Article: The physiological differences between articulate brachiopods and filter-feeding bivalves as a factor in the evolution of marine level-bottom communities

Relevant physiological differences are discussed to determine their potential influence on the change in dominance within marine level-bottom communities from articulate brachiopods in the Palaeozoic to filter-feeding bivalves in the Mesozoic and Cenozoic. It is shown that (1) the two groups of animals have overlapped greatly in requirements for food and space during the Phanerozoic, (2) the bivalves have considerably more energy per unit biomass to spend on the production of offspring, (3) filter-feeding bivalves are potentially able to exploit a larger number of habits and to cope more effectively with most environmental factors than are the articulates, and (4) that these bivalves have considerably greater abilities to colonize and to expand their distribution than have the articulates. Empirical evidence is cited for the importance of competition in Recent marine level-bottom communities in general, and amongst Recent filter-feeding bivalves in these communities in particular. Although competition may occur only occasionally, it seems to be important in shaping long-term structures of level-bottom communities. Competition on the level bottom appears to have been considerably intensified and its effects greatly enhanced at certain times during the Phanerozoic. It is suggested that partitioning of space contributed greatly to the ability of articulate brachiopods and filter-feeding bivalves to share the near-shore region during the Palaeozoic. The fact that filter-feeding bivalves became established close to shore early in the Palaeozoic is attributed to frequent unpredictable physical disruptions of shallow-water communities. A lack of severe physical disturbances offshore probably enabled community structures to be maintained there for considerable periods of geological time; when severe physical changes disrupted these structures at the end of the Permian, bivalves invaded and replaced the articulates as the dominant off-shore invertebrates. The decline of articulate brachiopods and increase in importance of filter-feeding bivalves occurred in a series of steps. It is suggested that because of physiological differences between the two groups, filter-feeding bivalves suffered less than articulates at both the Permian-Triassic and Triassic-Jurassic extinctions; that after each extinction filter-feeding bivalves were able to invade numerous vacant or partially vacant habitats earlier and faster than the articulates; and that competition, particularly from the established bivalves, then prevented articulates from reoccupying many of the habitats that they had previously held. The Cenozoic decline of articulate brachiopods resulted from a loss of their preferred habitats (Ager in litt. 1977).