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Aspects of the middle and external ear of early South African hominids.

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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.
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