Middle–Late Ordovician organic- walled phytoplankton from Sweden: diversity and early radiation

The Borenshult-1 core, drilled in the vicinity of Motala, east of Lake Vättern in south central Sweden, comprises a well-dated and nearly complete succession of marine marly carbonates deposited relatively close to land. The 34 core samples analyzed for palynology encompass the upper part of the Darriwilian (Furudal Limestone), the entire Sandbian (Dalby Limestone, the Kinnekulle K-bentonite and the lower Skagen Limestone) and the lower part of the Katian (Skagen Limestone). The age of this interval is well-constrained to the late Darriwilian (Stage slice Dw3)– early Katian (Stage slice Ka1 ) , based on conodonts and 206Pb/238U dating of volcanic ash deposits. The samples yielded predominantly marine organic-walled phytoplankton, mainly acritarchs, with subordinate chitinozoans, scolecodonts and fragments of graptolites. Sparse terrestrial palynomorphs, represented by cryptospores and trilete spores, were also found in 23 of the samples. A total of 154 acritarch species corresponding to 53 genera were identified, as well as low percentages of abnormal forms ( teratological forms) of acritarch species at some levels. The strati graphic distribution of genera and species allowed for three palynological assemblages to be distinguished: Assemblage A of a late Darriwilian age, Assemblage B of a Sandbian age ( further subdivided into sub-assemblages B1 and B2 ) , and Assemblage C dated as Katian.