Sprecher
Beschreibung
The Middle Ordovician of the palaeocontinent Baltica provides some of the best preserved and most diverse acritarch assemblages of the lower Palaeozoic. Acritarch research started with the pioneering studies in the 1930s by Alfred Eisenack, who first collected erratics from the southern coast of the Baltic Sea (Eisenack, 1938). Being fully aware of the morphological variability, Eisenack subsequently published further descriptions from erratic boulders (Eisenack, 1958), but also from outcrop material (Eisenack, 1959, 1968), including the Hälludden section of Öland (Eisenack, 1976). A great number of widely used acritarch genera and species have been described from this area since the 1960s (e.g. Staplin et al., 1965; Kjellström, 1972), usually with the main objective to provide biostratigraphically useful key-taxa. Here, we document that progressive changes of key morphological characters, such as the average process length, as well as the relative abundance of different morphotypes, are continuous throughout the sections. The progressive variations, observed from deeper- to shallower-water environments, demonstrate changing proportions of acritarch taxa and a high degree of morphological plasticity among acritarch morphotypes, thereby challenging their use in biostratigraphical correlation. The morphotypes remain useful, however. Here we present evidence that proportions of acritarch morphotypes and selected morphological criteria can be used for ecostratigraphical applications. Over half a century ago, Alfred Eisenack, together with other pioneers, noticed the extremely high variability of the acritarchs, and already noticed at that time that the morphology was related to the palaeoenvironment. Eisenack was right!