Environmental covariation of metazoans and microbialites in the Lower Ordovician Boat Harbour Formation, Newfoundland

An antagonistic view of the relationship between microbialites and metazoans has long been inferred, in part because of the large scale anticorrelation of these two groups through geologic time. The nexus of this relationship occurs in the Early Paleozoic Era: stromatolites declined in abundance as complex animals and algae diversified, but thrombolites, a type of microbialite little known before the Proterozoic-Cambrian boundary, proliferated for the first time. Well-preserved parasequences in the basal portion of the Lower Ordovician Boat Harbour Formation, western Newfoundland, contain a succession of stromatolites and thrombolites that permit an investigation into the role metazoans played in shaping the nature and abundance of microbialites in Early Paleozoic carbonate seas. Sessile benthic animals colonized thrombolite surfaces, but are nearly absent from stromatolites. Bioturbation rarely co-occurred with microbialites, but is widespread in clastic carbonates that lack microbialites. Our results, thus, support the hypothesis of ecological antagonism between microbial communities and motile benthic animals, but also demonstrate biological facilitation between thrombolites and both sessile benthic animals and nekton.