Gastropods from upper Pliensbachian–Toarcian (Lower Jurassic) sediments of Causses Basin, southern France and their recovery after the early Toarcian anoxic event

Abstract

A gastropod fauna has been studied from upper Pliensbachian -upper Toarcian deposits of two sections of the Causses Basin (southern France) in order to investigate the mode of recovery after the early Toarcian anoxic event. The fauna consists of 15 species, one of which is new (Bathrotomaria kronzwilmesorum sp. nov.). Their stratigraphical distribution shows two peaks of diversity -in the Bifrons Zone (Bifrons Subzone) and in the Aalensis Zone (Mactra Subzone) -which reflect brief times during which the oxygen content and bottom consistency favoured the settlement of a relatively diversified fauna. In the Variabilis-Pseudoradiosa zones, gastropods are only represented by two species. This probably indicates more severe and unstable environmental conditions, only allowing the survival of gastropod taxa with wide adaptive capacities. The very low species diversity and the discontinuous and slow faunal recovery were probably determined by physiographic factors. The Causses area was a small basin confined by exposed lands and open towards the central part of western Tethys. Gastropods described here occur exclusively in the Toarcian -early Aalenian communities of the European epicontinental seas, whereas species from the central region of western Tethys are absent. Geographic isolation and marginal location of the Causses Basin restricted faunal exchange with the western European epicontinental seas, preventing fast recovery after the anoxic event. Gastropods of the central region of the western Tethys were probably unable to settle and colonize that area due to the strongly different environment.

Gastropods from the Jurassic neptunian sills of Rocca Busambra (north‐western Sicily, Italy): Patellogastropoda, Pleurotomarioidea, Scissurelloidea, Fissurelloidea and Eucycloidea

“…Preliminary studies on the whole material (Conti et al 1993(Conti et al , 2004Szabó et al 1993) demonstrated that the gastropods from neptunian sills of Rocca Busambra are closely related to a faunal stock named by Szabó (1988) as Alpine type. As mentioned above, this stock inhabited topographically elevated areas of the central region of western Tethys characterized by condensed pelagic deposition (Pelagic Carbonate Platforms sensu Santantonio 1993Santantonio , 1994Santantonio & Carminati 2011) and was very different in taxonomic structure and composition from the faunas of the European epicontinental seas, the intra-Tethyan shallow water carbonate platforms and the North African belt (Szabó 1988(Szabó , 1992(Szabó , 1994Conti & Monari 2001;Gatto & Monari 2010). Its history started from the Sinemurian and can be traced at least up to the Bajocian (Intratethyan faunal unit in Conti & Szabó 1988;Conti et al 1993Conti et al , 2004Conti & Monari 2001).…”

“…4, figs 1-3;Conti & Monari 1986, p. 182, pl. 2, figs 1-2, 5;Gatto et al 2015a, p. 219, text-figs 2-3), type species of the genus Laevitomaria Conti & Szabó, 1987, differs from Laevitomaria babalusciae sp. nov. in having an evenly conoidal teleoconch outline.…”

“…Uppermost Lower Bajocian -Upper Bajocian (Stephanoceras humphriesianum -Parkinsonia parkinsoni biozones), Bakony Mountains (Hungary). Conti & Szabó, 1987 has been revised by Gatto et al (2015a) to which paper the reader is referred for further details. The species described below extends the stratigraphical range of the genus to the Lower Kimmeridgian in the central region of western Tethys.…”

“…Laevitomaria daytiai (Das, Bardhan & Kase, 2005) (p. 341, text-fig. 8a-d), from the Early Callovian -Oxfordian of Kutch (western India), is the only species currently known that is sub-coeval with L. babalusciae (see Gatto et al 2015a). It differs in its wider spiral angle, cyrtoconoidal shell, more convex whorls separated by deeper suture, and dominant spiral sculpture.…”

“…Elymicyclus has an early teleoconch of eucycloscalid type (see below). In the species of Lokuticyclus (see Szabó 1995Szabó , 2009Conti et al 2007;Gatto & Monari 2010), including the type species Lokuticyclus urkutensis (Szabó, 1995) (p. 70, pl. 7, figs 7-9;Szabó 2009, p. 78, text- fig.…”