“Keratose” sponge fossils and microbialites: a geobiological contribution to the understanding of metazoan origin

Abstract

Molecular clock and the fossil record at the Precambrian-Cambrian boundary indicate that animals probably had a long evolutionary history in the Precambrian which is still poorly explored and understood. This PhD study was aimed to contribute to the exploration of the early animal fossils based on the concepts that 1) sponges, especially the non-spicular taxa, are probably the best model for early animals according to the modern biological and phylogenomic studies; 2) special attention should be paid to the fossilized microbial bioconstructions in spite of readily recognizable animal remains when looking for the fossils of early animals, because the early animals probably had close association and intensive interaction with the microbial systems. Reliable fossils of non-spicular sponges were previously rarely known, except the single family Vauxiidae Walcott, 1920 reported from few Early Paleozoic shale Lagerstätten. This doctoral work revealed that non-spicular keratose demosponges (sensu Minchin 1900) have a higher preservation potential in fossil record than previously evaluated. Especially in carbonates, these sponges are actually recorded through the whole Phanerozoic with considerable abundance and certain diversity. The other type of non-spicular demosponge—the myxospongids which are devoid of any skeletons (sensu Minchin 1900) — was probably also but less commonly fossilized in carbonate facies. Furthermore, keratose demosponges were found being able to construct microbialite-like bioconstructions together with microbes. All these findings imply that the Precambrian ancestral sponges should have the potential to be recorded in carbonate microbialites even though they were not capable of biomineralization. The study on the microstructures of Precambrian microbialites is still in its beginning. Although the observation hitherto did not find any structure which is immediately comparable with the Phanerozoic sponge fossils, it showed that a few complex and enigmatic microbialite microstructures did start to appear in the Early to Middle Neoproterozoic (Tonian to Cryogenian). These include the organized “fluid canals” in microbial mats, massive occurrences of peloidal fabrics, chambered structures and chambered grains. To determine whether these structures are related to the advent of animals requires further research.