BRIEF COM MUNI CATION Aspects of the Middle and External Ear of Early South African Hominids YOEL RAK AND RONALD J . CLARKE Department of Anthropology, University of California, Berkeley, California 94720 and Transvaal Museum,P.O. Box 413, Pretoria, South Africa K E Y WORDS Swartkrans anatomy . Australopithecus robustus . Ear ABSTRACT A brief description of the exposed middle ear from two Swartkrans specimens demonstrates the feasibility of examining the inner auditory structures of existing fossil hominid material. These Swartkrans fossils provide the first clear view of the early hominid middle ear, and their structures appear to be morphologically distinct. The bony anatomy of the external ear of Australopithecus robustus is found to be unique among the South African hominids. This report seeks to draw attention to the hitherto uninvestigated auditory region of early hominids and to demonstrate the feasibility of examining the ear structures in the existing fossil material. The first clear view of the early hominid middle ear was made possible by minimal cleaning and preparation of two specimens from Swartkrans, SK 46 and SK 879. Their detailed anatomy is shown in figure 1. The petrous portion of the right temporal bone of SK 46,which in its present condition is compressed towards the left side, bears an exposed tympanic cavity (fig. 1, right). Specimen SK 879 consists of a fragmentary petrous portion that displays all the structures of the medial wall of the middle ear (fig. 1, left). Although the topography of the middle ear is generally varied, there may be some significance to the presence in both specimens of a large, inflated promontory (especially as compared with the gorilla and the chimpanzee), a spacious tympanic cavity, and a deeper, more pronounced groove - the promontory sulcus - of the tympanic nerve. Examination of another Swartkrans specimen, SK 848, revealed an ear ossicle (Rak and Clarke, '79). I t differs substantially from that ofHomo sapiens; in fact, SK 848 shows less resemblance to modern man than the latter does to the African great apes (fig. 2). The tympanic sulcus, site of attachment of the tympanic membrane, separates the middle AM. J. PHYS. ANTHROP. (1979) 51: 471-474. and the external ear. Australopithecus robustus has a wide, shallow tympanic sulcus. In specimen SK 46 this groove is almost on the same level as the floor of the external auditory meatus. However, modern man and the Homo specimen SK 847 display a narrow groove that is elevated on a sharp, distinct ridge (the tympanic ring). The raised appearance of the ridge is accentuated by the excavation of the meatus floor immediately lateral to the tympanic membrane. Such excavation is found neither in A . robustus (SK 46,14003, and 8481, nor in Australopithecus africanus (STS 25 and 67). Both A. africanus and A. robustus are characterized by a mediolaterally long external auditory meatus which contrasts with the short meatus found in modern man and the SwartkransHomo specimen, SK 847. I t is only the external auditory meatus of A. robustus, however, that forms a trumpet shape which is laterally wide and tapers toward the middle ear from a mean diameter of 13 mm to 8.7 mm (SK 46, 848, and 52/SKW 18). The trumpetshaped meatus contrasts with the parallelwalled meatus seen in A. africanus. Finally, it has been noted (Clarke, '77) that the roof of the lateral opening of the external auditory meatus hangs over the lateral margins of the tympanic plate in A. africanus, Homo erectus, and H. sapiens. No overhang exists in theA. robustus specimens SK 46,48,83, 848, and TM 1517, and in a sixth specimen (SK 471 Fig. 1 View of the right middle ear of specimen SK 46 bight) and the left middle ear of specimen SK 879 (left). t, tympanic sulcus; p, promontory; tt, tensor tympani; pc, cochleariform process; a, mastoid antrum; s, lateral semicircular canal; tn, tympanic nerve; b, prominence of the lateral semicircular canal; f, facial canal; y, pyramidal process; 0,fenestra ovalis; ic, internal carotid canal; r, fenestra rotundum. EARLY SOUTH AFRICAN HOMINID EAR 473 Fig. 2 Comparison of the right incua ofA. robustus (left) with that of modem man (right). Top row, medial view of the hone; middle row, lateral view; bottom row. view of the articular surface. 521, the overhang amounts to only 4 mm. The lack of an overhang in A . robustus is surprising, because the flaring of the zygomatic arch and the formation of the supraglenoid gutter would logically imply its presence. The lateral opening of the auditory meatus in A. robustus might be explained as a result of functional change of the glenoid fossa, necessitated by specializations in the masticatory system. Evaluation of these initial observations will require extensive comparison with a wide range of extant primates and mammals in which the morphology of the ear is already understood. ACKNOWLEDGMENTS We thank Dr. E. Vrba for access to the Transvaal collection, Richard Dreiman and Carla Echols for editorial assistance. This study was supported by grants from the Louis B. Leakey Foundation and the Boise Fund. LITERATURE CITED Clarke, R. J. 1977 The Cranium of the Swartkrans Hominid, SK 847 and Its Relevance to Human Origins. Unpublished thesis, University of the Witwatersrand, Johannesburg. Rak, Y., and R. J. Clarke 1979 Ear ossicle ofAustrulopithecus robustus. Nature, 279: 62-63.