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Rougier, G. W., Apesteguia, S. & Gaetano, L. C., 2011: Highly specialized
mammalian skulls from the Late Cretaceous of South America.
–Nature: Vol. 479, #7371, pp. 98-102 [doi: 10.1038/nature10591]

http://www.nature.com/nature/journal/v479/n7371/abs/nature10591.html
http://www.nature.com/nature/journal/v479/n7371/abs/nature10591.html#supplem
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Abstract:
“Dryolestoids are an extinct mammalian group belonging to the
lineage leading to modern marsupials and placentals1,2. Dryolestoids
are known by teeth and jaws from the Jurassic period of North
America and Europe2,3, but they thrived in South America up to the
end of the Mesozoic era and survived to the beginnings of the
Cenozoic2,4–7. Isolated teeth and jaws from the latest Cretaceous of
South America provide mounting evidence that, at least in western
Gondwana, dryolestoids developed into strongly endemic groups by
the Late Cretaceous4–9. However, the lack of pre-Late Cretaceous
dryolestoid remains made study of their origin and early diversification
intractable. Here we describe the first mammalian remains
from the early Late Cretaceous of South America, including two
partial skulls and jaws of a derived dryolestoid showing dental
and cranial features unknown among any other group of
Mesozoic mammals, such as single-rooted molars preceded by
double-rooted premolars, combined with a very long muzzle,
exceedingly long canines and evidence of highly specialized
masticatory musculature. On one hand, the new mammal shares
derived features of dryolestoids1–3 with forms from the Jurassic of
Laurasia, whereas on the other hand, it is very specialized and
highlights the endemic, diverse dryolestoid fauna from the
Cretaceous of South America. Our specimens include only the
second mammalian skull known for the Cretaceous of Gondwana,
bridging a previous 60-million-year gap in the fossil record, and
document the whole cranial morphology of a dryolestoid, revealing
an unsuspected morphological and ecological diversity for nontribosphenic
mammals.”

–Mikko