Diplograptus, genus of graptolites, small, extinct colonial marine animals thought to be related to the primitive chordates and restricted to ancient marine environments. Forms or species of Diplograptus are useful index, or guide, fossils for the Ordovician period (which occurred from 505 to 438 million years ago) and thus allow the correlation of sometimes widely separated rock units. Diplograptus is characterized by a caplike float from which featherlike assemblages of graptolite organisms were suspended. It was relatively large for a graptolite; several kinds of individuals were present in a colony.