CAMBRO-ORDOVICIAN KERATOSE SPONGE-MICROBIAL ‘KERATOLITE’ CONSORTIA MIMIC STROMATOLITES

Some late Cambrian and early Ordovician keratose sponge-microbial domes and columns macroscopically closely resemble classic stromatolites. These include late Cambrian (~485 million years) Cryptozoön from New York State, USA, which is widely regarded as the first stromatolite to have received a taxonomic name, and Cambro-Ordovician columns and domes in Newfoundland, Canada. All of these structures are interlayered consortia of keratose (‘horny’) sponge (keratolite) and laminated microbial carbonate (stromatolite) in roughly equal proportions. Repeated alternation of sponge and microbial mat created well-defined layering of these stromatolite-like structures. Frequent thin alternation of microbial mat and sponge layers suggests their cooperation in keratolite-stromatolite construction as follows: 1. Mats/biofilm surfaces provided sponges with favorable substrates for larval settlement and possibly contributed bacterial symbionts. 2. Keratosans relatively rapidly created extensive enveloping layers but were prone to mortality events that tended to be dome-wide. 3. Microbial mats colonized dead keratosan surfaces, benefiting nutritionally from tissue decay and recreating a substrate suitable for sponge larval settlement. Repetition of this cycle created distinctive alternating layering that macroscopically closely resembles classic stromatolites. Contrary to the perception of Phanerozoic stromatolites as anachronistic survivors in a eukaryotic world, these examples suggest mutualistic behavior 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 since the Proterozoic.