THE ROLE OF CONSTRAINTS AND CHARACTER INTERDEPENDENCE IN FUNCTIONAL MORPHOLOGY WITH AN EXAMPLE FROM PRODUCTIDE MUSCLE SYSTEMS

Organismal morphology is a function of mechanical, constructional, developmental and phylogenetic constraints, as well as adaptation. Because a brachiopod must open its valves to feed and respire, the arrangement of the diductor muscle system is expected to influence, and perhaps constrain, the evolution of brachiopod shape. Carlson (1989, 1992) demonstrated that strophomenate brachiopods manifested two functional trends of the diductor system through time: (1) increasing maximum gape of the valves; and (2) increasing opening moment about the hinge axis. Upper Paleozoic strophomenates were capable of generating significantly larger opening moments than were Lower Paleozoic strophomenates.

Opening moment is the product of length of the lever arm (LA) and the cross-sectional area of the muscle field (MF). Thus, opening moment can be increased by increasing either LA or MF. However, both the LA and the MF of primitive strophomenates were limited by spatial and functional constraints. The evolution of strophomenate shell and cardinal process morphology can be viewed as pathways to overcoming these constraints, and there are multiple "solutions" to this functional problem.

We explore two such solutions, both involving elongation of the cardinal process shaft, within the Aulostegidae and Echinoconchidae, two derived productide families. Although similar in function and appearance, these solutions can be shown to be convergent, using phylogenetic and geometric morphometric analyses. We then demonstrate a quantitative approach for testing character dependence, which reveals that the following features covary significantly: (1) angle of the cardinal process shaft; (2) shape of the umbo; (3) presence/absence of the pedicle valve interarea; and (4) the position of the diductor attachments on the pedicle valve. Examination of character covariance has great potential for providing insight into constraints on functional systems. Moreover, such analyses are also important for phylogenetic reconstruction, as a suite of functionally and constructionally related characters perhaps should be treated as a single character.