The Taxonomic and Phylogenetic Affinities of Bunopithecus sericus, a Fossil Hylobatid from the Pleistocene of China

Abstract

Fossil hylobatids are rare, but are known from late Miocene and Pleistocene sites throughout East Asia. The best-known fossil hylobatid from the Pleistocene of China is a left mandibular fragment with M2-3 (AMNH 18534), recovered from a pit deposit near the village of Yanjinggou in Wanzhou District, Chongqing Province. Matthew and Granger described this specimen in 1923 as a new genus and species, Bunopithecus sericus. Establishing the age of Bunopithecus has proved difficult because the Yanjinggou collection represents a mixed fauna of different ages, but it likely comes from early or middle Pleistocene deposits. Although the Bunopithecus specimen has featured prominently in discussions of hylobatid evolution and nomenclature, its systematic status has never been satisfactorily resolved. The present study reexamines the taxonomic and phylogenetic relationships of Bunopithecus by carrying out a detailed comparative morphometric study of its lower molars in relation to a large sample of modern hylobatids. Our results show that differences in M2 and M3 discriminate extant hylobatids fairly well, at least at the generic level, and that AMNH 18534 is not attributable to Hylobates, Nomascus or Symphalangus. Support for a close relationship between Bunopithecus and Hoolock is more equivocal. In most multivariate analyses, Bunopithecus presents a unique morphological pattern that falls outside the range of variation of any hylobatid taxon, although its distance from the cluster represented by extant hoolocks is relatively small. Our results support the generic distinction of Bunopithecus, which most likely represents an extinct crown hylobatid, and one that may possibly represent the sister taxon to Hoolock.

Fig 1. Photograph of the Bunopithecus sericus specimen (AMNH 18534) represented by a left mandibular fragment with M_2-3.

A) Lateral (buccal) view. B) Medial (lingual) view. C) Occlusal view (lingual to the right).

Fig 2. Hylobatid left lower molar showing dental variables taken.

A) Linear dimensions: 1) mesiodistal length, 2) buccolingual width at mesial cusps, and 3) buccolingual width at distal cusps. B) Angles: 1) position of mesiobuccal cusp, 2) position of mesiolingual cusp, and 3) position of hypoconulid. C) Absolute cusp areas: 1) protoconid, 2) metaconid, 3) entoconid, 4) hypoconid, and 5) hypoconulid.

Fig 3. Plot of the first two discriminant functions (DF1 and DF2) of the M₂ analysis using linear measurements, cusp angles and absolute areas.

Eigenvalues: 4.19 (DF1) and 0.52 (DF2); variance: 84.44% (DF1) and 10.41% (DF2).

Fig 4. Plot of the first two discriminant functions (DF1 and DF2) of the M₃ analysis using linear measurements, cusp angles and absolute areas.

Eigenvalues: 40.22 (DF1) and 0.31 (DF2); variance: 87.07% (DF1) and 6.80% (DF2).

Fig 5. Plot of the first two discriminant functions (DF1 and DF2) in the analysis of size variables, M₂ and M₃ combined.

Eigenvalues: 4.79 (DF1) and 0.73 (DF2); variance: 74.32% (DF1) and 11.41% (DF2).

Fig 6. Plot of the first two discriminant functions (DF1 and DF2) in the analysis of shape variables, M₂ and M₃ combined.

Eigenvalues: 0.95 (DF1) and 0.62 (DF2); variance: 42.88% (DF1) and 27.96% (DF2).

Fig 7. Map of East and Southeast Asia showing the historical and present distribution of gibbons (Hoolock, Hylobates and Nomascus).

The black star indicates the location of the village of Yanjinggou (Wanzhou District, Chongqing Province, China), where Bunopithecus sericus was found. Adapted from Gu [7], Gao et al. [30] and Geissmann [80].