Pyrumthecina n. Infraord.: Revisiting the Morphological Diversity and Identifying the Phylogenetic Home of Argynnia Within Arcellinida (Tubulinea: Amoebozoa).

Arcellinida is a diverse lineage of testate amoebae within Amoebozoa, whose evolutionary history has been clarified through phylogenomics, resulting in a stable classification into suborders and infraorders. Yet, some taxa, such as the genus Argynnia, remain unplaced due to ambiguous morphology and uncertain positions in single-gene phylogenies. In this study, we explore Argynnia by presenting new records of species sampled across Brazil and address its longstanding taxonomic uncertainty through phylogenomics including the Protist 10,000 Genomes isolate P10K-MW-000941. Previous single-gene analyzes suggested a close phylogenetic affinity between this isolate and Argynnia. Here, we confirm this relationship and using a curated, contaminant-free 227-gene dataset generated through the PhyloFisher workflow, we reconstructed the phylogeny of Arcellinida and confirmed the placement of P10K-MW-000941. Our results recover this isolate as a well-supported long-branch, sister to the infraorder Longithecina. Considering this phylogenomic placement, the distinct morphology of Argynnia, and its consistent placement outside existing infraorders based on single-marker analyzes, we propose Pyrumthecina n. infraord. and Argynniidae n. fam. to accommodate P10K-MW-000941 and all Argynnia species. The recognition of Pyrumthecina refines our understanding of shell evolution and diversification within Arcellinida, while also underscoring the presence of additional deeply branching lineages among taxa currently classified as incertae sedis.