Ostracod Associations Research Articles

Paleoecology of ODP LEG 122 Triassic Ostracodes (Wombat Plateau, NW Australia)

ODP Leg 122 drilled in the Indian Ocean off NW Australia reached the Triassic for the first time. Ostracodes wererecovered in 74 samples of 4 sites (122–759, 122–760, 122–761, 122–764). The ostracode associations show variations in the paleoenvironment. In Hole 122–759B paleoecological analysis shows an intertidal to infralittoral environment. In Hole 122–760B, abundance of Healdiidae should indicate a shallow warm water environment. The sediments of Hole 122–761C and Site 764 were deposited in open marine environment. A slight deepening is thus indicated during the Upper Triassic of this area.

Associations d'ostracodes et paléoécologie du Crétacé (Barrémien à Turonien) le long d'une transversale Agadir-Nador (Maroc)

Different depositional environments have been recognized by studying ostracod associations in the Barremian-Turonian interval along a section from Agadir to Nador (Morocco). The lacustrine environment is characterized by a moderate faunal density, a very low generic and specific diversity, and good-sized carapaces (smooth, punctuated or spinely) of Cyprideinae. In the supralittoral environment, the specific diversity is low and the quantity of ostracods is middle to high. Typical markers are the following genera: Virgatocypris, Mantelliana(?), Loxoconcha, Fabanella(?). The carapaces are large, smooth, punctuated or striated lengthwise. Antepaijenborchella and Loxoconcha are rather euryhaline. Marker genera are numerous and diversified in the infralittoral environment. Specific diversity and the quantity of ostracods can be low or high, but always higher than in the other environments. Some euryhaline ostracods are living on the flatform; some disappear because they are not adapted to salinity variations. The bottom is important for the spatial distribution of the species. Some species are living on a carbonate bottom, others on muddy bottoms. Monospecific Cytherella or Dolocytheridea appear in quiet littoral slikkes. In these environments instars of Paracypris pullutate. The decrease of dissolved oxygen kills a great number of species; some survive a small decrease, some a very great decrease ( Cytherelloidea, Paracypris, Veenia). In carbonate environments, the carapaces are strong, large, costulated, spinely and tuberculate. The ornamentation is clear and the fossae are wide, polygonal; the solum is flat or weakly concave. A secondary ornamentation occurs in the Trachyleberididae ( Cythereis, Spinoleberis, Veenia, Oertliella ?). Many opportunist genera appear: Antepaijenborchella, Cytherella, Paracypris, Dolocytheridea, Loxoconcha, Eocytheropteron, Cythereis, Veenia, Oertliella (?). Currents frequently introduce globular ( Conchoecia,Buntonia) and small-sized ( Pseudomonoceratina, Nigeroloxoconcha, Procytherura) ostracods from the basin or the outer shelf. Marker genera are less numerous in the circalittoral environment. Specific diversity is low to middle and the faunal density is low to high. The carapaces are smaller than in the infralittoral environment. The ornamentation is disordered and confused (?), punctuated, pitted, reticulated, sometimes absent or covered with a secondary ornamentation. The feeding is less abundant than in the infralittoral environment, and the ostracods live on sciaphilic plants. The decrease of dissolved oxygen leads to the monospecifism of Oertliella ? tarfayaensis. Different ways of life and moving can be found. Some ostracods are epibiontic ( Eocytheropteron, Cythereis, Protocythere, Bythoceratina), some can swim ( Nigeroloxoconcha, Procytherura, Pseudomonoceratina, Schuleridea, Dordoniella, Pontocyprella, Macrocypris), some are pelagic ( Conchoecia). Introduction from the infralittoral environment is caused by saltation among thick ostracods and by suspension among globular and brittle forms.

Biostratigraphy of the Middle Eocene succession at Gebel Mishgigah, Wadi Rayan, Libyan Desert, Egypt

The present study deals with the biozonation of the exposed section at Gebel Mishgigah, which is mainly composed of highly fossiliferous limestones, dolostones and marls. These deposits belong to Mishgigah Member of Wadi Rayan Formation and are of Late Lutetian to Bartonian (Late Middle Eocene) age. The collected samples were investigated for their microfaunal and nannofloral content. Different associations of calcareous nannoplankton, planktonic foraminifers, ostracods and bryozoans were detected. The depositional environments and paleoecologic factors that prevailed during the sedimentation were also interpreted.

The Silurian and Early Devonian biogeography of ostracodes in North America

Abstract Silurian and Early Devonian ostracode associations in North America represent at least three ecotypes, a leperditicopid association, a large beyrichiacean association, a mixed association, and possibly a fourth, spinose podocopid association, or Thuringian ecotype. Comparison of the large beyrichiacean association and mixed association ecotypes indicates the presence of three informal ostracode provinces, the Appohimchi, Baltic-British and Cordilleran, which remained relatively constant in geographical position throughout the Silurian and Early Devonian. Plotting the provinces on palaeogeographic maps suggests that temperature was not an important factor in delimiting the provinces, and land barriers, or possibly deep-water troughs as barriers, were the cause of provincial development. Benthic ostracodes differed from other benthic invertebrates such as brachiopods and corals in developing provincialism in the late Llandovery and having it start to decline in the Pragian through the Emsian.

Ostracodes maastrichtiens et paléocènes du Djebel Dyr, près de Tebessa (Algérie orientale)

Ostracode associations from Maastrichtian and Paleocene levels in Saharian Atlas, Eastern Algeria, are here in analysed. Those associations can be compared to those of the Kef section, Tunisia ; Kef section belonging to the same paleogeographical division "Sillon Tunisien" Algerian fauna seems to be poorer than the Tunisian one ; this fact can be explained by confined environment. During the envisaged period, in details, the bathymétrie evolution is not concomitant in the two localities. Those differences can be useful to separate stratigraphical species and more sensible to environment species. We also shortly present the dispersion of some Algerian species in South Tethysian province and in West Africa ; the "Sillon Tunisien" is probably the main way for faunal exchange between those provinces.

Le genre Ceratopsis (Ostracoda, Palaeocopa) dans l'Ordoviciende l'Europe et de l'Amérique du Nord: Phylogenèse, paléoécologie et implications paléobiogéographiques

CeratopsisUlrich is present in the Ordovician(Upper Llanvirn to Ashgill) of Europe and North America. 16 species are known. Ceratopsis calatravensis nov. sp. from Spain and C. krutai nov. sp. from Bohemia are described. HastatellinaPřibyl sp. is considered as a junior synonym of Ceratopsis. The antero-dorsal process (sperum) of Ceratopsis is shown to be a homeomorphic feature of the L-S sculpture. Detailed morphological comparisons lead to the recognition of: a) phylogenetical links between species from different regions, b) evolutionary trends throughout the Ordovician (development of velar dimorphic features and secondary fimbriate structures of the sperum). Phylogeny of Ceratopsis support other biogeographic evidence (i.e. ostracode distribution) that faunal connections occurred during the Caradoc a) between the British Isles and North America across the closening Iapetus Ocean, b) between the British Isles and the peri-gondwanan regions (including Ibero-Armorica and Bohemia) across the opening Rheic Ocean. Low diversity ostracode associations dominated by Ceratopsis are widely distributed in Europe and America during the Caradoc; they are related to mud-silt or carbonate sediments and probably to relatively high energy shelf environments under unstable conditions. The Possible function of the sperum and the habitat (benthic/endobenthic) of Ceratopsis species are discussed.

Ostracode fauna, stratigraphy and definition of the Beyrichienkalk sequence

An essential clue to the uppermost Silurian in the submarine Baltic arca is the Beyrichienkalk. According to recent investigations it constitutes a glacial drift boulder sequence characterized by four ostracode associations with about sixty species. In the correlation of ostracode zones of Baltoscandia these associations form a connecting link and thus are also of biozonal character. In the terms of regional stratigraphic units (East Baltic arca). the lower stratigraphic boundary of this sequence lies approximately at the beginning of the Kuressaare beds and the top in the upper Ohesaare beds. The term Beyrichienkalk should be used for those glacial erratics that are characterized by typical ostracodes of the four associations defined here.

Les microfaunes (foraminifères -ostracodes) du Coniacien -Santonien de Provence (S.E. France) : biostratigraphie, paléoécologie

A biozonation of the Coniacian -Santonian in Provence has been established by mean of planktonic Foraminifera. Ostracods which are mainly news species have been integrated in this framework. Although relatively reliable at the regional scale, this zona-tion needs to be compared with zonations established in other areas of the mesogean province. On the paleoecological level, it is possible to analyse with accuracy the repartition of the associations of benthic Foraminifera and Ostracods in various types of paleoenvironments. Microfau-nas from «margino-littoral» environments and the transition to the non marine formations at the end of the Santonian are also discus¬ sed.

Biogeography of the Saharan Gretaceous and Paleocene epicontinental transgressions

The first trans-Saharan epicontinental transgression began in the Late Cenomanian and peaked in the Early Turonian. The sea entered through rifts (Nigeria) and subsiding areas (north Africa) and, at maximum extent, spilled out onto stable craton. The central Sahara was land in late Early Turonian time; the sea seems to have remained in the central part of the Benue rift, to sally forth again during the minor Coniacian transgressive pulse. The Coniacian transgression went no further than eastern and central Niger Republic. For a short time during the Cenomanian-Turonian link-up between the Tethys and the South Atlantic, some organisms (mainly ammonites and pelecypods) passed across the Sahara. The Coniacian marine deposits of eastern Niger contain several elements typical of the Mungo River Formation of Cameroun, and coastal Nigeria. These faunistic indications imply that the topographical obstacle formed by the Zambuk ridge of northeastern Nigeria was swamped at maximum transgression. A third, very extensive epicontinental transgression began in latest Campanian time and reached its acme in the Early Maastrichtian. In the central Sahara, at least, there was a retreat of the sea in later Maastrichtian times. A final transgression peaked during the Paleocene, after which the sea withdrew permanently from the northwest African hinterland. The same marine ostracod associations occur in the Paleocene of coastal Nigeria, the Sokoto embayment, Mali and Libya, thus providing strong evidence of a marine connexion, albeit brief, between the South Atlantic and the Tethys, despite the lack of outcrop evidence in the crucial Niger valley region.

Découverte de microfaunes de l'Albienmoyen et supérieur dans la vallée de Joux (Jura Vaudois, Suisse)

The discovery of Foraminifera and Ostracodesin the dark green, sandy-glauconitic marl clay in the Joux Valley (in the Vaudois Jura) suggests to date these sediments Middle and Upper Albian. The Ostracode associations are similar to those found in the Paris Basin, but different from those which characterize the subalpine region.

The Silurian-Devonian Boundary in the Barrandian

ABSTRACT This report presents details of the development of the Silurian-Devonian boundary in the Barrandian area in Central Bohemia. It begins with a summary of previous investigations, followed by a survey of the general facies development of the region. The most important stratigraphic sections in which the boundary occurs are described and illustrated. Details of biostratigraphy concentrate primarily on the occurrence of index fossils. The second part of the paper is concerned with a discussion of individual faunal groups and their importance in delineating the boundary, their dependence on facies, and their value in correlation. Finally, the general development of the Silurian-Devonian boundary in the Barrandian is discussed, the stage and zonal subdivisions of the boundary beds are revised, and stratotypes are defined or specified in greater detail. Principal conclusions may be summarized as follows: 1. The Silurian-Devonian boundary is defined at the base of the Monograptus uniformis Zone which is coincident with the Pridolian-Lochkovian boundary. 2. Although unaccompanied by facies change, the boundary is well defined bio-stratigraphically on the basis of graptolites and other faunas. 3. Recent investigations confirm that there is no demonstrable stratigraphic hiatus or facies change at the boundary, which can be defined confidently only through biostratigraphy. In the vicinity of the boundary, however, mainly in the Lower Lochkovian, sedimentation was locally condensed and a relatively long time-interval may be represented by a small thickness of sediments. 4. The base of the Devonian is marked by the sudden appearance of Monograptus uniformis. Two subspecies, M. uniformis uniformis and M. uniformis angustidens, appear simultaneously or separated only by a very short interval. 5. The first occurrence of the trilobite Warburgella rugulosa rugosa is a suitable auxiliary indicator of the boundary, and is a prominent marker horizon in the Lower Lochkovian. It appears essentially simultaneously with M. uniformis. Because of its wider facies tolerance, the species is used for correlating between platy limestones and shales and the purely carbonate development. 6. The boundary can also be identified by using faunas that occur in the upper part of the Pridolian. These include: abundant Monograptus transgrediens, Dayia bohemica (particularly in the carbonate facies), Spathognathodus steinhornensis eosteinhornensis, and Boucia ornatissima (in the shale-rich development). The trilobite Prantlia (Tetinia) minuta is a significant horizon marker at the Pridolian-Lochkovian boundary in the carbonate facies. In addition to graptolites, trilobites, and conodonts (especially Icriodus woschmidti), the sudden increase of an ostracod association including Acanthoscapha bohemica serves to identify the lowest Lochkovian in the platy-limestone facies. The latter association, however, may begin in the latest Pridolian. 7. Accumulations of remains occur at various stratigraphic horizons within the Pridolian and Lower Lochkovian. They reach a maximum in the platy-limestone and shale facies in the southeast limb of the Barrandian in the basal Lochkovian, where they are referred to as the Scyphocrinites Horizon. The occurrence of the genus cannot, therefore, be used to delineate the Silurian-Devonian boundary or to define the stratigraphic units: Budnanian, Pridolian, or Lochkovian. End_Page 104------------------------ 8. Recent work has shown that almost all faunal groups in the boundary beds are facies-controlled. The fauna of the platy limestones and shales differs markedly from that of the pure carbonate facies. 9. The following chronostratigraphic division may be recognized: the Budnanian (equals Upper Silurian) Series is divided into the Ludlovian and Pridolian stages; the Lochkovian is the earliest Devonian stage. 10. The Pridolian-Lochkovian or Budnanian-Lochkovian boundary is redefined on the basis of graptolites, and stratotypes are revised accordingly.

Considerations biostratigraphiques sur les ostracodes des couches alternantes de l'Eocene moyen et superieur au sud de Bordeaux

Abstract Variations in associations of ostracods in middle and upper Eocene deposits of the Talence borehole, south of Bordeaux, France, are discussed in terms of facies changes. The data are used for defining the stratigraphy of the section and for interpreting paleoecologic conditions.