A Morphometric Analysis of Early Eocene Euprimate Calcanei from Gujarat, India

Abstract

Postcrania dating back to the early Eocene from the Cambay Shale Formation of Gujarat, India, provide a unique window for understanding early primate behavior. Previous metric analyses of postcrania assigned to Asiadapinae indicate that these primates were generalized arboreal quadrupeds. Examination of long bone cross‐sectional properties, however, complicates the initial behavioral interpretations because it shows that these fossils overlap with primates of differing locomotor behaviors. The purpose of this study is to re‐examine calcaneal morphology of the Indian material in order to refine the locomotor reconstructions of these fossils.Surface models of fossil calcanei (n=6) were generated from micro‐computed tomography scans while those (n=73) representing 23 extant primate species were downloaded from MorphoSource. A weighted spherical harmonics analysis was carried out on the total sample. This morphological analysis represents genus‐zero surfaces using a common set of spherical harmonic functions. The harmonic coefficients associated with each calcaneus serve as shape descriptors that can be summarized via principal components (PC) analysis. Bivariate plots and models representing plus and minus two standard deviations of each PC were created to visualize morphological differences.The first three PCs explain 52.4%, 9.7%, and 9.3% of the morphological variation in the sample respectively (71.4% combined). The first PC differentiates primates with elongated calcanei that are more likely to engage in vertical clinging and leaping, from more quadrupedal primates. The second PC distinguishes calcanei of slow climbers from primates that engage in other forms of locomotion (e.g., leaping and quadrupedalism). Extant primate morphology seems to follow a continuum in which relative anteroposterior calcaneal elongation negatively correlates with slow climber calcaneal morphology. As expected, the calcaneal morphology of Marcgodinotius indicus aligns with that of extant lemuriforms, and most closely resembles that of a generalized arboreal quadruped with some leaping ability. There are two fossils tentatively attributed to Asiadapis cambayensis included in the analysis. One groups with other generalized arboreal quadrupeds, while the other falls outside the range of calcaneal variation observed in extant primates. The outlier does not follow the general pattern of calcaneal morphology observed in extant primates; instead it is characterized by some aspects of slow climber morphology, but with a relative anteroposterior calcaneal elongation. These results may reflect the generalized morphology of this fossil, and consequently the behavior of this primitive euprimate.Support or Funding InformationSupported by NSF DGE‐1746891, National Geographic Society, Leakey Foundation