The modern sponge is a most undistinguished-looking animal, but it has a fossil record behind it that is probably not exceeded in length by any other multicellular animal group. The earliest definite fossil sponges are from Ediacaran rocks, but there are occassional reports of "body fossils" from Upper Paleozoic deposits.

Modern sponges are of many shapes and sizes, from small, amorphous and encrusting forms to meter-tall, meter-wide, erect vases. Fossil sponges exhibit the same variety. Since most sponges are soft and structurally weak though, the fossilization process usually destroys a considerable amount of morphological detail. Often the inner hollow of the animal (spongocoel) was filled with sediment which was later lithified (an internal mold), sometimes with impressions of the spicules on the outer surface. In many, if not most cases, fossil sponges are known only from small collections of loose spicules.

The classes of Phylum Porifera are characterized by spicule composition and form. On the face of it, this classification based on hard parts is ideal for paleontologists, but there are drawbacks. Spicules are easily dissolved, leaving only external molds, or they can be replaced by several different minerals. A classification based strictly on spicule morphology may someday prove more useful.

Sponges are exclusively aquatic and are found today in marine and fresh water. Since they are mostly sessile and benthic, and have a delicate water circulation system, sponges cannot tolerate environments with a high rate of sedimentation-their pores are easily clogged and the animals themselves have no defense against burial. Consequently, we find sponges most often in those areas where the water is "clean" and the sedimentation rate low.

Sponge skeletons are secreted by specialized cells (spongocytes and sclerocytes), and can be composed of CaCO₃, SiO₂, or spongin (a proteinaceous material; the spongy stuff). Nomenclature of spicules depends on the number of axes (monaxon, triaxon, tetraxon). Correspondingly, the numbers of rays (needlelike terminations that correspond to a direction of growth) depend upon how many end points are on a spicule. The terms monactine, diactine, triactine are examples of rays. Remember, the suffix "axon" refers to axes and the suffix "actine" refers to rays (or terminations).

6.1.2 Archaeocyatha

Archaeocyathans are a short-lived, Early Paleozoic group of enigmatic fossils. The organisms were constructed as a set of two concentric cylinders connected by transverse septa. The cylindrical walls were imperforated. Archaeocyathans were apparently benthic, sessile, suspension feeders. Becuase of their ecological habitat and trophic habits, they have been compared frequently to the Porifera, but it is unknown if the two phyla are closely related.

6.2 Taxonomy

Phylum Porifera* (Cambrian-Recent)

• Solitary or colonial animals having a cellular (non-tissue) grade of construction; body bears many pores, canals and chambers, but no mouth or internal organs; calcareous, siliceous or proteinaceous (songin) skeletal supports (spicules) generally present.

• Skeleton composed of calcareous spicules or porous calcareous walls lacking spicules. Aspiculate sponges beadlike or cystose linear, branched and massive. Lacking spongin.

• Skeleton composed of siliceous hexactines, hexactine-derived spicules, or stauracts (rays at right angles). Some with adjacent spicules fused to form rigid skeletons.

• Eight-sided, conical, nodous sponge. Fossils are commonly found to be fillings of the spongocoel, sometimes showing molds of regularly oriented spicule straps in a rectangular pattern.

• Skeleton composed of spongin spicules or siliceous spicules that do not meet consistently at right angles (no triaxons)- they are monaxon or tetraxon.

• Skeletons are of laminar calcite or aragonite with some siliceous spicules.

• Laminated calcareous deposits with pillars between the laminae; usually a system of elevations (monticules) and shallow grooves (astrorhizae) on the top surface. Some experts consider the stromatoporoids to be sclerospongids.

• Class Regulares
• Class Irregulares