Craniomandibular signals of diet in adapids

Objectives The craniomandibular morphology of the adapid primates of Europe, especially Adapis and Leptadapis (sensu lato), suggests that they possessed enormous jaw adductor muscles. The goal of this study is to estimate jaw adductor muscle mass, physiological cross-sectional area (PCSA), and fiber length in adapid primates from the Eocene of Europe. We also estimated muscle leverage, bite force, and gape parameters. Materials and Methods We use bony morphology and osteological correlates of soft tissues in a sample of extant strepsirrhines to estimate these soft-tissue and performance variables in Adapis and Leptadapis. Results Our results suggest that, compared to a broad sample of extant strepsirrhines, Adapis and Leptadapis had relatively great jaw adductor muscle mass, PCSA, and bite force. They had moderately great jaw adductor leverage but no sign of adaptation for wide gapes. There is no support for the hypothesis that either adapid was a gouger. Discussion Our estimates support the inference that Adapis and Leptadapis were primarily folivorous, perhaps also consuming small to medium-sized tough fruits, nuts, and seeds. Explanations for the likely extreme development of the jaw adductor muscles in adapids remain speculative. These include (1) foods that were generally tougher and/or of higher yield strength than those eaten by strepsirrhines today, (2) using the muscles "in shifts" to avoid muscle fatigue in the context of an obdurate diet, and (3) potential constraints on reshaping of the skull for more efficient food processing.