Prestonellinae, van Bruggen, Herbert & Breure, 2016Prestonella Connolly, 1929

Bruggen, A. C. Van, Herbert, David G. & Breure, Abraham S. H., 2016, Prestonellinae-validation of the name as a new subfamily of Bothriembryontidae (Mollusca, Gastropoda, Orthalicoidea), Zootaxa 4084 (4) : -

Superfamily Orthalicoidea Albers, 1860 Family Bothriembryontidae Iredale, 1937 Subfamily Prestonellinae subfam. n. Prestonellidae van Bruggen, 1978: 893. Not available.

Diagnosis. Shell ovate-conical to elongate-globose, thin; aperture lacking dentition; outer lip not thickened; penis sheath long, covering most of the penis; vas deferens strongly attached to upper part of penis with connective tissue.

Type genus. Prestonella Connolly, 1929 ; type species by original designation: Bulimus bowkeri G.B. Sowerby III, 1890.

Constituent genera. Prestonella Connolly, 1929 , Discoleus Breure, 1978 , and Plectostylus Beck, 1837 .

Remarks. The taxonomic history of Prestonella has been elaborated by Herbert (2007) and molecular sequence data presented by Herbert & Mitchell (2009) demonstrated a well-supported sister-group relationship between Prestonella and an Australasian clade comprising Bothriembryon and Placostylus . Based on extensive comparison with published morphological data, Herbert & Mitchell (2009) chose to refer Prestonella to the Bulimulidae s.l., employing this family name for a broadly interpreted assemblage of orthalicoid taxa not belonging to the Orthalicidae or Amphibulimidae . While Herbert & Mitchell (2009) had only a limited set of nine orthalicoid species for molecular studies, Breure et al. (2010) confirmed their results with a dataset of 22 orthalicoid taxa. They concluded that Prestonella clustered in a monophyletic clade including the Australasian Placostylus , Bothriembryon and the South American Plectostylus . This clade was treated as Placostylidae s.l. Pilsbry, 1946. Subsequently, using an extended data set of 74 taxa representing 30 genera, Breure & Romero (2012) were able to confirm that this clade also includes the South American Discoleus , and used the oldest available family name Bothriembryontidae Iredale, 1937 for this Gondwanan group.

Earlier, Neubert et al. (2009) had already demonstrated that Placostylus and allied genera from New Zealand and Melanesia are anatomically distinct from other bothriembryontids and similarly they appeared as a monophyletic group in the study of Breure & Romero (2012), who considered them as the subfamily Placostylinae . Likewise, molecular data show the Australian genus Bothriembryon to be a well-supported monophyletic entity, which Breure & Romero (2012) regarded as the subfamily Bothriembryontinae . The remaining bothriembryontids in the analysis of Breure & Romero (2012), namely Prestonella , Discoleus , and Plectostylus , formed a separate clade for which there is currently no name available. For this we here propose to validate the name Prestonellinae , choosing to rank the taxon at subfamily rather than family level. Although this clade currently lacks significant molecular support, Cuezzo et al. (2010), based on cladistic analysis of external and internal morphological characters from a number of orthalicoid species, including representatives of Bothriembryon , Prestonella , Discoleus , and Plectostylus , found two synapomorphies for the latter three genera that comprise the Prestonellinae . The morphological synapomorphies relate to characters of the male distal genitalia and are identified in bold above as the primary diagnostic characters. It should be noted that the statements of Breure (1978, 1979) on the lack of a penis sheath in Discoleus and Plectostylus need to be corrected here, and might be described more properly as “no externally visible penis sheath (pseudo-sheath) in both genera”. The lack of support for the Prestonellinae in the molecular studies may be a result of limited data and needs to be explored further with expanded taxon sampling. This may clarify whether the subfamily as here interpreted requires division into two separate entities of African and South American distribution respectively.

The Bothriembryontidae are distributed across the southern continents and Breure & Romero (2012) hypothesized that vicariance caused by break-up of Gondwana could explain the relationships between the different bothriembryontid clades. Using ancestral area reconstruction and divergence time analysis they showed that the split between the Australian Bothriembryon and the South American Discoleus and Plectostylus is according to the known geological data of break-up of Gondwana. However, the split between Prestonella and the South American genera could not be attributed to vicariance with the data available and remains a topic for future research.