Archaeocyaths: alternatively explained as consortia of siphonous algae and cyanobacteria-like microbes in shallow Cambrian seas

In order to test the systematic position of archaeocyaths, a group of Cambrian, exclusively marine fossils of unclear biological affinity, currently ascribed mostly to sponges (Porifera), we present an alternative explanation supporting the early idea of siphonous green algal affinity of archaeocyaths. The new arguments are based on a study of the morphology, microstructure, and microtaphonomy of early Cambrian (Tommotian and Atdabanian) archaeocyathan specimens from northeastern Siberia. Light microscopic and scanning electron microscopic (SEM) observations revealed that in many specimens the outer, often microlaminated, skeletal fabric grades continuously into the microfabric similar to that of microbial bush-like and dendritic microfossils such as Angulocellularia and Renalcis. The polyhedral to subglobular units characterizing the skeletal microfabric of the studied archaeocyaths and their microbial outgrowths are most similar to recent and fossil mineralized (calcified) colonies of benthic coccoidal cyanobacteria. This means that what had heretofore been interpreted as skeletons secreted by archaeocyathan organisms represent calcareous crusts precipitated by epibiontic cyanobacteria-like microbes, probably associated with a variety of other bacteria, colonizing surfaces and internal spaces of soft-bodied archaeocyathan organisms. Our study shows that a number of the morphological traits of archaeocyaths conserved by the mineralized microbial cover are similar to those of of modern siphonous green algae, like the marine genus Codium Stackhouse (Bryopsidales). Identifiable in well-preserved archaeocyaths are remains of such traits characteristic for Codium species as utricles, medullary threads, juvenile stages (primordia), and coalescence structures. The taxonomic and palaeoenvironmental implications of the recognition of archaeocyaths as possible consortia of siphonous green seaweeds (Chlorophyta) and cyanobacteria-like microbes are briefly discussed.