Updated August 4, 2022,
with both of these taxa joining the extant water opossum, Chironectes.
the tiny, Eocene, short-legged, ‘basal perissodactyl’ Hyopsodus (Figs 1, 2) nests as an Eocene ancestor to the massive Miocene ‘bear dog’ Amphicyon major (Figs 1, 3, 4) at the base of the Placentalia in the large reptile tree (LRT, 2122 taxa, Fig 5). Basal placentals have three incisors, three premolars and four molars. Members of the Carnivora tend to lose molars.
This isn’t the first time
leptictid clade members were identified as ungulates. Basal odontocetes were also identified as artiodactyls by whale experts.
(H. paulus type, Leidy 1870; Late Eocene, 56-45 mya; ~20 cm in length; AMNH 143783, Fig 1, 2) was originally considered a condylarth or a perissodactyl, an odd-toed ungulate, despite having a five-clawed manus and a four-clawed pes. Here it nests as a sister to Miacis and Amphicyon major (Figs 1, 3). Four molars are present.
(Lartet 1836, Bergounious and Crouzel 1973, Argot 2010, Miocene) 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. Note the gracile cheek arch.
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.
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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 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
Wesley-Hunt GD and Flynn JJ 2005. Phylogeny of the Carnivora: Basal Relationships Among the Carnivoramorphans, and Assessment of the Position of ‘Miacoidea’ Relative to Carnivora. Journal of Systematic Paleontology, 3: 1-28.