Monograptus, extinct genus of graptolites (small aquatic colonial animals related to primitive chordates) found as fossils in Silurian marine rocks (formed about 444 million to 416 million years ago). The most common Silurian graptolite genus, Monograptus is characterized by a single branch, or stipe, in which distinctive features of the structure occur. Monograptus descended from the genus Diplograptus, a two-branched form. Several forms or species are known and are useful for correlating Silurian rocks in widely separated areas and for further subdividing Silurian time.