METABOLIC RISE OF BIVALVES INDEPENDENT OF BRACHIOPOD DIVERSITY DECLINE

Bivalves replaced brachiopods as the most diverse and abundant benthic marine animals in the fossil record 250 million years ago. However, the extent to which bivalves succeeded at the expense of brachiopods remains poorly understood. Here we use newly compiled body size data for 3981 fossil genera of bivalves and brachiopods to approach this long-standing question in terms of energy metabolism. We find that bivalves have accounted for a larger share of metabolic activity of the past 465 million years. Our data further indicate that the metabolic activity of bivalves has increased by more than two orders of magnitude over this interval whereas brachiopod metabolic activity has declined by more than 50%. Given the low metabolic rates of early Paleozoic brachiopods, nearly all of the subsequent metabolic increase in bivalves must have occurred via the acquisition of new resources rather than through the displacement of brachiopods. The canonical view of a mid-Phanerozoic transition from brachiopod to bivalve dominance is a product of historical focus on taxonomic diversity and numerical abundance as measures of biological success. From a metabolic perspective, the oceans have always belonged to the clams.