FOSSIL INVERTEBRATES/Brachiopods 303 Figure Brachiopod shell structure based on sections through living Notosaria Adapted from Clarkson ENK (1998) Invertebrate Palaeontology and Evolution, 4th edn Cambridge: Blackwell Science have phosphatic material combined into the shell fabric The rhynchonelliformean brachiopods are composed of low-magnesian calcite; these organocarbonate shells may have fibrous, laminar, or crossbladed laminar shell fabrics in their secondary layers Many shells are perforated by small holes or punctae, in life holding finger-like extensions of the mantle or caeca Some strophomenides have pseudopunctae, with fine inclined rods or taleolae embedded in the shell fabric Classification and Phylogeny The traditional split of the phylum into the Inarticulata and Articulata has been discarded in favour of three subphyla: the Linguliformea, Craniiformea, and Rhynchonelliformea (Table 1) All three have quite different body plans and shell fabrics (Figure 5) The linguliformeans contain five orders united by organophosphatic shells; the inclusion of the paterinides is the most problematic, as 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 accept eight articulated orders, mainly based on the nature of the cardinalia 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, and the latter includes the spire bearers Cladistic-based investigations have developed a phylogenetic framework for the phylum, supporting the three subphyla (Figure 6); their defining characters are based on shell structure and substance The mutual relationships between these groups are still unclear, as are the relationships between the many primitive articulated and nonarticulated groups that appeared during the Cambrian explosion During and following the Ordovician radiation, the relationships between taxa are more clearly defined Origin and Affinities To date, no definite brachiopods have been described from Precambrian rocks Molecular data and the inclusion of the brachiopods within the other lophophorates (the bryozoans (see Fossil Invertebrates: Bryozoans) and phoronids) suggest that the phylum belongs to the protostomes This would support the evolution of the Brachiopoda from a slug-like ancestor, such as Halkieria (with anterior and dorsal shells), by the folding of the mantle along a line perpendicular to the length of the animal On the other hand, morphological evidence suggests that brachiopods are in fact deuterostomes The ancestral brachiopod may have possessed a planktotrophic larva with a straight gut with the adult form attached by a pedicle The group diverged to form the organophosphatic linguliformeans with a displaced anus, and the craniiformeans and rhynchonelliformeans with calcitic shells; the rhynchonelliformeans lost the anus and acquired complex hinging mechanisms Ecology Living and fossil brachiopods have developed a wide range of life styles (Figure 7) The majority were attached by a pedicle cemented to a hard substrate or rooted into soft sediment A number of quite different inarticulated and articulated taxa were cemented to the substrate, whereas some groups