New fossil mussels (Bivalvia: Mytilidae) from Miocene hydrocarbon seep deposits, North Island, New Zealand, with general remarks on vent and seep mussels

Bathymodiolus (sensu lato) heretaunga sp. nov. and Gigantidas coseli sp. nov. are described from Miocene-age hydrocarbon seep carbonates of North Island, New Zealand, adding to only four described fossil hydrocarbon seep mussel species. Both new species are small compared to their modern congeners, and it is suggested that an evolutionary trend toward gigantism occurred in vent and seep mussels since the first known fossil species of the group appeared in the Middle Eocene. Bathymodiolus heretaunga is highly variable in morphology, potentially reflecting population variation over a wide geographic area. Gigantidas coseli is the first named species, fossil or modern, of its genus known from hydrocarbon seeps, although several closely related species in the same group, the Bathymodiolus childressi clade, have been reported from these environments.