Brachiopoda

Abstract

Enclosed in shells with ventral and dorsal valves, extant brachiopods (meaning “arm” and “foot”) are classified into three major subphyla: the Rhynchonelliformea, the Linguliformea, and the Craniiformea. Rhynchonelliform brachiopods encompass what were once referred to as the “articulate” brachiopods, so named for the mineralized hinge that connects the calcite valves of their shells. No such hinge is found in members of the other two subphyla, rather their valves are held together only by various muscles and connective tissues. Craniiform brachiopods (e.g., Novocrania) also have calcitic shells, but the shells of linguliform brachiopods (such as the lingulid Glottidia and the discinid Discinisca) are composed of apatite, a phosphatic mineral, with an outer layer of chitin. Most brachiopod morphotypes have a smaller dorsal and a larger ventral valve, the latter of which often bears a muscular or rigid attachment structure called the pedicle. Rhynchonelliform brachiopods are often attached to hard substrata by the rigid pedicle with their ventral valves oriented upward. The shells of linguliform brachiopods such as Glottidia and Lingula generally have equally sized valves and their pedicles are long, muscular structures modified for burrowing into soft sediments. Craniiform brachiopods have lost the pedicle and cement directly to hard substrates.

Chapter vignette artwork by Brigitte Baldrian.© Brigitte Baldrian and Andreas Wanninger.