Biostratigraphical and palaeobiogeographical implications of Lower Silurian Radiolaria from black cherts of the Armorican Massif (France)

Abstract

Abstract. A moderately well-preserved radiolarian assemblage was recovered from organic-rich black chert in a Llandovery (Lower Silurian) sequence that crops out in southern Brittany (Chalonnes-sur-Loire section, Armorican Massif, France). The assemblage is composed of two families (Rotasphaeridae and Haplotaeniatidae), four genera (?Diparvapila, Secuicollacta, Orbiculopylorum, Haplotaeniatum) and 13 species. Some were identified from whole specimens preserved in silica and extracted following dilute hydrofluoric acid processing, while others were recognized in thin-section preparations, as they are beautifully preserved as ‘carbonized’ microfossils. The age range suggested by conodonts and chitinozoans yielded after HF processing from one of the 27 studied samples is in good agreement with the previously published age based on graptolites. The recovered radiolarians are discussed, documented and compared with known Rhuddanian, Aeronian and lower Telychian assemblages in the literature. The stratigraphic ranges are extended for the species Secuicollacta bipola, S. hexactinia, S. parvitesta, Orbiculopylorum granti and O. splendens based on our new data. A significant number of radiolarians found in our samples occur in Llandovery sections from Alaska, Nevada, Arctic Canada and Sweden. These similarities are used to discuss the palaeodistribution of Lower Silurian Radiolaria and our observations support the hypothesis of a wide geographical distribution for these Palaeozoic species.

Micro-CT analysis of Katian radiolarians from the Malongulli Formation, New South Wales, Australia, and implications for skeletogenesis

“…Micro-CT reconstructions show that the labyrinth compactness of H. spongium is less than that of mid-to lowermost Upper Ordovician forms of this genus (Maletz and Bruton, 2008;Pouille et al, 2014;Kachovich and Aitchison, 2020;Perera et al, 2020;Perera and Aitchison, 2022). However, the original morphology seems to return by the early Silurian for a majority of species (Noble et al, 1998;Noble and Maletz, 2000;Won et al, 2002;Umeda and Suzuki, 2005;MacDonald, 2006;Tetard et al, 2014). It is worth considering whether the simpler body plan of Katian haplotaeniatids was advantageous during less-favorable environmental conditions.…”

Thuringian affinity of the Silurian–Lower Devonian succession from the Eastern Taurus, Turkey

“…One of these weathering products is SiO 2 , which is mainly digested by the Radiolaria. The accumulation of radiolarian skeletons led to the formation of the chert-rich lower part of the Lower Graptolite Shale Formation (Jaeger, 1959;Tetard et al, 2014). Most obviously, the high radiolarian productivity is related to processes of upwelling, especially around the shelf-edges.…”