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

Abstract

The Homerian (Wenlock) to Ludfordian (Ludlow), late Silurian, retiolitine genera Neogothograptus and Holoretiolites and their relatives are revised. These are often considered as the youngest and smallest retiolitine taxa, characterised by a finite growth indicated by the appendix, leading to the extinction of the clade. The tubarium construction of the genera is analysed and compared, showing the loss of the pleural and parietal lists in Holoretiolites and the development of a central zigzag construction on the obverse and reverse sides of the tubarium. This construction is not homologous to similar constructions in earlier retiolitines, 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 development of 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.

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

“…The astogeny is an often neglected feature in graptolite taxonomy, but may be quite important for taxonomic purposes. Maletz (2023b) discussed the astogenetic changes for a number of species in the genera Holoretiolites and Neogothograptus and indicated the problems for the identification of juvenile specimens. Juvenile specimens of Paraplectograptus with less than four theca cannot be referred to a certain species, unless a growth series has been recovered from the sample.…”

“…Intraspecific variation may also play a role in the identification of retiolitines (Maletz 2023b), but has rarely been investigated. The intraspecific variation of Paraplectograptus has never been explored and most species have been described from very few specimens.…”