Cronopio dentiacutus (Rougier et al. 2011, early Late Cretaceous, 98 mya, MPCA PV 454) was originally described as a dryolestid mammal. Tradtionally dryolestids nest outside of the Theria. In the large reptile tree Cronopio nests between Juramaia and Didelphis (Linneaus 1758) which is indeed just outside the Theria. In Cronopio we are not seeing the origin of the diastema found in Macropus (LInneaus 1758), but yet another convergent appearance of this trait.
Cronopio in the Late Cretaceous demonstrates once again
the persistence of primitive mammalian taxa tens of millions of years after their origin.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Rougier GW, Apesteguía S and Gaetano LC 2011. Highly specialized mammalian skulls from the Late Cretaceous of South America. Nature. 479: 98–102. doi:10.1038/nature10591.