Hypelasma salevensis (FAVRE, 1913) from the Upper Kimmeridgian of the French Jura, and the Origin of the Rudist Family Requieniidae

The requieniid rudist species ‘Matheronia’ salevensis FAVRE, first described from the Tithonian of Mont Saleve, eastern France, is transferred to the genus Hypelasma PAQUIER, which is distinguished from Matheronia by possession of a posterior myophoral ledge in the left (attached) valve. Diminutive specimens from the Upper Kimmeridgian of the southern Jura are described and placed in this species. Hence, Hypelasma salevensis (FAVRE) is the stratigraphically oldest known member of the Family Requieniidae. It may also provide another example of phyletic size increase among rudists. Revised diagnoses are given for the family, genus and species. The main distinction between the requieniids and the diceratids, from among which they arose, concerns the angle between the coiling axis of the left valve and the commissural plane. In diceratids, this angle is large, such that the often sub-equal umbones tend to twist outwards from the commissural plane, so avoiding mutual interference. In requieniids, by contrast, this angle is small, such that the prominent umbo of the left valve tends to coil across the commissural plane in trochospiral to helicospiral fashion, while that of the right valve is suppressed in compensation, producing an exogyriform morphology. The requieniid modification of growth geometry, already present in H. salevensis, generated an extended basal surface on the flattened anterior wall of the left valve, implying specialized adaptation of these rudists as frictional or attached clingers. Requieniid ancestry should be sought among species of the pre-existing diceratid genera Epidiceras or Plesiodiceras, which also attached by the left valve. Although Plesiodiceras is favoured by its already more or less operculiform right valve and relatively small size, the derived condition of its posterior myophoral organisation is problematical. However, its juvenile shell shows some similarity of external form to H. salevensis, suggesting the possibility of paedomorphic evolution

Calcite/aragonite ratio fluctuations in Aptian rudist bivalves: Correlation with changing temperatures

Understanding how bivalves responded to past temperature fluctuations may help us to predict specific responses of complex calcifiers to future climate change. During the late-Early Aptian, aragonite-rich rudist bivalves decreased in abundance in northern Tethyan carbonate platforms, while rudists with a thickened calcitic outer shell layer came to dominate those of Iberia. Seawater cooling and variations in calcium carbonate saturation states may have controlled this faunal turnover. However, our understanding of how rudist lineages responded to changing environmental conditions is constrained by a lack of quantitative data on the evolution of thickness, size, and mineralogy of the shell. This study is based on volumetric measurements of the shell and shows the transition in lineages of the family Polyconitidae from aragonite-rich mineralogy in the earliest Aptian, to low-Mg calcite-dominated mineralogy in the middle Aptian, returning to aragonite-dominated composition in the latest Aptian. The platform biocalcification crisis that occurred at the Early-Late Aptian boundary in the Tethys was marked by a relative increase of calcite and a decrease in skeletal thickness and commissural diameters. The highest calcite/aragonite (Cc/A) ratios in polyconitid rudists accompanied the late Aptian cold episode, and the lowest values were reached during the warmer intervals of the earliest and latest Aptian. These results imply a correlation between Cc/A ratio values and temperature and suggest that some bivalves adapted to less favorable calcification conditions by changing calcite and aragonite proportions of their bimineralic shells and decreasing skeletal thickness, thereby reducing the metabolic cost of shell growth. GeoRef Subjec

Deep incision in an Aptian carbonate succession indicates major sea-level fall in the Cretaceous

Long-term relative sea-level cycles (0 5 to 6 Myr) have yet to be fully understood for the Cretaceous. During the Aptian, in the northern Maestrat Basin (Eastern Iberian Peninsula), fault-controlled subsidence created depositional space, but eustasy governed changes in depositional trends. Relative sea-level history was reconstructed by sequence stratigraphic analysis. Two forced regressive stages of relative sea-level were recognized within three depositional sequences. The first stage is late Early Aptian age (intra Dufrenoyia furcata Zone) and is characterized by foreshore to upper shoreface sedimentary wedges, which occur detached from a highstand carbonate platform, and were deposited above basin marls. The amplitude of relative sea-level drop was in the order of tens of metres, with a duration of 2 km wide and cut 115 m down into the underlying Aptian succession. With the subsequent transgression, the incision was back-filled with peritidal to shallow subtidal deposits. The changes in depositional trends, lithofacies evolution and geometric relation of the stratigraphic units characterized are similar to those observed in coeval rocks within the Maestrat Basin, as well as in other correlative basins elsewhere. The pace and magnitude of the two relative sea-level drops identified fall within the glacio-eustatic domain. In the Maestrat Basin, terrestrial palynological studies provide evidence that the late Early and Late Aptian climate was cooler than the earliest part of the Early Aptian and the Albian Stage, which were characterized by warmer environmental conditions. The outcrops documented here are significant because they preserve the results of Aptian long-term sea-level trends that are often only recognizable on larger scales (i.e. seismic) such as for the Arabian Plate

Revised lithostratigraphy of the Upper Cretaceous (Santonian) carbonate platform succession on the northern flank of Sant Corneli, southern Central Pyrenees

We describe a revised formal lithostratigraphy for the Sant Corneli Formation (Upper Cretaceous, Santonian) on the northern flank of the Sant Corneli Anticline, ENE of Tremp, southern Central Pyrenees. The formation comprises a carbonate platform succession, which overlies the platform deposits of the Montagut limestones, and is overlain by the basinal Herba-savina clays and marls. Five members are recognized in the formation, three of which are included in a newly defined type section in the western part of the outcrop. This section (196 m) exposes a lower and an upper tongue of the Aramunt Vell Member, separated by an eastward-thickening wedge consisting of the Sant Pere de Vilanoveta Member (new name) and overlying l’Aubagueta Member (new name). The Aramunt Vell Member comprises numerous coarsening-up cycles, each a few metres thick, of nodular silty marls and yellow-brown weathering, miliolid-rich bioclastic calcarenites. Its two tongues unite westwards to encompass the entire thickness of the formation. The Sant Pere de Vilanoveta Member contains coarsening-up cycles of coral-rich marls and marly limestones, rudist lithosomes and bioclastic floatstones and grainstones. The l’Aubagueta Member contains marly limestones and silty clays. Eastwards, two further members represent slope deposits that originally adjoined the platform, though now separated from it by a NW-SE trending normal fault. The Llau de Castellet Member (new name) comprises orange-brown marls and marly limestones laterally equivalent to the lower Aramunt Vell tongue. The overlying Llau de Joncarlat Member (new name) consists of white marls and marly limestones, laterally equivalent to the remaining platform units. Former names incompatible with this revised scheme, hence rejected herein, are the Carreu Formation and el Grau and Prats de Carreu members