Article: Species discrimination and evolutionary mode of Buchia (Bivalvia: Buchiidae) from Upper Jurassic–Lower Cretaceous strata of Grassy Island, British Columbia, Canada
Melissa Grey, James W. Haggart and Paul L. Smith
Buchiid bivalves are geographically widespread in Upper Jurassic and Lower Cretaceous strata of the Northern Hemisphere. They are often abundant and their short stratigraphic ranges make them ideal biostratigraphic index fossils; these characteristics also render them useful for study of evolutionary patterns. We used multivariate methods to determine if we could discriminate between species of Buchia and examine how morphological characters change through time within the genus. Using ten morphological characters to describe shell shape and size, we tested for taxonomic differences and morphologic change in populations of buchiids collected from a single stratigraphic section on Grassy Island, located along the west coast of Vancouver Island, British Columbia. Morphometric analysis utilized traditional morphological metrics and techniques, including linear and angular measurements as well as Fourier (outline shape) analyses. Phenetic discrimination revealed considerable overlap among the recognized species in the morphospace, as well as a fairly low discriminatory power between species when compared as a group using a step-wise canonical variate analysis. Step-wise discriminant analyses between species pairs gave rise to much higher classification rates, suggesting that different characters are important for distinguishing between different species pairs. Our results also indicate that single individuals and small sample sizes of Buchia specimens are insufficient for biostratigraphic discrimination (unless other rarely preserved features such as the hinge and bysuss ear are available) and that a number of previously described species variants may not be taxonomically valid. A biolog using the multivariate axis that best discriminates between species (CV1) and a random walk-based test using a Hurst estimate analysis indicate a gradualistic evolutionary mode for the Buchia species of Grassy Island. Shell shape and size of buchiids do not appear to be closely tied to lithofacies changes over the c. 10 myr time interval, suggesting that ecophenotypic variation (as it relates to substrate changes) probably had minimal influence on morphology.