similar to other marine mammals, such as seals and sea cows, due to convergent evolution. However, they evolved from originally terrestrial mammals. The most likely ancestors were long thought to be mesonychids—large, carnivorous animals from the early Cenozoic (Paleocene and Eocene), which had hooves instead of claws on their feet. Their molars were adapted to a carnivorous diet, resembling the teeth in modern toothed whales, and, unlike other mammals, have a uniform construction. The suspected relations can be shown as follows: Inner systematics Molecular findings and morphological indications suggest that artiodactyls as traditionally defined are paraphyletic with respect to cetaceans. Cetaceans are deeply nested within the former; the two groups together form a monophyletic taxon, for which the name Cetartiodactyla is sometimes used. Modern nomenclature divides Artiodactyla (or Cetartiodactyla) in four subordinate taxa: camelids (Tylopoda), pigs and peccaries (Suina), ruminants (Ruminantia), and hippos plus whales (Whippomorpha). The presumed lineages within Artiodactyla can be represented in the following cladogram: The four summarized Artiodactyla taxa are divided into ten extant families: The camelids (Tylopoda) comprise only one family, Camelidae. It is a species-poor artiodactyl suborder of North American origin that is well adapted to extreme habitats—the dromedary and Bactrian camels in the Old World deserts and the guanacos, llamas, vicuñas, and alpacas in South American high mountain regions. The pig-like creatures (Suina) are made up of two families: The pigs (Suidae) are limited to the Old World. These include the wild boar and the domesticated form, the domestic pig. The peccaries (Tayassuidae) are named after glands on their belly and are indigenous to Central and South America. The ruminants (Ruminantia) consist of six families: The mouse deer (Tragulidae) are the smallest and most primitive even-toed-ruminants; they inhabit forests of Africa and Asia. The giraffe-like creatures (Giraffidae) are composed of two species: the giraffe and the okapi. The musk deer (Moschidae) is indigenous to East Asia. The antilocaprids (Antilocapridae) of North America comprise only one extant species: the pronghorn. The deer (Cervidae) are made up of about 45 species, which are characterized by a pair of antlers (generally only in males). They are spread across Europe, Asia, and the Americas. This group includes, among other species, the red deer, moose, elk (wapiti), and reindeer (caribou). The bovids (Bovidae) are the most species-rich. Among them are cattle, sheep, caprines, and antelopes. The whippomorphans include hippos and cetaceans: The hippos (Hippopotamidae) comprise two groups, the