REASSESSING THE ROLE OF PRASINOPHYTE ALGAE IN MODERN AND ANCIENT OCEANS

Acritarchs are organic-walled microfossils of uncertain taxonomic affinity. Many smooth-walled acritarchs from the Paleozoic are inferred to be reproductive structures, called phycoma, made by a single genus within the green algal group Prasinophycea. The inferred green algal affinity of these acritarch forms is often used as the basis for various proxies, including diversity and primary production. However, these inferences are often made without detailed morphological comparisons between fossils and modern Prasinophycean analogs. As a result, implied diversity of photosynthetic taxa in the fossil record as well as any proxies derived from this diversity are not robust without a deeper understanding of modern Prasinophycean groups. Here, we present a comparison of the morphology and ecology of modern Prasinophytes and fossil taxa inferred or known to belong to the Prasinophycea, and demonstrate how knowledge of the biology of modern taxa is required for realistic interpretation of the fossil record of acritarchs. Examples include significant size differences between fossil and modern taxa, and differences between modern and paleo-environmental distribution. In addition, we will discuss the biology and function of modern Prasinophyte phycoma, which are a reproductive structure and not cysts, as often described in the literature. A more comprehensive understanding of the fossil record of Prasinophytes will not only help us to better understand the evolutionary history of the green algae, but also better understand ancient marine ecosystems.