Thylacosmiliformes

Thylacosmiliformes is an extinct clade of predatory metatherian mammals in the order Sparassodonta, related to the marsupials. Members of this clade are known from the Miocene and Pliocene epochs of South America (Argentina, Colombia, and Uruguay). The most notable thylacosmiliforms, such as Thylacosmilus, comprise the more exclusive family Thylacosmilidae and have prominent saber teeth.^[1][2]

Thylacosmiliformes Temporal range: Middle Miocene–Late Pliocene, (Colloncuran–Chapadmalalan) PreꞒ Ꞓ O S D C P T J K Pg N
Skull of Thylacosmilus
Lower jaw of Anachlysictis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Sparassodonta
Superfamily:	†Borhyaenoidea
Clade:	†Thylacosmiliformes Suarez et al., 2025
Type species
Thylacosmilus atrox Riggs, 1933
Subgroups
†Dimartinia †Thylacosmilidae

The clade was defined in 2025 to include thylacosmilids and their more "primitive" closest relatives such as Dimartinia.^[1]

Description

Members of the Thylacosmiliformes are characterized by several anatomical features of the mandible and teeth. These include the presence of two or fewer lower incisors, a lower canine that is compressed laterally, a small or absent first premolar, and an expanded symphyseal region of the dentary's anterior horizontal ramus.^[1]

Classification

In their 2025 description of Dimartinia, Suarez et al. performed a phylogenetic analysis to determine its position and relationships among other sparassodonts. They recovered Dimartinia as the basalmost member of the clade containing the Thylacosmilidae within the Borhyaenoidea. This group also contains an unnamed taxon from La Venta, Colombia.^[3] Their results are displayed in the cladogram below:^[1]

Borhyaenoidea	Lycopsis spp. Thylacosmiliformes Dimartinia IGM 251108 Thylacosmilidae Thylacosmilus Anachlysictis Patagosmilus UF 27881 (unnamed Honda Group sparassodont)[4] Prothylacynus Pharsophorus lacerans Eomakhaira Proborhyaenidae Borhyaenidae

The analyses of Suarez et al. (2025) placed Eomakhaira as a non-thylacosmiliform borhyaenoid, contrasting with its initial description,^[5] which suggested thylacosmilid affinities.^[1]