Vincelestes neuquenianus (originally Bonaparte 1986, Rougier, Wible and Hopson 1992, Early Cretaceous, 130 mya). Although Vincelestes is ancient and therefore primitive, the hyper-enlarged premolar fangs, loss of the canine, small tooth count and short rostrum indicate a highly derived taxon. Nine individuals are known. Skeleton is probably a chimaera of several specimens and the degree of completion is unknown. The tail is extraordinarily long and provided with deep chevrons.  µCT scan of the skull mage from Digimorph.org and used with permission.

Bi S-B, Zheng X-T, X, Wang X-L, Cignetti N-E, Yang SL and Wible JR 2018. An Early Cretaceous eutherian and the placental/marsupial dichotomy. Nature (advance online publication) DOI: https://doi.org/10.1038/s41586-018-0210-3
https://www.nature.com/articles/s41586-018-0210-3

Bonaparte JF 1986. Sobre Mesungulatum houusayi y nuevos mamíferos Cretácicos de Patagonia, Argentina [On Mesungulatum houssayi and new Cretaceous mammals from Patagonia, Argentina]. Actas del IV Congreso Argentino de Paleontología y Biostratigrafía 2:48-61.

Bonaparte JF and Rougier GW 1987. Mamiferos del Cretácio inferiorde Patagonia: IV Congreso Latinamericano de Paleontologia, Bolivia 1:343-359.

Bonaparte JF 2008. On the phylogenetic relationships of Vincelestes". Historical Biology. 20 (2): 81–86.

Rougier GW, Wible JR and Hopson JA 1992. Reconstruction of the cranial vessels in the Early Cretaceous mammal Vincelestes neuquenianus: implications for the evolution of the mammalian cranial vascular system. Journal of Vertebrate Paleontology 2:188–216. https://doi.org/10.1080/02724634.1992.10011449

wiki/Vinceleste
wiki/Ambolestes