Morphological variability in time and space: an example of patterns within buchiid bivalves (Bivalvia,Buchiidae)

Abstract: We studied morphological variation of the bivalve Buchia over its geographical and temporal range. Buchia was widely distributed during the Late Jurassic and Early Cretaceous, and, while previous quantitative studies have shown that species are characterized by large amounts of variation, there have been no prior attempts to measure how morphology varies geographically. We employed traditional morphometric techniques using nine linear/angular measurements on 1855 buchiid shells from eight localities taken from widely separated, but mostly coeval, sections across its range. Principal component and canonical variate analyses indicate that geographical morphospace of buchiids varied significantly, but we did not find evidence of a latitudinal gradient in shell shape. The amount of variation between localities was similar to the amount of variation between species, indicating the importance of geographical effects on morphology. Disparity (morphological diversity within a taxon, calculated by the sum of variances) and diversity (number of species) were calculated for each location and time period (age). Disparity and diversity reached ultimate lows just before the genus’ extinction in the Hauterivian, and is suggestive that extinction was morphologically selective. We did not find significant trends for either metric, but there were discordances throughout its temporal range. Latitudinal trends of disparity and diversity within Buchia are not apparent. This research adds to the growing body of work on geographical variation and is a preliminary step to understanding the nature and variation of buchiid species and of biodiversity in general.