Morphometric analysis of Skiagia-plexus acritarchs : towards a meaningful measure of phenotypic plasticity

The Cambrian evolutionary radiations are marked by spectacular biotic turnovers and the establishment of increasingly tiered food chains. At their base is primary productivity, which is chiefly represented by disparate assemblages of organic-walled microfossils. These unresolved remains have traditionally been attributed to an informal category of incertae sedis called ‘acritarchs’, and classified using a form-taxonomic scheme. As numerous acritarch form-species have been erected for biostratigraphic purposes, taxonomic inconsistencies and over-splitting have led to some degree of skepticism regarding the paleobiological significance of this classification. The acritarch form-genus Skiagia stands as an ideal case-study to explore these taxonomic challenges, since it encompasses a number of ‘species’ widely used in lower Cambrian biostratigraphy. Moreover, subtle morphological differences among these species are thought to underlie key evolutionary innovations towards complex reproduction strategies. Here we apply a morphometric approach, using multivariate ordination techniques to scrutinize the morphological variation of Skiagia-plexus acritarchs. Using an assemblage sourced from the Buen Formation (Cambrian Series 2, Stage 3–4) of North Greenland, our analysis showed that the specific-level classification of Skiagia discretizes a continuous spectrum of morphologies. Although this uninterrupted gradient brings important taxonomic and biostratigraphic hurdles to light, the unequal frequency distribution of life cycle stages among Skiagia species suggests that elements of phytoplankton paleobiology are captured by Skiagia form-taxonomy. These results demonstrate the value of using morphometric tools to explore acritarch plasticity, and for revealing potential drivers of morphological variation and their dynamics in Cambrian ecosystems.