This study includes a phylogenetic analysis that nests Thylacosmilus at the base of a clade of placental cats. Lacking here is a phylogenetic analysis that includes the closest marsupial relatives of Thylacosmilus: 1. Schowalteria, 2. Vincelestes + Conorytes, 3. Huerfanodon 4. Monodelphis + Chironectes. You need these taxa to decipher the phylogenetic bracketing of Thylacosmilus. The last two, BTW, are extant taxa.