The 'classic stromatolite' Cryptozoön is a keratose sponge-microbial consortium

Animal evolution transformed microbial mat development. Canonically inferred negative effects include grazing, disturbance and competition for space. In contrast, ancient examples of cooperation between microbial mats and invertebrates have rarely been reported. Late Cambrian (~485 million years) Cryptozoön is widely regarded as the first stromatolite to have received a taxonomic name and has been compared with present-day examples at Shark Bay, Australia. Here, we show that Cryptozoön is an interlayered consortium of keratose ('horny') sponge and microbial carbonate in roughly equal proportions. Cryptozoön's well-defined layering reflects repeated alternation of sponge and microbial mat. Its distinctive lateral growth is due to the ability of keratosans to colonize steep and overhanging surfaces. Contrary to the perception of Phanerozoic stromatolites as anachronistic survivors in a eukaryotic world, Cryptozoön suggests mutualistic behaviour in which sponges and microbial mats cooperated to gain support, stability and relief, while sharing substrates, bacteria and metabolites. Keratosan-microbial consortia may have been mistaken for stromatolites throughout the record of the past 500 million years, and possibly longer.