Mid-Cretaceous (Albian–Cenomanian) freshwater Mollusca from the Clarence Valley, Marlborough, New Zealand, and their biogeographical significance

Mollusca from the Warder Formation in the middle Clarence Valley, Marlborough, are the first well-preserved Cretaceous freshwater Mollusca recorded from New Zealand. They include a common mytilid (?Mytilus), a rare second mytilid (?Xenostrobus), a rare hyriid (?Echyridella), a common smooth ?Corbicula species and a rare commarginally sculptured one, an uncommon thiarid (?Melanoides), a rare lymnaeid (?Austropeplea), and abundant Clarenciella hippopedion n. gen., n. sp., a rissooidean gastropod referred to Cochliopidae. The Warder Formation underlies Gridiron Volcanics (97–95 Ma) and is laterally equivalent to the Bluff Sandstone, which contains a marine Ngaterian fauna (99.5–95.2 Ma, late Albian–Cenomanian). The freshwater fauna lived before New Zealand began to separate from the rest of Gondwana (85 Ma). Faunal elements lost from New Zealand since the Cretaceous include Cyrenidae and Cochliopidae, which are diverse in the South American Cenozoic and living freshwater faunas. In contrast, Hyriidae have a continuous Jurassic–Recent record in New Zealand and are among freshwater taxa that survived both the fragmentation of Gondwana and the putative Oligocene submergence. These and other archaic endemic freshwater taxa such as Latia (Gastropoda, Chilinoidea) remain a vestige of New Zealand’s Gondwanan heritage, modified by extinction, evolution and dispersal.