The ‘classic stromatolite’ Cryptozoön is a keratose sponge‐microbial consortium

Abstract

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.

Possible poriferan body fossils in early Neoproterozoic microbial reefs

“…The shape, size, branching style and polygonal meshworks of the Little Dal vermiform tubules closely resemble both spongin fibre networks of modern keratosan sponges (Fig. 2a-c) and vermiform microstructure either demonstrated or interpreted to be sponge-derived in diverse Phanerozoic microbial, reefal and non-reefal carbonate rocks [6][7][8]10,24,25,27,37 . The compositional and textural homogeneity of the microspar groundmass supports an origin through permineralization of a pre-existing biological substance 9 , rather than incremental accumulation of detrital sediment or microbial carbonate that passively incorporated complexly anastomosing tubular microfossils.…”

“…Three-dimensional reconstruction of vermiform microstructure has shown that tubule shape and branching configuration are too consistent and complex to be abiogenic (for example, compacted peloids), do not resemble the branching style of other possible organism types (microbial or fungal) 6 and are identical to the spongin meshworks of keratose sponges 6 . Although the existence of Proterozoic vermiform microstructure has been predicted 6,10,27 , published examples are rare 28,29 and difficult to understand.…”

Paleoecological implications of Lower-Middle Triassic stromatolites and microbe-metazoan build-ups in the Germanic Basin: Insights into the aftermath of the Permian – Triassic crisis

“…Furthermore, it is associated with ubiquitous occurrences of unusual sedimentary features, including microbialites (Woods 2014; Wu et al 2017; Heindel et al 2018; Chen et al 2019; Pei et al 2019). As stated before, microbe-metazoan build-ups are easily overlooked in geological records, but have received much attention and interests recently (Luo and Reitner 2014, 2016; Friesenbichler et al 2018; Heindel et al 2018; Baud et al 2021; Lee and Riding 2021; Pei et al 2021).…”

“…One possible explanation for the widespread occurrence of diverse microbial mats is a suppressed ecological competition with grazing metazoans that would prevent their development (Foster et al 2020). Although the details remain to be studied, it seems plausible that microbes and non-spicular demosponges had a mutualistic relationship (Luo and Reitner 2016; Lee and Riding 2021; Pei et al 2021), and it is tempting to speculate that the investigated microbial-metazoan build-ups reflect an ancient evolutionary and ecologic association.…”

“…However, pioneering work established robust criteria for the identification of non-spicular demosponges in ancient records and demonstrated the presence of these organisms in various Phanerozoic microbe-metazoan build-ups (Luo and Reitner 2014, 2016). Afterwards, much attention and interests have been aroused (Friesenbichler et al 2018; Heindel et al 2018; Lee and Riding 2021; Baud et al 2021; Pei et al 2021).…”