Family †Hemisyntrachelidae Order Artiodactyla: even-toed ungulates Suborder Suiformes Family †Raoellidae Family †Choeropotamidae Superfamily Suoidea Family Suidae: pigs Family Tayassuidae: peccaries Family †Santheriidae Family Hippopotamidae: hippos Superfamily †Dichobunoidea Family †Dichobunidae Family †Cebochoeridae Family †Mixtotheriidae Family †Helohyidae Superfamily †Anthracotherioidea Family †Haplobunodontidae Family †Anthracotheriidae Superfamily †Anoplotherioidea Family †Dacrytheriidae Family †Anoplotheriidae Family †Cainotheriidae Superfamily †Oreodontoidea Family †Agriochoeridae Family †Oreodontidae Superfamily †Entelodontoidea Family †Entelodontidae Suborder Tylopoda Family †Xiphodontidae Superfamily Cameloidea Family Camelidae: camels and llamas Family †Oromerycidae Superfamily †Protoceratoidea Family †Protoceratidae Suborder Ruminantia Family †Amphimerycidae Family †Hypertragulidae Family Tragulidae: mouse deer Family †Leptomerycidae Family †Bachitheriidae Family †Lophiomerycidae Family †Gelocidae Superfamily Cervoidea Family Moschidae: musk deer Family Antilocapridae: pronghorn Family †Palaeomerycidae Family †Hoplitomerycidae Family Cervidae: deer Superfamily Giraffoidea Family †Climacoceratidae Family Giraffidae: giraffe and okapi Superfamily Bovoidea Family Bovidae: cattle, antelope, and relatives Mirorder †Meridiungulata Family †Perutheriidae Family †Amilnedwardsiidae Order †Litopterna Family †Protolipternidae Superfamily †Macrauchenioidea Family †Macraucheniidae Family †Notonychopidae Family †Adianthidae Superfamily †Proterotherioidea Family †Proterotheriidae Order †Notoungulata: notoungulates Suborder †Notioprogonia Family †Henricosborniidae Family †Notostylopidae Suborder †Toxodontia Family †Isotemnidae Family †Leontiniidae Family †Notohippidae Family †Toxodontidae Family †Homalodotheriidae Suborder †Typotheria Family †Archaeopithecidae Family †Oldfieldthomasiidae Family †Interatheriidae Family †Campanorcidae Family †Mesotheriidae Suborder †Hegetotheria Family †Archaeohyracidae Family †Hegetotheriidae Order †Astrapotheria Family †Eoastrapostylopidae Family †Trigonostylopidae Family †Astrapotheriidae Order †Xenungulata Family †Carodniidae Order †Pyrotheria Family †Pyrotheriidae Mirorder Altungulata Order Perissodactyla: odd-toed ungulates Suborder Hippomorpha Family Equidae: horses Family †Palaeotheriidae Suborder Ceratomorpha Infraorder †Selenida Superfamily †Brontotherioidea Family †Brontotheriidae Family †Anchilophidae Superfamily †Chalicotherioidea Family †Eomoropidae Family †Chalicotheriidae Infraorder Tapiromorpha Superfamily Rhinocerotoidea Family †Hyracodontidae Family Rhinocerotidae: rhinoceroses Superfamily Tapiroidea Family †Helaletidae Family †Isectolophidae Family †Lophiodontidae Family †Deperetellidae Family †Lophialetidae Family Tapiridae: tapirs Order Uranotheria: elephants, manatees, hyraxes, and relatives Suborder Hyracoidea Family †Pliohyracidae Family Procaviidae: hyraxes Suborder †Embrithopoda Family †Phenacolophidae Family †Arsinoitheriidae Suborder Tethytheria Infraorder Sirenia: manatees, dugong, and sea cow Family †Prorastomidae Family Dugongidae: dugongs Family Trichechidae: manatees Infraorder Behemota Parvorder †Desmostylia Family †Desmostylidae Parvorder Proboscidea Family †Anthracobunidae Family †Moeritheriidae Family †Numidotheriidae Family †Barytheriidae Family †Deinotheriidae Family †Palaeomastodontidae Family †Phiomiidae Family †Hemimastodontidae Superfamily ‡Mammutoidea Family †Mammutidae: mastodons and relatives Superfamily Elephantoidea Family †Gomphotheriidae: gomphotheres Family Elephantidae: modern elephants Luo, Kielan-Jaworowska, and Cifelli classification Several important fossil mammal discoveries have been made that have led researchers to question many of the relationships proposed by McKenna and Bell (1997). Additionally, researchers are subjecting taxonomic hypotheses to more rigorous cladistic analyses of early mammal fossils. Luo et al. (2002) summarized existing ideas and proposed new ideas of relationships among mammals at the most basal level. They argued