Benthic (bryozoan) response to Silurian climatic changes
Final Report Abstract
In this project, Silurian Bryozoa, as example for sessile benthic organisms, were studied. Main question was how they responded to the climatic changes reflected in strong isotopic fluctuations. Most strikingly, at times of high isotope values, bryostromatolite reefs partly replaced the typical Silurian reefs, a feature not known so far. This reef type was mainly build of microbes and bryozoans instead of rugose corals, tabulates, and stromatoporoids. Furthermore, they were associated with abundant, otherwise rare components, such as phosphatic pearls and linings inside bryozoans, phosphate-shelled brachiopods (incl. Chynithele vexata which was only known from contemporaneous deposits in the Prague basin), gypsum pseudomorphs, and crust-like Palaeomicrocodium. In some bryostromatolites, Celticystis gotlandicus was found in high abundance. It is the only representative of Diploporita (order Cystoidea, Echinodermata) in the Silurian of Baltica and only known from Gotland. In addition, macrobioerosion traces are commonly observed, including the possibly oldest trace of deepboring bivalves known so far. The reefs probably formed in very shallow water, possibly right below the sea surface, indicated by Palaeomicrocodicum and the presence of gypsum and vadose silt (indicating phases of subaerial exposure). Studies of recent material from brackish ponds of Zeeland (Netherlands) show similar growth structures and water depths. It seams that the Silurian sea water in times of elevated d13 values was enriched by phosphate, a limiting nutrient in the ocean today, and generally supposed to be present only in low concentrations in sea water. It was apparently so much enriched that the bryozoans had to form phosphatic pearls or linings. The characteristic cauliflower-like surface acted as cryptic habitats that were used by rare organisms and at the same time promoted bryozoan diversity. On Gotland, such reefs only formed during times of high carbon isotope values. However, comparable Ordovician and Silurian reefs from other areas could be identified in the literature, and all of them occur at times strongly elevated carbon isotopes. This connection suggests that the formation of bryostromatolites was directly related to the changes that led to the elevated carbon isotope values. Besides the formation of bryostromatolites, bryozoan diversity seems to have increased during times of high isotope levels. Previously studied Silurian bryozoan assemblages were quite diverse, but because not many of them got studied so far, the knowledge about bryozoan diversity during the Silurian is very limited. The, in this project studied, assemblages also show a high diversity, and especially the Wenlock communities show a strong mixture of species from different palaeoprovinces. For example, the Sheinwoodian bryozoan community from Dolyhir (UK) consists of species known from different palaeoprovinces: (a) species from the Much Wenlock Limestone Formation (upper Wenlock of the province Avalonia), (b) from the Rochester Shale (lower Wenlock of the province Laurentia), and (c) from Gotland (upper Llandovery to Ludlow of the province Baltica). This unusual combination was probably induced by the narrowing of the Iapetus Ocean between the palaeoprovinces Avalonia and Laurentia. Formerly this ocean acted as barrier for larval drift. The closure possibly caused the absence of endemism in the Middle Silurian.
Publications
- (2019): The oldest deep boring bivalves? Evidence from the Silurian of Gotland (Sweden), Facies 65(3), 26, 1 – 15
Claussen, A.L., Munnecke, A., Wilson, M.A., Oswald, I.
(See online at https://doi.org/10.1007/s10347-019-0570-7) - (2021): The Silurian transgression of a palaeo-shoreline: the area between Old Radnor and Presteigne, Welsh Borderlands. Lithosphere, 2021(1), 7866176, 1 – 24
Ray, D.C, Jarochowska, E., Hughes, H., Claussen, A.L., Tingley, A.C., Moseley, J. and Bremer, O.
(See online at https://doi.org/10.2113/2021/7866176) - (2022) Stenolaemate bryozoans from the Graham Formation, Pennsylvanian (Virgilian) at Lost Creek Lake, Texas, USA. Palaeontologia Electronica, 25(2), a15, 1 – 56
Ernst, A., Claussen, A.L., Seuss, B. and Wyse Jackson, P.N.
(See online at https://doi.org/10.26879/1174) - (2022): Bryozoan-rich stromatolites (bryostromatolites) from the Silurian of Gotland and their relation to climate-related perturbations of the global carbon cycle. Sedimentology, 69(1), 162 – 198
Claussen, A.L., Munnecke, A. und Ernst, A.
(See online at https://doi.org/10.1111/sed.12863) - (2022): Modern brackish bryostromatolites („bryoliths“) from Zeeland (Netherlands). Palaeobiodiversity and Palaeoenvironments, 102(1), 89 – 101
Harrison, G.W.M., Claussen, A.L., Schulbert, C. und Munnecke, A.
(See online at https://doi.org/10.1007/s12549-021-00490-3)
