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Diagnostic Apomorphies of Foraminifera
The diagnostic apomorphies of Foraminifera include both morphological and molecular characters. One of the most recognizable apomorphies that characterizes the derived foraminiferal subclades, is the presence of a test, or shell, that grows by terminal addition, either by the addition of discrete chambers (resulting in a multi-chambered test), or by accretionary growth whereby new material is added to the test at the apertural margin of a single-chambered tubular test (Goldstein 2002, Hottinger 2000).
Additional apomorphies can only be observed in living taxa, and thus serve to distinguish Foraminifera from other crown clades (such as Gromida, Haplosporidia, Plasmodiophorida, and Radiolaria), within the more inclusive clade Rhizaria. The small subunit ribosomal RNA sequences of foraminiferans possess a unique helix in Domain III that is up to 350 nucleotides in length, and lies between Helices 41 and 39 in Domain III, and (Bowser et al. 2008: Fig. 5.2, Habura et al. 2004: Figs. 2, 3). The gene sequences that code for the two foraminiferal actin paralogs contain twenty unique spliceosomal introns (Flakowski et al. 2006: Fig. 2).
Foraminiferans possess a type 2 ß-tubulin isoform that is highly divergent relative to other eukaryotes (Habura et al. 2005: Figs. 3, 4). The reticulopodia of foraminiferans possess motility organizing vesicles (elliptical, fuzzy-coated vesicles) (Bowser and Travis 2002: Pl. 2, figs. 1, 2), and tubulin-containing helical filaments (Bowser and Travis 2002: Pl. 2, fig. 3; Pl. 3, Habura et al. 2005: Fig. 1).