MORPHOLOGICAL HISTORY OF PROTEROZOIC MACROSCOPIC CARBONACEOUS COMPRESSIONS: POSSIBLE IMPLICATIONS FOR THE ECOLOGY OF PROTEROZOIC MACROALGAE

Macroalgae are ecologically important in modern ecosystems. They form dense turfs and giant underwater forests where productivity rate (measured in biomass per unit area per unit time) is much greater than that of phytoplankton in the open ocean. These algal turfs and forests also form grazing, breeding, and encrusting substrates for animals. However, the morphological and ecological history of Proterozoic macroalgae is rarely discussed in the literature, partly because of the sporadic nature of the macroalgal fossil record and the difficulty in distinguishing algal from non-algal carbonaceous compressions. Despite these challenges, it is possible that a broad-scale picture can be reconstructed for Proterozoic macroalgal evolution and ecology. To test this possibility, we compiled a preliminary database of Proterozoic carbonaceous compressions that can be reasonably interpreted as macroalgal fossils. We then used quantitative methods to characterize their morphological complexity, modeled surface/volume ratio, and maximum canopy height. Our data show that the morphological history of Proterozoic macroalgae is broadly similar to that of Proterozoic acritarchs [Huntley et al., 2004, GSA Abstracts with Programs, 36(5): 522], showing stepwise increases in the Mesoproterozoic and Ediacaran with a stasis in between. We hypothesize that the Mesoproterozoic to early Neoproterozoic stasis of algal morphological complexity may be related to nutrient stress, although current geochronological resolution is insufficient to make a convincing case. The Ediacaran expansion of macroalgal morphospace was also accompanied by significant increase in surface/volume ratio and maximum canopy height. Insofar as the surface/volume ratio of living photosynthetic organisms is strongly correlated with their mass-specific productivity, the Ediacaran increase in macroalgal surface/volume ratio may be interpreted similarly. It is unclear whether the Ediacaran increase in algal complexity, surface/volume ratio, and canopy height was driven by bottom-up (e.g., nutrients or resources) or top-down (e.g., animal herbivory) ecological processes. Nonetheless, this exploratory study serves as a starting point for more extensive studies of Proterozoic macroalgae in the future.