WESTNEAT, M. W.; Field Museum of Natural History: Biomechanical Models of Feeding in Fishes: Testing for Convergence in Functional Design

Fishes use a sensational diversity of jaw mechanisms to capture and ingest their food. One of the hallmarks of this diversity is the complexity of the kinetic fish skull, which can have more than 20 mobile skeletal elements driven by numerous muscles. This complexity has often led to compelling examples of both divergence in form and function as well as convergence in design and behavior. How do we test for functional convergence in biomechanics? How do we define functional characters and determine when they are convergent or redundant? This study expands previous lever and linkage modeling and outlines more comprehensive models for analysis of fish feeding. Software for analysis of cranial levers and linkages allows for simulation of structure-function relationships in a wide range of taxa. Simulations show that structural changes in four-bar linkages map to unique sets of primary functional variables (vector direction and magnitude of motion) with little or no redundancy. A survey of skull mechanical designs throughout the phylogeny of fishes reveals that evolutionary changes in feeding mechanisms of fishes are accompanied by a wide range of strategies for transferring force and motion. Examples are described in which jaw functions have diverged due to linkage changes, and in which other morphology is modified but linkages remain static. A protocol for defining convergent and divergent functional characters is proposed. Linkage modeling leads to several conclusions regarding the use of lever and linkage theory: (1) Coordinate based shape analyses can yield functional insight using mechanically relevant landmarks; (2) The geometry and physiology of muscles are critical to accurate estimations of lever and linkage function; (3) Multiple mechanical variables and levels of design should be considered when defining convergent or redundant biomechanical systems.