Bryozoans are small colonial animals which can attain large sizes. The bryozoa colony contains thousands of individual cup-shaped polyp structures similar to a beehive.
Once considered intermediate between plant and animal and dubbed moss animals, bryozoans eventually were found to be animals, possessing a primitive digestive system but lacking other more advanced systems found in higher groups. Each bryozoan secretes a type of skeleton in which it lives, the grouping of thousands together produces a rigid colony which can be mistaken for a single animal. The colony can grow into brush and twig-like shapes that attach to rocks, shells, seaweeds, or any other hard object.
Within the colony, certain individuals often have different functions from the normal food gathering individuals. Instead, they are specialized for reproduction and are solely responsible for the propagation of the colony and receive food from the other polyps. The normal polyps have tentacles to capture microscopic food. Bryozoans are mainly marine with only a few groups having successfully adapted to fresh water. Fossil bryozoans are common there being approximately four thousand fossil species, which range from Late Cambrian to the present. They are very common in some Arizona strata.
Fossil bryozoa are often abundant in thin-bedded marine limestones or calcareous shales, usually most often found littering the ancient surfaces -- the bedding planes. Fossil bryozoa are rare or absent in noncalcareous rocks such as sandstones or conglomerates. Most easily collected on weathered surfaces of rock, they appear as slender, white, light or dark gray or yellow branches and fans. Many branching forms can be collected without matrix and later etched in very dilute acid to expose detail of frontal walls and pores.
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