The paper focuses on patterns of the evolution of the simplest and longest−ranging (approximately 18 Ma) Silurian graptolite Pristiograptus dubius. The Pristiograptus dubius species group consists of the P. dubius stem lineage represented by a sequence of a number of subspecies displaying only small morphological changes as well as derivative species produced from the stem lineage by means of iterative speciation. This long raging graptolite lineage is the only one, apart of one retiolitid, which survived the most severe environmental event for graptolites, the Cyrtograptus lundgreni Event. Based on three−dimensional, isolated material two P. dubius groups taxa are distinguished. One group has an obtuse angle between the thecal lip and the succeeding thecal wall, the second group has a right or acute angle. Other characters differentiating P. dubius forms are: the shape of the apertural lips, differences in rhabdosome shape and size, and a different number of sicular rings. Sixteen species and subspecies of Pristiograptus from the East European Platform, Poland, and Lithuania are discussed. Five new subspecies P. dubius magnus, P. dubius paezerensis, P. dubius praelodenicensis, P. dubius postfrequens, and P. dubius postmagnus are proposed.

Key words: Graptolithina, Pristiograptus dubius stem lineage, P. dubius species group, iterative speciation, Silurian, East European Platform, Poland, Lithuania.

Adam Urbanek [urbanek@twarda.pan.pl], Anna Kozłowska [akd@twarda.pan.pl], and Lech Teller, Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland; Sigitas Radzevičius [sigitas.radzevicius@gf.vu.lt], Department of Geology and Mineralogy, Vilnius University, M. K. Čiurlionio 21/27, Vilnius, LT2000, Lithuania.