of Kolponomos had downturned snouts and broad, heavy molars that would have been suited to a diet of hard-shelled marine invertebrates, and their narrow snouts and anteriorly directed eyes indicate that they would have had stereoscopic vision. Large neck muscle attachments and robust foot bones combine with these features to suggest that Kolponomos filled a unique niche among marine carnivores, approached today only by the very distantly related sea otter.”

Essentially desmostylians, like Kolponomos, are marine hippos
transitional to baleen whales (Mysticeti). The crushing cheek teeth of Kolponomos are desmostylian in architecture.

Tseng, Grohé and Flynn 2016 reported,
“Mammalian molluscivores feed mainly by shell-crushing or suction-feeding. The extinct marine arctoid, Kolponomos, has been interpreted as an otter-like shell-crusher based on similar dentitions. However, neither the masticatory biomechanics of the shell-crushing adaptation nor the way Kolponomos may have captured hard-shelled prey have been tested. Based on mandibular symphyseal morphology shared by Kolponomos and sabre-toothed carnivores, we hypothesize a sabretooth-like mechanism for Kolponomos prey-capture, whereby the mandible functioned as an anchor. This unique feeding system of Kolponomos exemplifies a mosaic of form-function convergence relative to other Carnivora.”

Perhaps taxon exclusion is the problem with prior studies. Kolponomos has a shorter face than related desmostylians (Fig 2) which has led to an ursid, pinniped, carnivore hypothesis of interrelationships in prior reports (see reference list).

References
Stirton RA 1960. A Marine Carnivore from the Clallam Miocene Formation, Washington: Its Correlation with Nonmarine Faunas. University of California Publications in Geological Sciences. 36 (7).
Tedford RH, Barnes LG and Ray CE 1994. The early Miocene littoral ursoid carnivoran Kolponomos: Systematics and mode of life (PDF). Proceedings of the San Diego Society of Natural History. 29: 11–32.
Tseng ZJ, Grohé C and Flynn JJ 2016. A unique feeding strategy of the extinct marine mammal Kolponomos: convergence on sabretooths and sea otters. Proceedings of the Royal Society B: Biological Sciences 283(1826)     DOI: 10.1098/rspb.2016.0044

wiki/Kolponomos