The acritarchs of the South Chinese Azygograptus suecicus graptolite Biozone and their bearing on the definition of the Lower–Middle Ordovician boundary

In southern China, well preserved acritarch assemblages have been recovered from numerous sections in the Yangtze Platform and the Jiangshan–Changshan–Yushan (JCY) area crossing the interval where the Lower–Middle Ordovician boundary should be defined, i.e., roughly at the base of the Chinese Azygograptus suecicus graptolite biozone. The acritarch taxa Aureotesta clathrata and Arbusculidium filamentosum first appear below the suecicus zone, while the genus Ampullula and the species Barakella felix have their First Appearance Datum (FAD) in the suecicus zone. These latter taxa, that are also present in Baltica and peri-Gondwana, respectively, thus probably indicate the base of the Middle Ordovician.