Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland—Ostreoidea

Abstract

The current work is the first part of a taxonomic revision of the highly diverse Kimmeridgian bivalve fauna of the              Reuchenette Formation of northwestern Switzerland (Canton Jura). It provides a taxonomic, paleoecologic and bibliographic review of the eight oyster species characterizing the northern Helvetic shelf: Circunula n. gen. cotyledon (Contejean, 1859) (Gryphaeidae, ?Pycnodonteinae), Nanogyra (Nanogyra) nana (J. Sowerby, 1822), Nanogyra (Palaeogyra) reniformis (Goldfuss, 1833), Nanogyra (Palaeogyra) virgula (Deshayes, 1831) (Gryphaeidae, Exogyrinae), Helvetostrea n. gen. sequana (Thurmann & Etallon, 1862) (Flemingostreidae, Crassostreinae), Praeexogyra dubiensis (Contejean, 1859), Praeexogyra monsbeliardensis (Contejean, 1859) (Flemingostreidae, Liostreinae), and Actinostreon gregareum (J. Sowerby, 1815) (Arctostreidae, Palaeolophinae).The paper proposes two new genera: Circunula and Helvetostrea. Palaeogyra Mirkamalov, 1963, is considered a subgenus of Nanogyra Beurlen, 1958. Lectotypes are designated for six species: C. cotyledon, Praeexogyra acuminata, P. dubiensis, P. monsbeliardensis, H. caprina, H. sequana. The figured types of H. oxfordiana (Rollier, 1917) and N. auricularis (Münster in Goldfuss, 1833) are considered holotypes by monotypy. All types are refigured in drawings and/or photographs.        Early phases of shell ontogeny in general and the generic characters of Praeexogyra are revisited. Larval shells or their internal moulds are shown for six species: N. nana, N. reniformis, N. virgula, N. cf. auricularis, Praeexogyra cf. sandalinoides (de Loriol, 1901), and Actinostreon marshii (J. Sowerby, 1814). All of them are “Crassostrea”-like suggesting a planktic-planktotrophic mode of development. Circunula n. gen. shows a relatively high incidence of prosogyry (up to ca. 20% of studied specimens) during very early postlarval development. To a lesser extent, prosogyry has also been observed in species of Catinula, Praeexogyra and Pernostrea. Chomata are typical of early ontogenetic stages of Circunula n. gen., but they disappear during later growth stages.        Circunula n. gen. cotyledon is a typical early settler on hardgrounds but occurs also in subtidal soft-bottom environments attached to large shells. Nanogyra (N.) nana attached itself to all kinds of biogenous hard and soft substrates including algal stems and thalli. It is regularly found in calm to moderately energetic shallow marine paleoenvironments. Nanogyra (P.) reniformis frequently settled on the interior of empty bivalve shells. Nanogyra (P.) virgula was esentially a secondary soft-bottom dweller of shallow marine marls and lime muds. The species is often found concentrated in widely distributed (par)autochthonous lumachelles (“virgula marls” of authors) in the Upper Oxfordian, Upper Kimmeridgian and Tithonian. Praeexogyra dubiensis and P. monsbeliardensis occur in marly, shallow marine paleoenvironments. Praeexogyra dubiensis appears to have preferred attachment to small objects in a moderately energetic facies. In the study area it is also associated with algal meadows. Praeexogyra monsbeliardensis was preferentially gregarious in somewhat deeper and calmer paleoenvironments. The strongly chambered and probably fast growing Helvetostrea n. gen. sequana was adapted to moderate to high energetic shallow marine, marly habitats. It is frequently associated with corals and forms ostreoliths or small oyster buildups. Actinostreon gregareum usually lived gregariously but was also able to attach itself to algae on soft substrates. The species is known from calm marly to higher energetic coralline paleoenvironments.