Comparative analysis of six common foraminiferal species of the genera Cassidulina, Paracassidulina, and Islandiella from the Arctic–North Atlantic domain

Abstract. Morphologically similar benthic foraminiferal taxa can be difficult to separate. Aside from causing issues in taxonomy, incorrect identifications complicate our understanding of species-specific ecological preferences and result in flawed palaeoenvironmental reconstructions and geochemical results. Over the years, a number of studies have grouped together several key Arctic–North Atlantic species in various combinations, despite their distinct environmental preferences and/or stratigraphical differences, causing great confusion in the literature. These species include Cassidulina laevigata, Cassidulina neoteretis, Cassidulina teretis, Paracassidulina neocarinata, Islandiella helenae, and Islandiella norcrossi. Here, we provide for the first time a detailed comparison of these taxa. We present a compilation of the original species descriptions, along with clear, illustrated guidelines on how to separate these taxa to circumvent taxonomic confusion. We acknowledge that some features cannot easily be seen with a standard low-powered microscope, especially if specimens are not well preserved. In those cases, we recommend the following actions: (i) always strive to make a precise identification and at least differentiate between the three genera; (ii) where C. neoteretis and C. teretis cannot be separated, and where the stratigraphical context does not make the species identification obvious, specimens belonging to these taxa should be reported as C. teretis/C. neoteretis; and (iii) where specimens in a sample cannot be confidently assigned to a specific species of Islandiella or Cassidulina, specimens should be grouped as Islandiella spp. or Cassidulina spp., followed by naming the most dominant species in brackets. The improved identification of Cassidulina, Paracassidulina, and Islandiella specimens will ensure development of a better understanding of the ecological affinities of these key Arctic–North Atlantic taxa, consequently resulting in more accurate palaeoenvironmental reconstructions and geochemical data.