Toothed whale porpoise as a fish. Toothed whales, as well as baleen whales, are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are closely related to the hippopotamus, sharing a common ancestor that lived around 54 million years ago (mya). The primitive cetaceans, or archaeocetes, first took to the sea approximately 49 mya and became fully aquatic by 5–10 million years later. The adaptation of echolocation and enhanced fat synthesis in blubber occurred when toothed whales split apart from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 mya. Unlike toothed whales, baleen
White-toothed woodrat data provided by Macedo and Mares (1988) for what was then thought to be a single species applies to both. White-toothed woodrat Neotoma leucodon is a species of rodent in the family Cricetidae. Although originally named from San Luis Potosí, Mexico, as a species by Merriam, the white-toothed woodrat was long considered to be a synonym of the white-throated woodrat ("Neotoma albigula"). Molecular data, however, indicate the populations east of the Rio Grande in New Mexico and Trans-Pecos Texas represent a different species than morphologically similar populations west of the river. The habitat preferences of the two species appear similar,