Systematic reappraisal of the diminutive Amphora thumensis and the transfer of Halamphora parathumensis to the genus Amphora

Since its description the amphoroid diatom Amphora thumensis has confounded simple taxonomic classification having been assigned to Amphora, Cymbella, and now Halamphora. Taxonomic ambiguity around this taxon stems from its small size and reduced, often contradictory, morphological features. In this investigation, we confirm the taxonomic position of A. thumensis by presenting a four-marker molecular phylogeny for a population collected from a small spring-fed lake in the Rocky Mountains of Colorado, USA. Our phylogenetic hypothesis places A. thumensis not within the genus Halamphora, but within the deep branching lineages of Amphora. We discuss this phylogenetic hypothesis in the context of the morphological and ecological diversity of the genus and evaluate the utility of putative diagnostic characters within Amphora. In light of this new evidence, the valid name Amphora thumensis (Mayer) Krieger is identified as the preferred designation for this taxon. Consequently, we transfer the morphologically similar Halamphora parathumensis to Amphora.