Evolutionary summary of the marsupials (mammlia: metatheria)

Abstract

The class of mammals have an evolutionary history that dates back to the end of the Triassic and throughout an extensive context guided by the incomplete fossil record that paleontology gathered over years of excavation and collection. The first mammals reasonably preserved in conditions are the Lower Jurassic (KERMACK et al., 1973, 1981; JENKINS e PARRINGTON, 1976), whose specimens are tiny, with skulls ranging from 20 to 30 mm body with about 150 mm and appearance similar to current shrews. Modern mammals are divided into placental, marsupials and monotremes and despite the scarce fossil material, marsupials have a peculiar evolutionary history and  complex phylogenetic relationships, in order to bring this group as a special focus of this work. Probably marsupials arose during the Early Cretaceous in Asia and, from this moment, dispersed to North America and, soon after, to South America and Australasia (SZALAY, 1994). It is generally accepted that the Australasian marsupials comprise a different side of those from the Americas, which is confirmed by recent molecular analysis (AMRINE-MADSEN et al., 2003). For many years, separate distribution of marsupials was established as a puzzle and thus have been proposed several biogeographical theories to explain it.