Palaeosinopa, Gobiohyus, Hyopsodus and Amphicyon major are water opossums

Recent work
on the large reptile tree (LRT, 2123 taxa, subset Fig 1) moves Palaeosinopa (Fig 3), Gobiohyus, Hyopsodus (Fig 4) and Amphicyon major (Fig 4) to the clade with the extant water opossum, Chironectes (Fig 2, 30cm snout to rump). The analytical work is not yet finished, but these changes appear to be robust.

In Chironectes both males and females have a pouch (= marsupium). The radiation of this clade had its genesis prior to the Middle Jurassic.

Figure 1. Palaeosinopa, Gobiohyus, Hyopsodus and Amphicyon major all join the extant water opossum, Chironectes after corrections were made to the LRT.

Figure 2. Chironectes minimus, the water opossum, in vivo. 30 cm snout-rump length

Palaeosinopa veterrima
(Matthew 1901, Gingerich 1980; Rose and von Koenigswald 2005; Late Paleocene, 56-48 mya; YPM VP 027216, Fig 3) was originally considered a pantolestid insectivore, but here nests with Gobiohyus and the water opossum, Chironectes. 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.

Figure 3. Palaeosinopa in situ with tail reconstructed from disturbed elements.

Amphicyon major
(Lartet 1836, Bergounious and Crouzel 1973, Argot 2010, Miocene, Fig 4) This bear dog is neither a bear nor a dog. It nests with tiny, Late Eocene Hyopsodus in the LRT, as if the two were adult and pup.

Figure 4. Hyopsodus and Amphicyon major are small and large water opossums respectively.

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

References
Argot C 2010. Morphofunctional analysis of the postcranium of Amphicyon major (Mammalia, Carnivora, Amphicyonidae) from the Miocene of Sansan (Gers, France) compared to three extant carnivores: Ursus arctos, Panthera leo, and Canis lupus. Geodiversitas 32 (1): 65-106.
Bergounious FM and Crouzel FC 1973. Amphicyon major Blainville du Miocene moyen de Sansan (Gers). Ann. Paleont., t. 59, fas. 1, pp. 3-76,47 fig. Paris.
Coombs MC and Coombs WP Jr. 1977. Dentition of Gobiohyus and a Reevaluation of the Helohyidae (Artiodactyla). Journal of Mammalogy58(3): 291-308.
Gingerich PD 1980. A New Species of Palaeosinopa (Insectivora: Pantolestidae) from the Late Paleocene of Western North America. Journal of Mammalogy 61 (3): 449.
Lartet E 1836. Nomenclature des mammiféres et des coquilles qu’il a trouvés dans un terrain d’eau douce prés de Simorre et de Sansan (Gers). Bulletin de la Société Géologique de France 7: 217–220.
Leidy J 1870. Remarks on a collection of fossils from the western territories. Proceedings of the Academy of Natural Sciences of Philadelphia 22: 109–110.
Matthew WD 1901. Additional observations on the Creodonta: Bulletin of the American Museum of Natural History, v. 14, p. 1–38.
Matthew WD and Granger W 1925. New mammals from the Irdin Manha Eocene of Mongolia. American Museum Novitates 198:1-10.
Orliac MJ, Argot C and Gilissen E 2012. Digital Cranial Endocast of Hyopsodus (Mammalia, “Condylarthra”): A Case of Paleogene Terrestrial Echolocation? PlosOne v.7(2); 2012PMC3277592
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 273: 55–96.
Zimmermann EAWv 1780. Geographische Geschichte der Menschen, und der algemein verbreiteten Vierfüssigen Thiere. Zweiter Band. Leipzig: Wenganschen Buchhandlung 2:6.

wiki/Water_opossun_Chironectes
wiki/Gobiohyus – not yet posted
wiki/Palaeosinopa
wiki/Hyopsodus
wiki/Amphicyon