Brachiopod classification

The classification of brachiopods is being discussed at present. The following is an overview of the different schemes which are proposed.

The "traditional" classification was defined in 1869. Two further approaches were established in the 1990s:

• In the "traditional" classification, the Articulata have toothed hinges between the valves, while the hinges of the Inarticulata are held together only by muscles.^87–93
• A classification devised in the 1990s, based on the materials of which the shells are based, united the Craniida and the "articulate" brachiopods in the Calciata, which have calcite shells. The Lingulida and Discinida, combined in the Lingulata, have shells made of chitin and calcium phosphate.
• A three-part scheme, also from the 1990s, places the Craniida in a separate group of its own, the Craniformea. The Lingulida and Discinida are grouped as Linguliformea, and the Rhynchonellida and Terebratulida as Rhynchonelliformea.

Principles

Underlying the discussion are some basic principles:

1. Traditional phylogeny (and hence taxonomy) was based on:
   1. overall body architecture, the idea of a body 'plan'.
   2. the idea that evolution went from simple to complex.
   3. certain basal features of embryology.
2. Phyla (et cetera) are man-made concepts which are meant to mirror our understanding of the reality we see. However, we now know that morphological characters (traits) are more changeable than was previously thought.
3. Molecular data are more objective, and can be decided with more certainty than data on morphology. Much more is now known about molecular sequences than is known about the evolution of morphology. Thus, evidence from molecular sequencing is used to develop classifications which better reflect evolution.

Three high-level classifications of brachiopods.

"Traditional" classification.	Inarticulata			Articulata
"Calciata" approach.	Lingulata		Calciata
Three-part approach.	Linguliformea		Craniformea	Rhynchonelliformea
Orders	Lingulida.	Discinida.	Craniida.	Terebratulida.	Rhynchonellida.
Hinge	No teeth			Teeth and sockets
Anus	On front of body, at end of U-shaped gut			None
Pedicle	Contains coelom (~cavity) with muscles running through		No pedicle	No coelom, muscles where joins body
	Long, burrows	Short, attached to hard surfaces	None, cemented to surface	Short, attached to hard surfaces.
Periostracum (surface layer of shell)	Glycoprotein and chitin		Chitin	Proteins
Primary (middle) mineralized layer of shell	Glycosaminoglycans and apatite (calcium phosphate)		Calcite (a form of calcium carbonate)
Inner mineralized layer of shell	Collagen and other proteins, chitinophosphate and apatite (calcium phosphate)		Calcite	Proteins and calcite
Chaetae (stiff hairs) around opening of shells	Yes.		No.	Yes.
Coelom fully divided	Yes.		No.	Yes.

About 330 living species are recognized, grouped into over 100 genera. The great majority of modern brachiopods are rhynchonelliforms (Articulata, but excluding Craniida).