An order of inarticulated brachiopods consisting of an exclusively marine group of lophophorate animals (i.e., animals with a tentacular food-gathering, respiratory, and protective organ): sessile benthic suspension-feeders possessing a chitino-phosphatic shell. Representatives occur throughout the Phanerozoic Era (from the Early Cambrian to the present). Three superfamilies are recognized: Linguloidea, with the family Lingulidae, extends from the Middle Ordovician to the present; Discinoidea, with the family Discinidae, extends from the Early Ordovician to the present; and Acrotheloidea, known from the Early Cambrian to the Early Ordovician.