Population dynamics of galeate acritarchs at the Cambrian–Ordovician transition in the Algerian Sahara

Abstract

Galeate acritarchs are a major component of Late Cambrian and Early Ordovician palynological assemblages. The populations of galeate acritarchs from the Cambrian–Ordovician transition section in borehole Nl‐2 of the Algerian Sahara show a gradual increase in process length and in complexity of the process structures through the succession. While the number of striate elements at the process base, the number of distal ramifications, and the number of membranes between the processes increases progressively, the central body diameter shows only minor variation within the succession. The galeate acritarchs are here interpreted as probably being resting cysts of a microorganism similar to modern dinoflagellates. Published laboratory culture experiments on living dinoflagellates document a considerable morphological variation with respect to the process morphology of the resting cysts that can be produced by a single biologically defined species. Based on these experiments, it is possible to interpret the morphological changes in the galeate acritarchs as being possibly related to changing ecological conditions. By analogy to the cyst distribution of modern dinoflagellates, changing salinity may have played an important role. Depending on environmental parameters, and perhaps on the maturity attained before rupture of the outer membrane during cyst formation, the galeate acritarchs may show a wide variety of process morphologies that have to date been used to describe four genera (morphogenera) and 84 species (morphospecies).