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Nutrients and sea-surface temperature were likely the most important paleoecological parameters that controlled the composition of Late Cretaceous calcareous nannofossil assemblages. The extinct nannofossil taxa Biscutum spp., Tranolithus orionatus and Zeugrhabdotus spp. are commonly thought to have preferred nutrient-rich surface waters. In order to test this hypothesis, calcareous nannofossils from two lower Turonian nearshore sections (Bochum, Herne; northwest Germany) were studied. These sections, which reflect a mesotrophic and perhaps cool paleoenvironment, contain well-preserved and highly diverse calcareous nannofossil assemblages yielding 92 species. Based on the first occurrences of Eprolithus eptapetalus, Quadrum gartneri, and Eiffellithus eximius, both sections have been assigned an early Turonian age. The most common taxa are Watznaueria barnesiae, Zeugrhabdotus spp., Biscutum constans, Prediscosphaera cretacea, and Tranolithus orionatus. In combination with previously published data from pelagic settings of the same area, these new findings allow the reconstruction of a nearshore-offshore transect. In proximal sections the abundances of B. constans and T. orionatus are higher than in coeval assemblages from distal localities. Zeugrhabdotus spp. shows no specific preference, whereas W. barnesiae is more common in open ocean settings. These findings suggest that B. constans and T. orionatus preferred more fertile and perhaps cooler nearshore habitats whereas W. barnesiae was adapted to oligotrophic and probably warmer paleoenvironments.
CARINA E. COLOMBI, RICARDO N. MARTÍNEZ, GUSTAVO CORREA, ELIANA FERNÁNDEZ, PAULA SANTI MALNIS, ANGEL PRADERIO, CECILIA APALDETTI, DIEGO ABELÍN, OSCAR ALCOBER, ANDREA AGUILAR-CAMEO
The first Triassic microfossil bonebed found in Argentina is located 80 meters from the top of the Quebrada del Barro Formation in the Marayes-El Carrizal Basin, in the province of San Juan. It consists of specimens from at least 63 individuals from an anomalously high concentration of fossils distributed laterally and vertically in a meter-thick fine-grained deposit. Two new taxa from the genera Sphenodontia and Eucynodontia had previously not been found in Argentine Triassic quarries. The bonebed is preserved in a mudflow deposit interbedded with calcic-Argillisols in the medial-distal zone of a distributary fluvial system (DFS). The accumulation is characterized by small-sized skeletal fragments (skulls, jaws and vertebra; all less than 50 mm), low degree of articulation, variable degrees of subaerial exposure, tooth marks, surface dissolution, and an alkaline authigenic mineral suite. Detailed paleoenvironmental and taphonomic characterization indicate that this accumulation underwent three stages in its taphonomic history: (1) biogenic extrinsic concentration; (2) local transportation and re-deposition by a mudflow on a swampy floodplain; and (3) drying and pedogenesis. Additionally, the deposit highlights biogenic activity as a way to concentrate a paleofaunal assemblage that likely represents the original community, and mudflow deposits from crevasse splays in DFS as a potential facies for microfossil preservation.
Shell beds are products of complex biological, taphonomic, and sedimentological factors. Paleoecological analysis of Pliensbachian shell accumulations from northeast Italy, Southern Alps, records successive phases of colonization and demise, each of which are characterized by specific taphonomic features and sedimentary fabrics. Field studies were augmented by serial sectioning and thin sections revealing microfacies and microtaphofacies, as well as scanning electro-microscopy (SEM) and electron backscatter diffraction (EBSD). The term bivalve carpet is introduced to describe the original geometry of the bivalve accumulation colonizing the substrate. Within a lagoonal depositional setting, small, thin-shelled Isognomonidae-like bivalves lived as endo-byssate, non-siphonate suspension feeders. These bivalves, interpreted as juveniles, constructed bivalve carpets with a primary lateral extension while lacking a distinct vertical growth. Bivalves colonized firm, dysoxic carbonate substrates consisting of a mixture of peloids, bioclasts, and chemolithotrophic iron-oxidizing bacteria. Oxygen-rich bottom currents and microbial activity stabilized the lagoon floor thus allowing for the episodic settlement of fixosessile bivalves and their autochthonous preservation. Short-term anoxic phases interpreted to reflect seasonal decreases in water energy resulted in the mass mortalities of the juvenile bivalves. The abundance of articulated, vertical orientated valve pairs in the shell accumulations indicate that the bivalve carpets were embedded soon after the death of the organisms without prior physical disturbance.
Fecal pellet-filled trace fossil Phymatoderma burkei from the Permian shallow-marine Teresina Formation (southern Brazil) was microscopically and geochemically investigated to reveal the significance of physicochemical processes in the preservation of invertebrate fecal pellets. Scanning electron microscope (SEM) analysis shows micron-sized spherules only in the pelletal infill of P. burkei, but not within the surrounding host sandstone. These spherules show highly uniform morphology and size (approximately 1 μm), and generally have hollow structures occasionally occurring as aggregates. Geochemically, such spherules are characterized by presence of Fe and Mn, occasionally with Ba. The evidence suggests that the spherules are mineral-replaced or mineral-encrusted microbial cells, which are most likely coccoid bacteria based on their morphological similarity. Within the fecal pellets excreted by the trace-maker of P. burkei, microbially mediated precipitation of Fe and Mn minerals probably occurred on or just near the outer surface of coccoid bacterial cells. Such microbial processes are important in the preservation of invertebrate fecal pellets excreted in shallow tiers, especially under wave- or current-influenced shallow-marine sedimentary conditions.
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