It is no valid objection to this conclusion, that certain brachiopods have been but slightly modified from an extremely remote geological epoch; and that certain land and fresh-water shells have remained nearly the same, from the time when, as far as is known, they first appeared.
Charles Darwin (1859) On the Origin of Species
The brachiopods are one of the most successful invertebrate phyla in terms of abundance and diversity. They appeared first in the Early Cambrian and diversified throughout the Paleozoic to dominate the low-level, suspension-feeding benthos; a wide range of shell morphologies and sizes characterize the phylum, from the tiny acrotretides
(microns in length) to the massive gigantopro-ductids (nearly 0.5 m wide). Although only about 120 genera of brachiopods, also known as lampshells, survive today, they occupy a wide range of habitats from the intertidal zone to the abyssal depths. The brachiopods are entirely marine, bilaterally symmetric animals with a ciliated feeding organ, or loph-ophore, contained within a pair of shells or valves. Internal structures such as teeth and sockets, cardinal processes and various muscle scars are all associated with the opening and closing of the two valves during feeding cycles. Brachiopods have featured in many paleoeco-logical studies of Paleozoic faunas, when they dominated life on the seabed in terms of numbers of both individuals and species. Their use in paleobiogeographic analysis is well documented (see Chapter 4). Nevertheless brachiopods have also been widely used in regional biostratigraphy and, during the Silurian, a number of orthide, pentameride and rhynchonellide lineages show good prospects for international correlation.
Despite their relative low diversity today, living brachiopods are actually quite widespread, represented mainly by forms attached by pedicles to a variety of substrates across a spectrum of water depths. At high latitudes brachiopods range from intertidal to basinal environments at depths of over 6000 m. They are most common in fjord settings in Canada, Norway and Scotland and in the seas around Antarctica and New Zealand. The association of the brachiopod Terebratulina retusa growing on the horse mussel, Modiolus modi-olus, a bivalve, is widespread in the northern hemisphere. In the tropics, however, many species are minute, exploiting cryptic habitats, hiding in reef crevices or in the shade of corals and sponges. Larger forms live in deeper-water environments, out of the range of predators, like sea urchins, that graze on the sumptuous meadows of newly attached larvae.
Morphology: brachiopod animal_
The brachiopod soft parts are enclosed by two morphologically different shells or valves that are opened and closed by a variety of muscles; this arrangement is modified differently across the three subphyla - the linguliformeans, craniiformeans and rhyn-chonelliformeans (Fig. 12.1a-f; Box 12.1). In contrast to the bivalves, where the right valve is a mirror image of the left, the plane of symmetry in brachiopods bisects both valves perpendicular to the plane along which the valves open, or the commissure. The larger of the two valves is generally the ventral or pedicle valve; in many brachiopods the fleshy stalk or pedicle pokes through the apex of this valve and attaches the animal to the substrate. The pedicle can vary from a thick, fleshy stalk to a bunch of delicate, thread-like strands, which can anchor the brachiopod in fine mud. Some extinct brachiopods lost their pedicles during ontogeny and adopted a free-living mode of life, lying recumbent on or partially in the sediments on the seafloor. The dorsal or brachial valve contains the extendable food-gathering organ or lophophore together with its supports. A number of types of lophophore have evolved (Fig. 12.1 g). The earliest growth stage, the trocholophe, is an incomplete ring of filaments, still retained by the pedomorphic (see p. 146) microbrachiopod Gwynia. By the schizolophe stage a bilobed outline has developed, which probably characterized many of the smaller Paleozoic taxa. The more complex plectolophe, ptycholophe and spirolophe styles are characteristic of the articulated brachiopods.
The linguliformeans (see Fig. 12.1a, b) have organophosphatic shells with pedicles that either emerge between both valves or through an opening called the foramen. The shells develop from a planktotrophic, or planktonfeeding, larval stage, and linguliformeans are characterized by an alimentary tract ending in an anus. In the lingulates, the opening and closing of the valves is achieved by a complex system of muscles and the pedicle emerges between both valves. Withdrawal of the soft parts posteriorly causes a space problem that can force the valves apart; relaxation allows the animal to expand again forwards allowing the valves to close. The paterinates are the oldest group of brachiopods, appearing in the lowest Cambrian Tommotian Stage. Although linked to the other linguliformeans on the basis of an organophosphatic shell substance, the shell structure of the group is quite different and the shells have true interareas, del-thyria and notothyria and apparently had a functional diductor muscle system.
The craniiformeans (see Fig. 12.1c) include a diverse, yet probably monophyletic, group of morphologies centered on Crania but including Craniops and the bizarre trimerel-lids. The shells consist of organocarbonate and the animal developed separate dorsal and ventral mantle lobes after the settlement of the larvae on the seabed during a nektoben-thonic stage.
The rhynchonelliformeans (see Fig. 12.1d-f) have a pair of calcitic valves that contain a fibrous secondary layer, with variable convexity, hinged posteriorly and opening anteriorly along the commissure. The mantle lobes are fused posteriorly, where the interareas are secreted; their margins form the hinge between the ventral and dorsal valves. Articulation was achieved by a pair of ventral teeth and dorsal sockets, and the valves were opened and closed by opposing diductor and adductor muscle scars. In the majority of rhyn-chonelliformeans, the valves were attached to the substrate by a pedicle, emerging through a foramen in the delthyrial region. The sub-phylum contains five classes, the Chileata, the Obolellata, the Kutorginata, the Strophome-nata and the Rhynchonellata. Already by the Early Cambrian, representatives of four of the five classes were present. However the two latter classes, containing respectively over 1500 and 2700 genera, dominated Phanero-zoic brachiopod faunas.
Brachiopods possess both planktotrophic and lecitotrophic larvae. The planktotrophic stage may have been the most primitive, spending some time in the plankton, whereas lecitotrophic larvae lurking in the benthos may have developed at least twice. This obviously has important consequences for brachiopod dispersion. Since many linguli-formeans are widespread it is assumed they had planktotrophic larvae in contrast to the more endemic rhynchonelliformeans with possible lecitotrophic larvae (Fig. 12.4).
Brachiopod shells can be very variable in shape. A single species can even mimic the outlines of a range of different orders. For example specimens of Terebratalia transversa from around the San Juan islands, western USA, show Spirifer-, Atrypa- and Terebratula-type morphs with increasing strengths of currents (Fig. 12.5). Moreover a number of brachiopods, such as the strophomenides, especially the productoids, may markedly setae shell —
lophophoral arm mouth gonad anus nephridium celomic cavity cuticle muscle shell muscles <
gut shell muscles <
lophophoral arm mouth gonad anus nephridium celomic cavity cuticle muscle pedicle
Adjuster muscle scar
Adductor muscle scar
Diductor muscle scar shell
Cardinal process Socket
Adductor muscle scars
Brachidium cle scar pedicle f(i)
Ventral valve f(ii)
Dorsal valve posterior adducti anterior adductor s oblique lateral scar brachial protractor muscle scar
Ventral valve Pedicle foramen Delthyrium
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