reptile of the order Sauria, 1819, from Modern Latin Sauria "the order of reptiles" (Brongniart, 1799), from Greek sauros "lizard" (see -saurus). As an adjective, "belonging to the Sauria," by 1829.
Sauropod for the suborder of the big plant-eating dinosaurs is by 1891, from Modern Latin sauropoda (Marsh, 1884), second element from Greek pous "foot" (from PIE root *ped- "foot"). Sauroid (n.) "a saurian animal" is by 1836, also as an adjective.
"a fin-footed mammal," one having feet like fins or flippers, especially of the group of fin-footed aquatic carnivorous quadruped mammals that includes seals, sea-lions, and walruses, 1842, from Modern Latin Pinnipedia, suborder of aquatic carnivorous mammals (seals and walruses), literally "having feet as fins," from Latin pinna in its secondary sense "fin" (see pin (n.)) + pes, genitive pedis "foot" (from PIE root *ped- "foot").