Named Peligrotherium tropicalis (“Tropical beast from [Punta] Peligro”), this animal was originally classified as an ungulate. This wasn’t too unreasonable; Punta Peligro has ungulates such as Carodnia, that may be related to modern tapirs, horses and rhinos. Punta Peligro ungulates were generally around the same size as Peligrotherium, adding more reasons to assume it was one.
But something amiss was noted early on. The molars and premolars of Peligrotherium, though superficially similar to those of ungulates, had a very strange cusp anatomy, so much so that it was almost as if it acquired complex cusps independently from any other therian mammal. Furthermore, unlike the other local ungulates its molar and premolar teeth had more than one root.
As it turns out, Peligrotherium wasn’t an ungulate, or even a therian mammal of any sort. Rather, it was a member of Dryolestoidea, an ancient lineage of mammals that lasted across the Jurassic and Cretaceous periods. Dryolestoids were fairly similar to therians, but still differed in a variety of details from tooth shape to brain anatomy.
A group of dryolestoids, Mesungulatidae, flourished in the South American Late Cretaceous, with forms like Mesungulatum and Coloniatherium being somewhat small herbivores with blunt skulls. Peligrotherium was closely related to these animals, and was essentially a much larger version, its molars dwarfing those of its Mesozoic relatives.
Mesungulatids, like many other mammals, became extinct alongside dinosaurs and other groups in the mass extinction at the end of the Cretaceous Period. However, Patagonia seems to have been less affected by it than other regions of South America, and a larger variety of animal species survive the catastrophe here. Among such animals were the ancestors of Peligrotherium, which in the absence of herbivorous dinosaurs and crocodilians grew to enormous sizes quickly.
The exact cause of extinction for Peligrotherium and kin isn’t fully understood. It has been suggested that competition with therian mammals might have corralled South American dryolestoids, but this hasn’t been formally tested, and it is just as likely that therian migrants from the north might simply have taken the niches emptied by the mass extinction event. Peligrotherium coexisted with various ungulates, so this might be of interest for future research.
Dryolestoids would continue to survive in South America for another 50 million years, with the mole-like digging Necrolestes occurring in the patagonian Miocene. More impressively, they might be still alive today in Australia, as one study found marsupial moles (Notoryctes) to not be actual marsupials, but dryolestoids related to Necrolestes. It isn’t peer-reviewed, however, and is yet to be formally addressed.
J. N. Gelfo and R. Pascual. 2001. Peligrotherium tropicalis (Mammalia, Dryolestida) from the early Paleocene of Patagonia, a survival from a Mesozoic Gondwanan radiation. Geodiversitas 23(3):369-379 [P. Mannion/P. Mannion]
G. W. Rougier, A. M. Forasiepi, R. V. Hill and M. Novacek. 2009. New mammalian remains from the Late Cretaceous La Colonia Formation, Patagonia, Argentina. Acta Palaeontologica Polonica 54(2):195-212
Agnolin, F.; Chimanto, N. (2014-12-22). "Morphological evidence supports Dryolestoid affinities for the living Australian marsupial mole Notoryctes". PeerJ Preprints. 2: e755v1. doi:10.7287/peerj.preprints.755v1. Retrieved 2015-12-30.