Doubled helix: Sagdoidea is the overlooked sister group of Helicoidea (Mollusca: Gastropoda: Pulmonata)

Abstract

We performed multi-locus, time-calibrated phylogenetic analyses of Jamaican Pleurodontidae to infer their relationships within pulmonate land snails. These analyses revealed that Sagdoidea, with about 200 species in the Caribbean Basin and neighbouring regions, is the sister group of Helicoidea with about 4700 species worldwide and that these superfamilies diverged 61-96 Ma. Morphological disparity in Sagdoidea is similar to that in Helicoidea despite its much lower species richness. Helicoidea originated in the New World and colonized the Old World 46-64 Ma. Pleurodontids and sagdids colonized Jamaica 15.0-18.4 and 12.8-16.5 Ma, respectively, consistent with geological estimates of Jamaican subaerial emergence by mid-Miocene. Allopatric convergence in shell morphologies required caution in using fossils from outside the geographic range of ingroup taxa to calibrate molecular clock estimates. Estimates of ages of clades varied by 24-55%, depending on the calibration points included. We use these results to revise Helicoidea and Sagdoidea. Pleurodontids from Jamaica and the Lesser Antilles were reciprocally monophyletic but other putative pleurodontids grouped basally in Helicoidea as Labyrinthidae (new family), or with Sagdidae. Newly recognized members of Sagdoidea are Solaropsinae and Caracolinae (Solaropsidae), Polydontinae (Sagdidae) and Zachrysiidae (new family). Pleurodontidae is restricted to two subfamilies, Pleurodontinae, in the Lesser Antilles, with Gonostomopsinae, a synonym, and Lucerninae resurrected for the Jamaican endemic genera Lucerna, Dentellaria, Thelidomus and Eurycratera. Lucerna and Dentellaria have been treated as subgenera of Pleurodonte, but rendered it paraphyletic in our analyses.