It's a shrew opossum!

Another virtual point goes to J. Velez-Juarbe for guessing that the skull belongs to a caenolestid.



Dusky shrew opossum
Caenolestes fuliginosus Tomes, 1863
Caenolestidae; Paucituberculata; Mammalia; Chordata
Cambridge Zoology Museum
June 2008

The caenolestids, shrew opossums, rat opossums, or flap-lips (!) are a family of three extant genera (Caenolestes, Lestoros and Rhyncholestes) in their own order of marsupials, the Paucituberculata. Shrew opossums, as I prefer to call them, are restricted to South America, much like the related opossums of Didelphimorphia, but even more so, not being found outside the Andes Mountains and southern 'boreal' forests of Chile, Argentina and the rest of western South America. As a result, they are poorly known, but exhibit a few distinctive features visible on the skull.

Marsupials can either be polyprotodont or diprotodont. The terms relate to the number of incisors in the upper jaw. The majority of Australian marsupials are diprotodont, having two large incisors at the front of the upper jaw, and two in the lower as well. Opossums and other related orders are all polyprotodon, with as many as five incisors on each side of the upper jaw. The Paucituberculata is one of these orders. All other polyprotodont families also have many incisors in the lower jaw, but the shrew opossums only have two, and these are extremely procumbent (forward-pointing), reminiscent of diprotodonts like kangaroos. There's also the inflected angular process (indented part of the lower jaw) present in all extant marsupials.



Dusky shrew opossum (stuffed)
Oxford Museum of Natural History
July 2008

The caenolestids are rather shrew-like in appearance, as can be seen in the above specimen. They are thought to occupy a similar niche, and since only a couple of shrew species of the genus Blarina have made it to South America, they have remained 'successful' (despite a reduction in the number of genera).