KT Mass Extinction

Metatherians, which comprise marsupials and their closest fossil relatives, were one of the most dominant
clades of mammals during the Cretaceous and are the most diverse clade of living mammals after Placentalia.
Our understanding of this group has increased greatly over the past 20 years, with the discovery of
new specimens and the application of new analytical tools. Here we provide a review of the phylogenetic
relationships of metatherians with respect to other mammals, discuss the taxonomic definition and diagnosis
of Metatheria, outline the Cretaceous history of major metatherian clades, describe the paleobiology,
biogeography, and macroevolution of Cretaceous metatherians, and provide a physical and climatic background
of Cretaceous metatherian faunas. Metatherians are a clade of boreosphendian mammals that must
have originated by the Late Jurassic, but the first unequivocal metatherian fossil is from the Early Cretaceous
of Asia. Metatherians have the distinctive tightly interlocking occlusal molar pattern of tribosphenic
mammals, but differ from Eutheria in their dental formula and tooth replacement pattern, which may be
related to the metatherian reproductive process which includes an extended period of lactation followed
by birth of extremely altricial young. Metatherians were widespread over Laurasia during the Cretaceous,
with members present in Asia, Europe, and North America by the early Late Cretaceous. In particular,
they were taxonomically and morphologically diverse and relatively abundant in the Late Cretaceous of
western North America, where they have been used to examine patterns of biogeography, macroevolution,
diversification, and extinction through the Late Cretaceous and across the Cretaceous-Paleogene (K-Pg)
boundary. Metatherian diversification patterns suggest that they were not strongly affected by a Cretaceous
Terrestrial Revolution, but they clearly underwent a severe extinction across the K-Pg boundary.