Reinterpretation of Voronocladus from the Silurian of Ukraine as a bryopsidalean alga (Chlorophyta): The outlines of a major early Paleozoic macroalgal radiation begin to come into focus

Voronocladus dryganti Skompski et al. was recently erected as a new genus and species of dasycladalean alga on the basis of material from the Ludlovian Konivka Formation of western Ukraine. The thalli of this noncalcified macroalga are remarkable not only for their exceptional preservation, but also for showing unusual structures at the top, which were interpreted as epibionts. Here, an alternative interpretation is proposed for this material. In it, the affinity of Voronocladus as a siphonous green macroalga is retained, but as a bryopsidalean rather than dasycladalean alga, and the graptolite-like structures at the top are considered to represent the uppermost siphons of mature thalli instead of epibionts. Detailed morphological comparisons with living bryopsidalean taxa, as well as taphonomic and evolutionary considerations and key basal divergence dates indicated by molecular clock studies, provide support for the new interpretation. Viewed in broader terms, the reinterpreted morphology of Voronocladus is important because it (1) provides novel insights regarding the phylogenetic affinity of Ordovician and Silurian macroalgae with a form broadly similar to Voronocladus, and (2) helps to resolve the outlines of a major radiation of bryopsidalean algae that began during the Ordovician and continued into the Silurian. This event, which is only now beginning to come into focus, includes the initial evolution of Caulerpa- and Codium-like thalli and coincides with a major radiation of dasycladalean algae.