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In the Silurian, the most common hosts of endobiotic rugose corals were stromatoporoids followed by tabulate corals. Here we describe the relationship between rugose corals and a bryozoan. Solitary rugosans and the cystoporate bryozoan Fistulipora przhidolensis Kopajevich, 1990 formed a symbiotic association in the late Silurian (Pridoli) of Saaremaa, Estonia. The syn vivo nature of the association is indicated by complete intergrowth of both organisms and the perpendicular orientation of the rugosans to the bryozoan growth surface. There are one to seven endobiotic rugosans per bryozoan colony. This is the first detailed study of coral-bryozoan symbiosis from the Silurian; all previous reports describe Late Ordovician, Devonian, or Cenozoic material. The lack of malformations and decrease in the size of bryozoan zooids near the rugosans suggest there were no strong negative effects of the rugosans on the bryozoan. The rugosans likely benefited from their association with the bryozoan, which served as an anchor to stabilize them in hydrodynamically active waters, and the bryozoan may have benefited by protection against some types of predators. The associations described here were most likely mutualistic.
Volcanic eruptions have an impact on the paleoecology of plant communities and their preservation in the fossil record. The aim of this contribution is to evaluate the influence of volcanism on plant preservation in lacustrine settings via systematic sampling and chemical analyses of fossil plants in the Upper Triassic Agua de la Zorra Formation, of the Argentinian Cuyana Basin. Plant remains are not uniformly preserved throughout the Agua de la Zorra Formation and their stratigraphic distribution reflects the original spatial pattern of the plant communities. SEM-EDX analyses was used to demonstrate how volcanism affected diagenesis of the plant remains. Eight taphonomic modes were determined from biostratinomic analysis, and three preservational modes from the chemical features observed in the SEM-EDX spectra. The three preservation modes recognized by SEM-EDX analysis are more likely linked to diagenesis and are not equivalent to the taphonomic modes described based on the biostratinomic features. The Agua de la Zorra paleolake preferentially preserved plant material because of anoxic conditions in deeper parts of the lake, and a high sedimentation rate in proximal near-shore facies. The plant remains are most abundant in sandy facies deposited by high-energy flows but there is no evidence that these flows were pyroclastic nor that the plant remains were generated by volcanic trauma.
The thickness of microbialite crusts in Holocene barrier and fringing reefs of Bora Bora was quantified in drill cores from windward and leeward settings to decipher possible spatial and temporal patterns as well as controlling environmental factors. Based on the analysis of 145 occurrences in nine rotary cores, microbialite thickness ranges from 0.1–11.0 cm with an average value of 1.97 cm (SD = 2.47). Microbialites occur only from 9.5–5.6 ka corresponding to a period of rapid sea-level rise and reef accretion in the early Holocene. However, there is no statistically significant correlation between microbialite thickness and reef accretion rate. Also, there is no correlation between microbialite abundance and age. The upcore increase in microbialite abundance, however, suggests that time available for carbonate accretion in shallow water plays a role in microbialite formation. Crust thickness is greater on windward as compared to leeward fringing reef settings indicating that flushing of pore space is a likely factor controlling microbialite accretion. Other environmental factors potentially being responsible for the Holocene decrease in microbialite abundance include climate, i.e., decreasing temperatures and precipitation (supporting nutrient input by runoff) as well as decreasing seawater alkalinity. At the mesoscale, structureless and laminated microbialites are by far the most common types. Coated debris, boring infill, and digitate types are less common. Textures at the microscale, including laminated, clotted, and peloidal, do not necessarily match mesoscale textures. The Bora Bora microbialites consist in more or less equal parts of high-magnesium calcite and aragonite. The δ13C values range from +3.0 to +4.1‰ and the δ 18O from -0.8 to +0.1‰. The contents of easy soluble sulfate (ESS) and carbonate associated sulfate (CAS) are relatively high. The δ 18OCAS (+11.0 to +12.7‰) and δ 34SCAS values (+21.9 to +23.6‰) exceed the seawater sulfate standard NBS-127 value and are in the same range as observed in other cryptic, Holocene reefal microbialites. The Bora Bora microbialites contain lipid biomarkers derived from sulfate-reducing bacteria (2–8 wt%), marine plankton, land plants, and unspecified bacteria. The former include branched, short-chain fatty acids and terminally branched fatty acids, whereby iso -fatty acids are more abundant than anteiso -fatty acids. Other compounds with terminally branched alkyl chains include iso - and anteiso -C15 and -C17 alcohols, which are interpreted as degradation products of monoalkyl glycerol ethers (MAGE). Collectively, the organic and inorganic geochemical data together with the sedimentological and mineralogical data suggest that sulfate-reducing bacteria were involved in microbialite development. The temporal and spatial distribution patterns suggest that factors such as exposure to waves and currents, time, nutrient availability, alkalinity, and climate play important roles, however, more quantitative data from other occurrences are needed to be able to better discriminate among them.