Poriferans, the pore-animals, are familiar sponges. Unlike the protozoans, sponges are multicellular, with cells that have specific functions, but they still lack well developed tissues and organ systems. Generally they are water residents, and attach themselves to objects, like barnacles. Other than bath sponges, which are not particularly typical of the phylum, many forms exist with every conceivable size and shape. All, though, are perforated by pores through which water is circulated and food particles are trapped and digested.

 The soft parts of sponges are supported by internal skeletons composed of hard fibers and/or mineralized rods called spicules. In most sponges the living tissue rots after death and the spicules fall apart and settle out into a formless mass or are scattered over large areas of the sea floor. Only rarely are complete sponge fossils found, and then it is because the spicules were fused into a rigid skeleton that is not easily destroyed. Sponges were present from the Cambrian period into the present but are rare as fossils.


Most commonly preserved are sponge spicules, which can be etched (if composed of silica) from calcareous rocks with the use of acids such as very dilute hydrochloric or acetic acids. If a collector is lucky, she or he may be able to remove an entire skeletal framework of spicules by using acid treatments.
With the exception of one fresh water sponge of the family Spongilhidae, all fossil sponges occur in marine sediments. Marine sponges are preserved in every variety of rock type, shale, limestone, and fine-grained sandstone, and they can either be preserved as calcite or silica. To the untrained eye, fossil sponges can look like odd lumps of mineral matter without much form. A closer look, under a 10X lens, will most often show telltale structures that give its nature away.
Nodules of chert in limestone can indicate the presence of silicisponges (sponges with a silica-quartz spicule skeleton), and can be the most common of fossils found in such concretions. Delicate, sack-like sponges, are frequently found pressed flat on bedding planes of very fine-grained rocks which were deposited in areas of little water movement. The more sturdy sponges often weather out of the matrix and can be collected whole, or can be collected as cross sections on weathered surfaces.
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