Divergent otolithic systems in the inner ear of Paranthropus robustus and Australopithecus africanus

The bony labyrinth of the inner ear houses the sensory end-organs responsible for balance (otolithic system in the utricle and saccule, and semicircular canal system) and hearing (cochlea). Study of the bony labyrinth has revealed considerable morphological diversity in the hominin lineage (semicircular canals and cochleae) and aided in reconstructing essential aspects of primate evolution, including positional behavior, audition, and phylogenic affinities. However, evidence of evolutionary change in the hominin otolithic system remains elusive. Such morphological variation in these gravitoinertial sensory end-organs may suggest functional differences as their geometry is linked with positional behavior. We approach the question of evolutionary morphological change in the hominin otolithic system by examining bony vestibule morphology in two South African hominin taxa Paranthropus robustus (n = 9) and Australopithecus africanus (n = 6), compared to extant hominids (Pongo pygmaeus, Gorilla gorilla, Pan troglodytes, and Homo sapiens). We use landmark-based shape analyses of 78 extant hominid inner ears by means of virtual three-dimensional models derived from micro-computed tomography scans. Thirty bony landmarks were chosen to approximate otolithic organ morphology and relative configuration. Results show a distinctive morphology in P. robustus compared to A. africanus and extant hominids. Specifically, P. robustus exhibits anterolateral–posteromedial compression in bony otolithic organ structure, reducing the size of the saccule and vestibular aqueduct. In contrast, A. africanus exhibits a modern-human-like otolithic system. This newfound morphological diversity identifies unique bony features of the P. robustus inner ear which: 1) offers potential evidence for differential positional behavior between P. robustus and A. africanus and 2) presents osteological markers to be used in taxonomic identification of P. robustus remains and in future assessments of Paranthropus classification.