Sponges are the simplest animals. Sponges do not have tissues or organs. However, the cells that make up a sponge are integrated and organized to filter feed, dispose waste, reproduce and secrete a shared porous skeleton. As the simplest form of multi-cellular life, sponges possess the property of cell recognition. A fine mesh can be used to separate sponge cells individually after which, they will recombine to form a new sponge. Sponges are sessile, filter-feeding organisms that live in both marine and freshwater environments.

Sponge Characteristics

Sponges are covered with small intake pores called ostium. A one-way current carries the water into an internal cavity(spongocoel) and out a larger opening, the osculum. The walls of a sponge are made of two cell layers separated by and embedded in a gel-like substance called mesoglea. Pinacocytes are leather-like cells that line the outside wall. Collar cells (Choanocytes) equipped with flagellum line the inside cavity and create the one-way water current. Collar cells trap small particles and plankton. Within the gel-like layer amoebocytes (amoeba-like cells) distribute food, remove waste, and build skeletal structures called spongin and spicules. Spicules are made of silica or calcite. Sponges secrete a calcite or aragonite base that serves to anchor the organism to a substrate. Sponges produce both sexually with sperm and eggs as well as asexually through budding. The larvae and the collar cells of sponges are evidence for evolutionary ties to the flagellated protists. When a sponge dies it disintegrates leaving the spicules behind. The mineralized spicules are the part of a sponge that is most likely to be fossilized.

Glass & Calcite

Traditionally, sponges have been divided into three classes. Sponges are known from Precambrian deposits and representatives from all three classes range from the Cambrian to the present. Glass sponges (class Hexactinellida) are represented by sponges with silica spicules. Members of this class helped to build massive reefs in the late Devonian. Calcareous sponges (class Calcarea) are represented by sponges with calcite spicules. Calcarea sponges were important reef builders in the Permian and Triassic. Common sponges have skeletons made of the protein spongin (class Demospongea).

Some fossils that have close affinities with sponges are stromatoporoids, which were important reef builders during the Silurian and Devonian periods and archaeocyathans, which were the first multi-cellular reef builders during the Cambrian.

Fossil sponges can be used as indicators of paleoenvironments. Sponges are sensitive to currents, turbidity and depth. Thus, species of sponges can be clues to the environmental conditions present during their lives.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes the two genera under the Sponges (Phylum Porifera) category: Hydnoceras (class Hexactinellida) and Astraeospongia or Astraeospongium (class Calcarea). Hydnoceras ranges from Devonian to Pennsylvanian. Astraeospongium ranges from Silurian to Devonian.

Astraeospongia meniscus
Silurian
Niagara Formation
Decatur County, Tennessee
6 cm diameter
Note spicules in red circle

Bibliography

Pinna, G. (1990). The Illustrated Encyclopedia of Fossils. New York: Facts on File.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Walker C. & Ward D. (2002). Smithsonian Handbooks: Fossils. New York: Dorling Kindersley