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Taxonomic uncertainty is not only confined to suprageneric and genus levels, as unravelling foraminiferal taxonomy at a species level is just as, if not more complex. Foraminiferal species are almost exclusively delineated based on their morphological characteristics. Each morphospecies is appointed a type specimen, which exhibits the key diagnostic morphological characters. This type specimen anchors the meaning of the name and the species concept, providing a standard point of reference. Historically, foraminiferal species were delineated by the application of predefined rules based on morphological characters, which reveal traits and patterns that are common to all individuals. These rules reduce reality into qualifiable and quantifiable traits, using a language of specialised terms to designate species of similar morphologies into the same group (Hottinger, 2000).

However, one of the principal taxonomic problems is that significant morphological plasticity is exhibited by the foraminiferal test. Uncertainty at species level has arisen due to a lack of scientific consensus on the interpretation of this morphological variability, which in turn led to the development of two parallel taxonomic schools. The first taxonomic school (practice) is often referred to as ‘lumping’, whereby few cosmopolitan species, which possess extensive morphological variability are recognised. The second taxonomic school focuses upon delineating foraminiferal species based on subtle morphological differences, whereby numerous species concepts with restricted biogeographical distributions are recognised. This taxonomic practice is commonly referred to as ‘splitting’. The assignment of individual specimens into a particular species is often a compromise; hence, species identification is often down to the personal and provincial bias of the researcher. Notably, the researcher’s access to the primary literature/ reference material often dictates which taxonomic practice (school) was employed (Buzas, 1966).

The traditional method of distributing taxonomic knowledge within the academic community has also compounded taxonomic uncertainty. For example, a fundamental limitation in the documentation of foraminiferal taxonomy is that many of the early descriptions of the most common species are often uninformative and the accessibility and quality of many of the type specimens is varied. For example, many of the holotypes have been lost or were never deposited (Fossiner and Hawksworth, 2009). Moreover, many of the original line drawings used to illustrate test morphology are often simplistic and neglect many of the key morphological features (Holbourn and Henderson, 2002). Additionally, the species descriptions and illustrations of foraminifera are dispersed amongst a wide range of publications (Holbourn and

Henderson, 2002). There is also a scarcity of studies which are devoted to quantitatively analysing the interspecific morphological boundaries of foraminifera, e.g. Brooks, (1967), Buzas (1966), Buzas et al. (1985), Debenay et al. (1998), Burgess and Schnitker, (1990) and Gooday et al. (2001). These studies have primarily focused upon quantifying a handful of line measurements (e.g. maximum test diameter) and presence/absence features (e.g. presence of umbilical boss) to assess structural features. Consequently, limited emphasis has been placed upon quantifying the full range of morphological variability exhibited by each species. Recent advancements in imaging technology such as the development of X-ray tomography and transmission electron microscopy (TEM) have presented opportunities to assess and quantify morphological variability within foraminifera (Spejjer et al., 2008; Briguglio et al., 2011; Prazeres et al., 2015). However, many of the recent studies utilising these new imaging techniques have primarily focused on quantification of growth rates (biometry), placing limited emphasis on using these new insights as an additional line of taxonomic evidence to elucidate interspecific boundaries. In contrast, the majority of studies within the foraminiferal literature have focused on delineating taxa based on qualitative morphological traits. However, no standardised terminology or consistent morphological features were analysed; this hinders the degree of comparability of species classifications between studies. Consequently, these factors made it difficult to compare taxa across time and space. Moreover, these classical morphological studies are primarily limited to a single line of taxonomic evidence. Thus, the value of the diagnostic features identified is hampered by the uncertainty surrounding whether the species boundaries identified correspond to genetically distinct species. As species delineation within classical taxonomy is often based on the judgement of an individual observer rather than unambiguous criteria, it is unsurprising that historically the identification of foraminifera at the species level is tentative at best (Murray, 2007).

Owing to the complex history of foraminifera, the determination of a reliable estimate of foraminiferal diversity is challenging. One of the original estimates of the total number of foraminifera recorded by Brady (1884) in the Challenger report estimated that there are 875 species. Whilst other authors have estimated foraminiferal diversity to be between 10,000 to 12,000 species (Boltovskoy and Wright, 1976), 10,000 species (Vickerman, 1992), 8000 species (Minelli, 1993), 6000 extant species (Jones, 1994) and 5,000 species were recognised by Debenay et al. (1996). One of the main challenges associated with compiling an estimate of foraminiferal biodiversity is the extensive synonymy found within the literature. Murray (2007) suggests that the number of living benthic foraminiferal species ranges from ∼3959 to ∼4280

species at 10% synonymy, and from ∼3210 to ∼3531 species at 25% synonymy. The latest global estimate lists 16,207 taxa, and 11,457 accepted species (combined extant and fossil estimates) in the world foraminiferal database (Hayward et al., 2015). This uncertainty of foraminiferal diversity and high levels of synonymy complicates the transfer of knowledge between researchers. Robust and consistent taxonomic names are required to create and provide the taxonomic placement of a species, without which the empirical data would have no meaning (Valdecasas et al., 2013). Thus nomenclatural instability may hamper the value of foraminifera in applied taxonomic investigations. Consequently, the clarification of species concepts and nomenclature is called for, to attain a more stable taxonomy.