brachiopods

Brachiopods are a phylum of marine invertebrates commonly known as lamp shells. They live inside two shells, or valves, that enclose a soft body. The valves are oriented along a dorsal–ventral axis rather than left and right, and they are hinged along the top edge. Most species attach to the substrate by a stalk-like pedicle that passes through an opening in the ventral valve, though some are free-living or cemented to hard surfaces.

The feeding apparatus is a crown of ciliated tentacles called the lophophore, which creates a water current to filter food particles from seawater. The lophophore sits inside a fold in the mantle and can be extended for feeding or withdrawn for protection.

Shell composition and major groups: brachiopod shells are typically calcareous, often made of calcite. There are two major clades: articulate brachiopods, with a toothed hinge and a curved row of teeth; and inarticulate brachiopods, which lack a toothed hinge and have a simpler articulating mechanism. Lingulids, inarticulate and often called living fossils, illustrate the long lineage of the group.

Ecology and distribution: brachiopods are mostly benthic filter feeders inhabiting shallow to deep seas; some attach to shells or other organisms, while others lie free on the seafloor. Modern diversity is limited compared with the Paleozoic, but they persist in many regions, often in cold or nutrient-rich waters.

Evolution and significance: Brachiopods first appeared in the early Cambrian and were highly abundant in the Paleozoic. They declined after the Permian mass extinction but survive today with a few hundred living species. Their abundant fossil record makes them important index fossils for Paleozoic strata and paleoenvironmental reconstructions.