Disentangling isolated dental remains of Asian Pleistocene hominins and pongines

Abstract

Scholars have debated the taxonomic identity of isolated primate teeth from the Asian Pleistocene for over a century, which is complicated by morphological and metric convergence between orangutan (Pongo) and hominin (Homo) molariform teeth. Like Homo erectus, Pongo once showed considerable dental variation and a wide distribution throughout mainland and insular Asia. In order to clarify the utility of isolated dental remains to document the presence of hominins during Asian prehistory, we examined enamel thickness, enamel-dentine junction shape, and crown development in 33 molars from G. H. R. von Koenigswald's Chinese Apothecary collection (11 Sinanthropus officinalis [= Homo erectus], 21 “Hemanthropus peii,” and 1 “Hemanthropus peii” or Pongo) and 7 molars from Sangiran dome (either Homo erectus or Pongo). All fossil teeth were imaged with non-destructive conventional and/or synchrotron micro-computed tomography. These were compared to H. erectus teeth from Zhoukoudian, Sangiran and Trinil, and a large comparative sample of fossil Pongo, recent Pongo, and recent human teeth. We find that Homo and Pongo molars overlap substantially in relative enamel thickness; molar enamel-dentine junction shape is more distinctive, with Pongo showing relatively shorter dentine horns and wider crowns than Homo. Long-period line periodicity values are significantly greater in Pongo than in H. erectus, leading to longer crown formation times in the former. Most of the sample originally assigned to S. officinalis and H. erectus shows greater affinity to Pongo than to the hominin comparative sample. Moreover, enamel thickness, enamel-dentine junction shape, and a long-period line periodicity value in the “Hemanthropus peii” sample are indistinguishable from fossil Pongo. These results underscore the need for additional recovery and study of associated dentitions prior to erecting new taxa from isolated teeth.

A distinguishing feature of Pongo upper molars and its implications for the taxonomic identification of isolated hominid teeth from the Pleistocene of Asia

“…Previous studies of tooth microstructure and dental tissue proportions have identified additional dental features that differ between pongines and hominins, such as the equal reduction of enamel and dentine in Pongo , but preferential loss of dentine in Homo during the process of tooth size reduction in these two groups (Grine & Franzen, ; Smith et al, ; with the exception of Neanderthals, see Olejniczak, Smith, Feeney, et al, ). Pongo molars also differ from those of humans in having lower dentine horns and an overall different shape of the EDJ with broader crowns, broad, and shallow dentinal intercuspal furrow pattern and less medially placed lingual dentine tips (Olejniczak, Martin, & Ulhaas, ; Olejniczak, Smith, Wang, et al, ; Smith et al, ; Zanolli et al, ). In addition, when only fossil individuals were considered, Smith et al () found that enamel has a more uniform distribution across the enamel cap in fossil Pongo than in H. erectus , although relative enamel thickness values overlap substantially in the two taxa (Smith et al, ).…”

“…In addition to specimens of known provenance discussed above, there are several isolated teeth from the von Koenigswald's Chinese Apothecary collection whose affinities have yielded varied opinions, but it appears that at least three hominoid groups are represented, including “ Hemanthropus peii ,” Sinanthropus officinal is [= H. erectus ], and Pongo sp. (reviewed in Smith et al, ).…”

“…(e.g., Tyler, , ). In particular, the taxonomic affinities of the following specimens from Southeast Asia have been the subject of intense debate among scholars: Trinil 11620 and Trinil 11621 from Trinil, Indonesia, and SMF‐8855, SMF‐8864, SMF‐8865, SMF‐8879, SMF‐8898, SMF‐100055, S5, S6a, S7‐9, S7‐20, S7‐53, S7‐62, S7‐64, S7‐65, and S7‐76 from Sangiran, Indonesia (reviewed in Smith et al, ; Zanolli et al, ).…”

“…Both pongines and hominins have relatively thick‐enameled crenulated molars with a low‐occlusal topography (Figure S1). Moreover, molars of these taxa overlap in crown formation times and daily secretion rates (Grine & Franzen, ; Martin, ; Olejniczak, Smith, Feeney, et al, ; Smith, Olejniczak, Reid, Ferrell, & Hublin, , Smith et al, ; Smith, ; but see Smith et al, for a recent comprehensive study on crown formation times in Pongo and Homo ). It has also been suggested that the occlusal outline of Pongo upper molars is more oval than in African great apes and more similar in this regard to that of Homo (Swindler & Olshan, ).…”

“…Specifically, we quantified patterns of intragroup and intergroup variation of these features in a large sample of upper molars encompassing a wide range of fossil and extant hominid taxa and examined how well they differentiate isolated teeth of Pongo from those of Homo and other hominids. To overcome the limitations introduced by dental wear, which can obliterate informative features at the level of the outer enamel surface (OES), data collection, and analyses were performed at the enamel‐dentine junction (EDJ; Olejniczak, Smith, Wang, et al, ; Ortiz, Bailey, Hublin, & Skinner, ; Skinner et al, ; Smith et al, ). The EDJ is the interface between the enamel cap and dentine crown and preserves the end point of growth of the inner enamel epithelium (Butler, ; Schour & Massler, ).…”