Article: Taxonomy, evolution and biostratigraphical importance of the Llandovery graptolite Spirograptus
Four species are assigned to the Llandovery graptolite genus Spirograptus. S. andrewsi from the upper Aeronian, is the earliest representative and is recorded here for the first time outside Australia. S. turriculatus and S. guerichi sp. nov. occur in the lower Telychian. S. guerichi is distinguished from S. turriculatus by its lesser dorso-ventral width, rhabdosome apical angle and whorl diameter. The subspecies S. turriculatus minor is here considered a junior synonym of S. turriculatus. S. minimus is found only in the middle Telychian of Russia and is not described here. Spirograptus probably evolved from Stimulograptus. Evolution within Spirograptus involved initially increased curvature of the sicula, attainment of a greater dorso-ventral width and modification of the thecae. A new Spirograptus guerichi Biozone is erected to replace the Spirograptus turriculatus minor Biozone of some authors.