By far the most diverse (taxonomically) and dominant (numerically) order of endemic South American ungulates are the Notoungulata, literally the "southern ungulates."  Though an estimate of their specific diversity is not possible (the number of valid species, inadvertently exaggerated by early workers, is still being sorted out), a reasonable estimate of their generic diversity includes more than 150 "genera" in 13 families.  They lived in South America throughout the Cenozoic, but only a single genus, Toxodon, ever emigrated from South America; it spread into tropical Central America during the Pleistocene after the formation of the Panamanian land bridge.  
     Notoungulates are united by characters of the ear region and the dentition, including the presence of a loph on the upper molars known as the "crochet".   A northern hemisphere group of Paleogene mammals known as arctostylopids have traditionally been included in the Notoungulata, but it now appears that the similarities in the dentition of these two groups were acquired independently.  Thus, with the exception of Toxodon, the notoungulate radiation was an exclusively South American phenomenon.