New Zealand late Middle Cambrian molluscs and the origin of Rostroconchia and Bivalvia

Abstract

Abstract Nine monoplacophoran taxa and one bivalve are described from a late Middle Cambrian limestone lens in New Zealand. Two new genera, Eurekapegma and Enigmaconus, and five new species, Obtusoconus foliaceus, Anabarella simesi, Eurekapegma cooperi, Enigmaconus parvus, and Protowenella cobbensis are named. Enigmaconus, a pegma-bearing monoplacophoran, is assigned to the new family Enigmaconidae within the order Cyrtonellida. Eurekapegma is assigned to the Stenothecidae Runnegar & Jell 1980. Pelagiellid monoplacophorans are assigned to a new order Pelagiellida. Aspects of the Pojeta-Runnegar model for the early evolution of the Mollusca are critically appraised. It is argued that Heraultipegma was not a rostroconch and that riberioid rostroconchs may have evolved from an Enigmaconus-like monoplacophoran as late as late Middle Cambrian. Evolution of bivalves from rostroconchs is rejected and replaced by the suggestion that bivalves are descended directly from monoplacophorans.