Morphometric variation in the hominoid orbital aperture: a case study with implications for the use of variable characters in Miocene catarrhine systematics.

Variable characters are ubiquitous in hominoid systematics and present a number of unique problems for phylogenetic analyses that include extinct taxa. As yet, however, few studies have quantified ranges of variation in complex morphometric characters within extant taxa and then used those data to assess the consistency with which discrete character states can be applied to poorly represented fossil species. In this study, ranges of intrageneric morphometric variation in the shape of the hominoid orbital aperture are estimated using exact randomization of average pairwise taxonomic distances (ATDs) derived from size-adjusted centroid, height-width, and elliptic Fourier (EF) variables. Using both centroid and height-width variables, 19 of the 21 possible ATDs between individuals representing seven extinct catarrhine taxa (Aegyptopithecus, Afropithecus, Ankarapithecus, Ouranopithecus, Paranthropus, Sivapithecus and Turkanapithecus) can be observed within a single extant hominoid subspecies, although generally with low probabilities. A resampling study is employed as a means for gauging the effect that this intrataxonomic variation may have on the consistency with which discrete orbital shape character states can be delimited given the small sample sizes available for most Miocene catarrhine taxa preserving this feature (i.e., n=1). For each type of morphometric variable, 100 cluster (UPGMA) analyses of pairwise ATDs are performed in which a single individual is randomly selected from each hominoid genus and analyzed alongside known extinct taxa; consensus trees are computed in order to obtain the frequencies with which different shape clusters appeared in each of the three analyses. The two major clusters appearing most frequently in all three consensus trees are found in only 57% (centroid variables), 49% (height-width variables), and 36% (EF variables) of these trees. If ranges of variation within represented extinct taxa could also be estimated, these frequencies would certainly be far lower. Hominoids clearly exhibit considerable intrageneric, intraspecific, and even intrasubspecific variation in orbit shape, and substantial morphometric overlap exists between taxa; consequently, discrete character states delimiting these patterns of continuous variation are likely to be highly unreliable in phylogenetic analyses of living and extinct species, particularly as the number of terminal taxa increases. Morphological phylogenetic studies of extant catarrhines that assess the effect of different methods (e.g., use of objective a priori weighting or frequency coding of variable characters, inclusion vs. exclusion of variable characters, use of specific vs. supraspecific terminal taxa) on phylogenetic accuracy may help to improve the techniques that systematists employ to make phylogenetic inferences about extinct taxa.