The main external difference lies in the symmetry of their shells. Brachiopods first appeared in the early Cambrian approximately 545 million years ago, and were the dominant benthic filter-feeders for most of the Palaeozoic Era. It was shortly after this that bivalve molluscs came to dominate this ecological niche.
Brachiopods still persist today, but are neither abundant nor diverse.
There are approximately 325 species of living brachiopods, which is a pale shadow of their former diversity of more than 12,000 described fossil species. All brachiopods are marine animals whose habitats range from the inter tidal zone to deep ocean. Most live permanently attached to a hard substrate, either by encrusting on the surface or cemented to it by a fleshy stalk called a pedicle.