Remains of sponges are known to occur in formations belonging to the Palaeozoic, Mesozoic, and Kainozoic epochs. The keratose or horny sponges are obviously incapable of leaving any evidence of their existence, otherwise than by the preservation of the spicula with which the skeleton is sometimes furnished; and such are occasionally found, though they are of rare occurrence. The calcareous sponges are found from the Silurian rocks upwards, though great obscurity still rests upon the true nature and affinities of many of the fossils which have been referred to this group.
The siliceous sponges are now known to possess a very high antiquity, both the sections of the Hexactinellidae and Lithistidae being developed as early as the Silurian period. Of the fossil Hexactinellidae, the best-known group is that of the Ventriculitidae, comprising a large number of beautiful Secondary sponges. Of the fossil Lithistids, the best-known genus is the widely distributed Siphonia of the Cretaceous period.