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New Hominids from the Lake Turkana Basin, Kenya
Barbara Brown Department of Orthopaedic Surgery, Northeastern Ohio Universities College of Medicine, P.O. Box 95, State Route 44, Rootstown, Ohio 44272, U.S.A.
Frank H. Brown Department of Geology and Geophysics, University of Utah, Salt Lake City, Ohio 84112, U.S.A.
New Hominids from the Lake Turkana Basin, Kenya New hominid fossils from the Lake Turkana Basin range in age from ca. 3·35 to ca. 1·0 Ma. Those recovered from sediments stratigraphically just above the Tulu Bor Tuff in the Lomekwi Member of the Nachukui Formation are best attributed to Australopithecus afarensis. This species is rare in Kenya, probably because of the scarcity of sediments deposited during its time span. Younger specimens are referable either to the megadont A. boisei or early Homo. Collectively the new fossils promote further understanding of morphological variation in East African Plio-Pleistocene hominids. 2001 Academic Press
Alan Walker Departments of Anthropology and Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. Received 7 March 2000 Revision received 5 December 2000 and accepted 10 January 2001 Keywords: hominid, Lake Turkana, Australopithecus, Homo.
Journal of Human Evolution (2001) 41, 29–44 doi:10.1006/jhev.2001.0476 Available online at http://www.idealibrary.com on
Introduction Three new Pliocene hominid taxa have recently been identified from sediments in the Lake Turkana Basin, Kenya, and the Middle Awash region of Ethiopia (White et al., 1994, 1995; Leakey et al., 1995, 1998; Asfaw et al., 1999; Ward et al., 1999a). Ardipithecus ramidus and Australopithecus anamensis occur between 4·4 and 3·9 Ma, while A. garhi is known so far only from 2·5 Ma deposits in the Middle Awash. The earliest of the new fossils reported here sample poorly known time horizons from the Lake Turkana Basin and help test phylogenetic hypotheses bearing on the relationships of 0047–2484/01/070029+16$35.00/0
Plio-Pleistocene hominids. However, a key element in identifying the affinities of the new taxa continues to be a comprehensive understanding of morphological and metric variation in previously known hominid species (Suwa, 1997). Paleontological research in the Lake Turkana Basin during the 1980s has resulted in the accumulation of new Plio-Pleistocene hominid fossils and many of these have been published previously (Brown et al., 1985; Walker et al., 1986; Walker & Leakey, 1993). This present paper reports the salient features of some undescribed specimens and provides new data documenting variation in early Australopithecus, A. boisei, and early Homo. 2001 Academic Press
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36° E
37° E
Omo group
Omo River
Mio-Pliocene Shungura Fm.
Nachukui Fm.
5° N
Ileret
Nariokotome Kokiselei Lomekwi
Koobi Fora
4° N Koobi Fora Fm.
Lake Turkana Kalodirr Lothidok Lodwar Turkwel River
3° N
Kanapoi
Ker
io R
iver
Lothagam
0
20 40 60 Kilometers
Figure 1. Map of the Turkana Basin showing the Nachukui and Koobi Formations: modified from Feibel et al. (1989) and Kibunjia et al. (1992).
Locality and age Hominid fossils described in this paper were recovered on both the eastern and western sides of Lake Turkana. Specimens found on the west side of the lake were recovered from the Natoo and Lomekwi Members of the Nachukui Formation (Figure 1). Those specimens from the Natoo sequence are from deposits ranging in age from 1·0 0·15 Ma to 1·80·1 Ma (Kibunjia et al., 1992). Those from the Lomekwi sequence are 3·260·01 Ma to 3·350·05 Ma
(Feibel et al., 1989). The Lomekwi sediments are contemporaneous with the recently reported South Turkwel hominid bearing sequence (Ward et al., 1999b). Hominid specimens from the Natoo Member are referable to H. erectus and derive from a level above that of the KNM-WT 15000 H. erectus skeleton (Walker & Leakey, 1993) found at Nariokotome. Two teeth of A. boisei are described from Kokiselei, where there is an archeological excavation site with stone tools including chopper-cores dated at 1·80·1 Ma (Kibunjia et al., 1992). Dates
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Table 1 List of hominids and stratigraphic and discovery information Accession number
Year
Discoverer
Locality
Member
Age (Ma)
KNM-ER 16842 KNM-ER 25520 KNM-WT 8556 KNM-WT 8557 KNM-WT 16001 KNM-WT 16002 KNM-WT 16003 KNM-WT 16006 KNM-WT 17396 KNM-WT 18600 KNM-WT 19700
1985 1992 1982 1982 1985 1985 1985 1985 1986 1987 1988
Wambua Mangao Mike Rogers Nzube Mutiwa Nzube Mutiwa Mwongela Muoka John Harris Musa Kyeva Nzube Mutiwa Nzube Mutiwa Wambua Mangao Kamoya Kimeu
Area 104 Koobi Fora Area 6 Ileret LO5 LO4 NK3 LO3 LO5 LO4 FxJh 5 FxJh 5 NC1
KBS KBS Lomekwi Lomekwi Lomekwi Lokalalei Lomekwi Lomekwi Natoo Natoo Nachukui
1·700·05 1·850·05 3·260·10 3·260·10 1·570·05 2·70·3 3·260·10 3·350·05 1·80·1 1·80·1 1·00·15
for the Lomekwi specimens were derived from their stratigraphic position relative to the Tulu Bor Tuff. Their overall morphology is most similar to A. afarensis from Ethiopia and Tanzania. This species is still rare at Kenyan sites and so additional specimens referable to this taxon are significant from a paleogeographic point of view. Two mandibles reported here were recovered from the east side of Lake Turkana. One is a relatively well preserved piece of mandible attributable to A. boisei from deposits dated about 1·850·05 Ma within the Koobi Fora Formation. The other is a fragmentary juvenile specimen from slightly younger deposits. The descriptions that follow are arranged by taxon, locality and age, beginning with the oldest material. Table 1 lists them with their discoverers and the year when they were found. Mesiodistal (MD) and buccolingual (BL) dental crown diameters are given in Table 2. Hominid specimens KNM-WT 16006 A. afarensis (Figure 2). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·350·05 Ma (Feibel et al., 1989). This is a left mandibular body with the M3 and the distal part of M2 in place. The body extends from just anterior to the mental
foramen to the angle. The roots of M1 and M2 and the alveoli of P4 are also present. The mandibular body is well preserved except for some cracks running on the lingual side. It is about 86 mm long. The single mental foramen is oval in outline and faces directly laterally. It is 2·6 mm MD by 1·6 mm superoinferiorly (SI) and situated 16 mm above the base of the mandible roughly in the center of the body. The base of the body is rounded. The posterior part of the anterior digastric attachment surface is present along the base to the level below P4/M1. It fades out about 15·5 mm from the anterior break. The region in front of M2 along the buccal body surface is swollen just before the takeoff of the ascending ramus. The mandibular thickness at the M1/M2 junction is 23·5 mm. The buccal groove is at least 8 mm wide but, because of the broken ramus takeoff, it is difficult to determine exactly how wide it is. On the flat lingual surface, the mylohyoid groove can be seen below M3 running directly to the toral area. The mandibular angle is sharply downturned and it is 8·9 mm thick at the masseter attachment. The takeoff of the ramus starts at the mesial end of M1. Posterior to M3 on the lingual surface the gonial angle is hollowed with strong medial pterygoid ridges. The lingual height of the corpus is 36·3 mm below M2 and 35·8 mm below M3.
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Table 2 Summary of tooth measurements
KNM-WT 8556
KNM-WT 8557 KNM-WT 16003 KNM-WT 16006 KNM-WT 17396 KNM-WT 18600 KNM-ER 25520 KNM-ER 25520 Crown height: RM2 RM3
LP3 RP3 RP4 RM1 RM2 RM3 LM2 RM3 LM3 LM3 LP3 RM2 RM3 Mesiolingual 7·2 8·8
BL
MD
11·5 12·2 12·8 13·1 (15) — (12) Trigonid 14·4 talonid 12·6 Trigonid 13·2 talonid 12·0 16·5 14·8 18·4 19·0
10·9 9·9 11·4 13·6 — (19) (14) 14 15·5 17·1 10·3 18·5 21·7
Distolingual 8·1 9·8
Mesiobuccal 7·4 7·3
Distobuccal 7·5 6·7
Hypoconulid — 7·0
All measurements in millimeters. Measurements in parentheses are estimates.
The M1 roots are broad and plate-like, and the distolingual tip of the distal root has a swollen apex. An estimated M1 root length is 17 mm. There is only the distal central piece of the M2 crown present. The occlusal surface is worn smoothly with only remnants of the posterior fovea and lateral cingulum remaining. A rough estimate of M2 crown length is 15 mm (Table 2), as judged by the roots and remaining crown fragment. The M2 roots are still in place. The mesial roots are 12·5 mm wide at the break and the distal are 11·1 mm wide at the break of the cervix. Its distal interstitial facet is oval in outline measuring 4·1 mm BL by 3·2 mm SI. The M3 crown is complete except for a tiny sliver of enamel from the distal border. The tooth is subrectangular in outline with only slight wear; the greatest wear is buccal. There is no dentine exposure. The interstitial facet is placed buccally off center. The protoconid is the largest cusp and it is broad and flattened by wear. The metaconid has a rounded apex and has many accessory grooves running distally from it. The entoconid and hypoconid also have accessory ridges and grooves. There is a very small hypoconulid. There is
also a single accessory cusp between the metaconid and entoconid. A large cingulum is strongest at the mesiobuccal corner of the crown. There is a 5 mm wide anterior fovea and a 2·5 mm wide posterior fovea. The anterior fovea is irregular because of grooves running down from the nearby cusps. In terms of its general corpus architecture, KNM-WT 16006 is a robust mandible like A.L. 333w-60 and A.L. 333w-1 a and b, and the teeth are similar in size to the specimens A.L. 145-35 and A.L. 266-1. However, the large cingulum of its M3 is not seen in the Hadar or Laetoli specimens LH 4 and LH 15. The mandible A.L. 198-1 shows the same lateral corpus relief, gonial angle, and anterior ramus morphology as KNM-WT 16006. Moreover, both mandibles have a mental foramen approximately midcorpus, the distance between the mental foramen and beginning of the ascending ramus is similar, and medially both lack a pronounced intertoral sulcus, a character which seems variable in the A. afarensis sample. On the other hand, KNM-WT 16006 is overall more robust than A.L. 198-1, and its teeth are larger and longer.
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Figure 2. KNM-WT 16006, left mandibular corpus showing (A) lateral, (B) medial, and (C) occlusal views.
KNM-WT 8556 A. afarensis (Figure 3). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·260·10 Ma (Feibel et al., 1989).
Figure 3. KNM-WT 8556, fragmentary mandible with symphysis and portion of right corpus showing (A) lateral, (B) medial, and (C) occlusal views with M2 and M3 re-approximated along the tooth row.
KNM-WT 8556 is a fragmentary mandible. It extends from the left P3 to midway along the right M2. The crowns of the left P3 and right M3 are preserved. The symphysis preserves alveoli for both the right and left incisors and canines. The specimen was found broken into many small fragments enveloped in matrix. The fragments have been disassociated and reconstructed in their original positions.
The symphysis is characterized by a straight anterior margin about 42 mm long. The postincisive planum (PIP) slopes 18 relative to the occlusal plane. The planum is 23 mm long and reaches a thick superior torus centered only 12 mm below the alveolar margin. The superior torus is separated from a very strong inferior mandibular
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torus by a double genioglossal pit whose width is 9 mm. A foramen is set in a tiny crest at the midline. The inferior torus is about 20 mm thick and takes the form of a smooth convex buttress extending to the base. There is a mild central keel coursing from the genial pit to a point near the base. If digastric impressions were present, they are now obliterated by cracks and abrasion. Mesiodistal widths of the right alveoli are 4 mm for I1, approximately 4 mm for I2, and about 6 mm for the canine. The mandibular body is moderately deep but the base is lost posterior to P3. The mental foramen is 3 mm in diameter. It is situated beneath the center of P4 about 20·5 mm above the base and about 18 mm below the alveolar margin. The mandible has an everted base posteriorly and a pronounced ramus takeoff at P4/M1. The tooth rows are moderately divergent at an angle of 28 to the midline. There is no dentine exposure on any of the teeth. Both P3 crowns are complete. They are irregular in outline with two main cusps. Each cusp is tall, conical, set 4 mm apart, and joined by a muted central crest. A crest separates an anterior fovea from a prominent posterior basin. A buccal groove runs superiorly to a mesial marginal ridge and almost to the cervical margin. A posterior buccal groove also extends from the cervical margin to the distal margin. Small accessory cusps are developed on the distomarginal ridge especially on the left tooth. The distal root of the left P3 is single and plate-like, about 10 mm wide at the cervix narrowing to 7 mm at the apex. It is joined to a small mesiobuccal root. The right P4 has two large primary cusps and very narrow anterior fovea. The large circular talonid is surrounded by a distomarginal ridge of even height around which are set several small blunt cuspules. A buccal groove runs from the base of the main cusp down to the cervix at a point where the main cusp meets the distomarginal ridge. The right M1 crown is complete except for a wedge on the mesial
side. There are five main cusps and wear is most pronounced on the buccal side. Buccal grooves are more prominent than lingual ones and only traces of a buccal cingulum remain. The small posterior fovea is positioned distolingually. Accessory cuspules run from major cusps down to a central basin. The right M2 is damaged but retains parts of the two mesial cusps and most of the two distal cusps. It is missing the distal and whole mesiobuccal portions of the crown. There is very slight wear which is more pronounced on the buccal side. The enamel is relatively thick, measuring 2·4 mm on the distolingual cusp tip and the same on the mesiolingual cusp tip. There are numerous puffy ridges extending into a broad central basin. The right M3 is mostly intact but is missing enamel around the cervical margin mesially and distally. There are five main cusps and an accessory (C6) cusp. There are small accessory cuspules between the two lingual cusps. There is no wear at all on this RM3, so it is unlikely that it was in occlusion. There are inflated ridges running radially down from the cusps to the central basin and accessory cuspules develop even inside the basin. The enamel is moderately thick—at least 2·4 mm on the hypoconid. Both P3s of KNM-WT 8556 appear most similar to the left P3 of A.L. 333w-1a. This is primarily because they are fully bicuspid teeth, whereas most of the Hadar specimens are either unicuspid, or they appear unicuspid after they have been worn. However, compared with the left P3 of KNM-WT 8556 the Hadar tooth has a deeper mesial fovea and less defined beading around the basin. The P3s of KNM-WT 8556 are also like those of A.L. 333w-58, A.L. 333w-60 and A.L. 266-1, except that the latter two premolars are worn down so that the cusps are at about the same level. The P3 of A.L. 333w-60 is nearly as broad as those of KNM-WT 8556 and the P3 of A.L. 333w-58 is of a similar size. Both of the P4s of A.L. 333w-1 are smaller but similar in
configuration to that of KNM-WT 8556. A Laetoli P4 (LH 3R) is about the same size as that of KNM-WT 8556, but it is less round in occlusal outline and its cusps are more compressed. The P4 of KNM-WT 8556 is more molarized, with a more expansive basin and anterior fovea that is quite compressed. The M1 of KNM-WT 8556 has a larger hypoconulid with wider grooves in between the cusps. The angle made by the grooves bordering the hypoconulid is obtuse like the M1s of AL 266-1, and more obtuse than the acute angles on M1s of Hadar, Laetoli, and OH 7. The M2 of KNM-WT 8556 is most like that of A.L. 266-1 primarily because the M2 of KNM-WT 8556 lacks a tuberculum accessorius medium (TAM or C7). It has a deflecting wrinkle like most of the Hadar specimens. The M3 is most like the unworn M3s of A.L. 266-1 and A.L. 400-1 in terms of size and numbers of cusps. The anterior symphyseal border of the mandible is relatively straight. In the Hadar mandibles some anterior symphyseal surfaces tend to be straight while others are curved. The PIP is more horizontal in KNM-WT 8556 than in most A. afarensis mandibles, which tend to slope down to the genial region. The position of the genial pit, halfway down the symphysis, contrasts with the Hadar, Maka and Laetoli mandibles where it is about two-thirds of the way down (White et al., 2000). The inferior torus is deeper and projects more strongly posteriorly than in other A. afarensis mandibles. KNM-WT 8556 also has a relatively pronounced intertoral sulcus similar to the mandibles of Hadar and Laetoli because of its prominent superior and inferior tori. The specimen resembles some A. boisei mandibles due to its flattened PIP, molarized P4, straight anterior aspect of the symphysis, large M3, and deep genioglossal pit. The mental foramina vary in position in the Hadar specimens, but generally, they are somewhat lower than in KNM-WT 8556.
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Figure 4. KNM-WT 8557, right mandibular second molar.
KNM-WT 8557 A. cf. afarensis (Figure 4). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·260·10 Ma (Feibel et al., 1989). This is most probably the distal threequarter of a mandibular left M2, as the distal root of M3s are more triangular. There is a trace of wear on the protoconid. KNM-WT 8557 is missing the whole of the mesial face, parts of the mesial cusps and the root tips. There is a pronounced irregular crack 1 mm thick through the mesial surface to the middle of the buccal surface. Two buccal grooves separate the buccal cusps. There are tiny accessory cusps on the lingual side. There is a large hypoconulid with a 4 mmwide distal fovea between it and the entoconid. All cusps have a series of puffy ridges running radially from them to the center. Although larger, it resembles the M2 of A.L. 128-23. The crown would not have been as long as the M3 of KNM-WT 16006, described above, which has two enamel folds (TAMs) that add to its length. KNM-WT 16003 A. cf. afarensis (Figure 5). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·260·10 Ma (Feibel et al., 1989). This is a right maxillary M3 with a trace of wear on the mesiolingual corner and along
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Figure 5. KNM-WT 16003, right maxillary third molar.
the ridge in front of the anterior fovea. The crown is roughly rectangular in outline and is complete except for one small piece of enamel at the mesial corner of the cervical margin and a small piece at the buccal margin. The crown is heavily crenulated and has swollen sides. The distance between the tips of the paracone and protocone is 7 mm. All main and accessory cusps have puffy ridges and grooves running radially from them. There is a 4 mm-wide anterior fovea and what would be a posterior fovea is interrupted by a small cuspule on the distal margin. A very strong lingual groove divides the main lingual cusps from the central fovea to the lingual cervical margin. More shallow grooves separate accessory cuspules and the paracone and metacone. There is a small contact facet low on the cervical margin of the crown mesially. It is 3·5 mm BL and its height is difficult to estimate. A double interproximal facet shows that the tooth had shifted after it came into occlusion. The posterior buccal root is missing. Two remaining roots are only partly developed. Their apices are open. The mesial root is 16 mm long and the lateral root is 8·5 mm long. The perforation at the apex of the mesiobuccal root is 4·2 mm BL by 2·5 mm MD; that on the lingual root is 6·2 mm MD by 2·5 mm BL. When compared with other non-robust australopithecine teeth, KNM-WT 16003 is most like A.L. 333x-1, although a little smaller.
Figure 6. KNM-WT 16002, right femur with neck and shaft showing (A) anterior, (B) posterior, (C) medial, and (D) lateral views.
KNM-WT 16002 Australopithecus sp. indet. (Figure 6). Locality: Lomekwi Drainage, Lomekwi Member. Age: 2·70·3 Ma. Based on new work carried out by one of us (F.H.B.) the geological age of this specimen
has been revised from that published in Feibel et al. (1989). The fossil was found in a block of exposures to the north of the Lomekwi sand river. This block is cut off by faults from exposures to the east and west. It does contain a single tuff that unfortunately does not correlate geochemically with any other known tuff. The strata to the east contain the Kokiselei Tuff (2·4 Ma) and the Kalochoro Tuff (2·34 Ma). The strata to the west contain the sequence above the Tulu Bor Tuff and a sandstone from the type section of the Tulu Bor can be correlated with one here. This means that the geological age estimate given in Feibel et al. (1989) should be sharply reduced. Without any correlate of the tuff, any age estimate given must have very wide uncertainties, but 2·70·03 Ma is far more appropriate than 3·25 Ma. The specimen is a right femur preserving the neck and most of the shaft. Clear outer lamellar bone and inner osteons were seen before the three main fragments were stuck together, and so this is from an adult individual. Overall preserved length is 235 mm. Midshaft dimensions are 20·0 mm mediolaterally by 17·5 mm anteroposteriorly. The neck measures 22 mm superoinferiorly and 16 mm anteroposteriorly, underscoring its anterior–posterior compression. The anterior surface of the neck is markedly hollowed, consequently the base of the greater trochanter overhangs it. The surface bone is damaged where an intertrochanteric line might have been present. However, preserved surfaces at the anterior base of the greater trochanter suggest that a pronounced femoral tubercle was present, making it likely that the iliofemoral ligament was well developed. Neck length can be estimated reasonably well because foramina associated with the reflected articular capsule fibers are preserved. The neck length to lateral shaft is estimated at 45 mm and neck length to center of shaft is 33 mm. The neck shaft angle is approximately 125. The shaft
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inferior to the lesser trochanter is 22·2 mm mediolaterally (ML) by 18 mm anteroposteriorly (AP). The shaft has a gentle anterior convexity and an almost straight linea aspera. There is a distinct gluteal attachment that includes a hypotrochanteric fossa 16·8 mm wide on the lateral surface approximately 34 mm below the base of the greater trochanter. The linea aspera begins as a crest just medial to this depression. The limb from the lesser trochanter does not join the linea until just above midshaft level. The linea continues as a single crest for 44 mm before dividing 80 mm from the bottom of the fragment. A major crack and anterior spalling above it lie inferior to the femoral neck and area of the lesser trochanter. A major crack just above midshaft is 127 mm superior to the lower break. Surface detail on the anterior inferior part of the shaft is lost by spalling especially near the distal break. The medial and lateral borders are almost straight except for a slight bulge adjacent to the gluteal attachment. The narrowest part of the shaft measures 19·5 mm about two-thirds of the way down. The maximum width is 21·5 mm at the gluteal swelling. The shaft appears platymeric and slightly anteverted. Anatomically, KNM-WT 16002 presents several features that can be matched in the femora of A. afarensis. It is more gracile overall than the A.L. 288 femur (A.L. 2881ap) and yet it is evident that KNM-WT 16002 was longer than A.L. 288-1ap based on comparison of the course of the linea aspera. At 125 the neck/shaft angle falls within the range reported for A.L. 288-1ap (123), and A.L. 333-3 (125) (Johanson et al., 1982; Stern & Susman, 1983). These values are lower than those reported for later australopithecines. The West Turkana femur also shares with A. afarensis femora a similar degree of compression of the neck, relatively short femoral neck, and lateral placement of the lesser trochanter. Although KNM-WT 16002 is not associated with
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craniodental remains, its overall morphological pattern conforms closely to that described for A. afarensis (Johanson et al., 1982; Lovejoy et al., 1982). The possibility is strong, however, that it belongs to another hominid taxon as it is probably several hundred thousand years younger than the youngest A. afarensis cranial remains. It does not compare well in detail with the femur from Bouri, Ethiopia at about 2·5 Ma that might possibly belong to A. garhi (Lovejoy, personal communication). It might also belong to the robust A. aethiopicus that is known from a cranium and a mandible from sites at Lomekwi of roughly the same age and near the femur site. Without any associated A. aethiopicus cranial and postcranial fossils, however, it is probably best referred only to the genus Australopithecus. KNM-ER 25520 A. boisei (Figure 7). Locality: Ileret, KBS Member. Age: 1·850·05 Ma (Feibel, personal communication). This is a right mandibular corpus with the crowns and roots of M2, M3, and roots of M1. It is broken posterior to the M3 at the ramus and anteriorly through the distal P4 root alveolus. Alveolar impressions of the distal P4 roots are visible. There is a flange of bone along the anterior part of the lingual surface of the corpus that bordered the P4. This flange is just beginning to curve medially to form the superior transverse torus. The mental canal is visible at the anterior break, about 20 mm from the estimated alveolar margin at P4. It is 3·3 mm wide by 2·2 mm high. Although the surface of the corpus is cracked, morphological details are well preserved and the lateral surface of the corpus displays sculptural relief. The lateral eminence covers the distal one-third of the M3. It begins at approximately two-thirds of the lateral corpus height inferior to the M2/M3. The lateral prominence borders a broad, relatively deep buccinator groove
Figure 7. KNM-ER 25520, right mandibular corpus showing (A) lateral, (B) medial, and (C) occlusal views.
which is about 13 mm wide. There are five to six carnivore toothmarks along the surface within the buccinator groove and at the buccinator and oblique lines. Two of these pits may have distorted the depth of the buccinator groove which seems especially deep at M2/M3. The oblique line appears at the base of the lateral prominence inferior to M2/M3 and extends anteriorly only to the midpoint of the M2 where it is obscured by surface cracking. The oblique line fades along the surface at an oval convexity inferior to M1. A large masseter attachment is
represented by an angular swelling near the corpus base inferior to the M2 mesial root. Two planar surfaces run along the base. One is posterior and inferior to the lateral prominence where the corpus curves medially for the base of the ramus. The other planar surface is at the level anterior to the most extreme swelling of the lateral prominence. This surface extends anteriorly to a swelling inferior to the M2 mesial root. The lateral corpus is vertically convex but becomes slightly concave anteriorly from the M2 mesial root towards the P4. A prominent alveolar ridge begins at a distinct retromolar triangle. It runs anteriorly along the buccal border and is 1·5–2 mm wide next to both M3 and M2 where it begins either to fade or be damaged by bone spalling at the M1 root. Lingually the alveolar margin is a distinct crest along the border of the distolingual M3 root. The crest becomes blunt at the middle of M3 and M2. The retromolar triangle is 12·4 mm from the edge of the M3 to the tip of the triangle. The corpus base is damaged inferior to M1. There are platysmatic striae inferior to M2 and M3 and anterior to the masseter attachment. The medial corpus has relatively large cracks running both vertically and horizontally. The cracks do not seem obviously to distort the corpus. There is a distinct vascular groove that runs along the medial aspect of the ramus. It splits into two branches about 15 mm from the broken edge, where they are separated by a ridge. The superior groove measures 28·8 mm; the inferior branch measures 30 mm from the broken edge. A shallow but broad intertoral sulcus extends along the medial corpus. The mylohyoid line begins posteriorly as a faint rugosity. It courses anteriorly along the superior border of the intertoral surface from the distal root area of M3 and ends anteriorly at a relatively large carnivore tooth pit approximately midcorpus, inferior to the M1 distal root.
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Figure 8. KNM-WT 17396, left mandibular third molar, occlusal view.
The teeth are minimally worn, and their crown heights are listed in Table 2. There is an exposed chip of M1 enamel with a thickness of 1·5 mm. Bone has spalled off the lateral surface at M1 so its mesiobuccal root is exposed. A large chip of bone is missing below the root. The M2 has a polished wear surface on the metaconid that extends at an angle across the central portion of the protoconid and on to the buccal half of the hypoconid. There is also wear along a shallow central fossa. A concave mesial M2 facet is 9·3 mm wide and about 4 mm high. The M3 has polished wear on its mesial border and protoconid. There are distinct interproximal facets approximately 7 mm wide for both the M3 and M2. There is no TAM on the M2, but the M3 has a TAM between the metaconid and entoconid, and between the entoconid and hypoconulid. Allowing for slight expansion cracks, this specimen is very similar in size and morphology to some of the bigger A. boisei mandibles from the Turkana Basin, such as KNM-ER 729 (Leakey et al., 1972). KNM-WT 17396 A. boisei (Figure 8). Locality: Kokiselei Drainage, Natoo Member. Age: 1·80·1 Ma (Kibunjia et al., 1992). This is most of a left mandibular M3 crown. The enamel is broken along the cervix and its thickness ranges from 1·4–2·9 mm. The
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Figure 9. KNM-WT 18600, upper left third premolar, occlusal view.
crown consists of four primary cusps, each of a similar height, a hypoconulid, and a pair of distolingual cuspules or enamel folds. There is a central groove 11·7 mm long with a mesial marginal ridge and four smaller grooves radiating from the mesial fovea. A buccolingual groove separates the four primary cusps, and another buccolingual groove runs mesial to the hypoconulid and distolingual cuspules. The distal fovea is shaped as an inverted Y with a small enamel fold in the distal tip at the distal end of the central groove. The crown is unworn and there appears to have been no root development. The broken enamel edge seems to be polished. Buccolingual distances between the tips of the cusps are 8·3 mm for the metaconid and protoconid and 9·8 mm between the entoconid and hypoconid.
Figure 10. KNM-ER 16842, juvenile left mandibular corpus showing (A) lateral, (B) medial, and (C) occlusal views. I2 and C are exposed in crypts mesial to roots of dm2 and M1.
KNM-WT 18600 A. boisei (Figure 9). Locality: Kokiselei Drainage, Natoo Member. Age: 1·80·1 Ma (Kubunjia et al., 1992).
the mesial surface. It runs diagonally to meet the occlusal margin. The main part of the facet is 3·3 mm by 1·6 mm. The faintest traces of anterior and posterior foveae and the median groove are evident.
This specimen is a left maxillary P3. It is oval in outline with a small area of dentine exposure on the buccal cusp and a 2·8 mm wide oval exposure on the lingual cusp. The roots are broken leaving a jagged edge around the cervical margin. There is a slightly hollow, oval interstitial facet for the P4. The facet measures 6·3 mm by 3·0 mm and is centered on the distal face nearly reaching the occlusal margin. A double, interstitial facet for a small canine is placed well buccally on
KNM-ER 16842 Homo sp. (Figure 10). Locality: Koobi Fora, upper KBS Member. Age: 1·700·05 Ma (Feibel, personal communication). This is a juvenile left mandible with roots of dm2 and M1. The P3 and C crowns are exposed in their crypts due to post depositional bone loss on portions of the lateral and medial surfaces. The medial aspect of the mandible is missing posterior to the M1 distal root so the buccal wall of the M2 crypt is
Figure 11. KNM-WT 16001 A, the largest portion of the associated parietal fragments.
exposed. The M1 distal root apex has two tiny holes that are not completely fused. The medial surface of the corpus is concave at P3 and C. An inferior transverse torus arises at C/P3. There is only a small concavity opposite M1 to suggest a buccinator groove. The lateral aspect of the corpus is damaged. A mental foramen is inferior to the dm2 mesial root approximately two-thirds down the corpus near the base. The main foramen is broad and two small accessory foramina are anterior to it. The C and P3 are weathered so perikymata are not well preserved. The relative sizes of the C, P3, and roots of the M1 seem similar to those of Homo specimens. More detailed comparison must await CT scanning of the mandible and the tooth germs. KNM-WT 16001 H. erectus (Figure 11). Locality: Nariokotome Drainage; Natoo Member. Age: 1·570·05 Ma (Feibel et al., 1989). The composite specimen consists of four skull fragments referable to H. erectus (H. ergaster to some). It was recovered from Nariokotome III, which is stratigraphically above the KNM-WT 15000 locality. KNM-WT 16001 A is a roughly rectangular parietal fragment about 45 mm by 45 mm. A temporal line runs down its center. The line is in the form of a very low, flattened,
41
10 mm-wide ridge sloping on one side and more steeply dipping on the other. Comparison with complete parietals indicates that the steeply dipping side is medial. Overall thickness of the bone at the temporal line is 11 mm. Elsewhere it varies from ca. 6–10 mm. The outer surface is slightly abraded and smooth, and the inner surface shows one vascular groove. KNM-WT 16001 B is the left lateral posterior corner of the parietal with the lambdoid suture. It is about 57 mm ML by 30 mm AP. The lambdoid suture defines the long axis of the fragment. The corner of the piece preserves asterion. Medially the thickness is 11·2 mm, it is 11·3 mm centrally and about 12 mm at asterion. This latter measurement is an estimate because a chip of bone is missing from the asterionic corner. Pegs of occipital and temporal bone interdigitate at the lambdoid suture. The outer surface of this piece is abraded, and the inner surface is smooth with six small emissary foramina. The uppermost part of the lambdoid suture is ‘‘rolled over’’ in such a manner that it would produce a ridge and a groove on the endocast. KNM-WT 16001 C and D are very badly abraded pieces of the lateral occipital bone. KNM-WT 16001 C is the right portion and KNM-WT 16001 D is the left. Both preserve sutural remnants. KNM-WT 16001 C is 45 mm ML by 32 mm AP and 16·5 mm thick at the occipital crest. KNM-WT 16001 D is 42 mm ML by 24 mm AP and 16 mm thick at the occipital crest. These small parts of an adult cranium match those of crania such as KNM-ER 730 and KNM-ER 3733 (Wood, 1991) and are clearly from a cranium of an early H. erectus. KNM-WT 19700 H. erectus (Figure 12). Locality: NC1, Nachukui Formation. Age: 1·00·15 Ma. This is the proximal quarter of a left tibia. It comes from sediments rather high in the
42
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Figure 12. KNM-WT 19700, left proximal tibia showing (A) anterior, (B) posterior, (C) medial, and (D) lateral views.
Nariokotome Member and was found 8–8·5 m below stromatolite horizon S3. This horizon lies about 45 m above the Lower Nariokotome Tuff (1·330·02 Ma). The only way to estimate how much younger this specimen is than this tuff is by using sedimentation rates. The distance from the Chari Tuff to the Lower Nariokotome Tuff at Nariokotome is 10 m, which represents about 0·06 Ma. If sedimentation rates were constant the tibia should be about 1·11 Ma. But farther to the North the Silbo Tuff (0·740·01 Ma) lies only 47 m above the
Lower Nariokotome Tuff. If we use this tuff pair to estimate sedimentation rates the age for the tibia is about 0·87 Ma. Because of this poor agreement across a relatively short distance, the age is best given as 1·00·15 Ma. The epiphysis is abraded circumferentially and the shaft is significantly damaged. The best preserved portion of epiphysis reveals that the intercondylar eminence had tubercles about 15 mm apart on a raised projection above the medial and lateral articular surfaces. The lateral articular surface is flat peripherally and concave near the tubercle.
The medial articular surface is concave in all directions. The anterior intercondylar area is roughened and hollow towards the eminence with a ridge running from the medial articular surface across to the tuberosity. There is a projection on the anteriolateral corner of the superior surface of the anterior intercondylar area which receives the insertion of the iliotibial tract. The 23 mm-wide tibial tuberosity does not project significantly, and begins about 35 mm distal to the tibial tubercles. The shaft is oriented asymmetrically with respect to the transverse axis of the epiphyseal surface so that it subtends an angle of 70 to the transverse of the condyles. The soleal line is poorly developed. The popliteal surface is not noticeably hollowed. At the distal break the shaft is 33 mm AP by 21·5 mm ML. The anterior border is rounded rather than sharp. This fragment compares closely, once the juvenile nature of the specimen is taken into account, with KNM-WT 15000 (Walker & Leakey, 1993), and with the adult tibia KNM-ER 741 (originally given as Australopithecus in Leakey et al., 1972). Especially obvious is the way in which the epiphysis is set at a strong angle to the anteroposterior axis of the shaft. Those firmly attributed to Australopithecus (e.g. A.L. 288-1 aq (Johanson et al., 1982) have the long axis of the epiphysis set at right angles to the anteroposterior axis of the shaft. Because of this we attribute this specimen to H. erectus. Summary The specimens described here expand the inventory of hominid fossils from the Turkana Basin between 3·4 and 1·0 Ma. Of particular importance is the recovery of material that we have referred to A. afarensis. This species is poorly represented at Kenyan sites (Kimbel, 1988) and this is most probably because of the paucity of sediments of the right age. There is some evidence from mandibular anatomy that
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A. afarensis was morphologically even more diverse than previously recognized. We surmise that possible regional and temporal differences in mandibular and dental anatomy, present in the Hadar, Maka and Laetoli samples noted by previous workers (e.g. White et al., 2000) might also show up in the Turkana Basin. The fact that the mandible of KNM-WT 8556 has a few characters foreshadowing those in later robust hominids might be consistent with phylogenetic hypotheses linking A. afarensis with A. aethiopicus and A. boisei. We believe, however, that more cranial material referable to A. afarensis from the Lake Turkana Basin and elsewhere will be needed before this issue can be resolved. The new specimens attributable to A. boisei add to the large sample of this taxon known from the Turkana Basin. Indeed, there was already sufficient mandibular material to evaluate patterns of mandibular variation in this taxon (Wood et al., 1994). The new A. boisei material from Konso in Ethiopia, shows that the craniofacial anatomy of this taxon is highly variable (Suwa, 1997). We endorse the comments of Suwa (1997) that evaluating interbasinal variation in hominid samples is a pressing issue, and should, to some extent, mitigate excessive ‘‘atomization’’ of anatomical features in taxonomic discussions. To date there is still considerable uncertainty concerning the origins of Homo. The recent discovery of A. garhi by Asfaw et al. (1999) certainly adds another potential ancestral species. However, the Lake Turkana Basin has yet to produce fossils at 2·5 Ma levels attributable to A. garhi. Only the isolated eight teeth, KNM-ER 5431, from Area 203 at Koobi Fora dated at 2·7 Ma (Feibel et al., 1989) have been found at roughly this time period. The specimens have been attributed to A. afarensis (Leonard & Hegmon, 1987; Boaz, 1988), but Suwa (1990) called them Australopithecus/Homo gen. and sp. indet.
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Wood (1991) was even more cautious, recording them as being from an indeterminate hominid. The uncertainty about the taxonomic attribution of KNM-ER 5431 in part reflects the paucity of comparative material between 2·7 and 2·5 Ma, particularly as it relates to the ancestry of early Homo. Future fieldwork in the Lake Turkana Basin should contribute to understanding the relationships of hominid species in the now critical period before the clear appearance of the genus Homo. Acknowledgements We thank the Office of the President, Republic of Kenya for permission to conduct research in Kenya. Field research led by R. E. Leakey was supported by the National Geographic Society, Washington, D.C. and The John D. and Catherine T. MacArthur Foundation. We especially thank Bw Kamoya Kimeu and the members of his team. We also thank Mrs Emma Mbua for curatorial help and Drs C. S. Feibel, M. G. Leakey, R. E. Leakey, C. O. Lovejoy, C. V. Ward, and S. C. Ward for helpful comments. Three anonymous reviewers greatly improved the manuscript. References Asfaw, B., White, T., Lovejoy, O., Latimer, B., Simpson, S. & Suwa, G. (1999). Australopithecus garhi: A new species of early hominid from Ethiopia. Science 284, 629–635. Boaz, N. T. (1988). Status of Australopithecus afarensis. Am. J. phys. Anthrop. 31, 85–113. Brown, F., Harris, J., Leakey, R. & Walker, A. (1985). Early Homo erectus skeleton from west Lake Turkana, Kenya. Nature 316, 788–792. Feibel, C. S., Brown, F. H. & McDougall, I. (1989). Stratigraphic context of fossil hominids from the Omo group deposits: Northern Turkana Basin, Kenya and Ethiopia. Am. J. phys. Anthrop. 78, 595– 622. Johanson, D. C., Lovejoy, C. O., Kimbel, W. H., Ward, S. C., Bush, M. E., Latimer, B. M. & Coppens, Y. (1982). Morphology of the Pliocene partial hominid skeleton (AL 288-1) from the Hadar Formation, Ethiopia. Am. J. phys. Anthrop. 57, 403–452.
Kibunjia, M., Roche, H., Brown, F. H. & Leakey, R. E. (1992). Pliocene and Pleistocene archeological sites west of Lake Turkana, Kenya. J. hum. Evol. 23, 431–438. Kimbel, W. H. (1988). Identification of a partial cranium of Australopithecus afarensis from the Koobi Fora Formation, Kenya. Am. J. phys. Anthrop. 17, 647–656. Leakey, M. G., Feibel, C. S., McDougall, I. & Walker, A. (1995). New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya. Nature 565–571. Leakey, M. G., Feibel, C. S., McDougall, I., Ward, C. V. & Walker, A. (1998). New specimens and confirmation of an early age for Australopithecus anamensis. Nature 393, 62–66. Leakey, R. E., Mungai, J. M. & Walker, A. C. (1972). New Australopithecines from East Rudolf, Kenya (II). Am. J. phys. Anthrop. 36, 235–251. Leonard, W. R. & Hegmon, M. (1987). Evolution of P3 morphology in Australopithecus afarensis. Am. J. phys. Anthrop. 73, 41–63. Lovejoy, C. O., Johanson, D. C. & Coppens, Y. (1982). Hominid lower limb bones recovered from the Hadar Formation: 1974–1977 Collections. Am. J. phys. Anthrop. 57, 679–700. Stern, J. T. & Susman, R. L. (1983). The locomotor anatomy of Australopithecus afarensis. Am. J. phys. Anthrop. 60, 279–317. Suwa, G. (1990). A comparative analysis of hominid dental remains from the Shungura and Usno Formations, Omo Valley, Ethiopia. Ph.D. Dissertation, University of California, Berkeley. Suwa, G. (1997). The first skull of Australopithecus boisei. Nature 389, 486–492. Walker, A. & Leakey, R. E. (1993). The Nariokotome Homo Erectus Skeleton. Cambridge: Harvard University Press. Walker, A., Leakey, R. E., Harris, J. M. & Brown, F. H. (1986). 2·5-Myr Australopithecus boisei from west of Lake Turkana, Kenya. Nature 322, 517–522. Ward, C. V., Leakey, M. G. & Walker, A. (1999a). A new hominid species Australopithecus anamensis. Evol. Anthrop. 7, 197–205. Ward, C. V., Leakey, M. G., Brown, B., Brown, F., Harris, J. & Walker, A. (1999b). South Turkwel: a new Pliocene hominid site in Kenya. J. hum. Evol. 36, 69–95. White, T. D., Suwa, G. & Asfaw, B. (1994). Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371, 306–312. White, T. D., Suwa, G. & Asfaw, B. (1995). Corregendum. Nature 375, 88. White, T. D., Suwa, G., Simpson, S. & Asfaw, B. (2000). Jaws and teeth of Australopithecus afarensis from Maka, Middle Awash, Ethiopia. Am. J. phys. Anthrop. 111, 45–68. Wood, B. (1991). Koobi Fora Research Project, Vol. 4: Hominid Cranial Remains. Oxford: Clarendon Press. Wood, B. A., Wood, C. & Konigsberg, L. (1994). Paranthropus boisei: an example of evolutionary statis? Am. J. phys. Anthrop. 95, 117–136.
Frank H. Brown Department of Geology and Geophysics, University of Utah, Salt Lake City, Ohio 84112, U.S.A.
New Hominids from the Lake Turkana Basin, Kenya New hominid fossils from the Lake Turkana Basin range in age from ca. 3·35 to ca. 1·0 Ma. Those recovered from sediments stratigraphically just above the Tulu Bor Tuff in the Lomekwi Member of the Nachukui Formation are best attributed to Australopithecus afarensis. This species is rare in Kenya, probably because of the scarcity of sediments deposited during its time span. Younger specimens are referable either to the megadont A. boisei or early Homo. Collectively the new fossils promote further understanding of morphological variation in East African Plio-Pleistocene hominids. 2001 Academic Press
Alan Walker Departments of Anthropology and Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. Received 7 March 2000 Revision received 5 December 2000 and accepted 10 January 2001 Keywords: hominid, Lake Turkana, Australopithecus, Homo.
Journal of Human Evolution (2001) 41, 29–44 doi:10.1006/jhev.2001.0476 Available online at http://www.idealibrary.com on
Introduction Three new Pliocene hominid taxa have recently been identified from sediments in the Lake Turkana Basin, Kenya, and the Middle Awash region of Ethiopia (White et al., 1994, 1995; Leakey et al., 1995, 1998; Asfaw et al., 1999; Ward et al., 1999a). Ardipithecus ramidus and Australopithecus anamensis occur between 4·4 and 3·9 Ma, while A. garhi is known so far only from 2·5 Ma deposits in the Middle Awash. The earliest of the new fossils reported here sample poorly known time horizons from the Lake Turkana Basin and help test phylogenetic hypotheses bearing on the relationships of 0047–2484/01/070029+16$35.00/0
Plio-Pleistocene hominids. However, a key element in identifying the affinities of the new taxa continues to be a comprehensive understanding of morphological and metric variation in previously known hominid species (Suwa, 1997). Paleontological research in the Lake Turkana Basin during the 1980s has resulted in the accumulation of new Plio-Pleistocene hominid fossils and many of these have been published previously (Brown et al., 1985; Walker et al., 1986; Walker & Leakey, 1993). This present paper reports the salient features of some undescribed specimens and provides new data documenting variation in early Australopithecus, A. boisei, and early Homo. 2001 Academic Press
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36° E
37° E
Omo group
Omo River
Mio-Pliocene Shungura Fm.
Nachukui Fm.
5° N
Ileret
Nariokotome Kokiselei Lomekwi
Koobi Fora
4° N Koobi Fora Fm.
Lake Turkana Kalodirr Lothidok Lodwar Turkwel River
3° N
Kanapoi
Ker
io R
iver
Lothagam
0
20 40 60 Kilometers
Figure 1. Map of the Turkana Basin showing the Nachukui and Koobi Formations: modified from Feibel et al. (1989) and Kibunjia et al. (1992).
Locality and age Hominid fossils described in this paper were recovered on both the eastern and western sides of Lake Turkana. Specimens found on the west side of the lake were recovered from the Natoo and Lomekwi Members of the Nachukui Formation (Figure 1). Those specimens from the Natoo sequence are from deposits ranging in age from 1·0 0·15 Ma to 1·80·1 Ma (Kibunjia et al., 1992). Those from the Lomekwi sequence are 3·260·01 Ma to 3·350·05 Ma
(Feibel et al., 1989). The Lomekwi sediments are contemporaneous with the recently reported South Turkwel hominid bearing sequence (Ward et al., 1999b). Hominid specimens from the Natoo Member are referable to H. erectus and derive from a level above that of the KNM-WT 15000 H. erectus skeleton (Walker & Leakey, 1993) found at Nariokotome. Two teeth of A. boisei are described from Kokiselei, where there is an archeological excavation site with stone tools including chopper-cores dated at 1·80·1 Ma (Kibunjia et al., 1992). Dates
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Table 1 List of hominids and stratigraphic and discovery information Accession number
Year
Discoverer
Locality
Member
Age (Ma)
KNM-ER 16842 KNM-ER 25520 KNM-WT 8556 KNM-WT 8557 KNM-WT 16001 KNM-WT 16002 KNM-WT 16003 KNM-WT 16006 KNM-WT 17396 KNM-WT 18600 KNM-WT 19700
1985 1992 1982 1982 1985 1985 1985 1985 1986 1987 1988
Wambua Mangao Mike Rogers Nzube Mutiwa Nzube Mutiwa Mwongela Muoka John Harris Musa Kyeva Nzube Mutiwa Nzube Mutiwa Wambua Mangao Kamoya Kimeu
Area 104 Koobi Fora Area 6 Ileret LO5 LO4 NK3 LO3 LO5 LO4 FxJh 5 FxJh 5 NC1
KBS KBS Lomekwi Lomekwi Lomekwi Lokalalei Lomekwi Lomekwi Natoo Natoo Nachukui
1·700·05 1·850·05 3·260·10 3·260·10 1·570·05 2·70·3 3·260·10 3·350·05 1·80·1 1·80·1 1·00·15
for the Lomekwi specimens were derived from their stratigraphic position relative to the Tulu Bor Tuff. Their overall morphology is most similar to A. afarensis from Ethiopia and Tanzania. This species is still rare at Kenyan sites and so additional specimens referable to this taxon are significant from a paleogeographic point of view. Two mandibles reported here were recovered from the east side of Lake Turkana. One is a relatively well preserved piece of mandible attributable to A. boisei from deposits dated about 1·850·05 Ma within the Koobi Fora Formation. The other is a fragmentary juvenile specimen from slightly younger deposits. The descriptions that follow are arranged by taxon, locality and age, beginning with the oldest material. Table 1 lists them with their discoverers and the year when they were found. Mesiodistal (MD) and buccolingual (BL) dental crown diameters are given in Table 2. Hominid specimens KNM-WT 16006 A. afarensis (Figure 2). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·350·05 Ma (Feibel et al., 1989). This is a left mandibular body with the M3 and the distal part of M2 in place. The body extends from just anterior to the mental
foramen to the angle. The roots of M1 and M2 and the alveoli of P4 are also present. The mandibular body is well preserved except for some cracks running on the lingual side. It is about 86 mm long. The single mental foramen is oval in outline and faces directly laterally. It is 2·6 mm MD by 1·6 mm superoinferiorly (SI) and situated 16 mm above the base of the mandible roughly in the center of the body. The base of the body is rounded. The posterior part of the anterior digastric attachment surface is present along the base to the level below P4/M1. It fades out about 15·5 mm from the anterior break. The region in front of M2 along the buccal body surface is swollen just before the takeoff of the ascending ramus. The mandibular thickness at the M1/M2 junction is 23·5 mm. The buccal groove is at least 8 mm wide but, because of the broken ramus takeoff, it is difficult to determine exactly how wide it is. On the flat lingual surface, the mylohyoid groove can be seen below M3 running directly to the toral area. The mandibular angle is sharply downturned and it is 8·9 mm thick at the masseter attachment. The takeoff of the ramus starts at the mesial end of M1. Posterior to M3 on the lingual surface the gonial angle is hollowed with strong medial pterygoid ridges. The lingual height of the corpus is 36·3 mm below M2 and 35·8 mm below M3.
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Table 2 Summary of tooth measurements
KNM-WT 8556
KNM-WT 8557 KNM-WT 16003 KNM-WT 16006 KNM-WT 17396 KNM-WT 18600 KNM-ER 25520 KNM-ER 25520 Crown height: RM2 RM3
LP3 RP3 RP4 RM1 RM2 RM3 LM2 RM3 LM3 LM3 LP3 RM2 RM3 Mesiolingual 7·2 8·8
BL
MD
11·5 12·2 12·8 13·1 (15) — (12) Trigonid 14·4 talonid 12·6 Trigonid 13·2 talonid 12·0 16·5 14·8 18·4 19·0
10·9 9·9 11·4 13·6 — (19) (14) 14 15·5 17·1 10·3 18·5 21·7
Distolingual 8·1 9·8
Mesiobuccal 7·4 7·3
Distobuccal 7·5 6·7
Hypoconulid — 7·0
All measurements in millimeters. Measurements in parentheses are estimates.
The M1 roots are broad and plate-like, and the distolingual tip of the distal root has a swollen apex. An estimated M1 root length is 17 mm. There is only the distal central piece of the M2 crown present. The occlusal surface is worn smoothly with only remnants of the posterior fovea and lateral cingulum remaining. A rough estimate of M2 crown length is 15 mm (Table 2), as judged by the roots and remaining crown fragment. The M2 roots are still in place. The mesial roots are 12·5 mm wide at the break and the distal are 11·1 mm wide at the break of the cervix. Its distal interstitial facet is oval in outline measuring 4·1 mm BL by 3·2 mm SI. The M3 crown is complete except for a tiny sliver of enamel from the distal border. The tooth is subrectangular in outline with only slight wear; the greatest wear is buccal. There is no dentine exposure. The interstitial facet is placed buccally off center. The protoconid is the largest cusp and it is broad and flattened by wear. The metaconid has a rounded apex and has many accessory grooves running distally from it. The entoconid and hypoconid also have accessory ridges and grooves. There is a very small hypoconulid. There is
also a single accessory cusp between the metaconid and entoconid. A large cingulum is strongest at the mesiobuccal corner of the crown. There is a 5 mm wide anterior fovea and a 2·5 mm wide posterior fovea. The anterior fovea is irregular because of grooves running down from the nearby cusps. In terms of its general corpus architecture, KNM-WT 16006 is a robust mandible like A.L. 333w-60 and A.L. 333w-1 a and b, and the teeth are similar in size to the specimens A.L. 145-35 and A.L. 266-1. However, the large cingulum of its M3 is not seen in the Hadar or Laetoli specimens LH 4 and LH 15. The mandible A.L. 198-1 shows the same lateral corpus relief, gonial angle, and anterior ramus morphology as KNM-WT 16006. Moreover, both mandibles have a mental foramen approximately midcorpus, the distance between the mental foramen and beginning of the ascending ramus is similar, and medially both lack a pronounced intertoral sulcus, a character which seems variable in the A. afarensis sample. On the other hand, KNM-WT 16006 is overall more robust than A.L. 198-1, and its teeth are larger and longer.
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Figure 2. KNM-WT 16006, left mandibular corpus showing (A) lateral, (B) medial, and (C) occlusal views.
KNM-WT 8556 A. afarensis (Figure 3). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·260·10 Ma (Feibel et al., 1989).
Figure 3. KNM-WT 8556, fragmentary mandible with symphysis and portion of right corpus showing (A) lateral, (B) medial, and (C) occlusal views with M2 and M3 re-approximated along the tooth row.
KNM-WT 8556 is a fragmentary mandible. It extends from the left P3 to midway along the right M2. The crowns of the left P3 and right M3 are preserved. The symphysis preserves alveoli for both the right and left incisors and canines. The specimen was found broken into many small fragments enveloped in matrix. The fragments have been disassociated and reconstructed in their original positions.
The symphysis is characterized by a straight anterior margin about 42 mm long. The postincisive planum (PIP) slopes 18 relative to the occlusal plane. The planum is 23 mm long and reaches a thick superior torus centered only 12 mm below the alveolar margin. The superior torus is separated from a very strong inferior mandibular
34
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torus by a double genioglossal pit whose width is 9 mm. A foramen is set in a tiny crest at the midline. The inferior torus is about 20 mm thick and takes the form of a smooth convex buttress extending to the base. There is a mild central keel coursing from the genial pit to a point near the base. If digastric impressions were present, they are now obliterated by cracks and abrasion. Mesiodistal widths of the right alveoli are 4 mm for I1, approximately 4 mm for I2, and about 6 mm for the canine. The mandibular body is moderately deep but the base is lost posterior to P3. The mental foramen is 3 mm in diameter. It is situated beneath the center of P4 about 20·5 mm above the base and about 18 mm below the alveolar margin. The mandible has an everted base posteriorly and a pronounced ramus takeoff at P4/M1. The tooth rows are moderately divergent at an angle of 28 to the midline. There is no dentine exposure on any of the teeth. Both P3 crowns are complete. They are irregular in outline with two main cusps. Each cusp is tall, conical, set 4 mm apart, and joined by a muted central crest. A crest separates an anterior fovea from a prominent posterior basin. A buccal groove runs superiorly to a mesial marginal ridge and almost to the cervical margin. A posterior buccal groove also extends from the cervical margin to the distal margin. Small accessory cusps are developed on the distomarginal ridge especially on the left tooth. The distal root of the left P3 is single and plate-like, about 10 mm wide at the cervix narrowing to 7 mm at the apex. It is joined to a small mesiobuccal root. The right P4 has two large primary cusps and very narrow anterior fovea. The large circular talonid is surrounded by a distomarginal ridge of even height around which are set several small blunt cuspules. A buccal groove runs from the base of the main cusp down to the cervix at a point where the main cusp meets the distomarginal ridge. The right M1 crown is complete except for a wedge on the mesial
side. There are five main cusps and wear is most pronounced on the buccal side. Buccal grooves are more prominent than lingual ones and only traces of a buccal cingulum remain. The small posterior fovea is positioned distolingually. Accessory cuspules run from major cusps down to a central basin. The right M2 is damaged but retains parts of the two mesial cusps and most of the two distal cusps. It is missing the distal and whole mesiobuccal portions of the crown. There is very slight wear which is more pronounced on the buccal side. The enamel is relatively thick, measuring 2·4 mm on the distolingual cusp tip and the same on the mesiolingual cusp tip. There are numerous puffy ridges extending into a broad central basin. The right M3 is mostly intact but is missing enamel around the cervical margin mesially and distally. There are five main cusps and an accessory (C6) cusp. There are small accessory cuspules between the two lingual cusps. There is no wear at all on this RM3, so it is unlikely that it was in occlusion. There are inflated ridges running radially down from the cusps to the central basin and accessory cuspules develop even inside the basin. The enamel is moderately thick—at least 2·4 mm on the hypoconid. Both P3s of KNM-WT 8556 appear most similar to the left P3 of A.L. 333w-1a. This is primarily because they are fully bicuspid teeth, whereas most of the Hadar specimens are either unicuspid, or they appear unicuspid after they have been worn. However, compared with the left P3 of KNM-WT 8556 the Hadar tooth has a deeper mesial fovea and less defined beading around the basin. The P3s of KNM-WT 8556 are also like those of A.L. 333w-58, A.L. 333w-60 and A.L. 266-1, except that the latter two premolars are worn down so that the cusps are at about the same level. The P3 of A.L. 333w-60 is nearly as broad as those of KNM-WT 8556 and the P3 of A.L. 333w-58 is of a similar size. Both of the P4s of A.L. 333w-1 are smaller but similar in
configuration to that of KNM-WT 8556. A Laetoli P4 (LH 3R) is about the same size as that of KNM-WT 8556, but it is less round in occlusal outline and its cusps are more compressed. The P4 of KNM-WT 8556 is more molarized, with a more expansive basin and anterior fovea that is quite compressed. The M1 of KNM-WT 8556 has a larger hypoconulid with wider grooves in between the cusps. The angle made by the grooves bordering the hypoconulid is obtuse like the M1s of AL 266-1, and more obtuse than the acute angles on M1s of Hadar, Laetoli, and OH 7. The M2 of KNM-WT 8556 is most like that of A.L. 266-1 primarily because the M2 of KNM-WT 8556 lacks a tuberculum accessorius medium (TAM or C7). It has a deflecting wrinkle like most of the Hadar specimens. The M3 is most like the unworn M3s of A.L. 266-1 and A.L. 400-1 in terms of size and numbers of cusps. The anterior symphyseal border of the mandible is relatively straight. In the Hadar mandibles some anterior symphyseal surfaces tend to be straight while others are curved. The PIP is more horizontal in KNM-WT 8556 than in most A. afarensis mandibles, which tend to slope down to the genial region. The position of the genial pit, halfway down the symphysis, contrasts with the Hadar, Maka and Laetoli mandibles where it is about two-thirds of the way down (White et al., 2000). The inferior torus is deeper and projects more strongly posteriorly than in other A. afarensis mandibles. KNM-WT 8556 also has a relatively pronounced intertoral sulcus similar to the mandibles of Hadar and Laetoli because of its prominent superior and inferior tori. The specimen resembles some A. boisei mandibles due to its flattened PIP, molarized P4, straight anterior aspect of the symphysis, large M3, and deep genioglossal pit. The mental foramina vary in position in the Hadar specimens, but generally, they are somewhat lower than in KNM-WT 8556.
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Figure 4. KNM-WT 8557, right mandibular second molar.
KNM-WT 8557 A. cf. afarensis (Figure 4). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·260·10 Ma (Feibel et al., 1989). This is most probably the distal threequarter of a mandibular left M2, as the distal root of M3s are more triangular. There is a trace of wear on the protoconid. KNM-WT 8557 is missing the whole of the mesial face, parts of the mesial cusps and the root tips. There is a pronounced irregular crack 1 mm thick through the mesial surface to the middle of the buccal surface. Two buccal grooves separate the buccal cusps. There are tiny accessory cusps on the lingual side. There is a large hypoconulid with a 4 mmwide distal fovea between it and the entoconid. All cusps have a series of puffy ridges running radially from them to the center. Although larger, it resembles the M2 of A.L. 128-23. The crown would not have been as long as the M3 of KNM-WT 16006, described above, which has two enamel folds (TAMs) that add to its length. KNM-WT 16003 A. cf. afarensis (Figure 5). Locality: Lomekwi Drainage, Lomekwi Member. Age: 3·260·10 Ma (Feibel et al., 1989). This is a right maxillary M3 with a trace of wear on the mesiolingual corner and along
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Figure 5. KNM-WT 16003, right maxillary third molar.
the ridge in front of the anterior fovea. The crown is roughly rectangular in outline and is complete except for one small piece of enamel at the mesial corner of the cervical margin and a small piece at the buccal margin. The crown is heavily crenulated and has swollen sides. The distance between the tips of the paracone and protocone is 7 mm. All main and accessory cusps have puffy ridges and grooves running radially from them. There is a 4 mm-wide anterior fovea and what would be a posterior fovea is interrupted by a small cuspule on the distal margin. A very strong lingual groove divides the main lingual cusps from the central fovea to the lingual cervical margin. More shallow grooves separate accessory cuspules and the paracone and metacone. There is a small contact facet low on the cervical margin of the crown mesially. It is 3·5 mm BL and its height is difficult to estimate. A double interproximal facet shows that the tooth had shifted after it came into occlusion. The posterior buccal root is missing. Two remaining roots are only partly developed. Their apices are open. The mesial root is 16 mm long and the lateral root is 8·5 mm long. The perforation at the apex of the mesiobuccal root is 4·2 mm BL by 2·5 mm MD; that on the lingual root is 6·2 mm MD by 2·5 mm BL. When compared with other non-robust australopithecine teeth, KNM-WT 16003 is most like A.L. 333x-1, although a little smaller.
Figure 6. KNM-WT 16002, right femur with neck and shaft showing (A) anterior, (B) posterior, (C) medial, and (D) lateral views.
KNM-WT 16002 Australopithecus sp. indet. (Figure 6). Locality: Lomekwi Drainage, Lomekwi Member. Age: 2·70·3 Ma. Based on new work carried out by one of us (F.H.B.) the geological age of this specimen
has been revised from that published in Feibel et al. (1989). The fossil was found in a block of exposures to the north of the Lomekwi sand river. This block is cut off by faults from exposures to the east and west. It does contain a single tuff that unfortunately does not correlate geochemically with any other known tuff. The strata to the east contain the Kokiselei Tuff (2·4 Ma) and the Kalochoro Tuff (2·34 Ma). The strata to the west contain the sequence above the Tulu Bor Tuff and a sandstone from the type section of the Tulu Bor can be correlated with one here. This means that the geological age estimate given in Feibel et al. (1989) should be sharply reduced. Without any correlate of the tuff, any age estimate given must have very wide uncertainties, but 2·70·03 Ma is far more appropriate than 3·25 Ma. The specimen is a right femur preserving the neck and most of the shaft. Clear outer lamellar bone and inner osteons were seen before the three main fragments were stuck together, and so this is from an adult individual. Overall preserved length is 235 mm. Midshaft dimensions are 20·0 mm mediolaterally by 17·5 mm anteroposteriorly. The neck measures 22 mm superoinferiorly and 16 mm anteroposteriorly, underscoring its anterior–posterior compression. The anterior surface of the neck is markedly hollowed, consequently the base of the greater trochanter overhangs it. The surface bone is damaged where an intertrochanteric line might have been present. However, preserved surfaces at the anterior base of the greater trochanter suggest that a pronounced femoral tubercle was present, making it likely that the iliofemoral ligament was well developed. Neck length can be estimated reasonably well because foramina associated with the reflected articular capsule fibers are preserved. The neck length to lateral shaft is estimated at 45 mm and neck length to center of shaft is 33 mm. The neck shaft angle is approximately 125. The shaft
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inferior to the lesser trochanter is 22·2 mm mediolaterally (ML) by 18 mm anteroposteriorly (AP). The shaft has a gentle anterior convexity and an almost straight linea aspera. There is a distinct gluteal attachment that includes a hypotrochanteric fossa 16·8 mm wide on the lateral surface approximately 34 mm below the base of the greater trochanter. The linea aspera begins as a crest just medial to this depression. The limb from the lesser trochanter does not join the linea until just above midshaft level. The linea continues as a single crest for 44 mm before dividing 80 mm from the bottom of the fragment. A major crack and anterior spalling above it lie inferior to the femoral neck and area of the lesser trochanter. A major crack just above midshaft is 127 mm superior to the lower break. Surface detail on the anterior inferior part of the shaft is lost by spalling especially near the distal break. The medial and lateral borders are almost straight except for a slight bulge adjacent to the gluteal attachment. The narrowest part of the shaft measures 19·5 mm about two-thirds of the way down. The maximum width is 21·5 mm at the gluteal swelling. The shaft appears platymeric and slightly anteverted. Anatomically, KNM-WT 16002 presents several features that can be matched in the femora of A. afarensis. It is more gracile overall than the A.L. 288 femur (A.L. 2881ap) and yet it is evident that KNM-WT 16002 was longer than A.L. 288-1ap based on comparison of the course of the linea aspera. At 125 the neck/shaft angle falls within the range reported for A.L. 288-1ap (123), and A.L. 333-3 (125) (Johanson et al., 1982; Stern & Susman, 1983). These values are lower than those reported for later australopithecines. The West Turkana femur also shares with A. afarensis femora a similar degree of compression of the neck, relatively short femoral neck, and lateral placement of the lesser trochanter. Although KNM-WT 16002 is not associated with
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craniodental remains, its overall morphological pattern conforms closely to that described for A. afarensis (Johanson et al., 1982; Lovejoy et al., 1982). The possibility is strong, however, that it belongs to another hominid taxon as it is probably several hundred thousand years younger than the youngest A. afarensis cranial remains. It does not compare well in detail with the femur from Bouri, Ethiopia at about 2·5 Ma that might possibly belong to A. garhi (Lovejoy, personal communication). It might also belong to the robust A. aethiopicus that is known from a cranium and a mandible from sites at Lomekwi of roughly the same age and near the femur site. Without any associated A. aethiopicus cranial and postcranial fossils, however, it is probably best referred only to the genus Australopithecus. KNM-ER 25520 A. boisei (Figure 7). Locality: Ileret, KBS Member. Age: 1·850·05 Ma (Feibel, personal communication). This is a right mandibular corpus with the crowns and roots of M2, M3, and roots of M1. It is broken posterior to the M3 at the ramus and anteriorly through the distal P4 root alveolus. Alveolar impressions of the distal P4 roots are visible. There is a flange of bone along the anterior part of the lingual surface of the corpus that bordered the P4. This flange is just beginning to curve medially to form the superior transverse torus. The mental canal is visible at the anterior break, about 20 mm from the estimated alveolar margin at P4. It is 3·3 mm wide by 2·2 mm high. Although the surface of the corpus is cracked, morphological details are well preserved and the lateral surface of the corpus displays sculptural relief. The lateral eminence covers the distal one-third of the M3. It begins at approximately two-thirds of the lateral corpus height inferior to the M2/M3. The lateral prominence borders a broad, relatively deep buccinator groove
Figure 7. KNM-ER 25520, right mandibular corpus showing (A) lateral, (B) medial, and (C) occlusal views.
which is about 13 mm wide. There are five to six carnivore toothmarks along the surface within the buccinator groove and at the buccinator and oblique lines. Two of these pits may have distorted the depth of the buccinator groove which seems especially deep at M2/M3. The oblique line appears at the base of the lateral prominence inferior to M2/M3 and extends anteriorly only to the midpoint of the M2 where it is obscured by surface cracking. The oblique line fades along the surface at an oval convexity inferior to M1. A large masseter attachment is
represented by an angular swelling near the corpus base inferior to the M2 mesial root. Two planar surfaces run along the base. One is posterior and inferior to the lateral prominence where the corpus curves medially for the base of the ramus. The other planar surface is at the level anterior to the most extreme swelling of the lateral prominence. This surface extends anteriorly to a swelling inferior to the M2 mesial root. The lateral corpus is vertically convex but becomes slightly concave anteriorly from the M2 mesial root towards the P4. A prominent alveolar ridge begins at a distinct retromolar triangle. It runs anteriorly along the buccal border and is 1·5–2 mm wide next to both M3 and M2 where it begins either to fade or be damaged by bone spalling at the M1 root. Lingually the alveolar margin is a distinct crest along the border of the distolingual M3 root. The crest becomes blunt at the middle of M3 and M2. The retromolar triangle is 12·4 mm from the edge of the M3 to the tip of the triangle. The corpus base is damaged inferior to M1. There are platysmatic striae inferior to M2 and M3 and anterior to the masseter attachment. The medial corpus has relatively large cracks running both vertically and horizontally. The cracks do not seem obviously to distort the corpus. There is a distinct vascular groove that runs along the medial aspect of the ramus. It splits into two branches about 15 mm from the broken edge, where they are separated by a ridge. The superior groove measures 28·8 mm; the inferior branch measures 30 mm from the broken edge. A shallow but broad intertoral sulcus extends along the medial corpus. The mylohyoid line begins posteriorly as a faint rugosity. It courses anteriorly along the superior border of the intertoral surface from the distal root area of M3 and ends anteriorly at a relatively large carnivore tooth pit approximately midcorpus, inferior to the M1 distal root.
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Figure 8. KNM-WT 17396, left mandibular third molar, occlusal view.
The teeth are minimally worn, and their crown heights are listed in Table 2. There is an exposed chip of M1 enamel with a thickness of 1·5 mm. Bone has spalled off the lateral surface at M1 so its mesiobuccal root is exposed. A large chip of bone is missing below the root. The M2 has a polished wear surface on the metaconid that extends at an angle across the central portion of the protoconid and on to the buccal half of the hypoconid. There is also wear along a shallow central fossa. A concave mesial M2 facet is 9·3 mm wide and about 4 mm high. The M3 has polished wear on its mesial border and protoconid. There are distinct interproximal facets approximately 7 mm wide for both the M3 and M2. There is no TAM on the M2, but the M3 has a TAM between the metaconid and entoconid, and between the entoconid and hypoconulid. Allowing for slight expansion cracks, this specimen is very similar in size and morphology to some of the bigger A. boisei mandibles from the Turkana Basin, such as KNM-ER 729 (Leakey et al., 1972). KNM-WT 17396 A. boisei (Figure 8). Locality: Kokiselei Drainage, Natoo Member. Age: 1·80·1 Ma (Kibunjia et al., 1992). This is most of a left mandibular M3 crown. The enamel is broken along the cervix and its thickness ranges from 1·4–2·9 mm. The
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Figure 9. KNM-WT 18600, upper left third premolar, occlusal view.
crown consists of four primary cusps, each of a similar height, a hypoconulid, and a pair of distolingual cuspules or enamel folds. There is a central groove 11·7 mm long with a mesial marginal ridge and four smaller grooves radiating from the mesial fovea. A buccolingual groove separates the four primary cusps, and another buccolingual groove runs mesial to the hypoconulid and distolingual cuspules. The distal fovea is shaped as an inverted Y with a small enamel fold in the distal tip at the distal end of the central groove. The crown is unworn and there appears to have been no root development. The broken enamel edge seems to be polished. Buccolingual distances between the tips of the cusps are 8·3 mm for the metaconid and protoconid and 9·8 mm between the entoconid and hypoconid.
Figure 10. KNM-ER 16842, juvenile left mandibular corpus showing (A) lateral, (B) medial, and (C) occlusal views. I2 and C are exposed in crypts mesial to roots of dm2 and M1.
KNM-WT 18600 A. boisei (Figure 9). Locality: Kokiselei Drainage, Natoo Member. Age: 1·80·1 Ma (Kubunjia et al., 1992).
the mesial surface. It runs diagonally to meet the occlusal margin. The main part of the facet is 3·3 mm by 1·6 mm. The faintest traces of anterior and posterior foveae and the median groove are evident.
This specimen is a left maxillary P3. It is oval in outline with a small area of dentine exposure on the buccal cusp and a 2·8 mm wide oval exposure on the lingual cusp. The roots are broken leaving a jagged edge around the cervical margin. There is a slightly hollow, oval interstitial facet for the P4. The facet measures 6·3 mm by 3·0 mm and is centered on the distal face nearly reaching the occlusal margin. A double, interstitial facet for a small canine is placed well buccally on
KNM-ER 16842 Homo sp. (Figure 10). Locality: Koobi Fora, upper KBS Member. Age: 1·700·05 Ma (Feibel, personal communication). This is a juvenile left mandible with roots of dm2 and M1. The P3 and C crowns are exposed in their crypts due to post depositional bone loss on portions of the lateral and medial surfaces. The medial aspect of the mandible is missing posterior to the M1 distal root so the buccal wall of the M2 crypt is
Figure 11. KNM-WT 16001 A, the largest portion of the associated parietal fragments.
exposed. The M1 distal root apex has two tiny holes that are not completely fused. The medial surface of the corpus is concave at P3 and C. An inferior transverse torus arises at C/P3. There is only a small concavity opposite M1 to suggest a buccinator groove. The lateral aspect of the corpus is damaged. A mental foramen is inferior to the dm2 mesial root approximately two-thirds down the corpus near the base. The main foramen is broad and two small accessory foramina are anterior to it. The C and P3 are weathered so perikymata are not well preserved. The relative sizes of the C, P3, and roots of the M1 seem similar to those of Homo specimens. More detailed comparison must await CT scanning of the mandible and the tooth germs. KNM-WT 16001 H. erectus (Figure 11). Locality: Nariokotome Drainage; Natoo Member. Age: 1·570·05 Ma (Feibel et al., 1989). The composite specimen consists of four skull fragments referable to H. erectus (H. ergaster to some). It was recovered from Nariokotome III, which is stratigraphically above the KNM-WT 15000 locality. KNM-WT 16001 A is a roughly rectangular parietal fragment about 45 mm by 45 mm. A temporal line runs down its center. The line is in the form of a very low, flattened,
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10 mm-wide ridge sloping on one side and more steeply dipping on the other. Comparison with complete parietals indicates that the steeply dipping side is medial. Overall thickness of the bone at the temporal line is 11 mm. Elsewhere it varies from ca. 6–10 mm. The outer surface is slightly abraded and smooth, and the inner surface shows one vascular groove. KNM-WT 16001 B is the left lateral posterior corner of the parietal with the lambdoid suture. It is about 57 mm ML by 30 mm AP. The lambdoid suture defines the long axis of the fragment. The corner of the piece preserves asterion. Medially the thickness is 11·2 mm, it is 11·3 mm centrally and about 12 mm at asterion. This latter measurement is an estimate because a chip of bone is missing from the asterionic corner. Pegs of occipital and temporal bone interdigitate at the lambdoid suture. The outer surface of this piece is abraded, and the inner surface is smooth with six small emissary foramina. The uppermost part of the lambdoid suture is ‘‘rolled over’’ in such a manner that it would produce a ridge and a groove on the endocast. KNM-WT 16001 C and D are very badly abraded pieces of the lateral occipital bone. KNM-WT 16001 C is the right portion and KNM-WT 16001 D is the left. Both preserve sutural remnants. KNM-WT 16001 C is 45 mm ML by 32 mm AP and 16·5 mm thick at the occipital crest. KNM-WT 16001 D is 42 mm ML by 24 mm AP and 16 mm thick at the occipital crest. These small parts of an adult cranium match those of crania such as KNM-ER 730 and KNM-ER 3733 (Wood, 1991) and are clearly from a cranium of an early H. erectus. KNM-WT 19700 H. erectus (Figure 12). Locality: NC1, Nachukui Formation. Age: 1·00·15 Ma. This is the proximal quarter of a left tibia. It comes from sediments rather high in the
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Figure 12. KNM-WT 19700, left proximal tibia showing (A) anterior, (B) posterior, (C) medial, and (D) lateral views.
Nariokotome Member and was found 8–8·5 m below stromatolite horizon S3. This horizon lies about 45 m above the Lower Nariokotome Tuff (1·330·02 Ma). The only way to estimate how much younger this specimen is than this tuff is by using sedimentation rates. The distance from the Chari Tuff to the Lower Nariokotome Tuff at Nariokotome is 10 m, which represents about 0·06 Ma. If sedimentation rates were constant the tibia should be about 1·11 Ma. But farther to the North the Silbo Tuff (0·740·01 Ma) lies only 47 m above the
Lower Nariokotome Tuff. If we use this tuff pair to estimate sedimentation rates the age for the tibia is about 0·87 Ma. Because of this poor agreement across a relatively short distance, the age is best given as 1·00·15 Ma. The epiphysis is abraded circumferentially and the shaft is significantly damaged. The best preserved portion of epiphysis reveals that the intercondylar eminence had tubercles about 15 mm apart on a raised projection above the medial and lateral articular surfaces. The lateral articular surface is flat peripherally and concave near the tubercle.
The medial articular surface is concave in all directions. The anterior intercondylar area is roughened and hollow towards the eminence with a ridge running from the medial articular surface across to the tuberosity. There is a projection on the anteriolateral corner of the superior surface of the anterior intercondylar area which receives the insertion of the iliotibial tract. The 23 mm-wide tibial tuberosity does not project significantly, and begins about 35 mm distal to the tibial tubercles. The shaft is oriented asymmetrically with respect to the transverse axis of the epiphyseal surface so that it subtends an angle of 70 to the transverse of the condyles. The soleal line is poorly developed. The popliteal surface is not noticeably hollowed. At the distal break the shaft is 33 mm AP by 21·5 mm ML. The anterior border is rounded rather than sharp. This fragment compares closely, once the juvenile nature of the specimen is taken into account, with KNM-WT 15000 (Walker & Leakey, 1993), and with the adult tibia KNM-ER 741 (originally given as Australopithecus in Leakey et al., 1972). Especially obvious is the way in which the epiphysis is set at a strong angle to the anteroposterior axis of the shaft. Those firmly attributed to Australopithecus (e.g. A.L. 288-1 aq (Johanson et al., 1982) have the long axis of the epiphysis set at right angles to the anteroposterior axis of the shaft. Because of this we attribute this specimen to H. erectus. Summary The specimens described here expand the inventory of hominid fossils from the Turkana Basin between 3·4 and 1·0 Ma. Of particular importance is the recovery of material that we have referred to A. afarensis. This species is poorly represented at Kenyan sites (Kimbel, 1988) and this is most probably because of the paucity of sediments of the right age. There is some evidence from mandibular anatomy that
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A. afarensis was morphologically even more diverse than previously recognized. We surmise that possible regional and temporal differences in mandibular and dental anatomy, present in the Hadar, Maka and Laetoli samples noted by previous workers (e.g. White et al., 2000) might also show up in the Turkana Basin. The fact that the mandible of KNM-WT 8556 has a few characters foreshadowing those in later robust hominids might be consistent with phylogenetic hypotheses linking A. afarensis with A. aethiopicus and A. boisei. We believe, however, that more cranial material referable to A. afarensis from the Lake Turkana Basin and elsewhere will be needed before this issue can be resolved. The new specimens attributable to A. boisei add to the large sample of this taxon known from the Turkana Basin. Indeed, there was already sufficient mandibular material to evaluate patterns of mandibular variation in this taxon (Wood et al., 1994). The new A. boisei material from Konso in Ethiopia, shows that the craniofacial anatomy of this taxon is highly variable (Suwa, 1997). We endorse the comments of Suwa (1997) that evaluating interbasinal variation in hominid samples is a pressing issue, and should, to some extent, mitigate excessive ‘‘atomization’’ of anatomical features in taxonomic discussions. To date there is still considerable uncertainty concerning the origins of Homo. The recent discovery of A. garhi by Asfaw et al. (1999) certainly adds another potential ancestral species. However, the Lake Turkana Basin has yet to produce fossils at 2·5 Ma levels attributable to A. garhi. Only the isolated eight teeth, KNM-ER 5431, from Area 203 at Koobi Fora dated at 2·7 Ma (Feibel et al., 1989) have been found at roughly this time period. The specimens have been attributed to A. afarensis (Leonard & Hegmon, 1987; Boaz, 1988), but Suwa (1990) called them Australopithecus/Homo gen. and sp. indet.
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Wood (1991) was even more cautious, recording them as being from an indeterminate hominid. The uncertainty about the taxonomic attribution of KNM-ER 5431 in part reflects the paucity of comparative material between 2·7 and 2·5 Ma, particularly as it relates to the ancestry of early Homo. Future fieldwork in the Lake Turkana Basin should contribute to understanding the relationships of hominid species in the now critical period before the clear appearance of the genus Homo. Acknowledgements We thank the Office of the President, Republic of Kenya for permission to conduct research in Kenya. Field research led by R. E. Leakey was supported by the National Geographic Society, Washington, D.C. and The John D. and Catherine T. MacArthur Foundation. We especially thank Bw Kamoya Kimeu and the members of his team. We also thank Mrs Emma Mbua for curatorial help and Drs C. S. Feibel, M. G. Leakey, R. E. Leakey, C. O. Lovejoy, C. V. Ward, and S. C. Ward for helpful comments. Three anonymous reviewers greatly improved the manuscript. References Asfaw, B., White, T., Lovejoy, O., Latimer, B., Simpson, S. & Suwa, G. (1999). Australopithecus garhi: A new species of early hominid from Ethiopia. Science 284, 629–635. Boaz, N. T. (1988). Status of Australopithecus afarensis. Am. J. phys. Anthrop. 31, 85–113. Brown, F., Harris, J., Leakey, R. & Walker, A. (1985). Early Homo erectus skeleton from west Lake Turkana, Kenya. Nature 316, 788–792. Feibel, C. S., Brown, F. H. & McDougall, I. (1989). Stratigraphic context of fossil hominids from the Omo group deposits: Northern Turkana Basin, Kenya and Ethiopia. Am. J. phys. Anthrop. 78, 595– 622. Johanson, D. C., Lovejoy, C. O., Kimbel, W. H., Ward, S. C., Bush, M. E., Latimer, B. M. & Coppens, Y. (1982). Morphology of the Pliocene partial hominid skeleton (AL 288-1) from the Hadar Formation, Ethiopia. Am. J. phys. Anthrop. 57, 403–452.
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