The tubarium construction in Holoretiolites, Neogothograptus and related taxa (Graptolithina, Retiolitinae): clues to their astogeny and species identification

The Homerian (Wenlock) to Ludfordian (Ludlow), late Silurian, retiolitine genera Neogothograptus and Holoretiolites and theirrelatives are revised. These are often considered as the youngest and smallest retiolitine taxa, characterised by a finite growthindicated by the appendix, leading to the extinction of the clade. The tubarium construction of the genera is analysed andcompared, showing the loss of the pleural and parietal lists in Holoretiolites and the development of a central zigzag constructionon the obverse and reverse sides of the tubarium. This construction is not homologous to similar constructions in earlierretiolitines, as it is based on the modification of the lateral apertural lists. It is also found in the closely related Semiplectograptus and Plectodinemagraptus. Useful details to understand the astogeny of these taxa include the developmentof the reticulum, the genicular processes and the outer ancora. These features are described for the first time for most species.Details of the outer ancora and its construction in the genus Neogothograptus are still poorly understood. Holoretiolites helenaewitoldi is synonymised with Holoretiolites atrabecularis. Holoretiolites manckoides is transferred to Semiplectograptus.