PATTERNS AND TRENDS IN NONCALCIFIED MACROALGAL (“SEAWEED”) MORPHOLOGY AND ECOLOGY DURING THE GREAT ANIMAL DIVERSIFICATION EVENTS OF THE EARLY PALEOZOIC

The Cambrian Explosion (CE) and Ordovician Radiation (GOBE) are two of the most important animal diversification events in the history of life. It is often overlooked, however, that the shallow seas in which these events unfolded were already long-populated by various megascopic photosynthetic eukaryotes, or macroalgae, which originated deep in the Proterozoic and achieved considerable morphological diversity by the Ediacaran. Here, we examine large-scale morphological and ecological patterns and trends in the early Paleozoic history of macroalgae, identified using a newly assembled and carefully vetted database comprising all known Proterozoic ­– early Paleozoic occurrences of noncalcified macroalgae, and consider what information they might convey about contemporary metazoan diversification events. The use of macroalgae as a proxy in this case would seem a reasonable proposition, given strong levels of mutual feedback observed between macroalgae and animals in modern seas. The database indicates no major changes for macroalgal assemblages in concert with the CE. No new morphogroups emerge during the Cambrian and all of the major morphogroups known from the late Proterozoic are present in the Cambrian. This in turn has three important implications. First, it provides a solid basis to remove morphological innovation among macroalgae as a potential trigger for the CE (i.e., no bottom-up effect). Second, it suggests that the event or events that precipitated the CE had limited impact on macroalgae. Finally, it indicates that changes in animal communities resulting from this event, although substantial, also had limited impact on the morphologies of macroalgae (i.e., no marked top-down effect), a finding consistent with results of recent ecological studies of animals in Burgess Shale-type deposits. In contrast, macroalgae show profound morphological changes roughly concomitant with the GOBE, ultimately resulting in replacement of the “Cambrian Flora” by the “Ordovician Flora”. Importantly, the nature and timing of these changes suggests that the major restructuring of metazoan communities into more recognizably modern frameworks that played out during the GOBE also led to the establishment of ecological selection pressures operating on macroalgae of a sort more similar to those operating today.