The earliest known possible fossils of eutherians come fromAsia. Juramaia from the late Middle Jurassic (approximately 160Ma) of China was described as the oldest eutherian mammal(Luo et al., 2011) but it might be a stem therian, originatingbefore the split of Metatheria and Eutheria. Eomaia from theEarly Cretaceous (approximately 125 Ma) of China (Ji et al.,2002) and slightly younger Prokennalestes from the Early Cre-taceous (approximately105 Ma) of Mongolia (Lopatin and Ave-rianov, 2017) are among the earliest known eutherian mammals.Prokennalestes and other Cretaceous eutherians show the typi-cal eutherian pattern of at most five upper and lower premolarsand three upper and lower molars. The last upper and lowerpremolars in the earliest eutherians as compared to metatheri-ans already show trends towards molarisation (i.e. adding extracusps found on molars). The labial (cheek side) of the uppermolars has a wide area called the stylar shelf which unlikein contemporary metatherians has few cusps. The back, lowermargin of the lower jaw, the dentary, has a projection (angu-lar process) that points backwards in eutherians but internally inmetatherians. These forms were all small, ranging in size froma shrew to an opossum. Eomaia is argued to show both scanso-rial (climbing) and arboreal (tree-living) adaptations, comparedto other Cretaceous eutherians that, when known, are terrestrialand sometimes cursorial (running). Diets were mostly carnivo-rous to insectivorous, but omnivory and probably even herbivoryoccurred in some eutherians by the time of dinosaur extinction 65Ma. Within approximately 15 million years of dinosaur extinc-tion, most of the 19 extant orders of placentals had appearedalong with some 16 other orders that are now extinct. This wasa truly explosive radiation and diversification (O’Leary et al.,2013).
North America and Eurasia are known to have served as cen-tres for much of the diversi fication of extant placental ordersthroughout much of the Tertiary. Although less is known aboutthe early radiation of extant placental orders in Africa, both cur-rent diversity on this continent and recent molecular studies ofendemic African clades indicate that this continent was also amajor centre for placental evolution. Eutherians probably did notreach South America until approximately 65 Ma. Except for pos-sibly Cingulata and Pilosa, no extant placental orders are believedto have originated in South America. This is not true of extinctorders. At least five extinct orders are endemic to South America.These mostly herbivorous taxa fiourished throughout much ofthe Tertiary, possibly rivalling the diversity among extant Africanherbivorous placentals. They ranged from rabbit- to rhino-sized.In Australia, except for bats, which reached Australia in theearly Eocene (approximately 55 Ma), eutherians are not defi-nitely known from this continent until approximately 5 Ma, whenrodents arrived. Today, bats and rats are the only placentals thatreached Australia without the aid of humans. Madagascar has anunusual placental fauna, the best known being lemurs and rela-tives, which hark back to an early Tertiary African fauna. Exceptfor bats, oceanic islands lack any nonmarine mammals, with theexception of a few species of rodents on the Galapagos. Even thelargest oceanic islands such as those comprising New Zealand,today totally lack nonmarine placentals, except for a few species of bats. See also:Diversity of Life through Time;FossilRecord; Geological Time: Principles
