Embryo fossilization is a biological process mediated by microbial biofilms

TLDR

Fossilized embryos with exceptional cellular preservation appear in the Late Neoproterozoic and Cambrian, coinciding with the first animal body fossils, and while microbial processes have been hypothesized to mediate this preservation, experimental evidence has been lacking. The study modeled the decay of cleavage‑stage marine embryos, similar in size and morphology to fossil embryos, and identified the major bacterial taxa forming the biofilms by 16S rDNA sequencing. Bacterial biofilms rapidly assemble on dead marine embryos, producing pseudomorphs that preserve cellular detail and, through a three‑step process of autolysis inhibition, biofilm replication, and bacterial mineralization, generate fossilization states that mirror those of Late Neoproterozoic and Cambrian embryos, demonstrating that soft‑tissue fossilization is mediated by microbial replacement and mineralization.

Abstract

Fossilized embryos with extraordinary cellular preservation appear in the Late Neoproterozoic and Cambrian, coincident with the appearance of animal body fossils. It has been hypothesized that microbial processes are responsible for preservation and mineralization of organic tissues. However, the actions of microbes in preservation of embryos have not been demonstrated experimentally. Here, we show that bacterial biofilms assemble rapidly in dead marine embryos and form remarkable pseudomorphs in which the bacterial biofilm replaces and exquisitely models details of cellular organization and structure. The experimental model was the decay of cleavage stage embryos similar in size and morphology to fossil embryos. The data show that embryo preservation takes place in 3 distinct steps: (i) blockage of autolysis by reducing or anaerobic conditions, (ii) rapid formation of microbial biofilms that consume the embryo but form a replica that retains cell organization and morphology, and (iii) bacterially catalyzed mineralization. Major bacterial taxa in embryo decay biofilms were identified by using 16S rDNA sequencing. Decay processes were similar in different taphonomic conditions, but the composition of bacterial populations depended on specific conditions. Experimental taphonomy generates preservation states similar to those in fossil embryos. The data show how fossilization of soft tissues in sediments can be mediated by bacterial replacement and mineralization, providing a foundation for experimentally creating biofilms from defined microbial species to model fossilization as a biological process.