The Lingula antigua is a little bivalve shell belonging at the bottom of the class Brachiopoda. The inarticulate brachiopod genus Lingula is the oldest, relatively unchanged animal known. The oldest lingulid fossils are found in Lower Cambrian rocks dating to roughly 550 million years ago. The origin of brachiopods is unknown. A possible ancestor is a sort of ancient "armored slug" known as Halkieria that had small brachiopod-like shields on its head and tail.