Maxilla of Siamopithecus eocaenus (Anthropoidea, Primates) from the Paleogene of Krabi, Thailand, and its taxonomic status

Detailed descriptions of the maxillae of Siamopithecus eocaenus, discovered from the latest Eocene/earliest Oligocene lignite mine in the Krabi basin of Peninsular Thailand, are presented. They include the morphology of P³–M³, the palate, a partial orbital region, and the zygomatic root. The specimen exhibits distinctive dental features including a single-rooted P² alveolus, a protocone on the P³ and P⁴, and a true hypocone on the upper molars, indicating its derived anthropoid dentition. Comparative studies and virtual reconstructions of the facial anatomy reveal close affinities with diurnal anthropoids rather than the notharctid strepsirrhines. The reconstructed facial morphology of Siamopithecus displays a short and subvertically oriented face, significant orbital convergence (72.1°), and frontation (81.6°), distinguishing it from both fossil and extant strepsirrhines. Moreover, the presence of a thin bony lamina extending from the distal part of the upper preserved area of the zygomatic suggests partial or complete postorbital closure. Phylogenetic analyses suggest an affiliation with amphipithecids, but recent morphological observations challenge this, leading to the proposal of an elevated family-group ranking, Siamopithecidae. Comparison with the most primitive known Afro-Arabian propliopithecid, the Taqah propliopithecid from Oman, reveals similarities in their molar structure but differences in dental formula (retention of P2) and premolar structure. The abrupt appearance of propliopithecids in the early Oligocene of Afro-Arabia, without a local ancestor, contributes to the debate on whether catarrhine origins were in Asia or Africa. However, alternative views, based on sister-group relationships with oligopithecids, support an African origin of propliopithecids from an undocumented Afro-Arabian region. This research provides new insights into the evolutionary history of early anthropoids, suggesting a complex biogeographical scenario involving both Asian and African lineages.