Multiscale approach reveals that <i>Cloudina</i> aggregates are detritus and not in situ reef constructions.

Abstract

The earliest metazoans capable of biomineralization appeared during the late Ediacaran Period (635-541 Ma) in strata associated with shallow water microbial reefs. It has been suggested that some Ediacaran microbial reefs were dominated (and possibly built) by an abundant and globally distributed tubular organism known as <i>Cloudina</i> If true, this interpretation implies that metazoan framework reef building-a complex behavior that is responsible for some of the largest bioconstructions and most diverse environments in modern oceans-emerged much earlier than previously thought. Here, we present 3D reconstructions of <i>Cloudina</i> populations, produced using an automated serial grinding and imaging system coupled with a recently developed neural network image classifier. Our reconstructions show that <i>Cloudina</i> aggregates are composed of transported remains while detailed field observations demonstrate that the studied reef outcrops contain only detrital <i>Cloudina</i> buildups, suggesting that <i>Cloudina</i> played a minor role in Ediacaran reef systems. These techniques have wide applicability to problems that require 3D reconstructions where physical separation is impossible and a lack of density contrast precludes tomographic imaging techniques.