The endemic mammals initially consisted of metatherians ( including marsupials , xenarthrans , and a diverse group of South American ungulates . Its endemic mammals initially consisted of metatherians ( marsupials and sparassodonts ) , xenarthrans , and a diverse group of native ungulates : notoungulates ( the `` southern ungulates '' ) , litopterns , astrapotheres ( e.g. Trigonostylops , Astrapotherium ) , and pyrotheres ( e.g. Pyrotherium ) .