MORPHOLOGIC VARIATION WITHIN THE BRACHIOPOD SPECIES LAQUEUS CALIFORNIANUS ALONG THE WEST COAST OF NORTH AMERICA

The range of morphological variability within and among populations and species of living brachiopods is surprisingly poorly known. In order to evaluate the boundaries of morphological species in the fossil record, our knowledge of recent variation must be improved. We used Elliptical Fourier Analysis (EFA) to study intraspecific variation among dorsal valve commissural outlines of Laqueus californianus specimens collected from along the west coast of North America. Latitudinal localities are divided into northern and southern populations with the southern populations (216 individuals) existing along the coast of California and the northern populations (51 individuals) residing from the Washington coast to the Aleutian Islands in Alaska. EFA uses dorsal valve outline data points to find harmonic coefficients which complete a series of sine and cosine curves which, when summed together, can reproduce the original outline, simplifying the outline into fewer shape variables. Prior to analysis, all shell outlines were translated to be invariant to starting position, location, rotation and size to remove variations due to outline alignment error. Analyses were performed with and without size invariance to allow separate comparisons of size and shape variations between populations.

Principal Component Analysis (PCA) of the EFA harmonic coefficients reveals that the northern and southern populations can be clearly differentiated in morphometric shape space. Moreover, size and shape variation along the PC axes reveal that while northern individuals are smaller in size, they still contain similar valve shapes as the southern populations. This suggests that growth rates and growth directions in the northern and southern populations of L. californianus vary considerably along a latitudinal gradient. With age and longevity information, determined from oxygen isotope analysis, these differences in size and shape have the potential to suggest intriguing hypotheses about the role of heterochrony in the evolution of this lineage.