Secular changes in colony-forms and bryozoan carbonate sediments through geological history



Ever since their first radiation in the Ordovician, bryozoans have contributed significantly to carbonate sedimentation. Most of the numerous colony-forms developed by bryozoans have evolved repeatedly in different taxonomic groups and vary in their sediment-producing potential. There are nine basic bryozoan colony-forms: encrusting, dome-shaped, palmate, foliose, fenestrate, robust branching, delicate branching, articulated and free-living. The proportion of these morphotypes in bryozoan faunas period by period is shown to change significantly through the Phanerozoic. Notable patterns include: (i) steady increase in the number and proportion of encrusting species through time, interrupted by a transient drop in the Late Palaeozoic; (ii) post-Triassic decrease in robust branching colonies; (iii) rise in the proportion of fenestrate colonies through the Palaeozoic, followed by their absence in the Triassic and Jurassic, rarity in the Cretaceous and reappearance in smaller proportions in the Cenozoic; and (iv) scarcity of articulated colonies and absence of free-living colonies until the Cretaceous. Most Palaeozoic bryozoan sediments come from two architecturally distinct groups of colonies: (i) domal, delicate branching, robust branching and palmate; and (ii) fenestrate. The former generate coarse particles both as sediment and components of stromatoporoid-coral reefs in the Early and mid Palaeozoic, whereas the delicate lacy fans of the latter create both prolific coarse sediment and form the cores of Late Palaeozoic deep-water, sub-photic biogenic mounds. Nearly all post-Palaeozoic bryozoan sediments comprise cyclostomes and cheilostomes with many of the same growth forms but with the addition of free-living colonies and significant numbers of articulated colonies. The latter produced sand and mud-sized bryozoan sediment via disarticulation for the first time. In contrast to the Palaeozoic, post-Palaeozoic bryozoans generated sediment varying more widely across the grain-size spectrum, from mud to sand to gravel. This article highlights the need to consider evolutionary changes in carbonate-producing organisms when interpreting facies changes through time.