When discrete characters are wanting: Continuous character integration under the phylospecies concept informs the revision of the Australian land snail Thersites (Eupulmonata, Camaenidae)

Species that are predominantly characterized by continuous instead of discrete morphological characters pose a challenge to species delimitation. Under the phylospecies concept, species are delimited by apomorphies, which are difficult to establish when characters are not discrete. In the present study, we address these challenges in the Australian land snail genus Thersites (family Camaenidae), where accepted species exhibit a scarcity of discrete distinguishing characters and differ in a limited range of continuous characters according to prior taxonomic studies. We integrate analyses of genome-scale molecular data with evaluations of several continuous, qualitative, and discrete morphological characters derived from landmarks to delineate species by identifying apomorphies. We found that dimensions of shell and genitals overlapped considerably among the currently accepted species. These overlaps may indicate morphospace saturation, which may affect species delineation through the gen-morph species concept. Statistical methods, such as Dunn’s test, failed to consistently delineate monophyletic taxa. Additionally, we could not derive apomorphies from discretized landmarks for the prevalence of outlier specimens. However, applying the Kruskal-Wallis test at certain nodes of the tree revealed significant differences in some continuous characters. We propose that these inferred differences represent apomorphies of taxonomic lineages. Ultimately, we suggest based on our findings that of the four currently accepted species, two should be synonymized. To maintain monophyly of taxa, we synonymize Thersites mitchellae with T. novaehollandiae and T. darlingtoni with T. richmondiana . Distinct character states of the umbilicus support the existence of two independent lineages: One with an open ( T . sp1 + T . sp2) and one with a closed umbilicus ( T. novaehollandiae + T. richmondiana ). Overall, the Kruskal-Wallis tests support four distinct species, T. novaehollandiae and T. richmondiana plus two newly described species.