The Role of Morphometrics and Cladistics in the Taxonomy of Fossils: A Paleornithological Example

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Abstract
Fossil species are often described on the basis of differences in linear dimensions among specimens. In addition, vertebrate paleontologists frequently refer individual skeletal elements to species described from different elements, based on some estimate of the relative proportions of these elements. Frequently, the variance of a particular dimension or the relative proportions among dimensions in fossil species are based on ranges and correlation structures in Recent taxa closely related to the fossil taxa. To describe a species according to size (i.e., linear distance between two landmarks), or to refer specimens to a species based on relative proportions of two or more dimensions, not only assumes that there is a historical component to morphometric data but also that population parameters in Recent species are applicable to the fossil species. In this paper, the assumption that linear dimensions of organisms reflect the evolutionary history of the species of which these organisms are a part is examined. Only those linear dimensions that reflect this history (as opposed to apomorphic dimensions reflecting drift or an adaptation) can be used in a phylogenetically based taxonomy of fossils. Methods for estimating linear dimensions and their variances in hypothesized common ancestors of Recent taxa and fossil species are described, and the variance of a dimension in a fossil species is estimated using parsimony procedures. However, calculating a size range for a fossil species is problematic because there is no theoretical basis by which the mean or any other measure of central tendency for that species can be estimated. Two methods for estimating the range for a particular dimension in a fossil species using reconstructed ancestral variances and the actual measurements of individual fossil specimens are proposed. Estimates of a linear dimension in a fossil species are made using existing phylogenetic hypotheses and the results from analyses of covariance and linear regression models. These methods and the assumptions used in the taxonomy of fossils are discussed using the morphometry and phylogenetic history of Recent and fossil Sulidae (Aves: Pelecaniformes).

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