Paleoecological Factors Research Articles

Climate-driven dinoflagellate cyst stratigraphy of the Oligocene-lower Miocene turbidite succession of the Lower Congo Basin (offshore NW Angola)

Biostratigraphic studies of cuttings from wells located in the Lower Congo Basin, offshore northwestern Angola, document with unprecedented wealth of data the organic-walled dinoflagellate cyst (dinocyst) assemblages of the Malembo Formation. The identification of a series of dinocyst events (highest and lowest occurrences, influxes and acmes) recurring in the same stratigraphic position in different wells and cross-calibrated by calcareous nannofossil data, provides a tool for age control and correlation of the Oligocene and lower Miocene in the equatorial and South Atlantic region.Proceeding downwards in the stratigraphy of the examined succession, the lowest occurrence of Pyxidinopsis pastilliformis, detected within the CN3 nannofossil Zone, approximates the Langhian-Burdigalian boundary, whereas the lowest occurrences of Sumatradinuim druggii and Sumatradinium hispidum occur at the base of the CN2 nannofossil Zone in the Burdigalian. The lowest occurrences of Sumatradinium soucouyantiae and Barssidinium taxandrianum fall within the Zone of acme of Hystrichokolpoma rigaudiae at the Burdigalian-Aquitanian transition, and immediately above the acmes of Cribroperidinium tenuitabulatum and Systematophora-Cleistosphaeridium group that are supposed to reflect climate cooling pulse Mi-1a. The presence of Chiropteridium spp. and Membranophoridium aspinatum is limited to the upper part of the Chattian. An influx of Tuberculodinium vancampoae, possibly related to a warmer interlude during a period of overall climate cooling, occurs in proximity of the Chattian-Rupelian boundary. The nearly synchronous highest occurrences of Gerdiocysta conopeum, Eocladopyxis peniculata and Heterosphaeridium sp. provide a robust infra-Rupelian biohorizon. Further down in the Rupelian, the influxes of Pentadinium laticinctutm and Distatodinium paradoxum are interpreted as responses to cooling pulses. The lowest occurrence of G. conopeum, detected in the basal part of the NP23 nannofossil Zone, corresponds to the global first appearance of this excellent Rupelian marker. Genera Deflandrea and Wetzeliella are noticeably missing in the studied material just as they lack in some coeval dinocyst assemblages reported in the literature from low paleolatitude Atlantic sections.The recorded alternation of influxes and acmes of Gonyaulacoid dinocyst taxa indicative of cooler and warmer water conditions was probably climate-driven, i.e. paced by cooling pulses and relatively warmer interludes characterizing the climate evolution during the Oligocene and early Miocene. Indeed, the lack of acmes of Peridinioid cysts suggests that climate, rather than constantly elevated nutrient levels ensured by the Congo River discharge, was the discriminant factor triggering the observed blooms of Gonyaulacoid cyst-forming taxa.The delivery of large volumes of sands into deepwater settings via gravity-driven fluxes was favored by sea-level drops and is ultimately associated with pulses of climate cooling and Antarctic ice-sheet growth, finally reflected by the composition of the dinocyst assemblages. In addition to paleoecological factors, hydraulic fractionation resulting from sedimentary processes that transported palynomorphs down the slope as part of turbidite fluxes, contributed to shape the palynological assemblages observed in this study.

First Pleistocene South American Teratornithidae (Aves): new insights into the late evolutionary history of teratorns

ABSTRACT The first unequivocal records of teratornithid birds from the Pleistocene of South America are here described, adding a new member, and the largest, to this highly diversified guild of large carnivorous flying birds that lived during these times in the Americas. The new specimens come from four fossiliferous localities of Central Argentina that range in age from the late middle to the early late Pleistocene, and agree with other known Teratornithidae taxa in size and morphology. We updated the taxonomy of the family and analyzed its fossil record in the Pleistocene of both Americas. The available evidence suggests that forms related to Teratornis lived in the South American Pampas around the time of the Last Interglacial (MIS 5), but they were restricted to North America during the latest Pleistocene (late MIS 3–early MIS 1). The contrasting latest Pleistocene record of teratorns between North and South America is not easy to understand, especially because the supposed flight capacity of these birds did not prevent them from crossing large geographical barriers. Although a bias in the fossil record cannot be ruled out, it is possible that the teratorns were limited in South America by paleoclimatic–paleoecological factors as yet undetermined, and/or that the northern and southern Pleistocene species had very dissimilar specializations. In relation to the latter, the previous inferences on the teratorn paleobiology without phylogenetic support are preliminarily questioned here.

Vertebrate fauna genesis in the Middle Paleozoic Podolian paleobasin

The formation of the Early Devonian ichthyofauna occurred under the combined influence of historical, paleogeographic and paleoecological factors. A consequence of evolutionary development at the beginning of the Devonian period arose a number of high-ranking taxa of sea vertebrates, which actively colonized the shallow water basins of new continent Laurussia. For representatives of local endemic faunas there was an opportunity to expand their habitats. There were exchanges of migrants between neighboring regional faunas. All this was accompanied by the evolution of ecosystems, the formation and development of new ecological niches. Early Devonian for most groups of vertebrates was marked by a sharp increase in the number of taxonomic units. The formation of Podillya regional ichthyofauna illustrates these trends of global biotic changes in that time.

Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part I: Snout and Masticatory Apparatus

Inquiring into the paleoecology of extinct forms is always a challenge, particularly when the taxa under study correspond to derived ecomorphs of ancient and completely extinct clades. In this contribution, the configuration of the masticatory apparatus and associated features of the Neogene pachyrukhines Paedotherium and Tremacyllus are studied in a detailed, mainly qualitative, comparative analysis of 36 specimens. Tooth morphology and the reconstructed muscular configuration of pachyrukhines indicate an important mediolateral component during chewing, and predominant crushing over grinding, as well as anteroposterior movements for the coupling and action of stronger gnawing incisors. These actions are more compatible with hard and brittle or turgid fruit food consumption than specialized folivorous, and particularly grazing, habits. The infraorbital and palatal foramina morphology and other rostral features indicate increased touch sensibility for object recognition and are congruent with the presence of infoldings of the lips protecting the gingiva during gnawing on hard foods. Additionally, there was a morphological gradient between Tremacyllus and P. bonaerense, from high selection of relatively soft and small food items, to specialized hard item consumption and higher resistance for abrasion and masticatory efforts (e.g., in eventual association with digging habits), respectively. Paedotherium typicum presents intermediate characteristics, with incisors designed for better cropping action or poorer selectivity during feeding. This more profound understanding of the feeding habits of pachyrukhines further allows the suggestion of paleoecological factors that could have contributed to niche segregation between these long-term coexisting rodent-like taxa.

The Brachiopod Dalmanella testudinaria across the End Ordovician Extinction Event in the Cuyania Terrane of Western Argentina

The species Dalmanella testudinaria (Dalman) was reported thirty years ago from the Hirnantian Don Braulio Formation of the Argentinean Precordillera, but its assignment was cast in doubt by a recent revision of the species. Through a detailed morphological analysis of a large collection of specimens, attribution of the Don Braulio Formation material to D. testudinaria is now confirmed. The overlying Rhuddanian sandstones of the La Chilca Formation yielded a dalmanellid which was previously referred to as Dalmanella aff. testudinaria. The minor differences existing between these specimens and the topotype material from Sweden is interpreted as intraspecific variation and consequently they are assigned to D. testudinaria. This implies that in the Precordillera basin this species survived the second pulse of the end-Ordovician mass extinction. In the Don Braulio Formation largest shells occur in shallow-water subtidal carbonate lenses and siltstones, near the base of the posglacial transgression, reflecting deposition in well-oxygenated, nutrient-rich waters. Shell size decreases upsection reaching a minimum in the organic-rich mudstones bearing the graptolite Metabolograptus persculptus (Elles et Wood). Overall evidence suggests that low-richness and low-density of this community could be attributed to unfavorable physical environmental conditions associated with decreased food availability, and such limiting paleoecological factors are considered to be the primary cause of dwarfing. Small specimens of D. testudinaria reappear a few meters above the “extinction level” in the lower part of the La Chilca Formation of Rhuddanian age. Before becoming extinct, D. testudinaria coexisted in the Cuyania terrane with the first representatives of the Afro-South American Realm such as Anabaia Clarke and Heterorthella Harper, Boucot, And Walmsley.

Paleobiogeography of the early Late Ordovician “Trentonian” (latest Sandbian to middle Katian) brachiopod fauna during a major marine transgression and colonization of the epicontinental seas in Laurentia

During the late Sandbian and early to middle Katian, a major marine transgression flooded the intracratonic basins and platforms of eastern North America, allowing brachiopods to disperse into the newly formed epicontinental seas. Two main brachiopod faunas can be differentiated using multivariate analysis of faunal data from localities across Laurentia during this time: a relatively diverse Scoto-Appalachian Fauna confined to the eastern margin of North America and peri-Laurentian terranes, and a less diverse but more widely distributed Epicontinental Fauna that expanded into the remainder of the flooded continent. The Epicontinental Fauna can be divided into four subfaunas with subtle compositional differences that correspond to different paleoenvironmental settings of eastern North America, influenced by such paleoecological factors as nutrient-rich cool-water upwelling onto the continent through the Sebree Trough, a greater range of habitable environments on the continental margin and in the adjacent deep Taconic foreland basin, and the interactions between pericratonic and intracratonic water masses of different physical and chemical properties. The Epicontinental brachiopod faunas dominated basins and platforms of continental interior until the Hirnantian extinction event.

Taxonomic and taphonomic signatures of mollusk shell concentrations from coastal lagoon environments in Belize, Central America

Faunal composition and distribution patterns of mollusk assemblages from 20 shell concentrations in cores collected in coastal lagoons, a mangrove-fringed tidal inlet, and the marginal marine area (shallow subtidal) along the central coast of Belize show considerable variation due to environmental heterogeneity and the interplay of several environmental factors in the course of the mid-late Holocene (ca. 6000 cal BP to modern). The investigated fauna ≥2 mm comprises 2246 bivalve, 789 gastropod, and 11 scaphopod specimens. Fifty-three mollusk species, belonging to 42 families, were identified. The bivalve Anomalocardia cuneimeris and cerithid gastropods are the dominant species and account for 78 % of the total fauna. Diversity indices are low in concentrations from lagoons and relatively high in the marginal marine and tidal inlet areas. Based on cluster analysis and nonmetric multidimensional scaling, seven lagoonal assemblages and three marginal marine/tidal inlet assemblages were defined. A separation between lagoonal and marginal marine/tidal inlet assemblages seen in ordination indicates a lagoon-onshore gradient. The statistical separation among lagoonal assemblages demonstrates environmental changes during the Holocene evolution of the coastal lagoons, which is probably related to the formation of barriers and spits. The controlling factors of species distribution patterns are difficult to rule out, probably due to the heterogeneity of the barrier–lagoon systems and the interaction of paleoecological and paleoenvironmental factors. In addition to the taxonomic analysis, a taphonomic analysis of 1827 valves of A. cuneimeris from coastal lagoons was carried out. There is no relationship between depth and age of shells and their taphonomic condition. Size-frequency distributions and right-left valve ratios of A. cuneimeris suggest that valves were not transported over long distances but were deposited parautochthonously in their original habitat. Shells from tidal inlet and marginal marine environments were also predominantly deposited in their original habitats.

Magnetic Susceptibility in the Evaluation of Spatial and Profile Heterogeneity of Soils Conditioned by the Paleoecological Factors

The objects of this study were the Holocene soils formed on different elements of relief and under conditions of different natural zones, as well as paleosoils including buried under the mounds. The aim was to identify the patterns and mechanisms of formation of the structure of spatial heterogeneity of soil properties and soil cover conditioned by the paleoecological factors, based on a measure of the magnetic susceptibility. For the first time regularities of spatial variability of the magnetic susceptibility indicator in soils of different elements of paleocryogenic microrelief were identified. The paleocryogenic microrelief role in the formation of strong magnetic minerals of iron is comparable with the effect of zonal conditions of soil formation and is expressed in differentiation of water-air conditions. The character of variation of these conditions determines heterogeneity of soil properties at the subtype level. Variography allowed revealing the characteristic dimensions of homogeneous structures, the presence of which was associated with paleocryogenic microrelief and its structure-forming elements. The spatial distribution of the magnetic susceptibility conditioned by paleocryogenic microrelief has the form of annular, rhythmically repeating structures. Profile distribution of the magnetic susceptibility in paleosoils of the steppe zone defined primarily the soil magnetite content can be used as a “magnetic recording” of paleoecological factors of steppes and provides quantitative characteristics of climate (precipitation). In addition, the topoisopleths of spatial distribution of the magnetic susceptibility can show both the stages of construction of ground burial structures (mounds), and the materials used. Results can be used in the study of soil cover structure and its mapping, monitoring of soil characteristics in connection with the development of technology “exact” agriculture.

Biostratigraphy and depositional environment evolution of the Asmari Formation at the Shajabil anticline, Iran

In the present research, biostratigraphy and paleoecology of the Asmari Formation at the Shajabil anticline, northwest Yasuj in the Izeh zone are discussed. The Asmari Formation at the studied area is Rupelian–Chattian to Burdigalian in age. In the study, 18 genera and 29 species of benthic foraminifera have been identified. Besides, the following biozones which reflect Rupelian–Chattian to Burdigalian age are identified for the Asmari Formation: (1) Lepidocyclina–Operculina–Ditrupa; (2) Archaias asmaricus–Archaias hensoni–Miogypsinoides complanatus; (3) Peneroplis farsenensis_Elphidium sp.14–Miogypsina sp.; (4) Borelis melo–Borelis melo curdica. Relying on fossils, Coral and Corallinacea have been detected depositional environment. The paleo-ecological factors are identified for the Asmari Formation at the study area: water salinity, 34 to more than 50 psu; water depth of 90 to 0 m; water temperature of 14 °C to more than 27 °C in a subtropical to tropical environment, oligotrophoic to mesotrophic conditions.

Biostratigraphy and paleo-ecological implications in microfacies of the Asmari Formation (Oligocene), Naura anticline (Interior Fars of the Zagros Basin), Iran

In this research, biostratigraphy and paleo-ecological implications related to the carbonates of the Asmari Formation located at the Naura anticline, Interior Fars of the Zagros Basin, Iran, are discussed. The Asmari Formation at the study area is Rupelian–Chattian in age. Age was determined by the occurrence of 22 genera and 41 species which led to identification of four faunal assemblages: (1, Globigerina–Tuborotalia cerroazulensis–Hantkenina; 2, Nummulites vascus–Nummulites fichteli; 3, Lepidocyclina–Operculina–Ditrupa; 4, Archaias asmaricus–Archaias hensoni–Miogypsinoides complanatus). Based on faunal associations and biofacies analysis with emphasize on large benthic foraminifera and coralline red algae communities, the following paleo-ecological factors are defined for deposition of the Asmari Formation at the study area: water salinity of 34 to more than 50 psu, depth zones of 0–35 m of inner and middle part of proximal open shelf, 35–150 m of middle open shelf and more than 200 m of outer open shelf, water temperature of 18–25°C in a tropical to sub-tropical environments, oligophotic to near mesotrophic conditions and grain association communities of foralgal and rhodalgal.

Paleoecological utility of insect trace fossils in dinosaur nesting sites of the Two Medicine Formation (Campanian), Choteau, Montana

Insect trace fossils, such as burrows, pupation chambers and nests, can provide broad paleoecological insights by helping to define paleohydrology, effects of seasonality or conditions of associated paleosols. Insect traces adjacent to nesting sites of the dinosaur Troodon formosus in the Cretaceous (Campanian) Two Medicine Formation near Choteau, Montana, demonstrate such paleoecological utility. One outcrop in particular contains an abundance of insect burrows and pupation chambers in a calcareous paleosol. Most trace fossils are interpreted as apocritan (wasps and bees) burrows, brooding chambers and cocoons. Apocritans prefer to construct burrows and brooding chambers in well-drained soils during relatively dry conditions (avoiding wet seasons). Their trace fossils are consistent with previous inferences of semi-arid conditions and seasonality for the Two Medicine Formation. Moreover, apocritan nesting is likely to have occurred in the same places and conditions as dinosaur nests: well above the local water table and during dry seasons. Such trace fossils hold the potential for more precise definitions of paleoecological factors in dinosaur nest sites. For example, within the Two Medicine Formation, the Celliforma ichnofacies is commonly associated with eggs of T. formosus and Continuoolithus, but not with those of Maiasaura peeblesorum, perhaps indicative of subtle nesting site preferences.

The flora of Early Permian coal measures from the Paraná Basin in Brazil: A review

This paper presents an updated overview integrating both previous and newly published data on the most important floras found associated with Early Permian coal seams in the Paraná Basin, Brazil. These floras occur within the Rio Bonito Formation and correspond to the Gondwana “Glossopteris Flora.” For this review, five floras are selected, in ascending stratigraphic order: the “São João do Triunfo,” “Figueira,” “Quitéria,” “Morro do Papaléo” and “Irapuá Bed” floras. They are the best-known floras of the basin in terms of taxonomic composition, paleoecology and environments of deposition. An early–mid Sakmarian to earliest Artinskian age is indicated for the Rio Bonito Formation based on absolute radiometric and relative biostratigraphic ages. Integration of available information about the selected floras allows evaluation of taphonomic and paleoecological factors to provide a more comprehensive understanding of the Early Permian floral record in the Paraná Basin. The variation observed in both the taxonomic composition of individual floras and in the frequency of occurrence of different plant groups is due to the broad range of environmental/edaphic conditions that prevailed in the many different depositional settings represented in the Rio Bonito Formation. A more precise age determination obtained for the plant-bearing deposits permits the establishment of a more confident correlation between the Early Permian floral succession in the Paraná Basin and other Early Permian floral successions in other basins. The Sakmarian global warming favored the appearance of pecopterid and sphenopterid ferns amongst the spore-producing plants, and the glossopterids amongst the pollen-producing plants.

Paleoenvironmental controls on the morphology and abundance of the coccolith Watznaueria britannica (Late Jurassic, southern Germany)

The coccolithophore species Watznaueria britannica is dominant in Middle-Upper Jurassic calcareous nannofossil assemblages and presents morphological variation, including different coccolith size, shape and length of the central area and of the bridge. Six morphotypes can be recognized in the polarizing light microscope. The aim of this work is to better understand the morphological variability of W. britannica and determine if this variability is controlled by paleoecological factors. In order to investigate the potential paleoecological controls on W. britannica morphology and abundance, we carried out a biometric study on a restricted temporal interval: the Late Oxfordian, in the Swabian Alb (southern Germany), characterized by increasing carbonate production linked to climatic changes. The Balingen–Tieringen section, where previous works on sedimentology, nannofossil assemblage composition, and δ 18O and δ 13C analyses were performed, was selected for this study. The variations in morphology and abundances of W. britannica were studied on 40 samples of the Balingen–Tieringen section, presenting variable lithologies and calcium carbonate contents. For each level, seven biometric parameters (coccolith length, width and ellipticity, central area length, width and ellipticity and central area proportion with respect to the coccolith) were measured or calculated on digitally captured images of the first 100 W. britannica coccoliths observed in the light microscope. The relationships between the different biometric variables were described using bivariate and Principal Component Analyses. Biometric parameters and Principal Component factors extracted from nannofossil assemblages as well as other paleoenvironmental proxies, were investigated using regression, and their stratigraphic trends were compared. Principal component analysis of the six biometric variables (3938 measurements) on W. britannica coccoliths shows a reduced morphological variability compared to a significant size gradient. An allometric trend recognized on the total placolith and on the central area within the W. britannica assemblages suggests that the different morphotypes may represent intra-specific variability rather than different species. The general trend through Late Oxfordian shows an increase in size of W. britannica coccoliths, mainly driven by an increase in the contribution of the large morphotypes. Increasing placolith size is associated with drier and warmer climatic conditions during the latest Oxfordian.

EPIBIONT PRESERVATIONAL AND OBSERVATIONAL BIAS IN FOSSIL MARINE DECAPODS

Epibionts on both living and fossil decapod crustaceans may serve as valuable proxies for paleoecological factors such as behavior and environment. Prevalence of epibionts, as observed primarily on the carapaces of fossil brachyurous decapod crustaceans, appears to be less than observed on living crabs, based upon observations in the literature, and from the study of a limited preserved biological and fossil collection. Among these factors, the three most important are that many extant epibionts do not possess skeletal structures and, therefore, are unlikely to be preserved; the epicuticle upon which epibionts attach to living brachyurans is lightly calcified and tends to be lost readily as a result of taphonomic processes; and the most common mode of preservation of fossil brachyurans is in concretions which tend to break open and leave a layer of cuticle on the counterpart, thus obscuring the potential surface of attachment of epibionts. Other factors such as the life habits of the crab, whether burrowing, burying, or remaining above the substrate; lifestyle, whether benthic or pelagic; and duration of the intermolt phase of the organism also play important roles in potential prevalence of epibionts. Careful preparation of part and counterpart remains of brachyurans as well as reference to the occurrence of epibionts in systematic studies will enhance knowledge of the host and epibiont.

African rain forest ecology and conservation: an interdisciplinary perspective

Extending from west Africa to Madagascar, from the vast lowland Congo Basin to the archipelago of forest islands on its eastern rim, the African rain forest is surpassed in size only by the Amazon. This book sheds light on the current efforts to understand and conserve the African rain forest, an area in need of urgent action to save its biological wealth, cultural heritage, and economic potential. Written by conservation scientists and practitioners based in the African rain forest, the book offers a multidisciplinary perspective that integrates many biological and social sciences. Early chapters trace the forces - from paleoecological factors to recent human actions - that have shaped the African forest environment. The next chapters discuss the dominant biological patterns of species ranging from the distinctive elephants, gorillas, and okapi to the less well known birds, butterflies, and amphibians. Other chapters focus on how such different groups as hunter-gatherers, forest farmers, bushmeat hunters, recent immigrants, and commercial foresters have used the forests. Several authors stress the need for tighter links between research and conservation action. The final section draws lessons from the collective experience of those working in an Africa wracked by political strife and economic hardship.

Carbon-isotope composition of Lower Cretaceous fossil wood: Ocean-atmosphere chemistry and relation to sea-level change

The carbon-isotope composition of fossil wood fragments, collected through a biostratigraphically well-constructed Aptian (Lower Cretaceous) shallow-marine siliciclastic succession on the Isle of Wight, southern Britain, shows distinct variations with time. The results indicate that the stratigraphic signature of {delta}{sup 13}C{sub wood} through the Aptian was influenced primarily by fluctuations in the isotopic composition of CO{sub 2} in the global ocean-atmosphere system, as registered in marine carbonates elsewhere, and was not governed by local paleoenvironmental and/or paleoecological factors. Negative and positive excursions in {delta}{sup 13}C{sub wood} through the lower Aptian occur in phase with inferred transgressions and regressions, respectively -- a pattern that contrasts with that observed in many previous studies for different time intervals. The relationship between {delta}{sup 13}C variations and relative sea-level change is tentatively interpreted as a response to various climatic and eustatic factors, relating to rapid sea-floor spreading, thermal uplift of ocean floor, emplacement of plateaus, volcanic CO{sub 2} emissions, weathering, and sedimentary rate.

Early Jurassic (Pliensbachian-Toarcian) Dinoflagellate Migrations and Cyst Paleoecology in the Boreal and Tethyan Realms

The discrepancies in the stratigraphical ranges of selected dinoflagellate cysts recorded in the Boreal and Tethyan realms have revealed two migrational events during the Early Jurassic. The first event occurred at the early-late Pliensbachian boundary and consists of mutual biotic exchanges between the two realms. This is linked to a major Early Jurassic transgression which improved marine communications between the Boreal and Tethyan areas. The second dinoflagellate migrational event occurred during the mid Toarcian and was driven by paleoenvironmental factors. The numerous available Lower Jurassic dinoflagellate cyst data from the Boreal and Tethyan realms indicates that phytoplankton distribution was profoundly affected by paleoecological factors. Information pertaining to the life strategies and the paleoecological requirements of the genera Luehndea, Nannoceratopsis and Valvaeodinium has also been determined. INTRODUCTION The distributions of dinoflagellate cysts are primarily related to paleoenvironmental regimes. Certain dinoflagellate cyst taxa exhibit heterochroneity over wide areas because of differences in paleoecological preferences and overall paleoenvironments (Goodman 1987). This situation has implications for the application of dinoflagellate cysts to intercontinental biostratigraphy (Bucefalo Palliani and Riding 1997a). During the Jurassic two faunal provinces, the Boreal and Tethyan realms, have been distinguished partly on the basis of different ammonite, foraminiferal, brachiopod and calcareous nannofossil assemblages (Arkell 1956; Gordon 1970; Voros 1977; Bown 1987). Biotic differences between these realms may be characterized by relative abundances rather than by the presence or absence of specific taxa (Hallam 1969). The Boreal Realm occupied the northern part of the Northern Hemisphere; the Tethyan Realm lay to the south (Hallam 1969). During the Early Jurassic a broad transitional area with both Boreal and Tethyan biotic characteristics was developed. This intermediate belt comprises southern France, Hungary and Portugal, and was also delineated on the basis of ammonites, calcareous nannofossils and dinoflagellate cysts (Zeigler 1980; Geczy 1984; Cariou et al. 1985; Gardin and Manivit 1994; Baldanza et al. 1995). Within the Tethyan Realm, the macrofaunas allow the Mediterranean and sub-Mediterranean bioprovinces to be distinguished. The former comprises the northern margin of the Tethys Ocean, thus is of European affinity; the sub-Mediterranean is of African affinity, being on the southern margin of Tethys (Pavia and Sarti 1987). Recently these two Tethyan provinces have been differentiated using calcareous nannofossils (Baldanza and Mattioli 1992; Baldanza et al. 1995; Mattioli 1995). The majority of publications on Lower Jurassic dinoflagellate cysts are from the Boreal domain; there are significantly fewer published data from north-west Tethys. The biogeographical distributions of Lower Jurassic dinoflagellate cysts have not been investigated in detail. On the basis of data by Davies (1985) from central Portugal, Tethyan Lower Jurassic microplankton assemblages are broadly similar in generic/specific content to more northerly floras. However, there are several taxa whose stratigraphical ranges differ profoundly from those in the Boreal Realm (Poulsen 1996; Riding and Ioannides 1996). Recently, palynological studies have been carried out on ammoniteand calcareous nannofossil-dated Lower Jurassic Tethyan sections in order to define the stratigraphical ranges of dinoflagellate cysts and to compare them with their ranges in the Boreal Realm (Bucefalo Palliani 1996). The aims of this study are to distinguish and interpret the stratigraphical discrepancies between the two paleogeographical realms, to utilize the stratigraphical data to identify the areas where the main Pliensbachian-Toarcian speciation events occured, to recognize the most important migrational events and consequently the biogeographical factors which drove the dinoflagellate cyst distributions. MATERIAL Samples were collected from several European Pliensbachianearly Toarcian successions (text-fig. 1). The correlations are based on the ammonite, calcareous nannofossil and dinoflagellate cyst content. The Portugese sections, Peniche, Rabacal and Brehna, are outcrop localities in central Portugal, north of Lisbon (text-fig. 2). The Peniche section ranges in age from the early Pliensbachian (Carixian) to the early Toarcian. It consists of intercalations of marls and more calcareous marls with interbedded black shales and detrital crinoidal limestone. The samples studied are from the Lower Pliensbachian (text-fig. 3). The late Sinemurian (Lotharingian) to early Bajocian Brenha road cutting section was dated by Mouterde et al. (1972). The Lower Pliensbachian portion of the section, made up by grey marls with marly limestone beds, has been investigated during this study (text-fig. 3). micropaleontology, vol. 45, no. 2, pp. 201-214, text-figures 1-12, appendix 1, 1999 201 This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 05:55:42 UTC All use subject to http://about.jstor.org/terms Raffaella B. Palliani and James B. Riding: Early Jurassic dinoflagellate migrations and paleoecology, Boreal and Tethyan realms

The pleistocene paleoenvironment of the Transcaucasus [Le paléoenvironnement pléistocène du Caucase

La Géorgie semble avoir offert aux hommes des conditions paléoenvironementales très intéressantes, car l'occupation peut être suivie pendant toute la période Paléolithique. Le site le plus ancien Dmanisi est daté de plus de 1,5 Ma d'années. Compte tenu de la localisation géographique, la Géorgie est maintenant considérée comme un couloir favorisant le passage des Hommes qui quittaient l'Afrique et le Proche Orient pour occuper l'Eurasie. Les études faunistiques et paléobotaniques indiquent, que ce gisement était entouré d'espace ouvert avec des forêts probablement localisées dans les vallées. De nombreux gisements archéologiques prouvent l'occupation continue des hommes dans cette région. En effet les grands changements climatiques du Quaternaire semblent plus atténués dans le Caucase, particulièrement dans le Caucase occidental, qui jouit d'un climat plus humide et tempéré à tendances méditerranéennes en raison de la diversité des paysages et de la proximité de la Mer Noire. Pendant le Pléistocène supérieur, il n'a pas été découvert de mammouths dans les nombreux sites étudiés. Les variations climatiques enregistrées lors de cette période correspondent à la redistribution des taxons mais pas à un changement de la structure. La seconde moitié de la dernière glaciation correspond a une descente des zones altitudinales. Le maximum de cette péjoration est atteint entre 20000 et 18000 B.P., pendant le dernier maximum glaciaire. L'augmentation de l'aridité du climat induit la réduction des paysages de forêt sans éliminer complètement les espèces thermophyles qui survivent et se redéploient largement juste après le maximum de la glaciation. Pendant ce dernier épisode glaciaire, plusieurs régions de Géorgie connaissent des ressources stables en approvisionnement d'eau, en matière première lithique, en abris et en grottes. C'est probablement ce qui a déterminé cette grande continuité de l'occupation humaine dans les paysages familiers et régulièrement exploités. En Géorgie, l'homme s'est adapté en fonction des zones altitudinales qui caractérisent cette région depuis le passé jusqu'à présent.

Biogeochemistry of Mono Lake, California

Modern sediments of Mono Lake show marked variation in lipid composition with depositional environment. Constituents derived from the drainage basin, characterized by high molecular weight alkane hydrocarbons (C 25–C 31), and the steroids β-sitosterol and brassicasterol, predominate in near-shore environments. In the deepest part of the lake, sediments exhibit a combination of externally-derived constituents, and lipids derived from the lake biota; the latter characterized by low molecular-weight alkanes and alkenes (C 15–C 17), phytane, and the steroids ergost-7-en-3β-ol and 24-ethylcholest-7-en-3-β-ol. Steranes, 4-methylsteranes, and the C 18 and C 19 isoprenoids appear to be forming in the intensely reducing bottom sediments at the present time. The compositions of samples from the Pleistocene succession of Mono Basin suggest that sample-to-sample variation within the same stratum is negligible so long as unweathered samples from the same depositional environment are compared. Sediments having equivalent lithologies may or may not have similar compositions, but sediments having similar fossil contents do show similar lipid compositions. Subaerial weathering of sediments causes a marked decrease in the amount of extractable organic material, as well as distinct changes in its hydrocarbon composition. Specifically, weathered sediments exhibit a decrease in relative content of low molecular weight hydrocarbons and a relative increase in nC 22. Organic composition of sediments from the Pleistocene stratigraphie column cannot be correlated with depth of burial. Compositional changes with stratigraphie position are probably related to paleo-ecological factors such as population or productivity rather than depth of burial. Lithology and organic composition provide mutually-corroborating evidence regarding glacial advances in the adjacent Sierra Nevada Mountains. During glaciations, the lake sediments are rich in sandstones, and the organic composition shows a predominance of externally-derived debris, with no evidence for contributions from the lake biota.

Paleoecological Factors Controlling Distribution and Mode of Life of Cretaceous Ammonoids in Texas Area

This paper touches upon the growing importance of paleoecology. The classification of marine environments now in vogue with biologists is briefly reviewed and modified for geological and paleontological purposes. Two classifications are recognized. The habitat classification (planktonic, nectonic, necto-benthonic, benthonic) is used without reference to depth of water involved. It is concluded that most or all shelled cephalopods were mobile benthonic and necto-benthonic. The bathymetric classification refers to depth of bottom. Divisions recognized are: 1. littoral, 2. neritic (a. mud flat, b. reef, c. epineritic, d. infraneritic), 3. bathyal (a. epibathyal, b. infrabathyal), 4. abyssal. It is suggested that the more obvious factors controlling ammonoid distribution may be interpreted in terms of morphology of shell, habits, and distribution of living cephalopods, associated fossil faunas, types of sediments, paleogeography, bottom, and factors of the water medium such as salinity, temperature, depth, oxygen content, waves, currents. Flotation of shells after death of the animal probably occurred, but was much less important as a factor of distribution than is commonly supposed. On the basis of shell morphology the principal Cretaceous ammonoid groups recognized are: 1. smooth, involute, obese (Phylloceras), 2. smooth, evolute, obese (Lytoceras), 3. smooth, ovate (Desmoceras), 4. sculptured, ovate to quadrate, a medium sculpture (Dufrenoya), b. robust sculpture (Mortoniceras), c. obese, spinose (Neocardioceras), 5. tenuous, involute, lightly sculptured (Oxytropidoceras), 6. tenuous, involute, smooth (Engonoceras), 7. uncoiled types (several subdivisions, Turrilites, Hamites, et cetera.) It is noted that ammonoids do not occur in littoral, mud flat, or reef deposits, and are rare with Orbitolina, rudistids, and corals. Only the tenuous ammonoids (Engonoceras, Oxytropidoceras) are numerous with the abundant echinoids, oysters, and thick-shelled gastropods and pelecypods of the epineritic zone. Sculptured forms of great variety and abundance occur in sediments deposited in the infraneritic zone (estimated at depths of 20-100 fathoms). Smooth, ovate genera (Desmoceras) lived in the epibathyal zone at or just below 100 fathoms. Smooth, obese genera (Phylloceras, Lytoceras and related forms) are found to the exclusion of other fossils in sediments of the infrabathyl zone. The Texas Cretaceous seas were too shallow to attract the smooth obese genera.