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I Banchi a Bivalvi del Giurassico Inferiore: Dinamica deposizionale, modalità di insediamento e sviluppo. Crisi biologiche, radiazioni adattative e dinamica delle piattaforme carbonatiche.
No full textMiddle Triassic (Anisian) benthic Foraminifera from the Monte Pra' della Vacca/Kühwiesenkopf section (Dont Formation, Braies Dolomites, Northern Italy
No full textThe recent discovery at Monte Pra' della Vacca/Kuhwiesenkopf (Braies Dolomites) of a significant plant deposit in the lower part of the Dont Formation, a basinal unit consisting of hemipelagic terrigenous-carbonate sediments, prompted an investigation of the benthic foraminiferal assemblage to provide a better chronostratigraphic definition of the deposit. The Anisian benthic foraminifera discovered in this section are widely distributed in the Tethys domain, and have been studied from the Dolomites region by several authors. The foraminiferal assemblage from Monte Pra' della Vacca/Kuhwiesenkopf has never been previously described or illustrated; it consists mainly of Ammodiscidae, Cornuspiridae and Duostominidae which are concentrated in the carbonate banks, while these are lacking in the stratigraphic interval corresponding to the plant deposit. The Pilammina densa, Meandrospira dinarica and Pilamminella semiplana association, typically Pelsonian in age, characterizes most of the section; towards the topmost layers the Paulbronnimannia judicariensis-Paulbronnimannella whirrakeri association occurs, which is referred to the upper Pelsonian. It is worth noting the occurrence of Paulbronnimannia judicariensis before the appearance of Meandrospira dinarica, and its co-existence with this latter. According to the foraminiferal association and distribution, the plant deposit can be referred to the Pelsonian
Upper Permian bivalves from the Bellerophon Formation (Dolomites, northern Italy) and biodiversity drop related to the end-Permian mass extinction
No full textThe Upper Permian Bellerophon Fm of the Dolomites (northern Italy) is an overall transgressive succession, which consists of a lower sulphate-evaporite unit, deposited in a barren basin, and an upper shallow-marine carbonate unit, deposited along a low-energy ramp setting. This succession, mainly Changhsingian in age, contains rich fossil assemblages, which have been studied since 19th century. Although some taxonomical groups have been object of recent revisions (e.g. nautiloids and brachiopods), the stratigraphic and systematic knowledge of the bivalves is not supported by recent studies, which are necessary to evaluate the survival rates after the end-Permian mass extinction. In this study, the biodiversity and stratigraphic distribution of the bivalves occurring in the Bellerophon Fm of western Dolomites is investigated with the aim to define the extinction patterns through the comparison with the taxa occurring in the Lower Triassic Werfen Fm. The specimens have been collected bed-by-bed from the Val Gardena and Val Badia sections aligned along a south west-north east transect, which records a slight deepening of the marine environment. Systematic assessment, based both on newly collected material and on historical collections analysis, has revealed 18 genera with 24 species. The studied material has been ascribed to the genera: Bakevellia, Chaenomya, Cosmetodon, Dyasmya, Edmonia, Eumorphotis, Palaeolima,?Palaeonelio, Permophorus, Pernopecten, ?Pleurophorella, ?Promytilus, Schizodus, Solemya (Janeia), ?Tambanella, Towapteria, Volsellina and Aviculopectinidae gen. nov. Infaunal suspension and detritus feeders suffered extinction rates higher than the semi-infaunal and epifaunal suspension feeders. The studied bivalves show a richness drop at genus level lower than other marine taxa. This higher survivorship can be related to the adaptation of the Upper Permian bivalves towards very shallow and stressed (e.g., high temperatures and low oxygen levels) marine environments, which characterized the western Dolomites during the deposition of the Bellerophon F
Permian–Triassic boundary enviromental and biotic changes: the Bulla parastratotype, Italy.
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Success and demise of exceptionally preserved terebratulide brachiopod accumulations in a Jurassic (early Pliensbachian) tropical lagoonal setting (Southern Alps, Italy): brachiopod response to environmental changes
No full textDuring the Early Jurassic, the shallow marine carbonate platforms of the western-Tethys margins were characterized by highly diverse benthos including larger foraminifera, sponges, bivalves, gastropods, brachiopods, echinoderms, and dasycladalean calcareous algae. In this paper, we document examples of such assemblages within the lower Pliensbachian part of the Rotzo Formation (upper Orbitopsella Zone) of the Southern Alps, Italy. This carbonate succession was deposited in a complex mosaic of marine and brackish habitats within a tropical lagoon of the Trento Platform area. Large terebratulide brachiopod shells form autochthonous accumulations comprising exceptionally well-preserved monospecific assemblages of Lychnothyris rotzoana. These brachiopod-bearing successions were analysed in terms of biotic components, microfacies analysis, shell biofabric (three-dimensional arrangement of skeletal elements), and taphonomic signatures to understand brachiopod response to changing conditions within a highly variable lagoonal palaeoecosystem. Findings show that terebratulide shell accumulations are dominated by adult specimens and juveniles are rare. The brachiopods thrived during low energy conditions that resulted in the accumulation of highly temporally-condensed shell beds. Stabilized by microbialite encrustations, the shells were not re-oriented during the subsequent rapid burial. The abrupt demise of these communities was possibly related to rapid environmental change, and causal factors are discussed. The medium-term response of brachiopods to the relatively instable ecosystem of the tropical lagoon shows that they were not able to adapt to continuous perturbations, and that continuing stress severely compromised the resilience of benthic taxa
Strontium isotope stratigraphic insights on the end-Permian mass extinction and the Permian-Triassic boundary in the Dolomites (Italy)
No full textThe Dolomites (Southern Alps, Italy) is a most significant region to investigate the evolution of shallow-marine ecosystems during the end-Permian mass extinction (EPME). Shallow-marine ecosystems are complex places from an oceanographic viewpoint and combine high biological productivity and ecological diversity. Therefore, establishing the timing and correlation of globally recognisable events in this region are fundamental to interpreting the biological crisis that followed. We took advantage of the Strontium Isotope Stratigraphy (SIS) concept to correlate our stratigraphic succession, ranging from the upper Bellerophon (Bulla Member, upper Changhsingian) to the lower Werfen (Tesero Member, upper Changhsingian - lower Induan) Formations of the Dolomites, to the Meishan (Zhejiang Province, China), the Global Stratotype Section and Point (GSSP) for the Permian–Triassic boundary. We integrated new and previously published 87Sr/86Sr data from well preserved brachiopods and show that the topmost part of the Bellerophon Formation (the Bulla Member) correlates with beds 24 e-f of the GSSP section, while the Tesero Member, where the Permian-Triassic boundary is located, is characterised by more radiogenic Sr isotope ratios than expected. Differences in Sr isotope ratios may be due to small-scale variations in local paleoenvironmental and paleogeographic conditions/settings. Continental weathering of sedimentary or igneous rocks, distance from terrestrial environment and nearby occurrence of radiogenic rocks might also produce locally variable Sr isotope signatures especially in shallow water environments complicating the application of SIS principles
Lopingian brachiopods from the Abadeh section (Central Iran) and their biostratigraphic implications
No full textThe Abadeh section is one of the most important Neotethyan sections, as it records a continuous marine sedimentary succession across the Permian-Triassic boundary. Although this succession yields a rich brachiopod fauna, this has never been studied in detail. Instead, most of the studies carried out so far have been focused on the position of the Permian-Triassic boundary, which is still debated. Here, we present a systematic study of the brachiopod fauna from the Hambast Formation, which comprises 30 species of the orders Productida, Orthotetida, Orthida, Athyridida and Spiriferida. Brachiopod biodiversity is relatively high and constant in the Wuchiapingian
part of the section. But in Changhsingian, just a single species occurs: Paracrurithyris pygmaea. Also, species of bivalves of the genus Claraia occur within an upper Griesbachian bed of the Elikah Formation.
Using the Unitary Associations method, we compared the stratigraphic distribution of the brachiopods in the Abadeh section with those in the Julfa section, another important Permian-Triassic succession in North-West Iran. Based on this analysis, three Wuchiapingian brachiopod biozones and one Changhsingian brachiopod biozone have been detected and correlated to the conodont biozonation; these biozones are the Araxilevis intermedius-Leptodus nobilis Biozone, the Permophricodothyris ovata-Araxathyris quadrilobata Biozone, and the Haydenella
kiangsiensis-Transcaucasathyris minor Biozone in the Wuchiapingian, and the Paracrurithyris pygmaea Biozone in the Changhsingian. Most of the biozones can be correlate
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