Snails of the genus Pachnodus (Mollusca; Gastropoda; Enidae): their origins and evolution

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Summary

Aim The phylogeny of the enid land-snail genus Pachnodus was determined in order to provide information on biogeographical patterns within the granitic Seychelles islands.

Location The genus Pachnodus is endemic to the granitic islands of Seychelles (Mahé, Silhouette, Praslin, La Digue and Fregate).

Methods Phylogeny was determined using a cladistic analysis of nineteen shell and soft-body anatomy characters. The outgroup used was the central-east African genus Cerastus.

Results The proposed phylogeny indicates that the genus divided into two distinct subgenera early in its history. Subsequent speciation occurred in parallel in the two subgenera, resulting in several islands supporting at least two distinct species representing the two subgenera.

Main conclusions The pattern of speciation is largely explicable by vicariance as a result of sea-level rises, followed by habitat specialisation and further speciation. The pattern is in contrast to previously published scenarios for other taxa in the islands and indicates significant differences between evolutionary and habitat factors in the biogeography of the Seychelles fauna

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