Evolutionary implications from the tubarium construction of the Retiolitinae (Graptolithina) across the Silurian Cyrtograptus lundgreni extinction event

Abstract

The Retiolitinae (Axonophora, Graptoloidea) experienced a dramatic extinction during the Homerian (Wenlock, Silurian) Cyrtograptus lundgreni extinction event. It resulted in a re-organization and re-construction of the surviving retiolitine graptoloids as demonstrated by the apparent survival of only the genera Gothograptus and Semigothograptus across this interval. The tubarium construction of Gothograptus is expressed through the merging of the lateral thecal walls with the ancora sleeve walls. The nema is incorporated into the obverse ancora sleeve and connected by horizontal connecting lists. This type of construction is unknown from any other retiolitine taxon. Gothograptus (upper Sheinwoodian to upper Homerian, Wenlock, Silurian) thus, is constructionally isolated from all other pre- and post-extinction Retiolitinae and is unlikely to show closer relationships to the Neogothograptus and Holoretiolites clade. Semigothograptus shows the preservation of the ancora sleeve walls with distinct subhorizontal parietal lists on both sides. It has a free nema and lacks indications of lateral thecal walls. The genus is constructionally similar to most post-extinction taxa, but also to the pre-extinction Sokolovograptus. A differentiation into two groups of retiolitines with pustulose ornamentation of their cortical bandages can be seen in the development of the seams indicating thecal and ancora sleeve membranes on either the outside or the inside of the tubaria. All taxa with outside seams disappear in the Cyrtograptus lundgreni extinction event, while only taxa with inside seams survived.