Evolutionary, Ecological, and Paleoenvironmental Implications of Acritarchs as Metazoan Resting Stages

Acritarchs, closed organic walled microfossils, have historically been considered phytoplankton because of their small size, abundant distribution, and general similarity to prasinophyte phycomata and dinoflagellate cysts.These affinities, however, do not necessarily form the most parsimonious categorization of acritarchs, especially the diverse taxa found in Ediacaran rocks. In terms of morphological and ultrastructural similarity, many Ediacaran acritarchs bear closer resemblance to the egg or diapause cysts of modern invertebrate groups, including those of certain crustaceans. Using Scanning Electron Microscopy, Focused Ion Beam Scanning Electron Microscopy and Transmission Electron Microscopy to analyze the morphology and ultrastructure of both fossil and modern forms illustrates this point and helps resolve issues of Ediacaran acritarch taxonomy.

Placement of some Ediacaran acritarchs within the Metazoa requires reinterpretation of the acritarch record and invites ecological and environmental hypotheses regarding the presence of recalcitrant cysts in Ediacaran seas. In modern oceans, many invertebrates create such cysts to protect their offspring from deleterious environmental conditions such as anoxia. The same approach was likely used by Ediacaran metazoans, regardless of specific taxonomic affinity. Modern analogs can also shed light on the adaptive utility of the abundant spines and processes characteristic of large Ediacaran acritarchs, which we hypothesize were not evolved solely for purposes of defense but rather enabled cysts to prolong suspension in the water column during periods of sediment disturbance. Such an ability woulc have enhanced the ability of organisms to sense changing oceanographic conditions. The Ediacaran acritarch record can thus be viewed as an evolutionary response to life in an ocean with spatiotemporal variations in oxygen and sulfide levels. This conclusion repositions our thinking about both the radiation and disappearance of large acanthomorphic acritarchs in Ediacaran oceans and has implications, as well, for the Paleozoic acritarchs record.