The identity of the Silurian retiolitine genera Paraplectograptus and Sagenograptoides (Graptoloidea, Retiolitinae)

Abstract

The genus Paraplectograptus includes a small number of retiolitine species with an identical development of the main thecal and ancora sleeve lists. A complete virgella-virga-nema development with connecting lists is formed on the obverse side of the tubarium, while the only thecal lists on the reverse side are the lateral apertural lists, connected through the pleural lists of the ancora sleeve. Obverse and reverse sides of the tubarium are connected by the thecal lips on the ventral sides. The characteristic zigzag thecal lists of the reverse side of many early retiolitines are not present in the genus. The development and extent of the reticular ancora sleeve lists on the obverse and reverse sides of the tubarium are quite variable and range from lacking entirely to dense. The genus Pseudoplectograptus is regarded as a junior synonym of Paraplectograptus. Pseudoplectograptus sagenus is transferred to the genus Sagenograptoides. Its type species ‘Sagenograptoides arctos’ was known only from small, immature or fragmentary specimens of the associated Sagenograptoides sagenus. This species bears a characteristic reticulum on the ventral thecal walls, not found in the genus Paraplectograptus, but which is present in other Cyrtograptus lundgreni Biozone retiolitines.