Phylum Brachiopoda Overview

Phylum Brachiopoda

• Brachiopods were dominant shelly marine invertebrates during the Paleozoic Era.
• They originated in the early Cambrian and diversified in the Ordovician.
• They survived the end-Permian mass extinction but declined through the Mesozoic.
• Classified into three subphyla: Linguliformea, Craniiformea, and Rhynchonelliformea.
• Exclusively marine filter feeders.
• Shell shape can indicate the substrate type.
• Communities can be used to study paleoenvironments.
• Some, like lingulids, have remained unchanged for 500 million years, called living fossils.
• Terebratulides are sometimes called lamp shells due to their resemblance to oil lamps.
• Most modern brachiopods live in marginal marine environments and have similar morphologies.
• Paleozoic brachiopods had a wider range of morphologies, from erect coral-like forms to flattened saucer shapes.

Morphology

• Brachiopod shells have one larger valve (ventral or pedicle valve).
• The ventral valve has an opening for a pedicle stalk to attach to the substrate.
• The opposite smaller valve is the dorsal (or brachial) valve.
• Lophophore (brachium) attaches to the dorsal valve, filling most of the interior.
• Coelomic cavity houses internal organs.
• Metanephridium (kidney-like structure) for waste excretion exists.
• Setae (bristles) around the margin sense the environment.
• Adductor muscles (connected to dorsal and ventral valves) close the shell.
• Diductor muscles are connected to the ventral valve pulling and opening,
• Pedicle foramen is the opening for the pedicle.
• Delthyrium is a notch in the ventral valve (rather than foramen in some cases).
• Notothyrium is a notch in the dorsal valve.

Morphology (additional)

• Pedicle foramen (opening for the pedicle) is often in the beak.
• Deltidial plates or a single deltidium may enclose the foramen.
• Delthyrium is a shallow notch instead of a foramen.
• Chilidium may be a plate enclosing the notothyrium.
• Pentameride brachiopods have a spondylium (spoon-shaped platform) in the ventral valve.
• Cruralium (spoon-shaped feature) is in the dorsal valve.
• Septum subdivides the shell into chambers.

Phylum Brachiopoda: Classification

• Traditional Classification:
  • Inarticulates: calcium phosphate shells, no hinges or teeth, held together by muscles, (e.g., Lingulida).
  • Articulates: calcium carbonate shells with hinges and teeth, (all other brachiopods).
• Modern Classification:
  • Subphyla include Linguliformea, Rhynchonelliformea, and Craniiformea.

Ecology and Paleoecology

• Benthic marine animals.
• Filter feeders; do not actively search for food; rely on currents for food/oxygen and waste removal.
• Life styles include burrowing, semi-infaunal, epifaunal, and encrusting.

Distinguishing Brachiopods and Bivalves

• Superficial similarities but different symmetry plane.
• Bivalves have left and right valves aligned with the line of symmetry along the valves' edges.
• Brachiopods have dorsal and ventral valves with the line of symmetry dividing each valve in half.
• Most brachiopods have a plane of symmetry that runs through both shells; the right valve is a mirror image of the left in bivalves.