Eutheria (Placental Mammals)

Abstract

Abstract Eutheria includes one of three major clades of mammals, the extant members of which are referred to as placentals. With more than 5000 extinct and extant genera, including 1220 extant (living) genera and over 6000 extant species arrayed in 19 extant orders, placentals (extant eutherians) are the most taxonomically diverse of the three branches of extant mammals. Eutherians appear in the fossil record by 125 Ma (million years ago). Extant orders of eutherians (placentals) do not occur in the fossil record until after dinosaur extinction 65–66 Ma. Molecular studies generally agree with these dates for the origin of placental orders but place the origin of Eutheria at approximately 190 Ma and groups related to living orders of mammals by 100 Ma. Placentals vary greatly in size (whales down to shrews), in locomotion (flying, swimming, climbing, burrowing, running), and diet (meat, leaves, fruit, termites). They have a high resting temperature (homiothermy) and produce this heat internally (endothermy). They have a chorioallantoic placenta that allows a long gestation for development. Key Concepts Eutherian mammals are the most ecomorphological and taxonomically diverse group of mammals. The evolutionary success of eutherian mammal is related to their mode of reproduction (euviviparity). Eutherian mammals originated in the Early Cretaceous but modern placental orders radiated in the Cenozoic, after extinction of dinosaurs. The four major groups of Placentalia are Xenarthra, Afrotheria, Laurasiatheria and Euarchontoglires. The Cretaceous stem placentals mostly retained primitive therian dental formula which was further modified in the Cenozoic crown placentals.