90 species) Parvorder Archaeoceti Family Pakicetidae Family Ambulocetidae Family Remingtonocetidae Family Basilosauridae Parvorder Mysticeti: baleen whales Superfamily Balaenoidea: right whales Family Balaenidae: greater right whales (four species) Family Cetotheriidae: pygmy right whale (one species) Superfamily Balaenopteroidea: large baleen whales Family Balaenopteridae: slender-back rorquals and humpback whale (eight species) Family Eschrichtiidae: gray whale (one species) Parvorder Odontoceti: toothed whales Superfamily Delphinoidea: oceanic dolphins, porpoises, and others Family Delphinidae: oceanic true dolphins (38 species) Family Monodontidae: Arctic whales; narwhal and beluga (two species) Family Phocoenidae: porpoises (six species) Superfamily Physeteroidea: sperm whales Family Kogiidae: lesser sperm whales (two species) Family Physeteridae: sperm whale (one species) Superfamily Platanistoidea: river dolphins Family Iniidae: South American river dolphins (two species) Family Lipotidae: Chinese river dolphin (one species, possibly extinct) Family Platanistidae: South Asian river dolphin (one species) Family Pontoporiidae: La Plata dolphin (one species) Superfamily Ziphioidea Family Ziphidae: beaked whales (22 species) Clade Ruminantiamorpha Suborder Ruminantia Infraorder Tragulina Family Amphimerycidae Family Prodremotheriidae Family Protoceratidae Family Hypertragulidae Family Praetragulidae Family Tragulidae: chevrotains (ten species) Family Archaeomerycidae Family Lophiomerycidae Infraorder Pecora Family Gelocidae Family Palaeomerycidae Family Antilocapridae: pronghorn (one species) Family Climacoceratidae Family Giraffidae: okapi and four species of giraffe (five species total) Family Hoplitomerycidae Family Cervidae: deer (49 species) Family Leptomerycidae Family Moschidae: musk deer (seven species) Family Bovidae: cattle, buffalo, goat-antelope, antelope, and others (135 species) Research history In the 1990s, biological systematics used not only morphology and fossils to classify organisms, but also molecular biology. Molecular biology involves sequencing an organism's DNA and RNA and comparing the sequence with that of other living beings—the more similar they are, the more closely they are related. Comparison of even-toed ungulate and cetaceans genetic material has shown that the closest living relatives of whales and hippopotamuses is the paraphyletic group Artiodactyla. Dan Graur and Desmond Higgins were among the first to come to this conclusion, and included a paper published in 1994. However, they did not recognize hippopotamuses and classified the ruminants as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based on casein genes, SINEs, fibrinogen sequences, cytochrome and rRNA sequences, IRBP (and vWF) gene sequences, adrenergic receptors, and apolipoproteins. In 2001, the fossil limbs of a Pakicetus (amphibioid cetacean the size of a wolf) and Ichthyolestes (an early whale the size of a fox) were found in Pakistan. They were both archaeocetes ("ancient whales") from