Mammals - Mammalia

Abstract

Extant mammals (~5400 sp.) are divided into three groups: Prototheria (monotremes; platypus and echidnas), Metatheria (marsupials), and Eutheria (placental mam- mals). Traditionally, monotremes are considered the closest relatives of Theria (marsupials + eutherians), but analyses of mitochondrial sequences have advocated a grouping of monotremes with marsupials. Recent analyses of nuclear data fi rmly support the Theria hypothesis. The mammalian timetree indicates that monotremes diverged from the- rians in the late Triassic, ~220 million years ago (Ma), and that marsupials and eutherians diverged ~176 Ma, between the early and middle Jurassic. However, estimates of diver- gence times vary among studies, which points to the need for further investigations.