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Seventy-nine fossil ostracode species were identified in 40 of 80 samples from 15 sites of the Lower Miocene Mizunami and Iwamura Groups (ca. 18 Ma), eastern Setouchi Province, central Japan. Six ostracode associations and seven biofacies were discriminated, mainly on the basis of R-mode and Q-mode cluster analyses, respectively. Species that lived in the innermost part of enclosed brackish bay habitats became extinct and their Recent related species are absent. Most Miocene species of the genera Schizocythere, Cornucoquimba and Loxoconcha inhabited similar environments to related living species. They lived in nearshore sand and silty sand bottoms with Zostera beds under the influence of subtropical-water currents as inferred from fossil molluscan assemblages. Pseudoaurila species, which are extinct, are estimated to have been dominant on seagrass or algae during the late Early to early Middle Miocene, but after that, they were replaced by diverse Aurila and Loxoconcha species. Species of the genera Palmenella, Kotoracythere, Acanthocythereis, Robertsonites, Celtia, Elofsonella and Laperousecythere are nearly identical morphologically to cryophilic or circumpolar species living in cold water masses of northern high-latitude seas or lower sublittoral to bathyal zones in the Sea of Japan. They occur in Miocene sediments, however, in association with subtropical to mild-temperate, upper to middle sublittoral molluscan species. Ancestors of Recent cold-water species have thus inhabited Japan since at least the Early Miocene. They expanded their habitats southward according with global cooling in chron C5Dr interval (late Early Miocene) correlated with the the oxygen isotope maximum period (the base of the Zone Mi 1b; ca. 18 Ma). Eleven new species are described. This study presents the earliest Miocene record of ostracodes in and around Japan and contributes to inferences on the origin, phylogeny and speciation of Recent ostracodes living in high-latitude seas in the Northern Hemisphere.
Growth-independent test characters of the genera Operculina, Planoperculina, and Planostegina were investigated to determine relations to water depth in form of morphoclines. All characters describing test form – thickness, initial vector of the test spiral that is correlated to proloculus size, and radius expansion rate describing test contour – are correlated. The investigated Operculina and Planoperculina species (O. ammonoides, O. elegans, O. complanata, P. heterosteginoides) demonstrate significant morphoclines in these characters, which can be used for gradient estimation using regression analyses. Thick forms of Operculina with intensively coiled spirals predominate in shallow water (−20 to −40m); in the deeper parts of the euphotic zone (−120m) they transform into thin forms possessing a weakly coiled spiral. Planoperculina heterosteginoides, with thin tests and therefore restricted to the deeper euphotic zone (starting at −80m), is the only symbiont-bearing larger foraminifer that can extend its distribution to just below the euphotic zone, where it develops extremely thin tests. Both Planostegina species, similar to shallow specimens of Planoperculina heterosteginoides in test form and thickness, do not show morphoclines and are thus restricted to depths between −30 and −90m (P. operculinoides) and −70 and −90m (P. longisepta). Chamber numbers, represented by the growth-independent parameters ‘chamber number of the first whorl’ and the ‘chamber number increase rate’ differ between Operculina species on the one hand and the Planostegina Planoperculina group on the other. Although both parameters are weakly correlated with characters describing test form, they do not show significant morphoclines either in the Operculina species or in the Planostegina Planoperculina group.
Benthic foraminifers collected in different regions of the high Antarctic (Weddell Sea, Mawson Sea, Davis Sea and some other locations in East Antarctic) on the shelf and upper part of the bathyal zone (2-2315m) were analyzed to determine regularities in their distribution. Most samples contained living organisms. The composition of the foraminiferal assemblages is influenced by many factors of water characteristics such as temperature, salinity, nutrient composition, availability of carbonate and variations in CCD, presence of near-bottom flows. The changes of water masses at different depths or mixing of the different water masses are clearly seen in the changes of the foraminiferal species composition. In some cases the local variations of water characteristics, seasonality and ice melting are also responsible for the foraminiferal distribution. Two faunas were identified: those of the shelf associated with the cold-water masses (−1° to 1.9°) and the upper slope with warmer water (−0.5°), the majority of the characteristic forms being circumantarctic. Species abundance displays two maximums at depths of 180–300m and 1500–2300m. At depths to 500, and even to 700m, both arenaceous and calcareous forms, especially miliolids, are abundant. Below this boundary some living calcareous forms were also found, though arenaceous ones were dominant. The presence of calcareous foraminifers at the majority of the stations in our material may be explained by the higher resistance of living calcareous forms to carbonate dissolution comparing the empty tests, dissolving more rapidly. The overall pattern of these our data and of data from previous studies allows to draw some general conclusions with regard to the Antarctic foraminiferal fauna: a high degree of endemism which, for the shelf depths of 2–50m, sometimes reaches 80%; the gigantism of many species; a wide range of vertical distribution of many species; introduction of deep water species of other oceans onto the shelf and the upper part of the bathyal zone; often high species diversity and high quantity abundance of many species in the community; abundance of circumantarctic species, often with a great number of species belonging to a single genus; the abundance (often predominance) of agglutinated forms compared to calcareous ones. Some taxonomic notes and morphological observations are given, as well as comparisons of some closely related Arctic and Antarctic forms.
Morphological characters of twelve Recent species of Nodosariinae (Foraminifera) are revised, using a large number of specimens collected in the North Atlantic. The species are assigned to four genera: DentalinaRisso 1826, LaevidentalinaLoeblich and Tappan 1986, NodosariaLamarck 1812, and PseudonodosariaBoomgaart 1949. One new species, Nodosaria haliensis is described. Previously unknown variation in the lamination pattern of the Nodosariinae test wall is described and some new terms are proposed to describe that variation. We continue to use the term monolamellar for septa made of a single layer, but use the term polylamellar, for septa made of two or more layers. The term plesio-polylamellar is proposed for forms with septa made of several layers, and which form secondary lamellae that envelope only some of the previous chambers. The term poly-monolamellar is replaced with a new term, ortho-polylamellar. We also discovered that only some, but not all of the primary layers in polylamellar septa continued to form secondary lamellae; this is referred to as a partial origin of secondary laminations. This is different from the well-known pattern in which all of the primary layers of a plesio-polylamellar septum continue and form what we refer to as a complete origin of secondary lamellae. We discovered that the lamellar structure of L. frobisherensis is somewhat variable even within the same specimen. The initial chambers are always ortho-monolamellar, but in later chambers the structure becomes plesio-monolamellar or even plesio-polylamellar, and the secondary lamellae can be either a partial or complete continuation of the primary lamellae of the septa. Species of Nodosaria are found to be atelo-lamellar, Laevidentalina are ortho-lamellar or plesio-lamellar and Dentalina and Pseudonodosaria are ortho-lamellar.
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