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The Lithothamnion Limestone constitutes the uppermost carbonate unit of the Bolognano Formation outcropping in the Majella structure (Central Apennines, Italy). It represents the northern extension of the large Apulia Carbonate Platform and preserves an excellent record of the progressive decay of trophic conditions due to the approach of foredeep systems characterized by turbiditic siliciclastic sedimentation during the early Messinian. Sedimentological and compositional analyses were used to reconstruct the depositional model and evolution of platform environmental conditions. The profile is consistent with a homoclinal carbonate ramp, with a wide middle-ramp environment in which coralline algae, mainly forming the maërl facies, dominated carbonate production. This facies was associated with seagrass meadows colonizing the inner ramp. The outer ramp was characterized by bioturbated hemipelagic marl with planktonic foraminifera and pectinids in the aphotic zone. Three main stages of ramp evolution have been identified. During the first stage, the ramp was subjected to high-energy wave-dominated conditions, which favored the development of deep rip channels in which accumulations of vertebrate bones have been identified. In the second stage, maërl facies and seagrass meadows developed, initially in an oligotrophic setting, later followed by a slight reduction in light penetration. The third stage involved a general increase in fine terrigenous sediments, together with a further decrease in light and also by the spread of coralline algal bindstone facies. This elevated terrigenous input was associated with increased trophic conditions, as also shown by the occurrence of abundant plankton and low-oxygenated foraminiferal assemblages.
The end-Triassic mass extinction was global, severe, and accompanied by worldwide disturbance to carbonate ramp and platform sedimentation. We examine the earliest known Jurassic carbonate ramp produced in the back-arc basin along NE Panthalassa following the extinction event to determine the biotic constituents and timing of local ecological recovery. Field observations, fossil surveys, and microfacies analysis focused on the Ferguson Hill Member (Hettangian and Sinemurian) of the Sunrise Formation in the New York Canyon area of west central Nevada, USA. In the Hettangian, post-extinction biosiliceous sedimentation extended to the inner ramp, where an ooid and grapestone shoal marked the outermost extent of a narrow belt of carbonate sedimentation. An early recovery phase in the late Hettangian is characterized by widespread, laterally homogeneous, demosponge-dominated level-bottom sedimentation across the mid- to inner-ramp, in addition to limited trophic tiering (sessile epifaunal suspension feeding and mobile infaunal deposit feeding), substantial ramp aggradation, and poor settling conditions for competitive benthic colonizers (e.g., corals, crinoids, infaunal bivalves). Within 1.6–2 Myr after the extinction (in the early Sinemurian), a late recovery phase is recognized by the appearance of epifaunal grazers (gastropods, echinoids) and suspension feeders (crinoids, solitary scleractinian corals), phototrophic microbialites (oncoids, and possibly photosymbionts within corals), and infaunal deposit or suspension feeders (bivalves). Although the late recovery faunas included more trophic levels than pre-extinction carbonate ramp habitats, development and progradation of the first Jurassic carbonate ramp still relied heavily on sponge, microbialite, and abiotic mineralization.
Trace fossils are important evidence of benthic activity, but they have received less study than body fossils for investigating the aftermath of the end-Permian extinction. There is therefore a need to document Lower Triassic ichnofaunas to understand their significance with respect to the end-Permian crisis. In light of this need, this paper describes a novel Lower Triassic ichnosite at Mount Pallone (Carnic Alps, Italy), where the Campil Member (Smithian) of the Werfen Formation (Griesbachian–Spathian) presents an abundant ichnofauna characterized by excellent preservation and low diversity. Documented ichnogenera include Asteriacites lumbricalis, Gyrochorte comosa, Diplocraterion habichi and Planolites beverleyensis. The ichnofaunal composition and the bioturbation style suggest a marginal marine paleoenvironment ranging from intertidal to shallow subtidal settings. Storm influence, hydrodynamic energy, sedimentation rate, freshwater input and/or water temperature played an important role in structuring the benthic ecosystem. Dense (300 specimens/m2) aggregations of the trace fossil Asteriacites lumbricalis reveals social behavior of their inferred brittlestar producers (Echinodermata, Ophiuroidea). In line with modern brittlestar beds, social behavior provided significant advantage because raised arms of brittlestars dampened hydrodynamic energy. This study suggests that Asteriacites beds may be considered ichnological proxies for marine settings, low bioturbation intensity, shallow tiering, high sedimentation rate and/or event-bed deposition, significant levels of hydrodynamic energy, and low predation pressure. The studied ichnofauna reflects stressed environmental conditions, but it is unclear whether this reflects local brackish conditions (‘Gazpacho model’) or global hot temperatures (‘Hot Soup model’).
Fossil microbes are generally preserved by authigenic minerals, including silica, apatite, iron minerals, clays, and carbonates. An alternative mode of preservation by entombment in calcite, without replacement, has been identified in carbonate cave pool microbialites that were etched and examined in the scanning electron microscope (SEM). Features identified include filaments, threads, and films that show excess carbon in energy dispersive X-ray (EDX) analyses, suggesting preservation of organic matter. Filaments are single smooth or reticulated strands with curving string-like morphology, often hollow, and with a uniform diameter of 0.5 to 1.0 μm. Threads, in contrast, are variable thickness, from several microns down to 0.1 μm, always solid, and commonly branch. Films are thin (< 1 μm) drapes associated with threads. Filaments are interpreted as microbial filaments, while threads and films are interpreted as preserved extracellular polymeric substance (EPS). In addition, microbial filaments and EPS are only revealed via acid etching, suggesting preservation of organic material by entombment, not by replacement with calcite. To determine whether entombed microbes are a common feature of carbonate microbialites that form in different environmental settings, samples of hot spring travertine, caliche soil, and reef microbialite were examined. Whereas the travertine samples were barren, entombed EPS was found in the caliche soil and the reef microbialite; the latter also contained a few entombed filaments. In addition, entombed microbial material has been reported from carbonate cold seep deposits. Such findings indicate that entombment of microbes and EPS in carbonates is not restricted to cave settings, but is more widespread than previously reported. Possible causes for the lack of preservation in travertines include rapid degradation of microbial material either by sunlight due to photolytic degradation, aerobic microbial degradation, detritivore consumption, or elevated temperatures. Rapid carbonate precipitation is ruled out as, somewhat surprisingly, preservation is better in slower growing cave carbonates than in rapidly growing travertines. Potential long-term preservation of organic material entombed in carbonate has implications for the characterization of fossil microbial communities using molecular biomarkers and the search for life on other planets.
The Norian Stage of the Late Triassic represents a long interval from which the benthic faunal succession is poorly understood, particularly from eastern Panthalassa. Fossiliferous bulk samples of shallow marine carbonates were collected from the Luning and Gabbs formations in west-central Nevada to evaluate the changes in faunal composition and paleoecological structure during the Norian Stage. Stationary epifauna dominated the early Norian faunal assemblages but gradually became less common by the late Norian, with the exception of cementing bivalves, which were common in the middle and late Norian. After the early Norian, mobile infauna also became increasingly abundant and diverse. These paleoecological trends are similar to those observed in Tethyan level-bottom carbonate deposits from the Lombardian Basin in northern Italy. Reclining epifauna remained uncommon in the local area until the Early Jurassic, and this Mesozoic decline in recliners preceded the end-Triassic mass extinction in Nevada.