Systematic reappraisal of the diminutive <i>Amphora thumensis</i> and the transfer of <i>Halamphora parathumensis</i> to the genus <i>Amphora</i>

Since its description the amphoroid diatom <i>Amphora thumensis</i> has confounded simple taxonomic classification having been assigned to <i>Amphora</i>, <i>Cymbella</i>, and now <i>Halamphora</i>. 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 <i>A</i>. <i>thumensis</i> 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 <i>A</i>. <i>thumensis</i> not within the genus <i>Halamphora</i>, but within the deep branching lineages of <i>Amphora</i>. 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 <i>Amphora</i>. In light of this new evidence, the valid name <i>Amphora thumensis</i> (Mayer) Krieger is identified as the preferred designation for this taxon. Consequently, we transfer the morphologically similar <i>Halamphora parathumensis</i> to <i>Amphora</i>.