Taxonomic descriptions and evolutionary implications of Middle Eocene pennate diatoms representing the extant genera Oxyneis, Actinella and Nupela (Bacillariophyceae)

Actinella F.W.Lewis, Oxyneis Round and Nupela Vyverman & Compère are three genera of pennate diatoms known exclusively from freshwater environments that are predominantly dilute and acidic (Round et al. 1990, Sabbe et al. 2001, Siver et al. 2007). Of the three, Oxyneis is the smallest genus and comprises only three species (Round et al. 1990, Metzeltin & Lange-Bertalot 2007), although only two have formally been described within the genus. Oxyneis is an araphid taxon, possessing one rimoportula per valve and a large number of girdle bands, each equipped with a septum (Round et al. 1990, Kingston 2003). In this regard, Oxyneis is closely related to Tabellaria Ehrenb. ex Kütz., a common araphid freshwater diatom. Actinella and Nupela both possess a raphe, although the morphology of this feature is very different, yielding placement of the two genera into separate subclasses (Round et al. 1990). Actinella is characterized by reduced raphe slits, one rimoportula per valve, and frustules that are asymmetric along all three axes. Nupela has a fully formed raphe on one (heterovalvar forms) or both (isovalvar forms) valves and symmetric frustules. Although Nupela was established by Vyverman & Compère in 1991, the majority of species now contained within the genus were originally described as Navicula, a genus that has been split into numerous taxa in recent years (see Metzeltin & Lange-Bertalot 1998, 2007 and references therein). Phylogenies based on gene sequences