980 million year old soft sponges may have been found in a reef associated with microbiolites in Canada, pushing back sponge fossils 300 million years. The lack of a fully mineralized skeleton in the earliest sponges may make fossilization and recognition difficult.
Turner, E.C. Possible poriferan body fossils in early Neoproterozoic microbial reefs. Nature (2021). https://doi.org/10.1038/s41586-021-03773-z
https://www.nature.com/articles/s41586-021-03773-z