A fossil assemblage including brachiopods, pelecypods, and bryozoans indicates which environment?

• A. Sublittoral
• B. Pelagic
• C. Bathyal
• D. Intertidal

Answer: (A)

Interpreting fossil assemblages is about matching the ecology of the organisms to the habitat that could support them. Brachiopods, pelecypods (bivalves), and bryozoans are all benthic, sessile filter feeders that attach to hard substrates in shallow water. Their co-occurrence indicates a stable, well-oxygenated marine environment that remains submerged most of the time, with enough light and circulation for filter feeding. That points to a sublittoral zone—below the intertidal band and down to the edge of the continental shelf. In contrast, the intertidal zone is periodically exposed to air and desiccation, and bathyal zones are deeper and host different, deeper-water communities; pelagic refers to the water column rather than the seafloor. So the assemblage best indicates a shallow, permanently submerged nearshore habitat.