Bryozoans are potentially useful as environmental indicators, although their application to this topic is rarely straightforward. A major problem is comparing modern, cheilostome-dominated assemblages with older faunas. Rare bryozoan species are tolerant of most conditions, so statistical analyses of diversity or abundance tend to be applied. In post-Palaeozoic rocks and in the modern oceans, bryozoans are dominant members of shallow benthic communities in temperate latitudes, with normal salinity and low to moderate rates of sedimentation. Abundance in the modern day peaks at water depths of 40-90 m, and in areas with sedimentation rates of less than 100 cm/1000 years. In common with most organisms, a high diversity usually represents environments close to the ideal range, while high abundances of a small number of species can typify more extreme conditions.
Individual shapes and sizes may also be used as environmental indicators in some cases. In general it has been observed that colony and zooid size decrease with increasing depth, although, paradoxically, zooids tend to be larger in cooler water. The shape of a bryozoan colony may be more or less favored in different environments. This means that environmental data may be buried in lists of the relative abundance of different morphological types. For example, encrusting forms are more common than erect shapes in shallow water in modern oceans, whilst rigid, erect colony shapes are most common from deep water assemblages. Free-living, mobile colonies are typical of sandy sea beds, where the bryozoan colony needs to be able to respond to a substrate that is itself mobile.
Stenolaemate bryozoan Ordovician-Permian
An erect, fan-shaped bryozoan with the branches of the colony stiffened by cross bars. The skeleton is lightly to moderately calcified. Colonies ranged in size up to 20 cm in height, and were locally rock forming. Each fan generated a one-way current of water through its apertures, from the side that bore lophophores towards the side that had none.
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