PHOSPHATIZED FOSSILS IN THE NEOPROTEROZOIC DOUSHANTUO FORMATION, CHINA, AND THE EARLY EVOLUTION OF FLORIDEOPHYTE RED ALGAE

The red algae comprise a diverse and ecologically widespread component of modern marine ecosystems that evolved complex multicellularity independently of plants and animals. Traditional classifications recognize two rhodophyte classes, the relatively undifferentiated Bangiophyceae and the more complex Florideophyceae; molecular phylogenies nest the diverse but monophyletic florideophytes within paraphyletic bangiophytes. Simple filaments in ca. 1200 Ma cherts are thought to be bangiophytes, but likely florideophyte fossils appear only toward the end of the Proterozoic. The ca. 600 Ma Doushantuo Formation, China, contains both carbonaceous compressions and anatomically preserved phosphatic fossils of probable florideophytes. Simple thalli characterized by pseudoparenchymatous growth are similar to the little differentiated thalli of extant Hildenbrandiales and Ahnfeltiales, but ordinal assignment based on morphological simplicity, rather than distinct features can be problematic. Thus, these fossils are best viewed as lineages that branched near the base of the florideophyte tree, before the divergence of more complex orders. They could be stem group florideophytes, stem groups to one of the earlier branching crown group clades, or simple members of early branching crown groups. In contrast, complex thalli that show evidence of thallus differentiation into an inner medulla and a thin peripheral cortex, as well as differentiated reproductive structures, compare more closely to fossil and living corallinaleans than to other florideophytes. That noted, the Doushantuo fossils lack features characteristic of living corallines, notably calcium carbonate skeletons. Thus, they may be uncalcified stem members of the coralline clade. Calcified Paleozoic fossils such Graticula and Archaeolithophyllum bridge the stratigraphic gap between these early fossils and crown group corallines, which diversified in the Mesozoic. Taken together, Doushantuo and other fossils suggest early divergence of the red algae from other eukaryotic clades, but notable diversification within the florideophytes only later, near the end of the eon. A moderate diversity of florideophytes lived in Paleozoic oceans, but the rich diversity of modern red algae may only have arisen in later Mesozoic and Cenozoic oceans.