Are Early Eocene Asiadapid Primates Primitive or Derived? Implications for Behavior and Morphology of the Ancestral Euprimate

The oldest known primates of modern aspect (euprimates) appear at the beginning of the Eocene across the Holarctic. At the time of their appearance they are already clearly divided into two distinct clades: Adapoidea (basal members of Strepsirrhini, lemurs, lorises, and bushbabies) and Omomyidae (basal Haplorhini, tarsiers, monkeys, and apes). However, postcranial remains from these earliest euprimates are rare and fragmentary, and most of our knowledge of postcranial adaptations within euprimate groups is based on younger, more derived fossils, which show postcranial specializations for leaping. This evidence has led researchers to suggest that leaping played a key role in the evolution of euprimates as a clade. Fossil adapoids (family Asiadapidae) and omomyids (the genus Vastanomys ) from the early Eocene of India are among the earliest fossil euprimates for which well‐preserved postcrania are known, making them ideal for addressing the question of the ancestral euprimate locomotor mode. These early euprimates are represented mostly by teeth and jaws that clearly place them within adapodiea or omomyidae, but the isolated postcranial elements are unique in showing little specialization for leaping. While it is generally accepted that the Indian primate postcrania lack derived traits seen in younger fossils, there is disagreement about whether the lack of specialization in asiadapids, specifically, reflects the ancestral euprimate condition or represents a reversal from a more specialized leaping ancestor. This talk reviews functional and comparative analyses of the Indian primate postcrania in a phylogenetic context to assess whether asiadapid morphology is likely due to plesiomorphy or homoplasy. The assertion that the non‐specialized morphology in asiadapids is due to homoplasy requires assumptions that are not well supported by the fossil record (that asiadapids evolved from a large‐bodied ancestor) and that reversals occur in body size and across the skeleton in both asiadapids and Vastanomys ; however, it is consistent with previous conceptions of the ancestral euprimate as a specialized leaper. If asiadapids are considered to represent an ancestral morphology, this would suggest that the ancestral euprimate was small, and that the early divergence of strepsirhines and haplorhines was driven by differences in hind‐limb behaviors, both of which are supported by our current knowledge of the fossil record. However, this scenario suggests that the ancestral euprimate was a generalized arboreal quadruped, rather than a specialized leaper, which necessitates an alternative explanation for the evolution of traits uniting euprimates. Support or Funding Information National Geographic Society, Leakey Foundation, Belgian Science Policy Office, Wadia Institute of Himalayan Geology