GeologyandpalaeontologyoftheLate Miocene Middle Awash ... - Nature

letters to nature 17. Dong, H., Hall, C. M., Peacor, D. R., Halliday, A. N. & Pevear, D. R. Thermal 40Ar/39Ar separation of diagenetic from detrital illitic clays in Gulf Coast shales. Earth Planet. Sci. Lett. 175, 309±325 (2000). 18. Hall, C. M. et al. Dating of alteration episodes related to mercury mineralization in the AlmadeÂn district, Spain. Earth Planet. Sci. Lett. 148, 287±298 (1997). 19. Jaboyedoff, M. & Cosca, M. A. Dating incipient metamorphism using 40Ar/39Ar geochronology and XRD modeling: a case study from the Swiss Alps. Contrib. Mineral. Petrol. 135, 93±113 (1999). 20. Bird, P. Formation of the Rocky Mountains, western United States: a continuum computer model. Science 239, 1501±1507 (1988). 21. Vrolijk, P., Covey, M. C., Pevear, D. R. & Longstaffe, F. Dating clay-rich thrust faults. Geol. Soc. Am. (Abstr. Progr.) 26, 466 (1994). 22. Pevear, D. R. & Schuette, J. F. in Computer Applications to X-ray Diffraction Analysis of Clay Minerals (eds Reynolds, R. C. & Walker, J. R.) 19±42 (Clay Minerals Society, Boulder, CO, 1993). 23. Srodon, J. & Eberl, D. D. Review in Mineralogy (ed. Bailey, S. W.) 495±544 (Mineralogical Society of America, Washington, DC, 1984). 24. Grathoff, G. H. & Moore, D. M. Illite polytype quanti®cation using Wild®re calculated X-ray diffraction patterns. Clay, Clay Mineral. 44, 835±842 (1996). 25. Reynolds, R. C. WILDFIRE: A computer program for the calculation of three-dimensional X-ray diffraction patterns for mica polytypes and their disordered variations (Hanover, New Hampshire, 1994). 26. Reynolds, R. C. & Reynolds, R. C. NEWMOD: A computer program for the calculation of onedimensional diffraction patterns of mixed-layered clays. (Hanover, New Hampshire, 1996).

Acknowledgements

by tectonic, volcanic, climatic and geomorphic processes. A similar wooded habitat also has been suggested for the 6.0 Myr hominoid fossils recently recovered from Lukeino, Kenya6. These ®ndings require fundamental reassessment of models that invoke a signi®cant role for global climatic change and/or savannah habitat in the origin of hominids. The western rift margin is more than 30-km wide, and drops in elevation from greater than 2,500 m on the plateau to about 600 m at the rift ¯oor. It is attenuated, with east-dipping, distinct arcuate antithetic morphology from fault displacement in a tectonic transfer zone between the NNW- and NNE-trending Red Sea and MER tectonic domains, respectively7,8 (Fig. 1, inset). Zones of broad warping along rift margins are typical of transfer zones in extensional regions such as the east African rift system7. The transfer zone is permeated by dike swarms9, and such magma ¯ux and dike injection along steep boundary faults during rifting probably increased geothermal gradient, ductile deformation and crustal separation in the southern Afar rift margin. The close association between rifting and development of transfer zones exerts signi®cant in¯uence on structural patterns and synrift sedimentation7. The

D. R. Pevear has retired from ExxonMobil Upstream Research Company. We thank D. R. Peacor for assistance and several Cordilleran geologists for discussion, and the National Science Foundation and ExxonMobil Upstream Research Company for support of our fault gouge research.

Red Sea

Correspondence and requests for materials should be addressed to B.v.d.P. (e-mail: [email protected]).

Afar rift

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Geology and palaeontology of the Late Miocene Middle Awash valley, Afar rift, Ethiopia

Bodo

Giday WoldeGabriel*, Yohannes Haile-Selassie², Paul R. Renne³, William K. Hart§, Stanley H. Ambrosek, Berhane Asfaw¶, Grant Heiken# & Tim White²

NATURE | VOL 412 | 12 JULY 2001 | www.nature.com

Digiba Dora

Amba East

Adu Dora

Western margin

The Middle Awash study area of Ethiopia's Afar rift has yielded abundant vertebrate fossils (< 10,000), including several hominid taxa1±4. The study area contains a long sedimentary record spanning Late Miocene (5.3±11.2 Myr ago) to Holocene times. Exposed in a unique tectonic and volcanic transition zone between the main Ethiopian rift (MER) and the Afar rift, sediments along the western Afar rift margin in the Middle Awash provide a unique window on the Late Miocene of Ethiopia. These deposits have now yielded the earliest hominids, described in an accompanying paper5 and dated here to between 5.54 and 5.77 Myr. These geological and palaeobiological data from the Middle Awash provide fresh perspectives on hominid origins and early evolution. Here we show that these earliest hominids derive from relatively wet and wooded environments that were modulated

Maka Aramis

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* EES-6/MS D462; and # Institute of Geophysics and Planetary Physics, MS C303, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA ² Department of Integrative Biology and Laboratory for Human Evolutionary Studies, Museum of Vertebrate Zoology, 3060 VLSB, University of California, Berkeley, California 94720, USA ³ Berkeley Geochronology Center, 2455 Ridge Road, and Department of Earth and Planetary Science, University of California, Berkeley, California 94709, USA § Department of Geology, Miami University, Oxford, Ohio 45056, USA k Department of Anthropology, University of Illinois, Urbana, Illinois 61801, USA ¶ Rift Valley Research Service, P. O. Box 5717, Addis Ababa, Ethiopia

Asa Koma

Alayla VP-2 Saitune Dora

Bouri

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Figure 1 Location map showing measured sections along the western rift margin of the Middle Awash region of the southern Afar rift. Map based on Landsat Thematic Mapper imagery. Complex linear and arcuate NE-trending and transverse faulting is apparent along the rift margin. The broad rift margin and rift ¯oor are shown by darker and lighter shades, respectively. Other hominid sites within the Middle Awash study area are located at Aramis (4.4 Myr; Ardipithecus ramidus), Maka (3.4 Myr; Australopithecus afarensis), Bouri (2.5 Myr; Australopithecus garhi) and Bodo (0.64 Myr; Homo).

© 2001 Macmillan Magazines Ltd

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letters to nature broad western margin is currently dominated by distinct northtrending long and narrow marginal grabens that are in the middle of this expansive tectonic zone. Faulting along the western Afar rift margin began during the 26±19 Myr interval. The Early Miocene (16.6±23.7 Myr) ¯uvial, alluvial and minor lacustrine red series sediments along the base of this escarpment to the north of the Middle Awash seem to be related to ancestral marginal basins9,10. Lithostratigraphic units of the Middle Awash study area were assigned to the Awash Group11,12. Previously published stratigraphy based on isolated sections was temporally controlled by biostratigraphy. Detailed ®eld studies, aided by 40Ar/39Ar dating, palaeomagnetic data and tephrachemistry have clari®ed the geological framework and palaeoenvironments of the largely Pliocene Sagantole Formation exposed in the Central Awash Complex (CAC)13,14. The older Adu-Asa Formation was originally divided into the Adu and Asa Members12. Subsequent work required that each of these former members be divided and the resulting four members be de®ned on the basis of associated tephra markers and distinctive lithofacies15. We divide the former Adu Member into the ¯uvial Saraitu and overlying lacustrine Adu Dora Members. The type section for the Saraitu Member is in the Saraitu drainage 3 km north of Alayla (Fig. 1). Here, and at Gaisale, Asa Koma and Adu Dora North, # 50 m of variegated ¯uvial deposits and thin (3± 5 cm) basaltic and silicic tephra are exposed (Fig. 2). Basaltic lavas yielding incremental heating plateau ages of 6.33 6 0.07 Myr (MA95-1) and 6.16 6 0.06 Myr (MA95-22) underlie the Saraitu Member. These dates provide a maximum age for the Adu-Asa

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Formation (see Supplementary Information). Vertebrate fossils in the Saraitu Member are rare and mostly aquatic. The contact with the overlying diatomite sequence is sharp and is de®ned by the Ankarara Basaltic Tuff (ANBT, MA00-22). The lacustrine Adu Dora Member (type section at Adu Dora) comprises $ 20 m of white ®ssile diatomite beds, silici®ed/cherty diatomite, diatomaceous silty clays, and thin (3±20 cm) silicic and basaltic tephra layers. The sequence directly underlies the Bakella Basaltic Tuff (BABT) of the overlying Asa Koma Member at Gaisale (MA99-72), Saraitu (MA99-95) and Asa Koma (MA99-98) (Figs 1 and 2). Widespread phreatomagmatic basaltic eruptions and associated ¯uvial sediment accumulation along ancestral marginal grabens superseded primarily lacustrine deposition of the Adu Dora Member, beginning at 5.77 Myr. These units belong to the # 40-m thick Asa Koma Member (type section at Asa Koma), which consists of bentonitic and sandy silty clays and at least ®ve tephra layers with thicknesses of several metres. The Dobaado Basaltic Tuff (DOBT) at the base of the overlying Rawa Member caps it. Asa Koma Member sediments contain abundant fauna, including all hominid remains collected from the western margin5 (Figs 1 and 2). Geochronology of the hominid-bearing Asa Koma Member is ®rmly grounded by the 5.77 6 0.08 Myr old Ladina Basaltic Tuff (LABT, MA97-15) near the base of the member. Higher in the member is the Witti Mixed Magmatic Tuff (WMMT). Splits of basaltic glass in this tuff from Asa Koma (MA96-30) yielded indistinguishable plateau ages of 5.63 6 0.12 and 5.57 6 0.08 Myr,

MA96-25

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Figure 2 The lithostratigraphic sequences of the Adu-Asa Formation along the western rift margin. The four de®ned members are shown. Named and unnamed basaltic and silicic tephra stratigraphic markers are labelled. Locations of the measured sections are shown 176

in Fig. 1. SIMA, Stable isotope samples; ANBT, Ankarara Basaltic Tuff; BABT, Bakella Basaltic Tuff; DOBT, Dobaado Basaltic Tuff; HABT, Hantuuta Basaltic Tuff; LABT, Ladina Basaltic Tuff; WMMT, Witti Mixed Magmatic Tuff.



letters to nature whereas plagioclase phenocrysts from the correlative MA95-4 at Digiba Dora yielded a weighted (by inverse variance) mean age of 5.68 6 0.07 Myr for 45 individually analysed crystals. At Saitune Dora, the VP-1 and VP-2 fossil localities are capped by basalt (MA95-7), yielding a plateau age of 5.54 6 0.17 Myr. Therefore, all of the vertebrate fossils discussed here are ®rmly bracketed between 5.54 and 5.77 Myr. Palaeomagnetic samples collected in this interval (Fig. 2) yield uniformly reverse polarity, as expected from the geomagnetic polarity timescale16. Waning phreatomagmatic eruptions of the Asa Koma Member marked the local transition from the Miocene to the Pliocene17. The post-phreatomagmatic sequence of the Rawa Member at the Asa Koma type section is < 75-m thick and consists of conglomerate, reddish brown silty clays and palaeosols. The Hantuuta Basaltic Tuff (HABT, MA98-45) marks the base of an overlying, distinct lithologic assemblage that de®nes the top of the Rawa Member. The sequence above the Rawa Member at the Asa Koma section contains silicic ashes and welded ignimbrite, which are widely distributed along strike of the frontal fault blocks. They are interbedded with widespread pedogenic carbonates ($ 5-m thick), palaeosols and coarse clastic deposits, re¯ecting changes in sedimentation processes and environmental conditions. A ®fth member may be designated for the Adu-Asa Formation when the detailed investigation of the post-Rawa Member sequence is completed. The succession described here records tectonic, volcanic and sedimentary processes along the western margin and adjacent basin ¯oor during the Late Miocene. Local and regional tephra correlations of isolated sections measured in the densely stepfaulted antithetic blocks of the escarpment allowed characterization of the local stratigraphy (Fig. 2; see also Supplementary Information for microprobe glass chemistry data from marker tephra). The earliest hominid remains in Ethiopia have been discovered among more than 60 identi®ed vertebrate species on the basis of over 1,900 fossils from the Middle Awash. Approximately 15% of this collection comes from the Kuseralee Member of the Sagantole Formation of the CAC (. 5.2 Myr14). The older fossils (5.54± 5.77 Myr) from the Asa Koma Member are primarily derived from ¯uviatile deposits. Biochronological assessment is consistent with the isotopic dating. Biochronologically important mammals include the suid Nyanzachoerus syrticus and the very primitive Primelephas. The thick ¯uvial lacustrine and phreatomagmatic units of the lower Adu-Asa Formation record environmental conditions much wetter than today. This is consistent with post-rifting depositional pro®les from the Gulf of Suez and seven locations along the western coastal areas of the Red Sea18, Late Miocene to Middle Pliocene (3.4 Myr) fossil ¯oral and isotopic evidence19,20, and low-resolution pollen data from Ocean Drilling Program (ODP) site 721 (refs 21, 22). There is strong potential for linking the Middle Awash record with marine homologues using silicic tephra recovered from Red Sea, Gulf of Aden, and the western Indian Ocean cores to understand the Neogene geological and palaeoenvironmental processes of the adjacent, evolving continental and oceanic rifts. Low carbonate carbon isotope ratios indicate woodland to grassy woodland23 (20±45% grass biomass) habitats at the Asa Koma and Digba Dora hominid sites. Low oxygen isotope ratios indicate cool, high altitude and/or humid habitats24 (see Supplementary Information for stable isotope information). Additional palaeoenvironmental indicators are vertebrate fossils from the four western margin localities that have yielded ten hominid specimens. These fossil assemblages indicate predominantly wet and closed woodland/forest habitats. The abundance of reduncine bovids also indicates the presence of open woodland or wooded grassland around lake margins. Among the micromammals, Tachyoryctes (root rats) and Thryonomys (cane rat) are abundant. Extant Thryonomys species are known to live along margins of rivers and lakes25. Extant Tachyoryctes species live in NATURE | VOL 412 | 12 JULY 2001 | www.nature.com

highland settings, consistent with the Adu-Asa Formation being deposited at a higher elevation during the end of the Miocene. Alcelaphines are absent in the formation, in contrast with their abundance in the Nawata Formation of Lothagam, Kenya. The rarity of lagomorphs (hares; only one species present), shows that open grassland habitats are not well sampled in the fauna of the Adu-Asa Formation. The dominant fauna from the younger Kuseralee Member of the CAC, 20 km to the east, indicates more open woodland and/or lake marginal habitat. Rich vertebrate assemblages are known from these more easterly, axial basin depositories. These sediments have yielded abundant fossils of Anancus and Nyanzachoerus. However, only one hominid specimen has been recovered from Kuseralee Member deposits despite intensive searching. This specimen from Amba East was found associated within a stratigraphically restricted, more subaerial deposit containing carnivores, bovids and cercopithecid monkeys. Isotopic results suggest a warmer, lower altitude and/or drier grassy woodland to woodland ¯oral habitat for the hominid-associated, stratigraphically restricted CAC assemblage. The end Miocene fauna from the Asa Koma Member is roughly contemporary with the Lothagam fauna from the upper Nawata Formation of Kenya, where a mosaic of habitats was present near the Miocene/Pliocene boundary26. However, hominids are nearly absent from the Nawata Formation. The demonstration that the earliest hominids consistently derive from strata bearing indicators of wooded environments may explain their rarity at some sites. It therefore seems increasingly likely that early hominids did not frequent open habitats until after 4.4 Myr13. Before that, they may have been con®ned to woodland and forest habitats. M Received 19 February; accepted 15 May 2001. 1. Conroy, G. C., Jolly, C. J., Cramer, D. & Kalb, J. E. Newly discovered fossil hominid skull from the Afar Depression, Ethiopia. Nature 275, 67±70 (1978). 2. White, T. D. et al. New discoveries of Australopithecus at Maka in Ethiopia. Nature 366, 261±265 (1993). 3. White, T. D., Suwa, G. & Asfaw, B. Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371, 306±312 (1994). 4. Asfaw, B. et al. Australopithecus garhi: a new species of early hominid from Ethiopia. Science 284, 625± 629 (1999). 5. Haile-Selassie, Y. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412, 178±181 (2001). 6. Pickford, M. & Senut, B. The geological and faunal context of Late Miocene hominid remains from Lukeino, Kenya. C.R. Acad. Sci. Ser. IIa 332, 145±152 (2001). 7. Morley, C. K., Nelson, R. A., Patton, T. L. & Munn, S. G. Transfer zones in the East African Rift System and their relevance to hydrocarbon exploration in rifts. Am. Assoc. Petrol Geol. 74, 1234±1253 (1990). 8. Morton, W. H. & Black, R. in Afar Depression of Ethiopia (eds Pilger, A. & Rosler, A.) 55±61 (Schweizerbart, Stuttgart, 1975). 9. Mohr, P. The Morton-Black hypothesis for the thinning of continental crust-revisited in western Afar. Tectonophysics 94, 509±528 (1983). 10. Tiercelin, J. J., Taieb, M. & Faure, H. Continental sedimentary basins and volcano-tectonic evolution of the Afar Rift. Atti Convegni Lincei 47, 491±504 (1980). 11. Kalb, J. E., Oswald, E. B., Mebrate, A., Tebedge, S. & Jolly, C. J. Stratigraphy of the Awash Group, Middle Awash Valley. Afar, Ethiopia. Newsl. Sratigr. 11, 95±127 (1982). 12. Kalb, J. E. Re®ned stratigraphy of the hominid-bearing Awash Group, Middle Awash Valley, Afar depression, Ethiopia. Newsl. Sratigr. 29, 21±62 (1993). 13. WoldeGabriel, G. et al. Ecological and temporal placement of early Pliocene hominids at Aramis, Ethiopia. Nature 371, 330±333 (1994). 14. Renne, P. R., WoldeGabriel, G., Hart, W. K., Heiken, G. & White, T. D. Chronostratigraphy of the Miocene-Pliocene Sagantole Formation, Middle Awash Valley, Afar Rift, Ethiopia. Geol. Soc. Am. Bull. 111, 869±885 (1999). 15. Hedberg, H. D. International Stratigraphic Guide (John Wiley and Sons, New York, 1976). 16. Cande, S. C. & Kent, D. V. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. B 100, 6093±6095 (1995). 17. Harland, W. B. et al. A Geologic Time Scale (Cambridge Univ. Press, Oxford, 1990). 18. Grif®n, D. L. The late Miocene climate of northeastern Africa: unraveling the signals in the sedimentary succession. J. Geol. Soc. Lond. 156, 817±826 (1999). 19. deMenocal, P. B. Plio-Pleistocene African climate. Science 270, 53±59 (1995). 20. Yemane, K., Bonne®lle, R. & Faure, H. Palaeoclimatic and tectonic implications of Neogene micro¯ora from the Northwestern Ethiopian highlands. Nature 318, 653±656 (1985). 21. Van Campo, E. Pollen transport into Arabian Sea sediments. Proc. ODP 117, 277±280 (1991). 22. Ruddiman, W. F. et al. Late Miocene to Pleistocene evolution of climate in Africa and the low-latitude Atlantic: overview of Leg 108 results. Proc. ODP 108, 463±484 (1989).. 23. Cerling, T. E., Quade, J., Wang, Y. & Bowman, J. R. Carbon isotopes in soils and paleosols as ecology and paleoecology indicators. Nature 341, 138±139 (1989). 24. Quade, J., Cerling, T. E. & Bowman, J. R. Systematic variations in the carbon and oxygen isotopic


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letters to nature composition of pedogenic carbonate along elevation transects in the southern Great Basin, United States. Geol. Soc. Am. Bull. 101, 464±475 (1989). 25. Kingdon, J. East African Mammals Vol. IIB (Academic, New York, 1974). 26. Leakey, M. G. et al. Lothagam: a record of faunal change in the Late Miocene of East Africa. J. Vert. Paleontol. 16, 556±570 (1996).

Supplementary information is available on Nature's World-Wide Web site (http://www.nature.com) or as paper copy from the London editorial of®ce of Nature.

Acknowledgements The Middle Awash Project is multinational, interdisciplinary research co-directed by B.A., Y. Beyene, J. D. Clark, T. D.W. and G.W.G. The research was supported by the National Science Foundation and the Institute of Geophysics and Planetary Physics of the University of California at Los Alamos National Laboratory. Additional contributions were made by the Graduate School, the Of®ce for Advancement of Scholarship and Teaching and the Geology Department at Miami University, and the Research Board of the University of Illinois. We thank H. Gilbert for ®eld and illustrations work; H. Saegusa for proboscidean identi®cations; D. DeGusta for primate identi®cations; F. C. Howell for carnivore identi®cations; H. Wesselman and M. Asnake for micromammal analysis and identi®cations; E. Vrba for bovid identi®cations; and L. Smeenk for palaeomagnetic analyses. We thank the Ministry of Information and Culture, the Authority for Research and Conservation of the Cultural Heritage, and the National Museum of Ethiopia for permission to conduct the research. We appreciate the support of the Afar regional government and the Afar people of the Middle Awash. Access to the Electron Microprobe Laboratory and additional support from the Earth Environmental Sciences Division, Los Alamos National Laboratory, and help from P. Snow is greatly appreciated. S. Baldridge internally reviewed the manuscript at Los Alamos. Correspondence and requests for materials should be addressed to G.W.G. (e-mail: [email protected]).

................................................................. Late Miocene hominids from the Middle Awash, Ethiopia Yohannes Haile-Selassie Department of Integrative Biology and Laboratory for Human Evolutionary Studies, Museum of Vertebrate Zoology, 3060 VLSB, University of California, Berkeley, California 94720, USA ..............................................................................................................................................

Molecular studies suggest that the lineages leading to humans and chimpanzees diverged approximately 6.5±5.5 million years (Myr) ago, in the Late Miocene1±3. Hominid fossils from this interval, however, are fragmentary and of uncertain phylogenetic status, age, or both4±6. Here I report new hominid specimens from the Middle Awash area of Ethiopia that date to 5.2±5.8 Myr and are associated with a wooded palaeoenvironment7. These Late Miocene fossils are assigned to the hominid genus Ardipithecus and represent the earliest de®nitive evidence of the hominid clade. Derived dental characters are shared exclusively with all younger hominids. This indicates that the fossils probably represent a hominid taxon that postdated the divergence of lineages leading to modern chimpanzees and humans. However, the persistence of primitive dental and postcranial characters in these new fossils indicates that Ardipithecus was phylogenetically close to the common ancestor of chimpanzees and humans. These new ®ndings raise additional questions about the claimed hominid status of Orrorin tugenensis8, recently described from Kenya and dated to ,6 Myr9. The western margin of the Middle Awash contains predominantly Late Miocene sediments mostly pre-dating the Kuseralee Member at the base of the Sagantole Formation of the Central Awash Complex (CAC)10. Palaeontological work since 1992 has yielded abundant vertebrate fossils, including hominids that date to 5.2±5.8 Myr (Table 1). Environmental indicators suggest a wooded habitat7. To date, 11 hominid specimens (Fig. 1) have been recovered at ®ve localities since the ®rst (a partial mandible) was recovered from Alayla in 1997. They represent at least ®ve individuals and 178

seem to represent a single taxon, a new subspecies of Ardipithecus (see Methods). The ®rst specimen recovered was the subspeci®c holotype, ALAVP-2/10, a right mandible with M3. (Note that subscripts indicate lower teeth, superscripts upper teeth.) Four isolated left lower teeth (I2, LC, P4 and M2) are associated by spatial proximity, colour, perimortem root fracture and wear. The left I2 is metrically and morphologically comparable to known later hominid incisors and distinctively narrower than the lateral incisors of chimpanzees (Pan troglodytes). The P4 has a well developed talonid and a Tome's root rather than the single roots reported for Aramis A. ramidus11. The associated lower canine is worn apically and distally. Its mesial crown shoulder is elevated relative to the condition usually seen in modern female apes. A distinct marginal ridge is formed on the mesial lingual face. Its distal face has an exposed dentine strip from apex to distal tubercle. The large distal tubercle is shared with Aramis homologues, but the posterior orientation of the wear facet is also shared by apes with a honing canine±premolar complex. However, the distal tubercle in apes is usually worn diagonally as the upper canine extends in full occlusion below the cervico-enamel junction of the lower canine. The distal tubercle in Ardipithecus is worn horizontally. The functional implication of this distinction is a possible absence of a fully functional honing canine±premolar complex in Ardipithecus. The M3 shows small occlusal wear facets on the buccal slopes of the spiky metaconid and entoconid. The buccal cusps are highly worn, with a deep, cupped, coalesced dentine exposure centred at the protoconid. The M3 of ARA-VP-1/128 (A. ramidus) shows a different wear pattern in which both protoconid and metaconid exhibit small apical perforations in the enamel. All later hominids have cusps that are more rounded before wear. The ALA-VP-2/10 and ARA-VP-1/128 lower third molars are similar in mesiodistal dimension. However, ALA-VP-2/10 is absolutely smaller than the known ranges of A. anamensis (n = 5) and A. afarensis (n = 14), and absolutely larger than homologues in a sample of 20 common chimpanzees. The M2 displays a buccal occlusal half deeply excavated by wear, with a large, oval, cupped dentine exposure spanning the protoconid and hypoconid and a separate deep, round exposure at the hypoconulid position. As with the M3, this wear pattern is different from that of later hominids owing to the extreme wear differential between the lingual and buccal cusps. A periodontal abscess affects the P4/M1 area, and consequent lateral corpus swelling resulted in only slight hollowing from P4 to posterior M1. The submandibular fossa is shallow anteriorly. The circular, anterosuperiorly opening mental foramen is positioned at or mesial to P4 at approximately midcorpus. The preserved corpus is comparable in absolute size to AL 288-1 (Australopithecus afarensis) but is less robust at the M2 and M3 levels than AL 288-1 or KNM-LT 329 (the Lothagam mandible). ASK-VP-3/160 is a left P3 crown at an early wear stage. The root is entirely missing. The occlusal crown morphology is similar to Aramis homologues, but the mesial fovea is shallower. In mesial aspect, the mesial marginal ridge of ASK-VP-3/160 is below midcrown level. Its lingual extension bears an occlusal facet suggesting a prominent P3 protoconid. It lacks the strong mesiobuccal crown extension commonly seen in Pan P3 teeth. STD-VP-2/61 is a narrow, pointed, unworn lower right canine with three strong horizontal buccal hypoplastic lines. The distal tubercle is less prominent than on ALA-VP-2/10. The mesial crown shoulder is lower (at midcrown) than the contemporary Alayla lower canine. One morphological feature that this canine shares with chimpanzees rather than later hominids is the ¯attening of the mesiolingual face with an absence of a distinct marginal ridge de®ned by a vertical mesiolingual groove. The weak development of later hominid lower canine traits on STD-VP-2/61, as well as the tall, narrow apex, makes this the most primitive hominid canine yet found.

