Cool-water carbonate production from epizoic bryozoans on ephemeral substrates

Abstract

Bryozoan skeletons are a dominant constituent of cool-water carbonate sediments in the Cenozoic of southern Australia. The primary substrate on much of the modern continental shelf is loose sediment that is reworked intermittently to 200 + m water depth by storm waves. Availability of stable substrate is a limiting factor in the modern distribution of bryozoans in this setting. As a result, a significant proportion of the sedimentologically important modern bryozoans (30-250 m water depth) live attached to sessile, benthic invertebrate hosts that possess organic or spicular skeletons. Hosts such as hydroids, ascidian tunicates, sponges, soft worm tubes, octocorals, and other lightly-calcified and articulated bryozoans provide ephemeral substrates; after death, host skeletons disarticulate and decay, leaving little or no body fossil record. The calcareous sediments produced by these epizoic bryozoans from ephemeral substrates result in loose particles that rarely preserve substratal relationships, but potentially retain diagnostic basal attachment morphologies. Although the best known examples of epizoic carbonate production on ephemeral substrates are from the southern Australian margin, this may be an important phenomenon both globally and in the fossil record. Bryozoan sediment production from epizoans on ephemeral substrates would seem, however, to have a scant record prior to the Cretaceous.