Lower Telychian (Silurian) species of Parapetalolithus from the linnaei, turriculatus and crispus biozones in the Prague Synform: taxonomy in the light of astogeny and intraspecific variability

The astogeny and intraspecific variability of the rhabdosomes of six species of the biserial graptolite genus Parapetalolithus are described, based upon rich and well-preserved material mostly from the classic Zelkovice and Litohlavy localities of the lower Telychian Litohlavy Formation (Prague Synform, Czech Republic). The previous systematic classification of Parapetalolithus species has been markedly influenced by the considerable morphological variability of rhabdosomes which is recorded herein within similarly preserved material from the same locality. This formerly resulted in excessive division into separate species and subspecies. Based upon morphometric measurements and subsequent revision, three species and subspecies (P. clavatus, P. elongatus linearis and P. conicus) are synonymized and the following six species are retained: P. ovatus, P. elongatus, P. hispanicus, P. palmeus, P. altissimus, and P. tenuis. This is the first detailed study documenting and comparing the astogeny of several Parapetalolithus species, which has a notable impact on their systematic classification and identification and for the first time enables species determination of early growth stages.