SEAWEED THROUGH TIME: THE EARLY PALEOZOIC

Nonbiomineralized marine macrophytes (“seaweeds”) comprise a key component of the Earth-Life system, impacting biogeochemical cycles on a planetary scale and playing an ecological role in shallow seas similar to that of plants on land. Yet, the bulk of their early evolutionary history, including the timing and nature of major morphological diversification events, has remained largely unknown, a consequence of a fossil record that is both sparse and difficult to interpret. SEM- and stable isotope-based analyses, together with modeling techniques and the discovery of numerous new fossil deposits rich in their remains, however, have greatly increased our ability to investigate seaweeds across the Proterozoic-Phanerozoic transition. Here we present results of the first comprehensive study of seaweeds during the early Paleozoic part of this critical interval. The study draws on a newly constructed database that includes taxonomic and morphometric information from across the early Paleozoic (Cambrian – Silurian), comprising some 134 species and 69 fossil deposits. During this study, some taxa were rejected as macrophytes based on new data, including Yuknessia from the Burgess Shale biota, new taxa were described, and others were revised. From a taxonomic perspective, the results largely mirror those obtained from previous studies of calcareous algae, with the entire Cambrian being characterized by one flora and the entire post-Cambrian part of the study interval by another. Importantly, these two early Paleozoic seaweed floras show fundamental differences in thallus architecture, the Cambrian flora comprising simple forms with little to no branching, and the post-Cambrian flora dominated by forms with complex thalli. An additional difference is evident with regard to size, modeled in terms of total thallus surface area, with the post-Cambrian flora showing significantly larger average thallus size relative to the Cambrian flora. Collectively, the results point to major morphological changes for seaweeds roughly concomitant with the Ordovician radiation (GOBE). Preliminary comparisons with Ediacaran macrophytes, however, do not suggest comparable changes in concert with the Cambrian Explosion of animals.