A)the Devonian period or "age of fishes." B)the early time of anaerobic prokaryotes. C)the Cambrian period and probably earlier. D)the time of the first prokaryotes but the sponges left no fossil evidence for lack of hard parts.Ancestral members of the phylum Porifera arose at least by the early Cambrian period and probably into Precambrian times.Later evolution resulted in diversification into the classes present today.