Mechanical couplings in the shell closing mechanism of articulate brachiopods: implications for the evolution of skeleto-muscular architecture

Abstract

Rapid shell closure in articulate brachiopods occurs by a twitch contraction of the “quick” adductor muscles. Rapid accelerations and decelerations of the valves induce large accelerations of the surrounding fluid, placing constraints on the speeds of closure; inertial forces govern the mechanics of the closing mechanism. The position, size, and physiological properties of the muscles, and the size and shape of the shell, govern the properties of the shell closing system. A numerical model predicts that, for given muscle and shell characteristics, there is a location of the adductor muscle which maximizes the shell's closing velocity, and that this position shifts allometrically with size. Positive allometry in the muscle's moment arm length is observed in both living and fossil brachiopods, although the observed coefficients are less than those predicted by the model.The brachiopod closing mechanism provides a model for understanding the biomechanical interrelationships between components of dynamic skeletal systems. Dynamic systems, where the muscles induce resisting inertial reactions, are characterized by mechanical feedback loops, or couplings. Mechanical couplings may represent an important class of constraints on the evolution of skeleto-muscular architecture.