Enamel thickness in the stem hylobatid Kapi ramnagarensis (Primates, Hylobatidae)

Enamel thickness has been extensively investigated in extant and fossil hominids, yet comparable data for hylobatids remain scarce. Existing studies are limited to Symphalangus syndactylus , Hylobates lar , and Hylobates muelleri and are based on relatively small samples. These studies have characterized enamel thickness as thin in Symphalangus and Hy. lar and intermediate‐thin in Hy. muelleri . The polarity of these conditions within Hylobatidae also remains unresolved because stem representatives of the clade have only recently been discovered. Here, we used microcomputed tomography to examine lower third molar (M 3 ) enamel thickness in the stem hylobatid Kapi ramnagarensis from the Miocene Lower Siwaliks locality of Ramnagar. We also provide new comparative data from 19 M 3 s of extant hylobatids representing all four living genera. Average (AET) and relative (RET) enamel thickness were quantified from two‐dimensional virtual sections passing through the dentin horns of the two mesial cusps. The M 3 of the single Kapi specimen exhibits enamel thickness values within the range observed in extant hylobatids, albeit near the lower end of that distribution, with RET most closely matching that of Hoolock leuconedys . Parsimony‐based ancestral state reconstructions of M 3 RET using gap‐weighted coding hypothesize an intermediate‐thin enamel condition at the base of Hominoidea. Within Hylobatidae, the ancestral state was likewise reconstructed as intermediate‐thin enamel. Although RET provides valuable phylogenetic information, its interpretation remains challenging due to the limited data for early catarrhines and hominoids and the pervasive influence of homoplasy. Larger samples will help clarify patterns of enamel thickness variation within hylobatids and across Hominoidea.