Figure 12.11 Lower Silurian depth-related paleocommunities developed across the Welsh and Anglo-Welsh region. (Based on Clarkson 1998.)

Ager Fossil
Figure 12.12 Mesozoic palaeocommunities developed across Alpine Europe. Numbers 1 to 7 refer to the seven different biotypes described on the figure. (Based on Ager, D.V. 1965. Palaeogeogr. Palaeoclimatol. Palaeoecol. 1.)

Box 12.4 Brachiopod predation

Brachiopods were eaten by gastropods, arthropods and other predators, and the best evidence is found in Paleozoic examples, especially in the Devonian. Many predatory gastropods feed by drilling into the shells of their prey, and two types of drill hole are commonly present in Paleozoic brachio-pods: small, cylindrical holes made by Oichnus simplex and larger, often beveled holes made by O. paraboloides; these are, of course, ichnogenera (see p. 525) and not actual brachiopods (Fig. 12.13). After the Devonian peak in drilling diversity, there was apparently a marked drop in the frequency of drilled shells, particularly after the Mid Carboniferous. Many Carboniferous and Permian groups such as the productides have thickened shells with an armor of frills, lamellae and spines, all perhaps acting as defense against marauders. Maybe the prey had won this early arms race or perhaps the introduction of mollusks into these communities provided fresh and preferable seafood for the predators. Nevertheless, if we use the Recent Antarctic benthos as a model for the Paleozoic fauna, there is a lack of fast-moving durophagous predators (Harper 2006). Some authors have speculated that the toxins within the flesh of some modern groups, such as the rhynchonellids, may have protected them from attack.

Boring Shell Fossil
Figure 12.13 Brachiopod predation: boring of Oichnus paraboloides in the conjoined valves of Terebratulina from the Pleistocene rocks of Barbados. Scale bar is in millimeters. (Courtesy of Stephen Donovan.)

the gape of the two shells. Thus an increased volume of nutrient-laden fluid may flow into the mantle cavity while grains of sediment with diameters exceeding the shell gape will still be excluded. So far so good.

During the Permian, a group of aberrant productoids, called the richthofeniids, mimicked corals and built biological frameworks that may be found as fossils in the Salt Ranges of Pakistan and the Glass Mountains of Texas. These brachiopods have a cylindrical pedicle valve attached to the substrate and a small, cap-like brachial valve. It is difficult to understand how these animals fed. A possible scenario involves the flapping of the upper, brachial valve to generate currents through the brachiopod's mantle cavity. Rudwick filmed the flow of water through the cylindrical, lower, pedicle valve as the upper valve was moved up and down. Fluid did in fact move efficiently through the animal, bringing in nutrients and flushing out waste. The paradigm, however, failed the test of field-based evidence. Specimens of the athyride Compos-ita apparently in life position occur attached to the upper valve of the richthofeniid. Vigorous flapping of the valve was thus unlikely and it would not have been an ideal attachment site for an epifauna. Rather, these aberrant animals may have developed lophophores with a ciliary pump action to move currents through the valves. One hypothesis has been rejected, and another stands as a possibility - we cannot prove how the richthofeniid brachiopods functioned, but the paradigm approach offers a reasonably objective way for paleontologists to approach these problems.

Distribution in space: biogeography_

The biogeographic patterns of the linguli-formean brachiopods were quite different from those of the craniiformeans and rhyn-chonelliformeans. The former had planktotro-phic larval phases (see p. 241) with a facility for wide dispersal; in contrast the lecithotro-phic larvae of the latter were short-lived and thus individual species were less widely distributed. Cambrian brachiopods were organized into tropical and polar realms. Linguliformeans developed widespread distributions in shelf and slope settings; rhynchonelliformeans were more diverse in the tropics, preferring shallow-water carbonate and mixed carbonate-siliciclastic environments. In the Ordovician, brachiopod provincialism generally decreased during the period. Provinciality was most marked during the Early Ordovician, when a range of platform provinces associated with the continents of Baltica, Gondwana, Lauren-tia and Siberia (see Appendix 2) were supplemented by centers of endemism associated with a range of microcontinents and volcanic arcs and island complexes.

Provincialism was reduced during the Silurian with the close proximity of many major continents. By the Wenlock, however, two broad provinces, the cool-water Clarkeia and the mid-latitudinal Tuvaella faunas, emphasized an increasing endemism, climaxing during the Ludlow and Pridoli epochs. Provinciality was particularly marked during the Mid Devonian coincident with peak diversities in the phylum. Clear biogeographic patterns continued into the Carboniferous, but the Permian was characterized by higher degrees of provinciality probably associated with steep climatic gradients.

During the Triassic, brachiopod faunas, following an interval of cosmopolitan disaster taxa, became organized into Boreal (high-latitude) and Tethyan (low-latitude) realms (Box 12.5). This pattern continued throughout the Mesozoic, but with centers of endemism and occasional modifications due to ecological factors such as the circulation of ocean currents and the local development of chemosynthetic environments. Biogeographic patterns among living forms reflect their Cenozoic roots: a southern area, the northern Pacific, and a northern area (Atlantic, Mediterranean, North Sea and the circumpolar northern oceans) are based on a variety of articulated brachiopod associations. The linguliformeans have more widespread, near-cosmopolitan distributions.

0 0

Post a comment