The origin and early evolution of metatherian mammals: the Cretaceous record.

Thomas E Williamson,Stephen L Brusatte,Gregory P Wilson

doi:10.3897/zookeys.465.8178

Abstract

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.

Highlights

Metatherian mammals, which include the extant marsupials, are the second most diverse major clade of living mammals (~334 species; Armati et al 2006)
We review the phylogenetic relationships of metatherians with respect to other mammals and the taxonomic definition and diagnosis of Metatheria, the Cretaceous history of major metatherian clades and the paleobiology and macroevolution of Cretaceous metatherians, and the physical and climatic background and biogeography of Cretaceous metatherian faunas
Salient differences are related to the inclusion of two additional basal eutherian taxa, Asioryctes and Ukhaatherium, which form a clade that is placed in a polytomy with Deltatheroida and Marsupialiaformes

Summary

Introduction

Metatherian mammals, which include the extant marsupials, are the second most diverse major clade of living mammals (~334 species; Armati et al 2006). The poorly known Holoclemensia from the Early Cretaceous of Texas may occupy a similar position close to the base of Metatheria (e.g., Luo et al 2003; Luo et al 2011; Rougier et al 1998; Rougier et al 2004), but alternatively it may be a basal eutherian or a stem boreosphenidan outside of Theria (e.g., Averianov et al 2010; Kielan-Jaworowska et al 2004; Vullo et al 2009).

Results

Conclusion