Recognition of fossil fresh water ostracodes: Fourier shape analysis

Abstract

Candonids are a group of unornamented ostracodes with smooth outlines. Taxonomic distinctions between groups are based on differences in appendages. Extending these taxonomic criteria to fossil ostracodes is a challenge because fossils lack soft tissue and distinct homologous hard parts. Shape of carapace, therefore, may be the primary taxonomic character for identification of candonid specimens. To test this hypothesis, Fourier analysis in closed form was used to measure the components of shape present in the two-dimensional outline of fossil specimens. When used in discriminant analysis, these components of shape proved to be statistically robust on three closely related species of Candona. Ninety-two percent (92%) of the cases examined were recognized successfully. This is true for all combinations of between species discriminations (e.g. if males or females of one species are compared with sexual dimorphs of another; or, if left and right valves of different species are compared), and for combinations of intraspecific sexual dimorphs. Based upon these results, shape descriptors provided by Fourier series are sufficient to effect correct taxonomic identification without dependence on homologous landmarks. Right-left valve distinctions within species can be identified, but the accuracy of these identifications ranges from 75% to 95%. The difficulty in accurate discrimination of right and left valves using this automated technique is peculiar because of the ease with which a trained worker can distinguish valves. However, results presented underline the observation that morphotypic variability that resides in valve shape may contain more complex information than has been recognized previously.