The Hadropithecus conundrum reconsidered, with implications for interpreting diet in fossil hominins

Abstract

The fossil 'monkey lemur' Hadropithecus stenognathus has long excited palaeontologists because its skull bears an astonishing resemblance to those of robust australopiths, an enigmatic side branch of the human family tree. Multiple lines of evidence point to the likelihood that these australopiths ate at least some 'hard', stress-limited food items, but conflicting data from H. stenognathus pose a conundrum. While its hominin-like craniofacial architecture is suggestive of an ability to generate high bite forces, details of its tooth structure suggest that it was not well equipped to withstand the forces imposed by cracking hard objects. Here, we use three-dimensional digital reconstructions and finite-element analysis to test the hard-object processing hypothesis. We show that Archaeolemur sp. cf. A. edwardsi, a longer-faced close relative of H. stenognathus that lacked hominin convergences, was probably capable of breaking apart large, stress-limited food items, while Hadropithecus was better suited to processing small, displacement-limited (tougher but more compliant) foods. Our suggestion that H. stenognathus was not a hard-object feeder has bearing on the interpretation of hominin cranial architecture; the features shared by H. stenognathus and robust australopiths do not necessarily reflect adaptations for hard-object processing.

Figure 1.

Dorsal views of the skulls of (a) H. stenognathus (Vienna cranium NHMW 1934 IV 1) and (b) Paranthropus boisei (KNM-ER 406). Both species have been interpreted as hard-object feeders, although contradictory signals exist for each [6,8,9,12,13].

Figure 2.

Estimated jaw positions at maximum gape and muscle attachment areas for (a) H. stenognathus and (b) A. sp. cf. A. edwardsi. (Additional details of muscle attachments are provided in the electronic supplementary material, figures S1 and S2.)

Figure 3.

The predicted distribution of von Mises stress in models of (a,b) A. sp. cf. A. edwardsi and (c,d) H. stenognathus during P4 biting at maximum gape. These models were scaled to predicted body size. Histograms illustrate the distribution of stress across the volume of each model. White areas indicate stresses that exceed the maximum value depicted in the stress histogram (10 MPa). These differences persisted but were smaller in magnitude when the models were scaled to equal body size and when equal ratios of applied force to total surface area were used in order to remove the effects of size on the stress results [34].

Figure 4.

The predicted distribution of von Mises stress in models of (a,b) A. sp. cf. A. edwardsi and (c,d) H. stenognathus during M2 biting at maximum gape. These models were scaled to predicted body size. Histograms illustrate the distribution of stress across the volume of each model. White areas indicate stresses that exceed the maximum value depicted in the stress histogram (10 MPa). These differences persisted but were smaller in magnitude when the models were scaled to equal body size and when equal ratios of applied force to total surface area were used in order to remove the effects of size on the stress results [34].