The small and odd ‘mustelid’, Leptarctus, enters the LRT

Korth and Baskin 2009
described Leptarctus (Fig 1, Leidy 1856) from the Late Miocene of Kansas. In the large reptile tree (LRT, 2340 taxa) Leptarctus nests (today) with a much larger, earlier Early Miocene Enhydrocyon (Fig 1), also from North America.

You’ll note
Leptarctus is unusual in having twin parasagittal crests, as in the totally unrelated Leptictis. In one specimen the twin crests are side-by-side. In another they are widely separated (Fig 1). Not sure what happened in the concavity between those crests, but laterally they anchored powerful jaw muscles. This trait is unique within Carnivora. Unique bony projections appear ventrally on the tympanic bullae = otic capsules.

Lim, Martin and Wilson 2016 reported,
“Leptarctus is a poorly known fossil carnivore that ranges through the Miocene of North America and Inner Mongolia, China (Lim, 1996; Zhai, 1964). Though it has been one of the least studied carnivores, more than 20 localities in North America have produced Leptarctus (Lim, 1999).”

From the AMNH publicity
“Results showed that the skull of Leptarctus was most similar to that of the American badger. Most likely, the extinct weasel had a similar diet, too: a carnivorous predator that could also be omnivorous when prey was scarce.”

Historically
Leidy 1856 thought the original tooth was similar to Nasua, the coatimundi. Wortmann 1894 thought Leptarctus was transitional between raccoons and the kinkajou. Gazin 1936 “conclusively establish the mustelid (weasel) affinities of Leptarctus.”

Tentatively = as of today,
Leptarctus and Enhydrocyon nest as basal members of the Carnivora, which splits at its origin. On one branch are Nandinia, Procyon (raccoon) and weasels, followed by Thylacosmilus, the sabertooth ‘marsupial’. On the other branch are Leptarctus and Enhydrocyon followed by cats, mongooses, wolves, civets, badgers, bears and pinnipeds (seals)… and their ancestors. Dissacus and Kopidodon are the current proximal outgroup taxa to the Carnivora.

This hypothesis of interrelationships is more than likely to change. Ordering within the Carnivora is the most difficult part of the mammal subset of the LRT to pin down. This is the first time the LRT has produced two parallel lineages of carnivores. Convergence and interpretation continue to be the issues.

Curious, I have not yet found what about sabertooth Thylacosmilus makes it a marsupial. I’ll keep looking for information and report it to you when found.

References
Korth WW and Baskin JA 2009. A new species of Leptarctus (Carnivora, Mustelidae) from the Late Clarendonian (Late Miocene) of Kansas. Annals of the Carnegie Museum 78(1):29-44.
Leidy J 1856. Notices of extinct Vertebrata discovered by Dr FV Hayden during the expedition to the Sioux country under the command of Lieut. GK Warren. Proceedings of the Academy of Natural Sciences of Philadelphia 8:31–317.
Lim J-D, Martin LD and Wilson RW 2001. A new species of Leptarctus (Carnivora, Mustellidae) from the Late Miocene of Texas. Journal of Paleontology 75(5):1043–1046.
Wang X 1994. Phylogenetic systematics of the Hesperocyoninae (Carnivora, Canidae). Bulletin of the American Museum of Natural History. 221: 1–207.
Wortman JL 1894. On the affinities of Leptarctus primus of Leidy. Bulletin of the AMNH 6(8):

wiki/Leptarctus in Italian
wiki/Enhydrocyon

Publicity
amnh.org/explore/news-blogs/extinct-weasel-american-badger