Zhang, G.; Cassis, G.; Koehler, F.

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 through detecting 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 certain continuous characters. We propose that these inferred differences represent apomorphies. Ultimately, our results suggest that of the four species currently accepted, both T. mitchellae and T. novaehollandiae should be synonymized as T. novaehollandiae is paraphyletic with respect to T. mitchellae. In addition, T. darlingtoni nests within the T. richmondiana lineage, and both taxa are also considered synonyms. 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). The Kruskal-Wallis tests support the recognition of four distinct species, T. novaehollandiae and T. richmondiana plus two new species.