INTRODUCTION TO THE BRACHIOPODA

The lampshells are bivalve sedentary marine animals that superficially resemble bivalve mollusks (Figures 1-2); however, their bivalve structure is a consequence of convergence. Furthermore, the brachiopods have a long and interesting history with good representation in the fossil record from the lower Cambrian (Zhang et al. 2003). Indeed, during the Paleozoic Period they were much more abundant and diverse than today (Figure 3). All brachiopods fall into one of two general morphological types: the inarticulates and the articulates, which was manifest as two major groups in traditional classification systems. Williams et al. (1996) divided the phylum into three higher taxa (they described them as subphyla; see Table 1): Linguliformea, Craniiformea, and Rhynchonelliformea, all of which have living representatives. In this structure, they recognized that two of the groups were inarticulate, but the Craniiformea was certainly intermediate between the inarticulate Linguliformea and the articulate Rhynchonelliformea.

Linguliformea: These animals have valves that are made of calcium phosphate, and are mirror-images of each other. The most obvious feature, besides the valves, is the large, robust peduncle (Figure 1), which has an extension of the coelomic cavity that extends into it. They have chaetae at the openings of the valves and the digestive tract is complete (mouth and anus). The larvae, which are valved, persist in the plankton where they feed by early developed lophophores.

Rhynchonelliformea: These are the articulate brachiopods (Figures 2 and 3). That is, they have two valves that are joined by a hinge with a pattern of teeth and sockets. Unlike Linguliformea, the two valves are unequal (dorsal and ventral), but each valve is bilaterally-symmetrical (see Figure 3). Also, the valves are mineralized with calcite. They do have a peduncle which emerges through a slit in the ventral valve, but, unlike Linguliformea, the peduncle does not have and extension of a coelom. The larvae are lecithotrophic, that is, although they disperse in the plankton, they do not feed , but rely on stored food in the yolk. Unlike the other groups of brachiopods, the articulate taxa do not have an anus.

Craniiformea: Although these animals are inarticulate, they resemble the articulate brachiopods in many ways: their valves are mineralized with calcite and have the same symmetry. Furthermore, their larvae are lecithotrophic. Like the linguloids, the have an anus. They do not have a peduncle but cement the lower valve to a hard surface, often the valves of other brachiopods.

The feeding apparatus is a lophophore, a ring of ciliated tentacles that creates a current directing food particles to the mouth, and some have distinctive structures that support the lophophores. They have long been allied with other lophophore-bearing taxa (e.g. Hyman 1959). The lophophorates have been problematic for some time. The problem is that the lophophore would seem to be a synapomorphic character, thus uniting all taxa with that morphological feature. Taxonomic systems based on morphology and development did just that (e.g. Ax 1995; Brusca and Brusca 2003). The problem was that the lophophore would seem to associate the all lophophorates, some of which clearly were deuterostomes (pterobranch hemichordates). Nielsen (2001) concluded that the lophophore must be a primitive character in the bilaterian line so that taxa having lophophores did not mean a monophyletic relationship. Nielsen (2002) used that argument again to group the brachiopods and phoronids within the deuterostomes and leave the other lophophorates (ectoprocts, and entoprocts) in the protostomes. Halanych et al. (1995) first suggested a relationship between Phoronida and Brachiopoda by analyses using 18S rDNA. The relationship held consistently such that (Cohen 2000) made the phoronids a valveless subclass of the brachiopods. We have kept the Phoronida as a sister group to the Brachiopoda in a clade called Brachiozoa (see Figure 4).