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:
- Traditional phylogeny (and hence taxonomy) was based on:
- overall body architecture, the idea of a body 'plan'.
- the idea that evolution went from simple to complex.
- certain basal features of embryology.
- 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.
- 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).
