Acritarch distribution in the Middle Ordovician of Öland, Sweden: Ecostratigraphical or biostratigraphical signal?

The Middle Ordovician of the palaeocontinent Baltica provides some of the best preserved and most diverse acritarch assemblages of the lower Palaeozoic. A great number of widely used acritarch genera and species have been described from this area since the 1960s and many of these taxa are considered as biostratigraphically useful for both regional and international correlation. Here, we document the changing proportion of acritarch morphotypes through two well-documented Darriwilian (Middle Ordovician) sections of Öland, Sweden. Progressive changes of key morphological characters, such as the standard process length, as well as the relative abundance of different morphotypes, are continuous throughout the sections. These changes in the acritarch assemblages parallel the transition from deeper to shallower water environments as deduced from local facies analyses and regional sedimentation patterns. Consequently, the previously suggested biostratigraphical signals in the acritarch assemblages could also be interpreted as ecophenotypical responses to the changing palaeoenvironment. Therefore, the morphological plasticity of acritarchs challenges their accuracy and validity for precise correlation if their palaeoecology is not addressed. • Morphological changes of acritarchs are documented from the Middle Ordovician of Öland, Sweden. • Previous biostratigraphical signals are here interpreted as ecophenotypical changes • The morphological plasticity of acritarchs can be used for ecostratigraphical studies.