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Renalcids are a problematic group of mostly Paleozoic microbial fossils. In the Upper Devonian reef complex of the Canning Basin, Western Australia, they grew in cryptic reef environments, such as the undersides of laminar stromatoporoids during Frasnian time and within arborescent thrombolites during Famennian time. Renalcids preferred a pendant growth habit and were often the final phase of microbial encrustation in reef crypts. The micritic chambers of the Upper Devonian renalcids, Shuguria and Izhella, have carbon isotope values 0.3–1.8‰ lower than contemporaneous early marine cements, suggesting incorporation of respired CO2 into the micrite. Based on growth environment and carbon-isotope geochemistry, renalcids are neither fossilized cyanobacteria nor marine algae.
Renalcids may be fossilized biofilm clusters, which calcified due to heterotrophic bacterial activity within biofilm microenvironments. The dendritic clots of the renalcids are similar in size and morphology to reported microbial clusters observed in laboratory biofilms and modern bacterial shrubs from travertine deposits. Cloudy microcrystalline cement and weakly laminated micrite immediately surrounding the renalcid chambers may have formed by calcification and sediment agglutination in extra-polymeric substances associated with the biofilm clusters. The biofilm model of renalcids explains their geochemistry, cryptic habitat, and morphology. Preservation of biofilms as renalcids may have required rapid calcite precipitation rates.
The framebuilding fauna of an Early Silurian (Aeronian, Llandoverian), Brassfield Formation reef from west-central Ohio is examined in detail. Brassfield reefs are among the very few Silurian reefs of the midcontinental United States that are not dolomitized. Species-level taxa identified in the framebuilding fauna include two favositid corals, two proporid corals, two halysitid corals, two colonial rugosan corals, and at least four stromatoporoids.
Favositids, halysitids, crinoid holdfasts, and bryozoans are distributed fairly evenly across the reef; colonial rugosans and solitary rugosans are more abundant along the windward side of the reef; and stromatoporoids are more abundant on the leeward side. The Brassfield reef fauna resembles that from the Jupiter Formation (Aeronian), Anticosti Island, and the Manitoulin Formation (Aeronian), Manitoulin Island; however, there are differences in total diversity and composition. Paleogeographic positioning, local water depth, and access to open-ocean circulation are inferred as reasons for the differences among the Aeronian reefs.
The Omingonde Formation of Central Namibia is a redbed succession infilling a half-graben that developed on the upland plateau of southern Gondwana in the early mid-Triassic. Field studies of the rocks and vertebrate fossils of these strata are used to reconstruct the changes that occurred in the rift valley landscapes over approximately 10 My, and show how these changes affected the life habits and preservation of terrestrial reptiles that inhabited the valley at that time.
The early rift basin contained extensive lakes that were filled rapidly with conglomeratic sands deposited on alluvial fans prograding from the active boundary fault. These fans subsequently drained into an axial braided river system. With cessation of downfaulting, the fault scarp retreated and gradients in the basin became gentle enough to convert the braid plain to one or more meandering rivers confined by extensive floodplains. The climate changed from sub-humid at the beginning of rifting, through semi-arid for most of the Omingonde times, to arid at the onset of the overlying Etjo Formation sedimentation. The influx of abundant loessic silt, possibly a peripheral effect of the Triassic “megamonsoon,” significantly increased floodplain accretion and the burial potential of surface bones on the Upper Omingonde floodplains.
The major control of fluvial style and floodplain accretion rates in the Omingonde basin was subsidence caused by episodic movements of the boundary fault. However, the association of desiccated and mummified carcasses with thick beds of loessic silt suggests that climatic aridity was the overriding factor controlling the preservation and taphonomic style of vertebrate remains.
Predominant equilibrichnial Rosselia socialis are present at up to thirty stratigraphic levels in middle Pleistocene, siliciclastic inner shelf deposits on the Boso Peninsula, Japan. The ichnofabric composed of R. socialis is interpreted to have formed by a considerably dense population of R. socialis animals (terebellid polychaetes?). The ichnofabric interval represents a transgressive inner shelf deposit strongly affected by a high-frequency, 5th- or 6th-order sea-level rise that probably was rapid enough to influence the ecology of benthic communities. The dense colonization by the R. socialis animals is interpreted to result mainly from rapid transgression caused by short-term sea-level rise. Coastal erosion induced frequent pulses of sedimentation in to the shelfal environment and probably prevented colonization by most benthic animals, except for R. socialis animals, which are thought to be tolerant of such conditions. Ravinement also provided organic detritus, derived mainly from organic-rich coastal deposits of salt-marsh origin, which enabled the detritus-feeding and stress-tolerant R. socialis animals to thrive.
Rhynchonelliform brachiopods were diverse and often dominant benthos of tropical seas in the Paleozoic. In contrast, they are believed to be rare in open habitats of modern oceans, especially at low latitudes. This study documents numerous occurrences of rhynchonelliform brachiopods on a modern tropical shelf, particularly in areas influenced by upwelling. Extensive sampling of the outer shelf and coastal bays of the Southeast Brazilian Bight revealed dense populations of terebratulid brachiopods (>103 individuals / m2 of seafloor) between 24° and 26°S. On the outer shelf, brachiopods are more abundant than bivalves and gastropods combined. However, brachiopod diversity is low: only four species belonging to the genera Bouchardia, Terebratulina, Argyrotheca, and Platidia were identified among over 16000 examined specimens. Brachiopods occur preferentially on carbonate bottoms and include two substrate-related associations: Bouchardia (40–70% CaCO3 weight content) and Terebratulina-Argyrotheca (70–95% CaCO3). All four species display a broad bathymetric range that contrasts with a narrow depth tolerance postulated for many Paleozoic rhynchonelliforms. The most abundant populations occur in the depth range between 100 and 200 m, and coincide with zones of shelf-break upwelling, where relatively colder and nutrient-rich water masses of the South Atlantic Central Water are brought upward by cyclonic meanders of the South Brazil Current (a western boundary current that flows poleward along the coast of Brazil). This is consistent with previous biological and paleontological studies that suggest upwelling may play a role in sustaining brachiopod-dominated benthic associations. The presence of abundant brachiopods in the open habitats of the tropical shelf of the western South Atlantic contrasts with current understanding of their latitudinal distribution and points to major gaps in our knowledge of their present-day biogeography. The ecological importance of rhynchonelliform brachiopods in modern oceans and their role as producers of biogenic sedimentary particles may be underestimated.
Taxon-specific stable carbon-isotope ratios were determined for 7 graptolite (consumer) and 4 alga (primary producer) taxa preserved as thin organic compressions within North American Silurian Konservat-Lagerstätten. Values range from −26.6‰ for a graptolite from the Goat Island Formation, New York, to −32.3‰ for an alga from the Cape Phillips Formation, Arctic Canada. Co-occurring graptolite taxa display nearly identical δ13C compositions (within 0.2‰). Algal taxa co-occurring with graptolite taxa have δ13C compositions which are 1–2‰ more negative than those of associated graptolites. The observed isotopic offset between associated graptolites and macroalgae, both of which are common sources of organic matter in Paleozoic strata, holds implications for the interpretation of time-series δ13Corg curves, and suggests that stable carbon-isotope composition could serve as a chemosystematic tool for discerning the affinities of organic macrofossil problematica.
A survey of the large number of scallops from the Plio-Pleistocene shell beds of Florida held in the collections of the Florida Museum of Natural History has revealed that a proportion have been penetrated by drillholes of the ichnospecies Oichnus ovalis, which may be attributed to predatory octopods. Only large individuals had been drilled. Drillhole positioning was highly stereotyped, most being located on the ‘upper’ left valve, between ribs and directly into the adductor myostracum. Such stereotypy indicates that the octopods concerned were highly accustomed to taking scallop prey and also that the complex predatory behavior seen in modern octopods was already in place by the Pliocene. Such stereotypic patterns suggest that scallops from these localities were frequently the victims of octopod attack.
Benthic foraminiferal species abundance in samples from three Mendeleyev Ridge box cores were analyzed by cluster analysis and the newer method of SHE analysis. Previously, the latter technique only has been used on foraminiferal data from depth transects of modern surface sediment samples. Unlike most methods, which initially compare all possible pairs of samples, the SHE procedure results in a linear pattern if a sequence of samples are from the same statistical distribution. A change in slope indicates a statistical change in community structure and/or a change in species composition.
The research reported herein is the first application of SHE for the purpose of identifying biozones in sediment core samples for the purpose of stratigraphic correlation. Both cluster analysis and the SHE method provided zonation within cores. However, the cluster method often produced clusters that were difficult to identify and also contained a mixture of samples without stratigraphic continuity. In contrast, SHE resulted in easily identifiable biozones and ensured temporal continuity within them. In general, the cluster analysis produced more zones than the SHE analysis. About 87% of the cluster zones and 64% of the SHE zones were correlated across more than one core. The average age range for correlated biozone boundaries among the three cores, based on radiocarbon dates, was 821 years using cluster analysis and 296 years using SHE. The sequential nature of the analysis, ease in choosing boundaries, and correlation of these boundaries across cores makes SHE the preferred technique.