sabertooth marsupials nest together with other carnivorous marsupials in a clade Cope 1875 called Creodonta and Ameghino 1895 called Saprassodonta (back when creodonts were considered archaic placentals).
Patagosmilus goini (Fig. 1; Forasiepi and Carlini 2010) is a large sabertooth marsupial from the Middle Miocene of south America traditionally considered a sister to the more famous and distinctly different and more famous sabertooth from South America, Thylacosmilus (Fig. 2). After testing in the large reptile tree (LRT, 1361 taxa), Patagosmilus nests with the ultra tiny basal sabertooth metatherian, Hadrocodium (Figs. 1,3) from the Early Jurassic. Evidently transitional taxa have yet to be discovered (or tested).
A little backstory:
Thylacosmilus atrox (Riggs 1933; Miocene, 40-3 mya; 1 m long) was a leopard-like predator, but plantigrade. Thylacosmilus was a sister to Early Cretaceous Vincelestes. The premaxillary teeth are either absent or were taphonomivally lost here. The canines are enlarged to curved fangs. Their long roots extend above the orbits. The dentary has deep ventral processes that guide and protect the canines. The lower canines are round stubs. The coronoid process is small. The mandible was able to open nearly 90º.
Hadrocodium wui (Luo, Crompton and Sun 2001; Early Jurassic; skull length: 1.2cm), known only from a tiny skull about the size of a thumbnail, Hadrocodium was originally considered a juvenile basal mammal, but later a tiny adult. Hadrocodium has a relatively larger brain size and more advanced ear structure than Megazostrodon. Hadrocodium nests with other basal metatherians with three (not two) molars, Morganucodon and Volaticotherium. The first molar was originally considered the third premolar.
Contra traditional studies
none of these taxa are related to marsupial creodonts and/or borhyaenids, like Borhyaena and Hyaenodon (Fig. 5), all of which have large canine teeth, none of which have saber teeth.
elephants, walruses, deer and other taxa with hyper-elongated teeth are never considered sabertooth proboscidians, sabertooth seals and sabertooth deer. Perhaps this is so because such teeth have a round cross-section, not a narrow, sword/saber-like morphology.
Ameghino F 1892. (Issued in 1894.) Enumeration synoptique deses pesces de mammiferes fossilesdes formation sócénes de Patagonie. Boletindela, Academia Nacionalde Cienciasen Cordoba, XIII, p.259 (p.108 in reprint).
Cope ED 1875. On the Supposed Carnivora of the Eocene of the Rocky Mountains. Proceedings of the Academy of Natural Sciences, Philadelphia. pp. 444–449.
Forasiepi AM and Carlini AA 2010. A new thylacosmilid (Mammalia, Metatheria, Sparassodonta) from the Miocene of Patagonia. Zootaxa. 2552, ss. 55–68.