The Classification of the Brachiopoda

The brachiopods have for a long time been traditionally divided into two classes, the Inarticulata and the Articulata.  The Inarticulata are so-called because they possess two valves that do not have an articulating hinge. Recently it has been suggested that hinge articulation is not a good way to classify brachiopods anyway, that the traditional organization of the Brachiopoda is erroneous, and that the "Inarticulata" is probably an artificial group and should perhaps be broken up into several classes. Cladistic analysis of the Brachiopoda has resulted in a major revision of the supra-ordinal taxa of the Brachiopoda, which has become widely accepted, although the traditional classification as used in the Treatise on Invertebrate Paleontology, is still standard.

The taxonomy of brachiopods is complicated by homeomorphy, or similarity due to convergent evolution. In any case newer cladistically-based arrangements (see Brachiopod Systematics and Classification des Brachiopoda) give a much more complex classification arrangement than the familiar Treatise on Invertebrate Paleontology, with a large number of higher taxa. These are compared in the following table:

It seems to me that this newer version, based as it is on Cladistics, gives a grossly inflated number of higher Linnaean taxa for the "Inarticulates". What was previously a simple class is now divided into no less than three subphylum and six classes! To give the idea of a subphylum, the entire Vertebrata (Vertebrate animals) are in the Linnaean arrangement a subphylum of the phylum Chordata. The taxonomic diversity of a very morphologically conservative group like the "Inarticulates" certainly does not seem to justify such a high ranking. This is typical of the sort of problems that come about when trying to integrate Cladistic and Linnaean systems. Such attempts often lead to a huge taxonomic inflation of ancestral ("basal") taxa.

The Articulates meanwhile are in the newer arrangement split into two classes and a greater number of orders, some of which may indeed be warranted (for example four rather one order of "Spirifid" brachiopods can be justified if it can be shown that the Spirifid condition evolved in four different and unrelated lineages).

The relationship of the Phoronida (burrowing worm-like lophophorates) is another problem. Although a number of authors still regard Phoronida as constituting a separate phylum. Several recent evolution trees show the Phoronida and Brachiopoda as closely interrelated, with the Brachiopoda possibly a paraphyletic group. It is becoming increasingly accepted that the constitute a single clade together with the Brachiopoda, possibly as a class within the phylum Lophophorata, which also includes the Bryozoa, and Brachiopoda, all three groups sharing a ciliated lophophore and a number of other features (although the status of the Bryozoa awaits molecular confirmation).

It has even been suggested that brachiopods are polyphyletic, and that is, that different brachiopod groups evolved separately from soft-bodied phoronid-like ancestors.

This might explain the way in which a number of distinct brachiopod lineages appear in the earliest Cambrian with no intermediate forms.

Note, in the following classification, diagnosis for the most part follows Sean Robson

Subphylum Linguliformea

Lingulata Linguliform brachiopods with smooth or pitted larval shell; marginal setae form continuous rows along mantle margin; digestive tract recurved with anteriorly placed, functional anus.				Paterinata
Lingulida	Discinida	Acrotretida	Siphonotretida	Paterinida

Subphylum Craniiformea

Craniata
	no image available yet	no image available yet
Craniida	Craniopsida	Trimerellida

Subphylum Rhynchonelliformea (including "Articulata")

Brachiopods with calcitic articulated shells. Teeth and sockets are usually present, except in some primitive "inarticulate" forms, but can be lost secondarily. The gut is blind and without an anus. The Articulata have calcareous shells whose valves, like Bivalve mollusk shells, are hinged articulated) by well-developed teeth and sockets. The shell is divided into two distinct layers, primary (outer) and secondary (inner). The outer layer is lamellar (consisting of layers) and the inner layer is fibrous, made up of calcareous fibers.

Primitive ("Inarticulate") forms

Chileata		Obolellata		Kutorginata
no image available yet	no image available yet		no image available yet
Chileida	Dictyonellida	Obolellida	Naukatida	Kutorginida

Advanced ("articulate") forms - the Strophomenids

Strophomenata Articulate brachiopods with variable profile in productids; commonly planar to weakly concavoconvex in strophonmenides. middle Cambrian - Late Permian
Billingsellida	Orthotetida	Strophomenida	Productida

Advanced ("articulate") forms - the rest of the Articulates

Rhynchonellata Articulate brachiopods with biconvex, strophic or astrophic shells articulated by deltidiodont or cyrtomatodont teeth and sockets buttressed by brachiophores. Early Cambrian - Recent
no image available yet		no image available yet		no image available yet
Protorthida	Orthida	Spiriferida	Pentamerida	Terebratulida
no image available yet	no image available yet		no image available yet	no image available yet
Rhynchonellida	Atrypida	Athyridida	Spiriferidida	Thecideida

Links

Brachiopods - this European Web site includes up-to-date information on Brachiopoda, references, systematics, diagnoses...)  a directory of specialists working on brachiopods (Brachiopodologists - yes, there is such a term!).

Brachio-Philes - Sean Robson's Brachiopod site - see his Brachiopod Systematics page

The classifications on both these sites are based on Williams, Carlson, Brunton, Holmer, and Popov, 1996: A Supra-ordinal classification of the Brachiopoda.   Philosophical Transactions of the Royal Society of London. Series B. Volume 351,  pgs.1171-1193.

Brachiopoda main page