Generic delimitation in Caryophyllaceae has been a challenge, and has been informed most recently by use of molecular phylogenetic data. In this study, analysis of 29 samples from the small segregate Mononeuria using nuclear ITS and plastid rps16 data revealed it to be polyphyletic. The type species Mononeuria patula as well as two others (M. muscorum and M. paludicola) were shown to belong to the Sabulina clade. The remaining species formed a clade that also included the previously monotypic Geocarpon and was sister to a heterogeneous group that included the Hawaiian Schiedea and three other monotypic genera, Honckenya, Wilhelmsia, and Triplateia. Although several nomenclatural options are available, we propose to place the species from this clade into a single genus, Geocarpon, which basically follows the most recent treatment after exclusion of Sabulina species, but with the necessary new genus placements. New combinations are proposed: Sabulina muscorum, Sabulina paludicola, Geocarpon carolinianum, Geocarpon cumberlandensis, Geocarpon glabrum, Geocarpon groendlandicum, Geocarpon nuttallii, and Geocarpon uniflorum. Analysis of the sequence data revealed remarkable variability among populations of Sabulina (formerly Mononeuria) patula, Sabulina (formerly Mononeuria) paludicola, and Geocarpon (formerly Mononeuria) groenlandicum, suggesting that cryptic species may be present. The data also suggested that broader sampling of Sabulina and Geocarpon could lead to increased understanding of the timing and origins of occupation of calcareous glades and rock outcrop habitats in eastern North America.

New sequences from Sanger sequencing; previously obtained sequences downloaded from GenBank.

A few samples lack data for the rps16 intron.