Evolutionary history of theGothograptuslineage of the Retiolitidae (Graptolithina)

Abstract

Evolutionary trends of the Gothograptus lineage (Graptolithina, Retiolitidae) from the lower Homerian to lower Ludfordian, Silurian are described. Gothograptids evolved towards simplification of the tubarium and decrease in the number of thecae. The lineage is characterized by finite growth, ending with a tubular appendix in most species, long sicula and singular genicular processes in some forms. Species belonging to the genera of the Gothograptus lineage (Gothograptus, Baculograptus, Neogothograptus and Holoretiolites) occur continuously from the Cyrtograptus lundgreni to the Saetograptus leintwardinensis biozones. Gothograptus nassa is the only retiolitid known to appear immediately after the lundgreni event. The oldest genera of the lineage, Gothograptus and Baculograptus, have a well-developed tubarium with a dense reticulum and nema incorporated into the lateral wall. Younger genera have a free nema and, in general, gradually reduced number of thecae, lists, reticulum and apertural processes, whereas the sicula becomes longer. The best-developed apertural processes occur in Gothograptus. Holoretiolites Eisenack, 1951, the last known representative of the lineage, has the most reduced tubarium with three pairs of thecae, few lists and no apertural processes. This is one of the last representatives of the retiolitids, which became extinct in the S. leintwardinensis Biozone.