Evolution by valves: morphological constraints, environmental change, and the replacement of brachiopods by bivalves

Brachiopoda and Bivalvia evolved the bivalved condition independently during the early Cambrian, but they have contrasting histories, with bivalves largely replacing brachiopods in warm, shallow, post-Paleozoic seas. Overlapping hypotheses have been proposed to explain this replacement, but it remains unclear how inherent constraints and evolutionary potentials of the bivalved form in the two groups interacted with environmental changes that favoured bivalves. Here I document the origins, fates, limitations, and versatility of 55 morphological and habitat traits, of which 13 are unique to brachiopods, 23 are unique to bivalves, and 19 evolved convergently in both. Only 6.3% of brachiopod traits are post-Paleozoic as compared with 24% of bivalve traits. Extinction affected 44% of brachiopod traits but only 2% of bivalve traits. Characteristics associated with post-Paleozoic bivalve proliferation evolved much earlier, including the early Cambrian origin of the extensible foot, hinge ligament, and morphological within-valve modularity, none of which evolved in brachiopods and all of which provided early versatility in bivalves. Constraints on form were already present in early Cambrian brachiopods and were never overcome. The full potential of bivalves was realized only when more intense biological activity made Mesozoic and Cenozoic seas more productive.