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Extensive study of modern Bahamian stromatolites has resulted in a comprehensive model for their formation. Modern Bahamian thrombolites—microbial deposits with a mottled, clotted fabric— have not, however, received the same degree of study. Current models link the Bahamian thrombolites with the presence of a mixed-bacterial-and-metaphyte benthic ecosystem, whereas stromatolite formation is linked with an almost exclusively bacterial benthic ecosystem. By focusing on the preserved fabrics of several specimens, including an entire column 1.5 meters tall, we have developed a new model for the genesis of the clotted, thrombolitic fabric. Our findings demonstrate that variations in the amount and style of penecontemporaneous diagenesis, rather than differences in surficial benthic ecosystems, are the predominate cause of the disparate carbonate fabrics present in the Bahamian microbialites examined. More specifically, the irregular, clotted fabric that characterizes the thrombolites is the result of remodeling a precursor fabric. This remodeling is caused by physical and metazoan disruption, penecontemporaneous micritization, secondary cementation, and localized carbonate dissolution. This new model of syndepositional remodeling of a laminated fabric to a well-cemented, clotted one may be applicable to some ancient thrombolites.
The Bissekty Formation, exposed on the Dzharakuduk escarpment (Kyzylkum Desert) in Uzbekistan, contains the richest and most abundant Turonian terrestrial fauna known from Eurasia. This study utilizes ordination analyses to identify spatial or temporal patterns (i.e., biofacies) in the distribution of spatially averaged skeletal elements (i.e., mix of unequivocally marine faunal elements with terrestrial ones) within the laterally extensive intraformational conglomerates (IFCs) of the Bissekty Formation. Ordination analyses were used to determine similarities among the IFCs based on their absolute taxonomic abundance and presence or absence. To determine the primary factor(s) driving the ordination pattern, taxonomic abundance, richness, environmental restrictions (aquatic, semi-aquatic, and terrestrial), and skeletal element size were examined. Relative Sørensen and Sørensen in combination with Euclidean and city block metrics were used in the analysis and results were consistent across methodologies. Ordination patterns were driven by aquatic taxa, which were dominated in abundance and richness by marine and brackish-tolerant taxa. By mapping the abundance of terrestrial, semi-aquatic, and aquatic taxa of individual IFCs onto ordination space, the relative position of the coastline with reference to the Dzharakuduk escarpment during the deposition of the Bissekty Formation can be inferred. These results indicate that ordination analyses are useful tools for examining taphonomically biased samples and should be utilized more frequently in vertebrate studies.
The silicified brachiopod faunas collected by G. Arthur Cooper and Richard E. Grant over decades of field work in the Permian Basin of western Texas provide a remarkable resource for studying change in the structure of fossilized ecological communities. Ordination analyses using 511 faunal lists that include 967 species (190 genera) from the Kungurian through the Capitanian Stages of the Guadalupe and Glass Mountains (∼15 myr) reveal four distinct, major stratigraphic clusters, each divisible into two to three subclusters. For the most part, these clusters coincide with third-order depositional sequences bounded by unconformities. Except for the youngest (Capitanian Stage), all the stratigraphic clusters show internal ordination patterns consistent with biogeographic and environmental (i.e., biotic gradients) control on the distribution of taxa. Despite this evidence for environmental sorting of taxa, Mantel tests indicate that patterns of generic co-occurrence break down from cluster to cluster despite sharing many of the same species and genera. This pattern cannot easily be explained by preservational or stratigraphic biases, suggesting that the processes that govern ecological communities may not lead to unique, strictly determined associations despite the presence of broadly similar species under similar environmental conditions. In contrast to the rest of the study interval, Capitanian clusters are less well structured (i.e., they do not show strong biotic gradients) than assemblages from any of the other clusters and do not show significant recurrence of generic associations. This may reflect evolution of carbonate margins in the basin from ramps to shelves with steep slopes over the duration of the study interval, a shift that may have changed the spatial distribution of (and covariation among) the environmental parameters that controlled brachiopod distributions.
Chemosynthetic ecosystems teeming with tubeworm colonies were discovered at hydrothermal vents in the Galapagos Ridge in 1977 and at cold seeps at the base of the Florida Escarpment in 1984. As a result of these reports a number of fossil examples were identified in the rock record. One such assemblage was recognized in the Western Interior Seaway, in the Middle Campanian Pierre Shale Formation, where previous researchers noted siboglinid (formerly vestimentiferan and pogonophoran) tubeworms in methane-derived nodular limestones with tubules. On the inside these tubules have an outer ring of micrite with microparticulate siliciclastic materials and a core of calcite cement. Alternatively, they have an outer annulus of calcite cement and a core of microparticulate siliciclastic materials with calcite cements. Interestingly, the cemented cores can contain meniscate and vesiculate fabrics in association with the microparticulate linings. With this evidence we infer that the tubules preserve fabrics of former gas bubbles; the microparticulate linings are deposits that accumulated on the walls of the tubules as fluids streamed through them. Methane bubbles would have carried adhered siliciclastic microparticles and bubble wakes would have held entrained microparticles. We, thus, interpret the tubules to be former, small, subseafloor conduits along which fluid and particulate transport occurred. Particle transport by gas bubbles is a well-known process in chemical and mineral industries. Our observations highlight this process for the first time in an ancient geologic conduit system and provide a mechanism for maintaining particulate plumes that accompany effusing methane streams at modern seeps and vents.
Here we document the occurrence of locally common oncoids in the Cedar Mountain Formation of Utah in the Woodside Anticline area of the San Rafael Swell and use them to understand changes in the Early Cretaceous landscape and their effects on the dinosaur fauna. Detailed facies analysis is required to understand the context of these changes within the broader patterns of Mesozoic tectonics and the fossil record. Oncolite crops out in the Cedar Mountain Formation, directly overlying the Buckhorn Conglomerate. Oncolite is not widely distributed outside of the Woodside Anticline area. The oncoids are found in a bimodal population with the majority in the 2–5-cm-diameter range and a smaller population >25 cm in diameter. Nuclei are mostly rounded chert clasts and also include litharenite, polymict conglomerate, limestone, and both abraded and nonabraded dinosaur bone and wood fragments. Cortices are 3–5 mm thick with distinct, penecint (laminae that completely enclose a body; Hofmann, 1969), low-relief laminae. Some laminae are crenulated and comprise ministromatolites. The petrography of the oncoids suggests formation along lake margins where large fragments of reworked sedimentary clasts and dinosaur bones came to rest and were coated by bacterial mats. Caliche cements and coats some of the oncolite; these define a lake shoreline affected by fluctuating lake level. The isotope geochemistry indicates a combination of primary and diagenetic signals consistent with oncoid formation in open, ephemeral freshwater lakes.
The Oligocene–Miocene Transition (OMT) was an interval of regional environmental and biotic change on Caribbean reefs. During the late Oligocene, a diverse Tethyan biota contributed to extensive reef building across the region, but by the early Miocene, reef building had declined, and a regional extinction had removed up to 50% of the late Oligocene diversity. The general decline in reef building in the Caribbean has been attributed to changes in regional water quality. New collections of scleractinian reef corals from four different units in the northwestern Falcón Basin of Venezuela include distinct late Oligocene and early Miocene assemblages. The distribution of fossil coral species and reef limestones suggests that the thick carbonates of the San Luis Formation were deposited during the late Oligocene and that changing water quality during the OMT resulted in the demise of San Luis coral-reef ecosystems. Previous studies have noted an increase in surface-water productivity after the OMT in the region and have suggested oceanographic reorganization as the primary cause; this interval, however, also coincides with a switch in the outlet position of an ancestral Orinoco River. A change in the terrestrial geography of South America might have caused the regional decline in reef building by altering surface-water characteristics, just as modern Orinoco and Amazon outflows exert strong control on shallow-water habitats off the coast of northeastern South America.
Although most investigations of crinoid-predator interactions have focused on nektonic vertebrates (fishes and sharks), slow-moving benthic animals such as cidaroid echinoids may also interact antagonistically with stalked crinoids. This was recently supported by observations of extant isocrinids in modern deep-sea environments near the west end of Grand Bahama Island. In this paper, we report on stalks of crinoids from the Late Jurassic of south-central Poland, which co-occur with remains of cidaroids and show characteristic holes, bite marks, and signs of breakage. By analogy with the modern example, we interpret this as evidence of predation by cidaroids on crinoids. These Late Jurassic data may indicate that benthic predation was intense during the mid-Mesozoic. Importantly, this discovery also strengthens the hypothesis that benthic predators may have exerted considerable influence on the evolution of stalked crinoids.