Materials and Method

Abstract

Geometric morphometrics is the analysis of geometric landmark coordinates points on specific parts of an organism (Bookstein 1991). Morphometric analyses were based on the use of landmarks to capture shape; landmarks are points representing the same location on each specimen, and can be assigned to three general categories (Bookstein 1991): Type I—discrete juxtapositions (e.g., meeting of three structures), Type II—functional equivalents (e.g., tips of extrusions and maximum curvatures) and Type III—extremal points (e.g., tip of beak). In this study, type I and II landmarks were used, as they are most likely to capture biologically meaningful shape change (Fig. 2.1). When selecting landmarks for analyses, we selected points that characterized not only body shape accurately, but also represent some aspect of the inferred ecological niche. While landmarks were not biologically homologous, they represent discrete points that correspond among forms (sensu Bookstein 1991), which is appropriate for analyses attempting to capture shape changes or function (as opposed to describing phylogenetic relationships). For all individuals, measurements were taken on the entire specimen (brachial and pedicle valve. Once coordinates were obtained, geometric morphometrics were applied to landmarks to convert them into Bookstein shape coordinates (see Bookstein 1991 for a full description of Bookstein shape coordinate equations and methods), thus rotating, translating and scaling all landmarks, while maintaining their geometric relationships; these scaled coordinates were used in all analyses.