MORPHOLOGICAL EVOLUTION OF THE SCAPULA IN TREE SQUIRRELS, CHIPMUNKS, AND GROUND-SQUIRRELS (SCIURIDAE) - AN ANALYSIS USING THIN-PLATE SPLINES

The mammalian scapula, like many bones, is a single structural element that serves as an attachment site for several muscles. The goal of th is study was to determine whether the scapula evolves as an integrated unit, or as a collection of distinct parts. Shape differences among t he scapulae of tree squirrels, chipmunks, and ground squirrels were de scribed using thin-plate spline analysis. This technique produces a ge ometric description of shape differences that can be decomposed into a series of components ranging in scale from features that span the ent ire form to features that are highly localized. Shape differences amon g tree squirrel scapulae were found only in large-scale features, indi cating spatially integrated shape change. Chipmunks and ground squirre ls differ from tree squirrels in several features, but shared differen ces reflecting divergence of their common ancestor were found only in the small-scale features. Divergence of ground squirrels from the comm on ancestor involved some large-scale changes but was dominated by sma ll-scale changes. Divergence of chipmunks was dominated by large-scale changes. Thus, the scapula evolved as an integrated unit during some transitions but as a collection of distinct parts during others. These results suggest that evolutionary patterns of the postcranial skeleto n may be as complex as the patterns that have been described for skull s and feeding mechanisms.