When did the phylum Porifera first appear on Earth?

Fossil records clearly show cubozoans dating back to about 350 million years ago. Most scientists think that cnidarians showed up perhaps 700 million years ago since their gelatinous bodies don't lend themselves well to fossilization. That is quite an ancient history for an animal that has the most complicated and sophisticated single cell now in existence, the nematocyte. That cell defines the phylum and can explosively launch a tubule in less than 3ms at an acceleration of about 40,000 times that of gravity, some of which can penetrate almost a millimeter (hundreds of times the length of the nematocyte).