Los rudistas (Bivalvia, Hippuritoidea) en el apenino centro-meridional (Italia): análisis de las asociaciones de radiolítidos en contexto de plataforma calcárea en el super-greenhouse climate del cretácico superior

Abstract

El estudio de las plataformas calcáreas del Mesozoico y de los organismos que han contribuido a su desarrollo ha recibido nuevas e importantes aportaciones como ejemplo de respuesta al supergreenhouse climate (clima de superinvernadero) que caracterizó gran parte del Cretácico.<br/>El trabajo se ha centrado en el análisis de las asociaciones de rudistas (Bivalvia, Hippuritoidea) del intervalo Turoniense superior-Campaniense, Cretácico superior, en 12 secciones estratigráficas de plataforma calcárea que afloran en localidades de significado paleontológico y estratigráfico particular de los Apeninos centro-meridionales, Italia y la revisión de colecciones en 12 museos.<br/>Se han estudiado cinco géneros de la Familia Radiolitidae: Biradiolites, Bournonia, Distefanella, Lapeirousella y Radiolites; y en particular nueve especies con especial significado para la estratigrafía del área considerada: Biradiolites angulosus (d'Orbigny, 1847), Biradiolites martelli (Parona, 1911), Bournonia excavata (d'Orbigny, 1842), Bournonia fascicularis (Pirona, 1869), Distefanella salmojraghii Parona, 1901, Distefanella montagnei Sli kovic, 1971, Lapeirousella samnitica (Parona, 1901), Radiolites dario (Catullo, 1834) y Radiolites trigeri (Coquand, 1859).<br/>Se han reinterpretado los caracteres morfológicos de algunas de estas especies y también de algún género, proponiendo, en consecuencia, nuevas descripciones taxonómicas.<br/>El análisis de la valva izquierda en los radiolítidos con valva derecha alargada, ha conducido a reconocer dos morfologías distintas, características de contextos sedimentarios diferentes (Tipo A y Tipo B). El análisis de la tasa de crecimiento de los radiolítidos de valva derecha aIargada y concha delgada y de su relación con la tasa de sedimentación ha conducido a estimar tasas realmente elevadas, por lo que estos radiolítidos se consideran indicadores de elevado acúmulo sedimentario.<br/>Se han distinguido dos tipos de biofacies con radiolítidos de valva derecha alargada, Slender Radiolitid Biofacies (SRB) que se desarrollaron en distintas áreas y ambientes de la zona considerada a lo largo del Cretácico superior, caracterizadas por distintas asociaciones de especies (SRB Tipo A y SRB Tipo B). La distribución de las biofacies con radiolítidos alargados (SRB) en los Apeninos centro-meridionales es asimétrica y está en íntima relación con la compleja fisiografía de las plataformas calcáreas aisladas que se desarrollaron en el área mediterránea central del Tethys del Turoniense al Santoniense. Su distribución permite reconocer un modelo paleoecológico general en el que deben considerarse algunos parámetros atmosféricos y oceanográficos ligados al flujo de la Corriente Circumglobal del Tethys.<br/>Finalmente, los mecanismos de biomineralización adoptados por los bivalvos actuales permiten suponer que la baja relación Mg/Ca y la asociación proteica específica, debe haber jugado un papel importante en la construcción de la concha de los radiolítidos, como lo hace en los bivalvos actuales.

The central Mediterranean area has been investigated by reconstructing the palaeoclimatologic context following the most recent data in literature. Data<br/>asses that surface boundary conditions in late Cretaceous were much different from that of today: sea level much higher (100-200 m higher), medium temperature (higher 14 °C), higher sea water acidity and pCO2 more than 700 ppmv (today 380 ppmv). All these factors deeply modified the global circulation regime. The palaeobiologic answer had to be much different from that of today's biota: rudists with thick calcite shell as radiolitids were well suited to this panorama and represent a keystone in assessing palaeobiologic conditions in a warm climate of the late Cretaceous.<br/>Radiolitid rudist bivalves are widely represented in carbonate successions cropping out in central and southern Italy. In order to analyse rudist assemblages, stratigraphic analysis has been performed in 12 successions from five distant areas.<br/>In the Monti d'Ocre the succession is condensate, spanning from the Aptian to the Campanian, with three datum planes, mainly of platform margin setting. Rudist assemblages give a good biostratigraphic control with caprinids of the Aptian "Urgonian facies", lagoonal facies with Sauvagesia and other molluscs in the middle Cenomanian, Biradiolites angulosus horizons overlied by grain-supported high-energy Distefanella salmojraghii and Lapeirousella samnitica facies.<br/>In the Monti Simbruini, limestones with prevalent mud-supported texture contain radiolitid assemblages composed of beds of elongate Biradiolites angulosus, Radiolites trigeri, Durania arnaudi of late Turonian-Coniacian age, overlaid by assemblages with elongate Radiolites dario and Bournonia excavata of Santonian age. The sedimentary setting is of inner platform. In the Matese Mountains, carbonates of middle Turonian-Campanian age crop out, this is an historical site where Distefanella was instituted. Bioclastic limestones with grain-supported texture bear Distefanella salmojraghii assemblages and other rudist-rich facies.<br/>In the Majella Mountain, the physical scalloped platform margin of the Apulian platform crops out. Well preserved Distefanella assemblages with very elongate specimens (up to 40 cm) in bioclastic grain-supported limestone have been studied in situ and found in reprised blocks as olistolithes in the pelagic scaglia.<br/>In Cilento area, radiolitids have been found in two distinct assemblages:<br/>Radiolites trigeri, Durania arnaudi, Biradiolites martelli, Bournononia gardonica,<br/>Biradiolites angulosus (CO Assemblage of late Turonian-Coniacian) and Radiolites dario, Bournonia fascicularis, Sauvagesia tenuicostata (SA assemblage of late Coniacian-Santonian age). Both associations are embedded in mud-supported limestone of inner platform setting and are composed of elongate specimens.<br/>Specimens collected in outcrops and others studied in museum's collections allowed to update palaeontologic information on five genera and nine species and to re-describe genus Distefanella. The biomineralization stategy adopted by these extinct bivalves with thick calcite shells, could be analogous to that of living molluscs, eventually involving protein associations with aspein and caspartin.<br/>During the Late Cretaceous, different sedimentary contexts show peculiar radiolitid assemblages; the rudist biofacies distribution is strongly influenced by the palaeogeographic setting and by the global palaeo-oceanic circulation pattern that, in the central Mediterranean area, was mainly east-to-west directed.<br/>R E S U L T S<br/>The study performed on rudist radiolitid assemblages of the late Turonian-Campanian interval, Late Cretaceous, in 12 stratigraphic sections sampled in the central-southern Apennines, Italy, and the revision of fossil collections in 12 museums, lead to some conclusions on the palaeontology of these bivalves. Nine species with particular significance for the stratigraphy of the considered area, belonging to five genera of the Family Radiolitidae have been studied: Biradiolites angulosus (d'Orbigny, 1847), Biradiolites martelli (Parona, 1911), Bournonia excavata (d'Orbigny, 1842), Bournonia fascicularis (Pirona, 1869), Distefanella salmojraghii Parona, 1901, Distefanella montagnei Sli kovic, 1971, Lapeirousella samnitica (Parona, 1901), Radiolites dario (Catullo, 1834) and Radiolites trigeri (Coquand, 1859).<br/>The morphologic features of some of these species, and also of some genera, have been reinterpreted. New taxonomic attributions have been proposed.<br/>- Bournonia fascicularis (Pirona, 1869). The species description has been completed as it was poorly known; it has been found in many Coniacian-Santonian successions of Italy. Its generic attribution has been justified.<br/>- Biradiolites martelli (Parona, 1911). Two morphotypes have been distinguished: morphotype A with elongate costae and morphotype B with short costae. Its generic attribution has been justified. The revision work lead to consider that the species of genus Milovanovicia Pol ak, 1967 can be ascribed to B. martelli for the whole of its features, so that this genus is unnecessary.<br/>- Distefanella Parona 1911. The type material has been revised, reinterpreting the diagnostic features of the genus. It has been ascertained synonymy among species originally ascribed. The study of the whole literature published after, and new collected material allowed to reduce the 25 known species to only two, Distefanella salmojraghii Parona, 1911 and Distefanella montagnei Sli kovi&#263;, 1971, that are different for the development and grade of invagination of the radial bands.<br/>- Its attribution to genus Lapeirousella Milovanovi&#263;, has been definitively ascertained. The analysis of the left valve in radiolitids with elongate right valve, thin shell, and deeply anchored in the substrate, together with the study of the sediment where they are contained, lead to recognize two distinct morphologies, each typical of different sedimentary context, of inner platform/ramp and platform margin respectively.<br/>- Type A, with left valve flat or smoothly concave, provided with a little developed myocardinal apparatus that is considered to be well adapted in environments with moderate to low energy.<br/>- Type B, with left valve high, cupuliform, provided with a myocardinal apparatus that, with two teeth very large and myophores laterally well developed, forms a nearly tubular structure that, inserted in the right valve, gives good mechanic strength at the area near the commissure. It is considered well adapted to prevent the accumulation of sediment on the valve itself, moreover to be resistant in an environment under moderate to high energy.<br/>The analysis of the growth rate of radiolitids with slender and thin shelled right valve, and their relation with the sedimentation rate, lead to approximate rates really high, up to >4 mm/yr, so that these radiolitids are considered as indicators of high sediment accumulation.<br/>Two types of biofacies with slender radiolitids have been recognized, (Slender Radiolitid Biofacies, SRB) that developed in different zones and environments of the considered area in the late Cretaceous, characterized by different species assemblages:<br/>- SRB Type A, in limestone with mainly mud-supported texture, frequently of algal-bacterial sedimentation, of inner platform and ramp in the Simbruini Mt., Ausoni Mts. and in Cilento. The carbonate productivity in this context is extremely high if comparing to actual models (up to 20 times higher).<br/>In the late Turonian-Coniacian the fossiliferous assemblage is marked by: Biradiolites angulosus d'Orbigny, Biradiolites martelli (Parona) morphotype A, Bournonia excavata (d'Orbigny), Durania arnaudi (Choffat), Radiolites trigeri (Coquand) and Sauvagesia sp. In the Coniacian-Santonian by: Bournonia fascicularis (Pirona) and Radiolites dario (Catullo).<br/>- SRB Type B, in limestone with texture mainly grain-supported, bioclastic, of platform margin in the Matese Mts. and in the Majella Mt. The carbonate productivity can be compared to that recorded in the actual most productive barrier reefs. In the late Turonian-Santonian the fossiliferous assemblage is marked by: Distefanella salmojraghii Parona and Lapeirousella samnitica (Parona).<br/>It has been ascertained that the slender radiolitid biofacies distribution (SRB) in the central-southern Apennines is asymmetric:<br/>- T- he SRB Type A crop out in the Simbruini Mts., Ausoni Mts. and in the Cilento area, localized in the western side of the Apenninic belt.<br/>- The SRB Type B is well developed in the bioclastic successions that crop out in the Monti d'Ocre, Matese Mts. and in the Majella Mt., localized in the eastern side of the belt.<br/>Both the SRB Type A and SRB Type B are intimately linked to the complex physiography of the isolate carbonate platforms that developed in the central Mediterranean area of the Tethys from the Turonian up to the Santonian. Its distribution allows to recognize a general palaeoecologic model where, to explain the asymmetry, some oceanographic and atmospheric parameters must be considered, linked to the Tethys Circumglobal Current flux that, in the Late Cretaceous, was from the East to the West in the latitude corresponding to the central Mediterranean area (between 15° and 25° of latitude).<br/>The examination of the biomineralization mechanisms adopted by the living bivalves allow to hypothesize that the low Mg/Ca ratio and the specific protein assemblage have had a key role in the formation of the radiolitid shell, as it is in the living bivalves. Particularly, aspein and caspartin might have acted as amplifiers in calcite shell biomineralization process and as palaeobiologic response of these bivalves to the supergreenhouse climate context of the Cretaceous.