Of related interest:
Poriferan Fossils

Phylum porifera, the sponges, are the most basal animals, having branched from other metazoans during Late Precambrian time. Some 900 genera are represented in the fossil record comprising some 5000 species.

Interestingly, and consistent with their early evolution, the cells of sponges are far more independent that those of other animals. Different cell types perform distinct bodily functions that together allow the composite organism to feed and live. Lacking mouths, special cells form small pores that filter food from the sea as other cells form flagella that maintain a constant flow through a series of bodily channels. Some sponges are radially symmetrical and others asymmetrical. Skeletal support comes from protein collagen and spicules that are calcareous or siliceous. Reproduction by sponges is by both sexual and asexual means.

The oldest (likely) sponge in the fossil record, Paleophragmodictya, was described in 1996, coming from the Vandian or Ediacaran of Southern Australia dating to the period of 650 to 543 million years ago, the latest time of the Proterozoic. Of this period of time Darwin wrote: "The difficulty of assigning any good reason for the absence of vast piles of strata rich in fossils beneath the Cambrian system is very great ….only a small portion of the world is known with accuracy."

Sponges are represented in the fossil record since the Precambrian, and the Phanerozoic sponges have been a critical component of reef ecosystem. While often rare, sponge fossils from the Cambrian Explosion are found in several Cambrian sites in North America, most notably the Burgess Shale of Canada, and the Cambrian strata of Utah. Sponges are also described from the Chengjiang biota of China. The Profera are believed to have undergone repeated radiations in the Paleozoic and Mesozoic eras, and probably attained their greatest diversity in the Cretaceous period.