crustaceans

crustaceans The crustaceans are a class of the phylum Arthropoda, a large and important phylum that includes animals with a segmented body, hard exoskeleton, and many jointed legs. The name ‘Crustacea’ was originally used to designate animals with a hard but flexible ‘crust’, but as this applies to most arthropods we now distinguish crustaceans as arthropods that breathe by means of gills and have two pairs of antennae (crabs, lobsters, barnacles, and ostracods). The first crustaceans are known from the Early Cambrian and were forms in which a bivalved carapace almost covered the body. These phyllocarids might have been ancestral to the more advanced shrimp- or lobster-like forms which appeared in the Late Devonian, and have become very important since then, particularly in the oceans.

Most crustaceans have little geological value despite their abundance; however, one group that does is the Ostracoda, small laterally compressed crustaceans enclosed within a protective shell which is formed of two chitinous or calcareous valves that are hinged above the dorsal region of the body. These animals are usually about a millimetre in length and are filter-feeders, consuming micro-organisms stirred up by their appendages. They mostly live in aquatic environments and are benthonic (bottom-dwelling) or planktonic (floating) organisms. The abundance of their shells in marine sediments makes them particularly useful for biozonation and also as indicators of palaeosalinities (ancient salinities). They have a long and well-documented fossil record from the Early Cambrian to the Present.

David K. Elliott