[thotpoizn]

Is the "oxygen-rich environment" universally true, though? It's not as though oxygen saturation in the ocean is perfectly homogeneous. There are plenty of naturally occurring areas of hypoxia, and one could easily imagine an adaptation which allowed better survival in or near these "dead zones" could be quite useful, at least to localized populations. Is there some obvious reason that dramatically reduces the likelihood that one or more pockets of sponges might have capitalized on such a mutation more recently?