Body size evolution of the Middle Jurassic-Early Cretaceous bivalves of Kutch, India

An organism's body size is interlinked with several ecological and physiological parameters and, therefore, has been widely used to detect and describe long-term macroevolutionary trends. One of those long-term trends is Cope's rule, a tendency to increase size over time. In the present study, we document long-term evolutionary trends in the body size of the Middle Jurassic - Early Cretaceous bivalves from the Kutch basin, India. It appears that the body size of the Kutch bivalves did not follow any gradual increasing trend or Cope's rule. Our data suggest a fluctuating pattern of change in bivalve body size from the Bathonian to the Aptian. The repeated transgressive-regressive cycles in the Mesozoic may have resulted in temporally volatile adaptive landscapes and adaptive optima, which may explain the observed patterns of change in bivalve body size. This oscillating pattern in body size is evident at higher (class and order) and lower taxonomic levels (family and genus). The most significant increase in bivalve size occurred across the Jurassic - Cretaceous boundary. We argue that a high abundance of certain groups, ecological interactions, and taphonomy may explain this significant increase in bivalve body size.