Taxonomic Assessment of the Trinil Molars Using Non-Destructive 3D Structural and Development Analysis.

Abstract

Two molars recovered at Trinil, Java, have been the subject of more than a century of debate since their discovery by Eugène Dubois in 1891–92. These molars have been attributed to several ape and human taxa (including Panand Meganthropus), although most studies agree that they are either fossil Pongo or Homo erectus molars. Compli-cating the assessment of these molars is the metric and morphological similarity of Pongo and Homo erectus mo-lars, and uncertainty regarding their serial positions within the maxillary row. Here we applied non-destructive conventional and synchrotron microtomographic imaging to measure the structure of these molars and aspects of their development. Comparisons were made with modern Homo and Pongo maxillary molars, as well as small samples of fossil Pongo and Homo erectus molars. Root spread was calculated from three-dimensional surface mod-els, and enamel thickness and enamel-dentine junction morphology were assessed from virtual planes of section. Developmental features were investigated using phase contrast X-ray synchrotron imaging. The highly splayed root morphology of the Trinil maxillary molars suggests that they are not third or fourth mo-lars. Trinil molar enamel thickness is most similar to Homo sapiens and Homo erectus first molar mean values, and is thicker than most modern Pongo molars. The shapes of their enamel-dentine junctions are outside the Pongorange of variation, and within the range of variation in Homo. moreover, the internal long-period line periodicity of these two teeth is most similar to fossil and extant hominins, and is outside of the known range of fossil and living Pongo. Taken together, these results strongly suggest that the two molars are in fact Homo erectus teeth, and Dubois’ original attribution to “Pithecanthropus erectus” (a junior synonym of Homo erectus) is correct.