Modes of pre-Ediacaran multicellularity

A multicellular grade of organization is widely distributed among extant organisms and widely represented in the pre-Ediacaran fossil record. A review of the pre-Ediacaran record identifies nine general categories of multicellular organization: (1) simple clonal colonies; (2) integrated coenobial colonies; (3) simple uniseriate filaments; (4) simple multiseriate filaments; (5) simple coenocytic filaments; (6) branched coenocytic filaments; (7) complex multicellular filaments; (8) complex multicellular vesicles; and (9) problematic macrofossils. A small subset of these fossils can be assigned to extant lineages based on taxonomically diagnostic patterns of cell division, including compelling evidence for chroococcacean, oscillatoriacean and pleurocapsalean cyanobacteria, bangiophycaceaen red algae and hydrodictaceaen green algae. The identification of pre-Ediacaran vaucheriaceaen xanthophyte algae and siphonocladalean green algae is almost as secure, whereas the case for early multicellular nostocalean/stigonematalean cyanobacteria and fungi requires further corroboration; a distinctively patterned acritarch is tentatively identified as an early Neoproterozoic poriferan. Despite this breadth of early multicellular experimentation, there is no evidence for organ-grade differentiation prior to the Ediacaran. Indeed, it is the absence of eumetazoans and embryophytes that distinguishes the pre-Ediacaran world from the fundamentally richer and more dynamic biosphere of the Phanerozoic.