REVUE DE MICROPALEONTOLOGIE最新文献

In this study, taxonomic criteria were revised for 64 spumellarians/collodarians from the Upper Miocene that were collected at the Ocean Drilling Program Site 1208. The taxonomic approach employed in this study allowed a relatively wide range of intra-species variation to promote palaeoceanographic studies on Upper Miocene fauna in the future. In addition, the nomenclature of species with controversial taxonomies, for which there is significant biostratigraphic interest, was also revised. One such example is Stylatractus universus Hays, which should be renamed as Xiphosphaerantha angelina (Campbell and Clark). In addition, a new species from the Upper to Middle Miocene core sequences was collected at mid-latitudes of the North Pacific Ocean. This species has been named Didymocyrtis noritoshii and can potentially be used as a biostratigraphic and environmental index in core sequences from the Upper Miocene.

Resumé

Dans cette étude, les critères taxonomiques de 64 spumellaires et collodaires du Miocène supérieur, collectés sur le site de l'Ocean Drilling Program 1208 sont révisés. Mon approche taxonomique utilisée dans le cadre de cette étude fut libérale, permettant une certaine marge de variations intra-espèces dans le but de promouvoir à l'avenir les études paléocéanographiques en se basant sur la faune radiolaire du Miocène supérieur. En permettant ainsi une plus large variation morphologique au sein d'une espèce, les taxonomies sont plus simples que si on applique une variation morphologique avec des critères assez stricts pour séparer les espèces. En outre, la nomenclature des espèces aux taxonomies controversées parmi lesquelles y figure une espèce à certain intérêt biostratigraphique a également été révisée. C’est le cas de Stylatractus universus Hays, que j'ai renommé Xiphosphaerantha angelina (Campbell et Clark). Enfin, une nouvelle espèce a pu être décrite dans les séquences sédimentaires datant du Miocène supérieur. Cette espèce, qui a été qui fut nommée Didymocyrtis noritoshii, pourrait potentiellement être utilisée comme indicateur biostratigraphique et environnemental dans l'océan Pacifique du nord.

Well-preserved Late Cretaceous (early Cenomanian) radiolarians were recovered from two sections in Crimea: Bodrak River and Sel’-Bukhra Mountain. Abundant and diverse Nassellaria were studied and four new species are described: Holocryptocanium pseudoastiense nov. sp., Diacanthocapsa bodrakiense nov. sp., Novixitus riedeli nov. sp. and Rhopalosyringium panovi nov. sp. Herein Archaeodictyomitra (?) speciosa Bragina is newly attributed to the genus Obeliscoites O’Dogherty and Dorypyle ovoidea (Squinabol) is tentatively attributed to the genus Homeoarchicorys Empson-Morin. The stratigraphic ranges of Diacanthocapsa urkutica Bragina, Distylocapsa squama O’Dogherty, Novixitus subtilis Bragina, and Siphocampe altamontensis (Campbell and Clark) are more precisely defined with their lower limits expanded to the lower Cenomanian and the upper limits of Pseudodictyomitra languida O’Dogherty and Thanarla conica (Squinabol) extended to the lower Cenomanian.

Two stratigraphic sections referable to the late Messinian have been identified and studied in eastern Tunisia; the first, south of Cape Bon (Salakta-SAL section), the second in the Sahel region (Wadi el Kebir–OK section). The analysis of the ostracod fauna content (assemblages) has revealed that in some stratigraphic levels some taxa are certainly attributable to the Lago-Mare fauna, while other taxa, observed in subsequent or previous stratigraphic levels, are referable to true shallow-water marine environments. From these latter shallow water marine levels, several ostracods species were recognized; some of which, due to their morphological particularities, have been described as new species.

Consequently, in this article, 24 species belonging to the genera Cytherella, Paijenborchellina, Cimbaurila, Pokornyella, Dorukella, Graptocythere, Capsacythere, Chrysocythere, Okadaleberis, Ruggieria, Cytheretta, and Neomonoceratina are described and illustrated as new.

Quaternary deep-sea ostracods supply invaluable data for the understanding of both hydrological and sedimentary processes in the continental slope. A huge portion of western South Atlantic bathyal regions is still poorly explored with respect to deep-sea ostracod research. This article contributes to the study of the assemblage ostracod of the Upper Pleistocene of five piston cores recovered in the Cone of Rio Grande, Pelotas Basin, Brazil. Taxonomic analysis revealed 21 species belonging to 13 genera: Cytherella, Macropyxis, Argilloecia, Paracytherois, Cytheropteron, Rimacytheropteron, Eucytherura, Loxoconchidea, Xestoleberis, Apatihowella, Henryhowella, Poseidonamicus and Krithe. The assemblages are quite similar to those registered in southwestern Brazilian bathyal regions. A taxon left in open nomenclature is probably a new genus and species. Eucytherura fossapunctata nov. sp. is here described.

Experimental culture studies of living radiolarians experience many difficulties because radiolarians are too sensitive to be maintained alive in artificial conditions. We here developed an improved experimental protocol in order to detect newly-formed parts of the polycystine radiolarian skeletons after they were labeled with fluorescent markers. Living samples were collected from surface seawater of the Kuroshio Current offshore from Kashiwajima Island, Kochi Prefecture on the island of Shikoku, Japan. The taxonomic composition of the Kashiwajima radiolarian samples varied markedly across the seasons and sampling sites. Spumellaria were usually abundant in samples collected near Kashiwajima, and had high diversity in shell morphology. Here, we performed experimental studies on living spumellarian radiolarians to clarify their siliceous skeletogenesis. Our culture-based research on spumellarians provided the following results.

(A) The pattern of silica deposition on the skeleton of Spongaster tetras tetras varies depending on the growth stage. Three ontogenetic stages were identified: (1) young, (2) progressively-growing and (3) fully-grown. Furthermore, novel varied types of skeletal growth modes were determined as follows: (i) rapid growth on the outermost shell with enlargement of shell size and (ii) internal deposition on skeletal components without change in shell size. From measurements of the growth rate of cultured radiolarians, it is assumed that S. tetras tetras cell has at least a month-long life cycle.

(B) Spherical spumellarians have their own shell-forming strategy that permits additional silicogenesis while maintaining the taxonomically-specific shape of their shell; the meshwork of their shell surface consists of polygonal-shaped pore-frames formed by heterogeneous patterns of added silica.

Our direct observation with fluorescence markers detected a skeletogenesis process of Haliommilla capillacea possessing a fragile spherical shell. We thus confirm the bridge-growth hypothesis for morphogenesis of the polygonal frame as proposed in previous studies. Spongosphaera streptacantha, having extremely long spines, exhibited a special growth pattern of spine elongation in a centrifugal direction, without a change in width. This was achieved by a particular temporal pattern of silicification characterized by an alternating pattern of patchy addition of new silica on the entire part of each blade from the base to the tip. Moreover, a thin layer of new silica was added by a flowing deposition along the edge of the spine. Based on these culture-based studies using living radiolarians, we conclude that Spumellaria control and maintain a particular shell shape by regulating where silicification takes place on the skeleton, particularly depending on the growth stage.

The Sirt Basin of Libya hosts significant hydrocarbon reservoirs in Paleocene shallow-marine carbonates (Upper Sabil Formation) with abundant larger benthic foraminifera. Benthic foraminifera proved to be an essential biostratigraphic tool for precise biostratigraphic dating of these carbonate platform/ramp deposits. A detailed systematic description of the Middle to Late Paleocene larger benthic foraminifera from the Sirt Basin is presented, which refines the biostratigraphic age of the Paleogene shallow-marine carbonates. Eight taxa are identified at a species level. Among them, Ranikothalia solimani is the most important taxon for the correlation of the Upper Sabil Formation. The taxa Miscellanea yvettae, Miscellanites primitivus, and Glomalveolina primaeva with co-occurrence of orthophragminid foraminifera constrain the age of the Upper Sabil Formation as Middle to Late Paleocene (Selandian-Early Thanetian).

The multi-proxy investigation of the deep-marine Kottafi Hill section (KHS), a part of the carbonate system of the Miocene Pakhna Formation, Cyprus, involved such proxies as calcareous nannofossil analysis, measurements of the oxygen and carbon isotope composition of the planktonic foraminifer Orbulina universa, and determination of the pollen and palynomorph contents, revealed the importance of these sedimentary sequences in the assessment of the impact of major global events during the middle Miocene on the regional scale. The KHS spans the 20.89–11.6 Ma time interval, during which eighteen OC-rich siltstone intercalated laminae have been deposited under warm and humid climate at 15.5–11.6 Ma. These layers can be possibly considered as the precursors of sapropelic layers mostly developed in the eastern Mediterranean Basin during Pliocene–Holocene. The global glacial events Mi3a–Mi5, traced by δ¹⁸O planktonic foraminifera records in the KHS, represent the stepwise cooling phase during the middle Miocene Climate Transition.

The current Special Issue of Revue de Micropaleontologie, entitled “Updating Micropaleontology and Stratigraphy in Greece and Eastern Mediterranean” includes contributions from the T1S1 special Session [conveners Maria Triantaphyllou (NKUA), Dimitris Kostopoulos (AUTH), George Iliopoulos (UPAT)] organized by the Hellenic Committee for Paleontology and Stratigraphy at the 15th Conference of the Geological Society of Greece (May 2019, Athens, Greece).

The current study performs a detailed micropalaeontological analysis, in terms of ostracods, benthic foraminifera and calcareous nannoplankton, in order to reconstruct the palaeoenvironmental history of the sedimentary sequence exposed in the central part of Corinth Isthmus. Twenty one samples from two lateral sections were studied, on both sides of a westwards dipping normal fault at the west end of the central horst, which are part of the same stratigraphical sequence. The calcareous nannofossil analysis provided for the first time dating evidence for the upper Corinth Marl sequence, revealing that Section A (at the footwall block of the fault/the upper part of the Corinth Marl) is attributed to the Early Pleistocene /Calabrian (nannofossil biozone MNN19f) and that Section B (at the hanging wall of the fault) is assigned to the Middle Pleistocene /Chibanian (MNN20). Both parts of the studied sequence not only differentiate in age but bear notably separated species composition of the main ostracod assemblages as well. The combined study of ostracod and benthic foraminiferal microfaunas indicated that at least since Early Pleistocene the Corinth Marl deposits at the central part of Corinth Isthmus corresponded to a shallow, restricted, oligohaline lagoon with salinity alternations expressed as layers enriched in nannofossil content. Towards Middle Pleistocene the lagoon presented increased salinity and signs of shoaling. The studied sequence closes upwards with a bed of Middle Pleistocene age, rich in marine molluscs and corals indicating a radical environmental change: the opening of the lagoon and the establishment of a shallow marine coastal environment.