Phylogenetic and functional signal in prezygapophyseal articular facet shape of the first post‐transitional vertebra in anthropoids

Objectives Previous studies of the prezygapophyseal articular facet (PAF) of the thoracolumbar vertebrae in primates have suggested that the morphology of this feature reflects relative mobility of the lower back, and therefore corresponds to locomotor behavior. Specifically, these studies suggest that the PAF morphology found in cercopithecoids reflects greater mobility of the lower back compared to a stiffer lower back adapted for forelimb-dominated suspensory behaviors in hominoids. In this study, we sought to re-examine this question in terms of both locomotor behavior and phylogenetic signal in a broad sample of anthropoid taxa. Materials and Methods The study sample consisted of 291 first post-transitional vertebrae of wild-caught individuals representing 27 extant anthropoid species (16 genera). Vertebrae were 3D scanned, and 19 landmarks were digitized. PAF shape was tested for the presence of a phylogenetic signal using the multivariate version of the K-statistic (K-mult), and a chronometric consensus phylogenetic tree was mapped onto the major axes of shape space using species means to produce a phylomorphospace. Results Results showed that phylogenetic signal is statistically significant in PAF shape (K-mult = 0.3; p < 0.0001), and phylogenetic separation is apparent in the phylomorphospace, with some exceptions. However, certain aspects of PAF shape also appear to be associated with locomotor behavior within major taxonomic groups, such as hominoids and platyrrhines. Discussion Our results suggest that both phylogenetic relatedness and function may contribute to PAF shape variation in anthropoids.