Presence of a posterodorsal ridge in Jurassic to Recent larval shells of arcoidean bivalves

Abstract

A posterodorsal notch is known from the larval shells (PII) of extant Ostreidae and Gryphaeidae. 1‐3 It results from a postanal ciliary tuft that deforms the shell edge and leaves a ridge-like growth track on the prodissoconch. 3 In fossil Ostreoidea this feature has been traced back into the Middle Jurassic, and was considered a probable autapomorphic character of the Ostreoidea. 4,5 The aim of this research note is, however, to document a posterodorsal ridge in some larval shells of Jurassic, Cretaceous, Eocene and Recent arcoid bivalves. All studied specimens were juveniles (� 3 mm in length), making determination of their taxonomic position below superfamily level beyond the scope of this study. The Recent material was collected in the lagoon and reef area of Foulpointe (also known as Mahavelona), 58 km north of Tamatave in northeastern Madagascar, from depths not greater than 5 m. The Eocene material was collected near Damery in the Paris Basin, France, and is of Lutetian age. 6 The Cretaceous material was collected in the vicinity of Ambatolafia in the Mahajanga Basin in northwestern Madagascar, and is of lower Albian age. 7 The Jurassic material was collected 3 km south of Zakaraha in southwestern Madagascar, and is of Bajocian age. 8 In all cases, sediment samples were taken and fractionated by sieving with mesh-sizes of 2.0, 1.0 and 0.5 mm. The Cretaceous and Jurassic samples were treated with a 5‐10% H2O2 solution using standard procedures prior to sieving. Specimens were mounted on stubs, sputter-coated with gold, and photos were taken with a Leo 1455VP scanning electron microscope. In all, 28 specimens belonging to seven species were examined (Table 1); in species E and G only right valves were available for study. The material is deposited in the palaeontological collection of the Museum fur Naturkunde in Berlin, Germany.