Phylogenetic analysis of the Silurian Retiolitidae

Abstract

A cladistic, phylogenetic analysis of the Retiolitidae, using 18 genera and parataxa and 23 characters, reveals that two distinct clades can be recognized within this family, as well as a primitive stem‐group that includes the genus Pseudoretiolites. One of the derived clades includes Pseudo‐plegmatograptus, Stomatograptus and Retiolites, which are assigned to the revised subfamily Retiolitinae. The other derived clade, the early part of which shows a paraphyletic relationship, includes the remaining known genera Rotaretiofites s.l., Paraplectograptus s.l., Sokolovograptus, Plectograptus, Agastograptus, Spinograptus, Gothograptus, Eisenackograptus, Neogothograptus, Semiplectograptus, Plectodinemagraptus and Holoretiolites. These genera are all included within an expanded Plectograptinae. The biostratigraphic distribution of these taxa suggests that these two clades diverged from a Pseudoretiolites‐like ancestor in mid‐Aeronian time.

The classification of the Pterobranchia (Cephalodiscida and Graptolithina)

“…They also recognized a few novel characters like the lack of the reticulum in most Plectograptinae and the small size of most colonies among others, but the authors did not use isolated material to work on that later workers used for their analyses (e.g. Lenz & Melchin 1997, Bates et al 2005, Kozłowska-Dawidziuk et al 2003. Plectograptinae has more recently been based on the presence of pustulose bandages with smooth to striated bandages present in Retiolitinae (Lenz & Melchin 1997).…”

“…Lenz & Melchin (1987) found the surface sculptures on the retiolitid lists to be diagnostic for the subfamilies Retiolitinae and Plectograptinae. An earlier cladistic interpretation of the Retiolitidae by Lenz & Melchin (1997) recognized the Retiolitinae and Plectograptinae as monophyletic taxa and a stem-group taxon Pseudoretiolites, but included "Rotaretiolites" and Rotaretiolites as basal members in the Plectograptinae. In the accompanying diagram (Lenz & Melchin 1997, fig.…”

“…An earlier cladistic interpretation of the Retiolitidae by Lenz & Melchin (1997) recognized the Retiolitinae and Plectograptinae as monophyletic taxa and a stem-group taxon Pseudoretiolites, but included "Rotaretiolites" and Rotaretiolites as basal members in the Plectograptinae. In the accompanying diagram (Lenz & Melchin 1997, fig. 4), however, the retiolitines (in black) are shown as a paraphyletic group from which the monophyletic plectograptines originate.…”

“…Therefore, cladistic approaches are still limited to a few well-known groups, such as the retiolitids (Lenz & Melchin 1997, Bates et al 2005 and the Ordovician to lower Silurian biserial axonophorans (Mitchell 1987, Mitchell et al 2007, Štorch et al 2011. In these groups, enough taxa are available as isolated or relief specimens, yielding the morphological details necessary for a reasonable cladistic analysis.…”

“…The alternative would be to define an [informal] paraphyletic stem group that includes the group formerly called Retiolitinae and separate the derived Plectograptinae from these. Lenz & Melchin (1997) took a slightly different route and regarded Pseudoretiolites as a paraphyletic stem group and recognized two monophyletic subfamilies, the Retiolitinae and Plectograptinae. A similar "trick" was used by Maletz et al (2009) for several groups of early Graptoloidea, by routinely identifying informal, paraphyletic stem groups of formally defined monophyletic taxa.…”