The inner ear of the Paranthropus specimen DNH 22 from Drimolen, South Africa

Abstract

Objectives: Morphological variation within the southern African hypodigm of Paranthropus has been the focus of major interest since the earliest discoveries in the "Cradle of Humankind." Given the relevance of the bony labyrinth for investigating fossil primate paleobiodiversity, this article aims to provide additional evidence for assessing the degree of regional variation within Paranthropus through the comparative analysis of the inner ear of DNH 22.Materials and methods: As comparative material, 18 southern African hominin specimens from Sterkfontein, Swartkrans, and Makapansgat (plus published data from Kromdraai B), attributed to Australopithecus, early Homo or Paranthropus, as well as 10 extant human and 10 extant common chimpanzee specimens are investigated. A landmark-based geometric morphometric method is applied for quantitatively assessing labyrinthine morphology. Additionally, cochlear parameters and oval window area are measured.Results: In terms of semicircular canal and cochlear shape, DNH 22 most resembles the Paranthropus specimen SKW 18 from Swartkrans. Both specimens differ from the other Paranthropus specimens investigated in this study by an anteroposteriorly large posterior semicircular canal and a cochlea with loose turns in the apical portion.Conversely, the oval window area in DNH 22 closely fits the range observed in Paranthropus from Swartkrans and Kromdraai B.Discussion: The inner ear of the DNH 22 specimen represents a unique opportunity to provide further insight into the early hominin labyrinthine variation pattern. In particular, the description of DNH 22 raises critical questions on the diversity of the vestibular system and evolutionary pattern of the auditory apparatus in Paranthropus. K E Y W O R D Scochlea, Cradle of Humankind, early hominins, Plio-Pleistocene, semicircular canals

Cochlear shape distinguishes southern African early hominin taxa with unique auditory ecologies

“…The developmental stability 4 and species-specific features 5 – 7 of the bony labyrinth, and the relationships between cochlear morphology and auditory perception among mammalian species 8 – 10 , suggest that cochlear shape has the potential to provide evidence about the sensory ecology of extinct hominin taxa. Previous investigations have not detected significant cochlear differences among Plio-Pleistocene australopiths 1 , 2 , 11 . However, the complex shape of the cochlea, comprising a curve of decreasing radius that approximates but does not conform precisely to a logarithmic spiral with different degrees of torsion along its length, does not lend itself to being captured by the linear methods and 3D geometric morphometric (3DGM) approaches that were previously employed.…”

“…Neither linear measurements nor 3DGM morphometric techniques have been able to discern clear-cut differences between A. africanus , P. robustus and early Homo with respect to their middle ear ossicles 31 , bony labyrinths 1 , 2 , 11 or cochleae 2 , 11 . The present study using diffeomorphometry has shown that the cochlear shapes of both A. africanus and especially P. robustus are distinct from those of modern humans and the African apes.…”

Reassessment of the phylogenetic relationships of the late Miocene apes Hispanopithecus and Rudapithecus based on vestibular morphology

“…The morphology of the semicircular canals (SCs), which partly constitute the inner ear's bony labyrinth, has been classically related to locomotion (36)(37)(38)(39)(40)(41)(42). However, several studies have highlighted the possibility of inferring phylogenetic relatedness based on this portion of the inner ear morphology (43)(44)(45)(46)(47) Recently, it has been shown that this anatomical structure also embeds a strong phylogenetic signal among catarrhine primates by means of 3D geometric morphometric (3DGM) analyses (5,(47)(48)(49), thus being potentially useful to test phylogenetic hypotheses for extinct hominoids. Previous studies relied on the SC radius of Rudapithecus hungaricus and Hispanopithecus laietanus to infer slow and deliberate arboreal locomotion for these species (41).…”