Which four index fossils were living organisms during the middle Ordovician?

During the Middle Ordovician period, notable index fossils include Didymoceras, a distinctive ammonite, Graptolites, which are important for dating and correlating rock layers, Trilobites such as Asaphus, and Brachiopods like Orthoceras. These organisms are widely distributed in sedimentary rock layers from this time, making them useful for identifying and dating Middle Ordovician strata. Their presence helps paleontologists understand the biodiversity and environmental conditions of that era.