The “inoceramus limpet”Gigantocapulus problematicus (Nagao & Otatume, 1938) in New Zealand (Late Cretaceous Gastropoda or Monoplacophora, Gigantocapulidae n. fam.)

Abstract

Limpets up to at least 200 mm long occur widely in New Zealand early Haumurian (Campanian) rocks. Three specimens are in life position, attached to articulated specimens ofIsognomon. The limpet has several unique characters — an apparently anterior marginal apex (all gastropod limpets have a central or posterior apex), an enormously thickened apical area, and a tongue-like projection, with a gape at each side, below the margin at the anterior — demonstrating that it belongs inGigantocapulus Hayami & Kanie. The low shape, weak sculpture, and marginal apex indicate that it belongs inG. problematicus (Nagao & Otatume), previously reponed only from Japan and Kamchatka. New Zealand material supports previous interpretations that the apex was anterior, and thatGigantocapulus was an epiparasite or, more probably, a filter feeder living sedentarily on bivalves. The shell is, uniquely, composed of calcite in multiple complex crossed-lamellar layers, very different from the aragonite shell with inner nacreous and outer prismatic layers of both early Palaeozoic and present-day tergomyans. Nevertheless, as the anteriori?) tongue-like projection, thickening and gapes closely resemble those of early Palaeozoic tergomyans, no other limpet-shaped molluscs are known with an anterior apex, and not all modem monoplacophorans are minute (Neopilina reaches at least 40 mm long), a position in Monoplacophora (= Tergomya) remains a possibility. However,G. giganteus (Schmidt) has a subcentral apex and is clearly cyclomyan rather than tergomyan.Gigantocapulus is probably a vanikoroidean gastropod, but could also be a tergomyan, a helcionelloidan, or a member of another, now extinct group of gastropods. Gigantocapulidae n. fam. is proposed.