Colony growth strategies, dormancy and repair in some Late Cretaceous encrusting bryozoans: insights into the ecology of the Chalk seabed

Bryozoans are among the most common macrofossils in the Late Cretaceous Chalk. They include many species that encrusted hard substrates, notably echinoid tests, forming habitat islands on the Chalk seabed. The growth strategies adopted by these bryozoans, as well as the occurrence of reparative structures, provides evidence of the conditions experienced by bryozoans and other benthic animals during the accumulation of this unique pelagic sediment deposited over large areas of the continental shelf. Here, we use historical material in the Natural History Museum, London, to provide qualitative evidence that whereas available substrates, including irregular echinoids, were long-lasting, most individual bryozoan colonies were probably short-lived. Some cheilostome species produced heavily calcified polymorphic zooids at the outer edges of the colony that persisted after loss of the feeding autozooids and became the source of regenerative colony growth. Short-term (possibly annual) periodicity is suggested in the benthic environment experienced by encrusting bryozoans, which may have possibly been a result of cyclical variations in dinoflagellate food supply and/or swamping by unpalatable and potentially poisonous coccolithospheres.