Bryozoan: Archimedes

Bryozoans have a very ancient lineage going back to the Ordovician with common forms still around today, though most living bryozoans are of the encrusting type rather than the erect and branching types that were so common in the past.

All Bryozoans are aquatic, and most are marine forms though there are some fairly common freshwater forms as well. Most are colonial animals, much like the corals that they are sometimes confused with.

JPS version

click for larger version

This particular type is known as Arhimedes because of its screw shape, an analogy which should be obvious to anyone who has heard of the Archimedes screw, a type of water pump which inspired modern ship propellers. Archimedes forms are pretty common as fossils (this one was in an $8 fossil set!) but have been extinct since the Triassic.

In life the individual animals, "Zooids", formed sheets that were attached to the central skeletal structure shown here. The whole structure would be attached to the seafloor or a shell.

Freshwater forms are in the class Phylactolaemata and seldom fossilize so it is not known how common they were in the past. Sometimes large colonies clog water intakes in lakes.

Marine forms can also be pests, encrusting the bottoms of ships much like barnacles do. In fact a lot of the "barnacles" removed from the bottom of ships are actually bryozoans.

measurements of Archimedes bryozoan 
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Though some live several miles deep in the ocean, most bryozoans are found in much shallower habitats. Stationary colonies are the norm, but some colonies can move around slowly. There are noncolonial forms which move around between sand grains at the bottom of the ocean.

Though there are several classes of bryozoans and over 5000 living species most people are unaware of their existance and would have a hard time distinguishing colonial marine bryozoans from coral.

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