Still, I'll grant you that sponges are pretty weird as animals go – probably because they branched off very early from the rest of the animal family tree.
The major groups of animals we see around us today -- from sponges to arthropods to chordates -- all showed up in this little window of time in the Cambrian.
Coral reefs, for example, support over 4000 types of fish, along with sponges, crustaceans, and many other organisms. So, exactly when and how did the Notothenioids come to dominate the southern ocean?