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A negative stable carbon-isotope excursion (CIE) has been identified at sites across the globe in strata that span the Triassic–Jurassic boundary. Different studies have suggested that this negative CIE could be the result of either a change in vegetation or a massive perturbation in the global carbon cycle at this time. To determine which, 84 hand-picked leaf cuticle fragments from plant macrofossils previously identified to genus level were analyzed for stable carbon-isotope values. The samples were taken from known heights in nine plant beds spanning the Rhaetian–Hettangian (Upper Triassic–Lower Jurassic) at Astartekløft, East Greenland. We have constructed taxon-specific stable carbon-isotope curves for Ginkgoales and Bennettitales and compared these to an existing δ13C curve based on fossil wood from the same section. This study reveals that taxon-specific carbon-isotope curves based on the leaf data from these two seed-plant groups both record the same negative CIE as the fossil wood, despite having different ecological roles and different relative abundances in the section. Correspondence analysis of the macrofossil abundance data, where the plants are considered in their ecological groups, shows that the δ13C values bear no relationship to changes in vegetation. This result further suggests that vegetation change had little role in determining the δ13C values at this time. Considered together, the bulk cuticle and taxon-specific δ13C record indicate that the negative CIE at the Triassic–Jurassic boundary is likely to have been caused by a massive perturbation of the global carbon cycle and not by vegetation change.
Since population studies are most reliable when applied to census assemblages, edrioasteroid paleoecology can best be understood by examining catastrophically buried obrution communities. This paleoecologic study examines a carbonate hardground surface encrusted with four species of isorophid edrioasteroids: Curvitriordo stecki, Carneyella ulrichi, Carneyella pilea, and Streptaster vorticellatus. Analysis of edrioasteroid diameters, a proxy for age, shows a bimodal distribution for Curvitriordo stecki, suggesting a hiatus in recruitment or multiple spatfalls. Low juvenile mortality may explain a left-skewed distribution among individuals of Carneyella ulrichi. Lack of juvenile individuals of S. vorticellatus suggests that this population matured from a single spatfall; there were too few specimens of C. pilea for analysis. Edrioasteroids on this surface exhibit no preferred ambulacral orientation. Spatial analysis (SA) shows an inter-specific clustered distribution at several spatial scales. Intraspecific SA indicates a clustered distribution for Curvitriordo stecki and Carneyella ulrichi; there were too few specimens of S. vorticellatus and C. pilea for analysis. Examination of inter- and intraspecific edrioasteroid taphonomy reveals that thecal collapse, disarticulated cover plates, and disarticulated interambulacral plates occur in nearly half of the population, suggesting brief post-mortem exposure on the paleoseafloor without protection of sediment cover. Individuals of S. vorticellatus suffered thecal collapse, yet all plates and ambulacra remained intact, suggesting that robust thecal elements may inhibit thecal disarticulation.
The Cambrian–Ordovician boundary interval is a critical moment in the ecology of trilobite communities. To understand this transition, we studied—at three different spatial scales—changes in the structure of olenid-dominated communities included in the Parabolina fauna, which flourished in the latest Cambrian, largely storm-dominated, successions of northwestern Argentina. At the local (∼meter) scale, species-poor communities occur in shoreface deposits. Relatively flat species-abundance distributions (SADs) and high evenness characterize upper offshore to offshore transition settings of the early highstand systems tract (HST), whereas uneven SADs in species-poor communities are typical of the lower offshore and shelf environments of the transgressive systems tract (TST). This pattern is unlikely to be caused by a change in time averaging and is consistent with the intermediate disturbance hypothesis predicting unimodal diversity gradients. The pattern is thus interpreted to be related to a trend in intensity and frequency of storm disturbance along local shallowing-upward gradients. At the regional scale (∼100 km), the diversity trend across the sampled west-east transect is rather variable and does not match the depth or oxygen-related gradients. At the biogeographic scale, the patterns of abundance of two key taxa (Parabolina and Asaphellus) show contrasting abundance and occupancy patterns between the Cordillera Oriental siliciclastic settings and the more carbonate-rich settings of Famatina (Argentina) and Oaxaca (Mexico). The presence of these genera in settings spatially adjacent, but environmentally different from their preferred habitats can represent a signature of source-sink dynamics. Low sample evenness values for the Cordillera Oriental contrast with those of coeval Laurentian communities, implying that a secular increase in evenness took place earlier in Laurentia than in Gondwana.
Black shales deposited in epeiric seas preserve dynamic, bottom-water oxygen conditions as inferred from high-resolution trace- and body-fossil data. Nearly all paleoproxies are imperfect; however, this study utilizes the strength of paleobiological proxies to capture variability in bottom-water oxygen levels through aerobic and anaerobic, as well as dysaerobic (reduced but non-zero oxygen) conditions. Trace- and body-fossil data were collected at high resolution from twelve localities through central and western New York State, United States, exposing Devonian black shales. The combination of relative amount of bioturbation, estimated as ichnofabric index (ii), maximum burrow diameter measurements, and body-fossil species diversity allows relative bottom-water oxygen levels to be interpreted on a millimeter scale. Overall, trace-fossil size and diversity through the dysaerobic zone are much reduced compared with those described from younger strata. The decreased depth of bioturbation compared with younger strata, and resulting reduced overprinting of the sedimentological record by deep burrowing infauna, provide the opportunity for records of high frequency fluctuations in relative oxygen levels to be preserved. The biological signal preserved in these units reveals that bottom-water oxygen levels fluctuated considerably within a narrow stratigraphic range (on a centimeter scale) and that the dysaerobic zone is not temporally stable.
Neoichnological experiments with freshwater ostracodes document different morphological types of traces, their associated behavior in various water depths and media ( = substrates) in controlled microcosms, and the potential for their identification in the fossil record. In uncompacted, very fine- to medium-grained sand, the nektobenthic freshwater ostracode Heterocypris incongruens produced seven trace types: three crawling trails, a swimming trail, a resting trace, a burrow, and a self-righting trace. The most common crawling trails (Type 1) are randomly sinuous with infrequent looping. Two other crawling trails are observed infrequently and were self-looping (Type 2) and zigzagging (Type 7). Swimming trails (Type 3) are straight to sinuous and composed of a parallel set of appendage scratch marks. Crawling and swimming trails likely indicate exploratory or foraging behaviors associated with locomotion. Oval-shaped depressions (Type 4) commonly occur and represent a resting behavior. Burrowing produced asymmetrical U-shaped burrows (Type 5) and represents hiding from a stimulus or a foraging behavior. A fan-shaped trace of appendage scratches (Type 6) was produced when H. incongruens righted itself after landing upside down in the sediment. No traces were observed in coarse or very coarse sand, but ostracodes were observed pushing and toppling sand grains. Preservation potential of ostracode traces in freshwater environments is likely low. Traces were observed to degrade and fill in with sediment within 24–48 hours after formation or were destroyed immediately in water currents that disrupted the sediment surface. Ostracode traces are likely preserved best when formed below storm wave base and buried rapidly, or prior to desiccation in an ephemeral environment. After desiccation, only gross morphology of the traces is observed and desiccation cracks tend to follow crawling trails.