The graptolite Pseudoretiolites Bouček & Münch, 1944 and the early evolution of the Retiolitinae

The Retiolitinae (Graptolithina) of the Aeronian to early Telychian (Llandovery, Silurian) are very incompletely known. The genus Pseudoretiolites is one of the few taxa with often completely preserved thecal floors surrounded by the reticulum of the ancora sleeve. Chemically isolated specimens from Arctic Canada and Scandinavia provide important information for an understanding of the thecal membranes and ancora sleeve development. The genus probably originated in the Demirastrites triangulatus Biozone of the basal Aeronian (lower Silurian) and ranged into the Spirograptus guerichi Biozone of early Telychian (lower Silurian) age. The proximal end of the tubarium bears a deep, rounded ancora umbrella with two spiral lists ending in the umbrella rim. The thecal membranes are often preserved, bearing a characteristic mid-ventral list with a characteristic wide zigzag construction the fuselli at the aperture. The ancora umbrella with the two spiral lists, together with the thecal shape and the presence of mid-ventral lists is similar to the early retiolitines of the genera Hercograptus, Aeroretiolites and Eorograptus. All species of Pseudoretiolites are evaluated and their taxonomic value is discussed.