Box Brachiopod classification

Recent cladistic and molecular phylogenetic analyses have shown that the traditional split of the phylum Brachiopoda into the Inarticulata and Articulata is incorrect, and instead there are three subphyla, the Linguliformea, Craniiformea and Rhynchonelliformea. All three have quite different body plans and shell fabrics (Fig. 12.2). The linguliformeans contain five orders united by organo-phosphatic shells; the inclusion of the paterinides is the most problematic since the group shares some morphological characters with the rhynchonelliforms. The craniiformeans include three rather disparate groups with quite different morphologies but which together possess an organocarbonate shell. Most scientists now accept 14 articulated orders in the rhynchonelliformeans, not counting the chileides, dictyonellides, obolellides and kutorginides, mainly based on the nature of the cardi-nalia and the morphology of the other internal structures associated with the attachment of muscles and the support of the lophophore. Recently the more deviant chileides, obolellides and kutorginides have been added to the subphylum. In addition, the articulated taxa have been split into those with deltidiodont (simple) and cyrtomatodont (complex) dentitions; the former group includes the orthides and strophomenides whereas the latter include the spire bearers.

Cladistic-based investigations have developed a phylogenetic framework for the phylum (Williams et al. 1996), supporting the three subphyla (Fig.12.2); their defining characters are based on shell structure and substance. The mutual relationships among these groups are still unclear as are the relationships between the many primitive articulated and non-articulated groups that appeared during the Cambrian explosion together with the origin of the phylum as a whole (Box 12.2).

A data matrix containing all the data from Williams et al. (1996) is available at http://www. blackwellpublishing.com/paleobiology/.

Subphylum

Order

Key characteristics

Stratigraphie range

Craniiformea

Discinida Siphonotretida Paterinida Craniida Craniopsida Trimerellida Rhynchonelliformea Chileida

Dictyonellida

Linguliformea Lingulida Spatulate valves with pedicle usually emerging between both shells Acrotretida Micromorphic forms with conical ventral valve; dorsal valve with platforms

Subcircular shells with conical ventral valve and distinctive pedicle foramen Subcircular, biconvex valves with spines and elongate pedicle foramen Strophic shells with variably developed interareas

Usually attached by ventral valve; dorsal valve with quadripartite muscle scars Small oval valves with internal platforms and marked concentric growth lines Commonly gigantic, aragonitic shells, with platforms and umbonal cavities Strophic shells lacking articulatory structures but with umbonal perforation Biconvex valves with large umbonal opening commonly covered by a colleplax

Naukatida Biconvex shells with articulatory structures and apical foramen Obolellida Oval valves with primitive articulatory structures

Kutorginida Strophic valves with interareas but lacking articulatory structures Orthotetida Biconvex shells, commonly cemented, with bilobed cardinal process Billingsellida Usually biconvex with transverse teeth and simple cardinal process Strophomenida Concavoconvex, usually with a bilobed cardinal process; recumbent life mode; cross-laminar shell structure with pseudopunctae Productida Concavoconvex valves with complex cardinalia; recumbent or cemented life mode; often with external spines Protorthida Well-developed interareas, primitive articulation and ventral free spondylium

Orthida Biconvex, usually simple cardinal process; pedunculate; delthyria and notothyria open Pentamerida Biconvex, rostrate valves with cruralia and spondylia variably developed Rhynchonellida Usually biconvex, rostrate valves with variably developed crurae Atrypida Biconvex valves with dorsally-directed spiralia and variably developed jugum Athyridida Usually biconvex valves with short hinge line and posterolaterally-directed spiralia

Spiriferida Wide strophic valves with laterally-

directed spiralia; both punctate and impunctate taxa Thecideida Small, strophic shells with complex spiralia including brachial ridges and median septum

Terebratulida Biconvex valves with variably developed long or short loops

Cambrian to

Recent Cambrian to Devonian

Ordovician to

Recent Cambrian to Ordovician Cambrian to Ordovician Ordovician to

Recent Ordovician to

Carboniferous Ordovician to

Silurian Cambrian

Ordovician to Permian

Cambrian

Cambrian

Cambrian

Ordovician to

Permian Cambrian to Ordovician Ordovician to Permian

Ordovician to Triassic

Cambrian to Devonian

Cambrian to Permian

Cambrian to Devonian

Ordovician to Recent

Ordovician to Devonian

Ordovician to Jurassic

Ordovician to Jurassic

Triassic to Recent

Devonian to Recent

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