Lethaia最新文献

Wellin patch reefs are small Upper Eifelian build?ups within the fine?grained argillaceous limestone of the Hanonet Formation. Whereas the reefs themselves are not well exposed, their fossil assemblage is accessible in the hills near the town of Wellin, approximately 40xA0km SE of Dinant in Belgium. It is especially rich in massive stromatoporoids, heliolitids and other tabulate corals. They exhibit predominantly domical and bulbous morphologies. This paper focuses primarily on the palaeoautoecology of the heliolitid corals and their relationships with other organisms. Cases of mutual overgrowth between heliolitids, other corals and stromatoporids suggest a high degree of competition for space on the reefs, possibly related to the scarcity of hard substrates. Coral and stromatoporoid growth forms, as well as the prevalence of micritic matrix, point to a relatively low energy environment. However, abundant growth interruption surfaces, sediment intercalations and rejuvenations of corals suggest episodically increased hydrodynamic regime and sediment supply. It is inferred that the patch reefs developed in a relatively shallow environment, where the reefal assemblage was regularly affected by storms. Heliolitids exhibited high sediment tolerance and relied on passive sediment removal for survival. They also could regenerate effectively and commonly overgrew their epibionts, after the colony’s growth was hampered by the sediment. This is recorded in extremely abundant growth interruption surfaces, which allow the analysis of the impact of sediment influxes on the heliolitid corals.

A primary study of bioerosional structures and skeletobionts associated with an almost monotypic assemblage of free-lying bryozoan colonies of Celleporaria? sp., from the middle Miocene succession of the Siwa Oasis, Egypt, revealed four groups of encrusters (serpulids, sheet-like cheilostome bryozoans, balanoid barnacles and oysters) and eight different ichnotaxa. Undersides of the colonies have basal exterior walls, which are relatively richer in encrusters than are the convex sides. The dominant bioerosional structures were due to the boring activities of non-predatory organisms. The diversity of macro-bioerosional ichnotaxa is moderate and includes Trypanites, Gastrochaenolites, Maeandropolydora, Caulostrepsis, Renichnus, Spirolites and Centrichnus. In addition, some conical borings perpendicularly penetrating the shells could belong to the predatory tracemakers of Oichnus. Despite a moderate total ichnodiversity, the observed sample diversity of ichnofossils is low (about 35% of the total specimens studied). This is explained by a relatively high sedimentation rate with only short periods of exposure on the seafloor. The dome-shaped bryozoan occurrence is interpreted to represent a parautochthonous assemblage, which may have been influenced by shallow-water processes in response to storm activity.

Thecideide brachiopods are an anomalous group of invertebrates. In this study, we discuss the evolution of thecideide brachiopods from the Triassic to the Holocene and base our results and conclusions on microstructure and texture measurements gained from electron backscatter diffraction (EBSD). In fossil and Recent thecideide shells, we observe the following mineral units: (1) nanometric to small granules; (2) acicles; (3) fibres; (4) polygonal crystals; and (5) large roundish crystals. We trace for thecideide shells the change of mineral unit characteristics such as morphology, size, orientation, arrangement and distribution pattern. Triassic thecideide shells contain extensive sections formed of fibres interspersed with large, roundish crystals. Upper Cretaceous to Pleistocene thecideide hard tissues consist of a matrix of minute to small grains reinforced by acicles and small polygonal crystals. Recent thecideide species form their shell of mineral units that show a wide range of shapes, sizes and arrangements. We find from Late Triassic to Recent a gradual decrease in mineral unit size, regularity of mineral unit morphology and orientation and the degree of calcite co-orientation. While crystallite co-orientation is the highest for fibrous microstructures, it is strikingly low for taxa that form their shell out of nanogranular to acicular mineral units. Our results indicate that Upper Jurassic species represent transitional forms between ancient taxa with fibrous shells and Recent forms that construct their shells of acicles and granules. We attribute the observed changes in microstructure and texture to be an adaptation to a different habitat and lifestyle associated with cementation to hard substrates.

The impact of volcanic activity and trophic state on the preservation of plant remains from two Triassic palaeolake deposits in western Gondwana is investigated. Six taphonomic modes are identified for the Potrerillos–Cacheuta sequence and the Los Rastros Formation, along with proposed taphonomic pathways for each mode. The taphonomic pathways were compared with those proposed for the Agua de la Zorra Formation, a coeval lacustrine-deltaic succession. In all three cases, plant preservation in off-shore lacustrine facies was enhanced by anoxic bottom conditions. In the more proximal near-shore facies, a high sedimentation rate would have been factor that enhanced the preservation. Variation in relative abundance and taxonomic diversity between the three successions are attributed to ecological differences rather than taphonomic differences among the lake systems. No evidence was recorded that the preserved plant material was produced by volcanic trauma. The preservation of the cuticle recorded in the Cacheuta Formation was likely enhanced by the apparently low thermal maturation of the rock and the presence of tuffaceous horizons. In the three successions, the abundance and distribution of remains throughout the sections suggest that the trophic state of the lake did not influence the preservation of the plant.

Planktonic foraminifera census data have been widely used to reconstruct changes in ocean ecosystems as well as ocean chemistry and circulation through calibration functions. Here we analyse new core-top census data from 22 sites in the western South Atlantic, improving the geographical coverage and spatial resolution of the environmental gradients from the region covered by the Brazil–Malvinas Confluence (˜38°S–53°W). We combine them with previous data to provide an up-to-date compilation of the western South Atlantic planktonic foraminifera–calibration data set. We study the relationship between the assemblages present in the core-top samples and the most dynamic environmental variables from the region to establish the environmental variable(s) more resolved by the assemblages. Therefore, we develop and assess a new calibration function using the data set and testing several statistical models at different water depths. Our results reveal that the distribution and abundance of the species in the Brazil–Malvinas Confluence region mainly reflect the influence of two environmental variables: the mixed layer temperature and mixed layer depth (57% of total variance). The most precise reconstructions were obtained when using sub-surface temperatures between 40 and 50 m water depth. The application of the calibration function to a Holocene sediment core at ˜37°S–53°W revealed mean annual sub-surface temperature reconstructions between 8°C and 11°C, confirming the northward displacement of the Brazil–Malvinas Confluence during the onset of the Holocene and suggesting a major influence of the Malvinas Current during the entire Holocene at the studied site.

In addition to a marked rise in marine biodiversity, the Ordovician witnessed the most profound increase in the complexity of marine ecosystems in the history of the earth, including the expansion of tiering and enhanced biotic interactions. In order to understand these changes, it is important to study palaeoecological relationships among organisms at the commencement of the Great Ordovician Biodiversification Event (GOBE). Here, we describe sclerobionts associated with one of the earliest bryozoans in the fossil record, the oldest known trepostome, Orbiramus from the Fenhsiang Formation (late Tremadocian) of Hubei Province, China. These sclerobionts are diverse and include borings (e.g. Trypanites and Sanctum), bioclaustrations of vermiform organisms, and syn-vivo fouling by the putative black coral Sinopathes and other bryozoans. Diverse sclerobiotic associations and intricate palaeoecological relationships had already been established between bryozoans and other metazoans by the Tremadocian, showing the early onset of the ‘hard substrate revolution’ during the GOBE.