Orthida's an extinct order of Brachiopods which appeared during the Early Cambrian period and became very diverse by the Ordovician, living in shallow-shelf seas. Orthids are the oldest member of the Rhynchonelliformea subphylum, and's the order from which all other brachiopods of this group stem. Physically they're usually strophic, with well-developed interareas. They also commonly have radiating ribs, sulcus, and fold structures. Typically one valve, often the brachial valve,'s flatter than… (More on Orthida)