Reconstructing Anthropoid Facial Skeletal Evolution, and Predicting the Facial Skeletal Morphology of the Last Common Ancestors of Apes and Humans
Open AccessThree-dimensional geometric morphometric (3DGM) methods are accessible and efficient tools for biologists to characterize and analyze variation in complex shape data. Increasingly, novel evolutionary modeling approaches are used (often with 3DGM data) to study evolutionary mode and tempo, predict ancestral morphologies, and test hypotheses for the pathways to extant and fossil anatomical trends. This study evaluates how critical decisions made along different workflow steps affect the results of analyses downstream. The hominoid facial skeleton offers an opportunity to test new methods for achieving this understanding. Long-standing gaps within this clade, and the uncertainty of phylogeny for older fossils within it, elicit new and improved methods for creating and testing evolutionary hypotheses. To this end, we applied evolutionary models to 3DGM data for the facial anatomy of a primate cranial sample representing extant anthropoid primate variation. Ancestral state reconstructions (ASR) of morphometric data for key clades varied significantly in their Procrustes distances from descendant taxa when different Brownian motion models for evolution were employed; however, this disparity was only revealed under one of the two semilandmark sliding methods applied to the 3DGM data. In fact, semilandmark sliding method was the only methodological factor to consistently and significantly affect Procrustes distances from hypothetical last common ancestors to respective descendant taxa. The ancestral morphologies reconstructed in this study are by no means considered reflective of the actual ancestral states for the primate clades investigated, as the analysis neglects important fossil data. Rather these results highlight the need to carefully consider different methodological approaches. This may prove especially important when using ASR to ascribe phylogenetic and phenetic affinities to fossils, such as the Miocene apes whose positions among hominoids remain questions of intense debate.
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