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| 2. |
- Bremer, Oskar, 1985-, et al.
(författare)
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Vertebrate remains and conodonts in the upper Silurian Hamra and Sundre formations of Gotland, Sweden
- 2020
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 142:1, s. 52-80
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Tidskriftsartikel (refereegranskat)abstract
- A long history of geological research on the island of Gotland, Sweden, has resulted in a detailed biostratigraphy based on conodonts for the Gotland sedimentary succession, but the relation between the Hamra and Sundre formations, the youngest strata on southern Gotland, has remained poorly resolved. These formations have also remained relatively poorly described in terms of vertebrates compared to other parts of the succession. A survey of museum collections and newly sampled material reveal that the taxonomical compositions and richness of vertebrate faunas remain similar compared to the underlying Burgsvik Sandstone and Oolite members. However, the relative abundance of the respective groups changes: Paralogania ludlowiensis and rare osteostracan remains of Tahulaspis sp. only occur in samples from the lower Hamra Formation, while Thelodus sculptilis becomes more common in samples from Sundre Formation. Conodont and isotope data give support to previous suggestions that the Hamra and Sundre formations may be largely isochronous, and it is possible that the observed differences in vertebrate faunas reflect changes in depositional setting. This interval on Gotland has been suggested to represent a hiatus in the East Baltic sections, where younger strata show an increased importance of acanthodians in the vertebrate faunas. Gotland could therefore give insights into the early stages of this diversification of gnathostomes during late Silurian times. However, this has to be done in combination with data from other areas, as well as with a review and revision of the scale-based taxonomy of Silurian acanthodians from the Baltic Basin.
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- Jarochowska, Emilia, et al.
(författare)
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Conodonts in Silurian hypersaline environments : specialized an dunexpectedly diverse
- 2017
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 45:1, s. 3-6
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Tidskriftsartikel (refereegranskat)abstract
- Hypersaline environments are commonly assumed to be barren of metazoans and therefore are avoided by paleontologists, yet a number of early Paleozoic jawless vertebrate groups specialized to live in such settings. Sampling bias against restricted settings resulted in substantial underestimation of their diversity. Rare studies venturing into such environments yielded multiple new species of conodonts, suggesting that the diversity and habitat range of these hyperdiverse predators of the early oceans are equally underestimated. We describe here autochthonous conodont fauna from evaporite-bearing horizons from the middle Silurian of Estonia that provide evidence for efficient osmoregulation in this group. Based on a global compilation of coeval conodont assemblages, we show that marginal-marine, periodically emergent environments were characterized by higher conodont diversity than open-marine shallow settings. This diversity is due to a high number of species occurring in these environments only. The high degree of specialization is also reflected by the highest within-habitat variability (β diversity) in marginal settings. Most conodont species had narrow environmental niches and, unlike in marine invertebrates, extreme environments were inhabited by the most specialized taxa. Such environments represent a large proportion of early Paleozoic tropical epicratonic basins. Our analysis allows quantification of the degree to which mid-Silurian conodont diversity is underestimated as a result of sampling bias against marginal-marine settings.
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| 7. |
- Jarochowska, Emilia, et al.
(författare)
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Revision of thelodonts, acanthodians, conodonts, and the depositional environments in the Burgen outlier (Ludlow, Silurian) of Gotland, Sweden
- 2021
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Ingår i: GFF. - London : Taylor & Francis. - 1103-5897 .- 2000-0863. ; 143:2-3, s. 168-189
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Tidskriftsartikel (refereegranskat)abstract
- Ludfordian strata exposed in the Burgen outlier in eastern Gotland, Sweden record a time of initial faunal recovery after a global environmental perturbation manifested in the Ludfordian Carbon Isotope Excursion (LCIE). Vertebrate microfossils in the collection of the late Lennart Jeppsson, hosted at the Swedish Museum of Natural History, hold the key to reconstruct the dynamics of faunal immigration and diversification during the decline of the LCIE, but the stratigraphic relationships of the strata have been debated. Historically, they had been placed in the Burgsvik Formation, which included the Burgsvik Sandstone and the Burgsvik Oolite members. We revise the fauna in the Jeppsson collection and characterize key outcrops of Burgen and Kapellet. The former Burgsvik Oolite Member is here revised as the Burgen Oolite Formation. In the Burgenoutlier, back-shoal facies of this formation are represented and their position in the Ozarkodina snajdri Biozone is supported. The shallow-marine position compared to the coeval strata in southern Gotland isreflected in the higher δ13C carb values, reaching +9.2‰. The back-shoal succession includes high-diversity metazoan reefs, which indicate a complete recovery of the carbonate producers as the LCIE declined. The impoverishment of conodonts associated with the LCIE in southern Gotland might be a product of facies preferences, as the diverse environments in the outlier yielded all 21 species known from the formation. Fish diversity also returned to normal levels as the LCIE declined, with a minimum of nine species. In line with previous reports, thelodont scales appear to dominate samples from the Burgen outlier.
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| 8. |
- Ray, David C., et al.
(författare)
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The Silurian Transgression of a Palaeoshoreline : The Area between Old Radnor and Presteigne, Welsh Borderlands
- 2021
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Ingår i: Lithosphere. - : GeoScienceWorld. - 1941-8264 .- 1947-4253. ; 2021:1
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Tidskriftsartikel (refereegranskat)abstract
- Quarries between Old Radnor and Presteigne, Welsh Borderlands, expose a Silurian nearshore succession, which onlaps a rocky palaeotopography of the Neoproterozoic basement that had been uplifted along the Church Stretton Fault Zone. The succession documents the Aeronian to Sheinwoodian transgression of an island or islands, with the following sequence of events: deposition of shallow marine sandstones (Folly Sandstone Formation), regional uplift, preservation of a rocky shoreline and associated deposits (Dolyhir Rudite Member), deposition of limestones characterized by a profusion of coralline algae and the abundant remains of reefs (Dolyhir and Nash Scar Limestone Formation), and finally deposition of trilobitic silty mudstones (basal Coalbrookdale Formation). Facies analysis, carbon isotope (delta C-13(carb)) values, sequence stratigraphy, and collections of bryozoans, conodonts, thelodonts, and trilobites have been used here as a means of refining our stratigraphic understanding of this unique succession. The revised stratigraphy demonstrates many similarities with the adjoining Midland Platform and the wider Silurian world. Notable features include the globally recognized early Sheinwoodian carbon isotope excursion and sealevel changes of regional and global extent. As one of the best examples of its kind, the palaeoshoreline and nearshore succession of Old Radnor and Presteigne acts as a depositional model for ancient rocky shores worldwide.
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| 9. |
- Sproson, Adam D., et al.
(författare)
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Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations
- 2022
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X. ; 577
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Tidskriftsartikel (refereegranskat)abstract
- The Ordovician (∼487 to 443 Ma) ended with the formation of extensive Southern Hemisphere ice sheets, known as the Hirnantian glaciation, and the second largest mass extinction in Earth History. It was followed by the Silurian (∼443 to 419 Ma), one of the most climatically unstable periods of the Phanerozoic as evidenced by several large scale (>5‰) carbon isotope (δ13C) perturbations associated with further extinction events. Despite several decades of research, the cause of these environmental instabilities remains enigmatic. Here, we provide osmium (187Os/188Os) and lithium (δ7Li) isotope measurements of marine sedimentary rocks that cover four Silurian δ13C excursions. Osmium and Li isotope records resemble those previously recorded for the Hirnantian glaciation suggesting a similar causal mechanism. When combined with a new dynamic carbon-osmium-lithium biogeochemical model we suggest that astronomical forcing of the marine organic carbon cycle, as opposed to a decline in volcanic arc degassing or the rise of early land plants, resulted in drawdown of atmospheric CO2, triggering continental scale glaciation, intense global cooling and eustatic sea-level lows recognised in the geological record. Lower atmospheric pCO2 and temperatures during the Hirnantian and Silurian glaciations suppressed CO2 removal by silicate weathering, driving 187Os/188Os and δ7Li variability, supporting the existence of climate-regulating feedbacks.
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| 10. |
- Terrill, David F., et al.
(författare)
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Sr/Ca and Ba/Ca ratios support trophic partitioning within a Silurian conodont community from Gotland, Sweden
- 2022
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Ingår i: Paleobiology. - : Cambridge University Press. - 0094-8373 .- 1938-5331. ; 48:4, s. 601-621
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Tidskriftsartikel (refereegranskat)abstract
- Conodonts were a highly diverse and abundant vertebrate group whose fossils are found in marine Paleozoic and Triassic strata around the world. They inhabited environments ranging from lagoons to open oceans and are represented by a wide variety of dental morphologies. Conodonts may have filled many different ecological niches and represent a significant proportion of nekton before the Devonian. Despite this, very little is known about trophic ecology of conodonts. While morphological diversity suggests a complex trophic structure within conodont communities, there is little evidence to support dietary niche partitioning among conodonts. We tested the hypothesis that individual conodont taxa occupied different trophic niches, using Sr/Ca and Ba/Ca ratios preserved in the dental elements of assemblages from Silurian strata of Gotland, Sweden. Sr/Ca and Ba/Ca have been shown to vary in vertebrate skeletal tissues depending on trophic positioning, although biological and environmental conditions can affect these ratios. Environmental influences were minimized by examining entire conodont communities from a tropical epeiric sea and by measuring strontium isotope ratios using thermal ionization mass spectrometry in the most metropolitan taxon (Ozarkodina confluens). Composition of white matter, a tissue unique to conodonts, was also analyzed using microprobe analysis, revealing significantly lower Sr concentrations than in surrounding lamellar tissue, suggesting taxon-specific histology should be considered when analyzing conodonts for geochemical data. Excluding taxa with highly variable quantities of white matter, the results show that each taxon preserves different Sr/Ca and Ba/Ca ratios with limited overlap, indicating variation in trophic positioning.
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