Family Argynniidae Porfirio-Sousa, Gomes E Souza, Jones, Brown, Lahr, and Tice 2025

Diagnosis: Can be diagnosed by its specific sequences of the nuclear and mitochondrial DNA markers (SSU and COI) and by its phylogenetic placement. 

Node-based definition: the clade originating with the first ancestor of Argynnia and isolate P10K941 that is not the ancestor of Longithecina. 

Type Family: Argynniidae Porfirio-Sousa, Gomes E Souza, Jones, Brown, Lahr, and Tice 2025. 

Remarks: The genus Argynnia Vucetich 1974 has remained a taxonomic enigma within Arcellinida due to its distinct shell morphology and lack of phylogenetic resolution based on singlemarker reconstructions. Originally classified in the family Nebelidae because of its pear-shaped, laterally compressed test composed of mineral particles and euglyphid plates, features resembling Nebela, Argynnia genus establishment was initially invalid due to the lack of a type species. This was later corrected with the designation of Argynnia schwabei as the type (Vucetich 1974; Meisterfeld 2002; Kosakyan et al. 2025). Despite morphological similarities to nebelid genera, Argynnia also exhibits traits typical of Difflugia (Sphaerothecina), and molecular phylogenies based on SSU and COI have consistently failed to place Argynnia within Hyalospheniformes or any recognized infraorder. Consequently, Argynnia’s evolutionary relationships remained unresolved. Moreover, Argynnia exhibits a distinctive combination of traits that supports its differential diagnosis: it lacks an organic rim around the pseudostome, distinguishing it from genera within Hyalospheniformes, and it has a laterally compressed shell, in contrast to members of the infraorder Longithecina (including Difflugidae and Lesquereusia), which typically possess tubular or cylindrical tests. The close relationship between Argynnia and the P10K941 isolate, combined with the phylogenomic placement of the P10K941 isolate, illuminates the enigma and clarifies why single-gene markers have failed to place Argynnia; they comprise an independent deeper branch within Arcellinida, accommodated by the erection of Pyrumthecina n. infraord. as a sister infraorder to Longithecina (From: Porfirio-Sousa, Gomes E Souza, Jones, Brown, Lahr, and Tice 2025).