Mio-Pliocene mammals from the Middle Awash, Ethiopia

Geobios 37 (2004) 536–552 www.elsevier.com/locate/geobio Original article Mio-Pliocene mammals from the Middle Awash, Ethiopia Mammifères mio-pliocène du moyen Aouache, Éthiopie Yohannes Haile-Selassie a,*, Giday Woldegabriel b, Tim D. White c, Raymond L. Bernor d, David Degusta c, Paul R. Renne e, William K. Hart f, Elisabeth Vrba g, Ambrose Stanley h, F.C. Howell c b a Cleveland Museum of Natural History, Physical Anthropology Department, 1 Wade Oval Drive, Cleveland, Ohio 44106, USA EES-6/MS D462, Institute of Geophysics and Planetary Physics, MS C303, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA c Laboratory for Human Evolutionary Studies, Museum of Vertebrate Zoology, and Department of Integrative Biology, University of California, Berkeley, CA 94720, USA d College of Medicine, Department of Anatomy, Laboratory of Evolutionary Biology, Howard University, Washington, DC 20059, USA e Berkeley Geochronology Center, 2455, Ridge Road, Berkeley, CA 94709; Department of Geology and Geophysics, University of California at Berkeley, Berkeley, CA 94720, USA f Department of Geology, Miami University, Oxford, OH 45056 USA g Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA h Department of Anthropology, University of Illinois, Urbana, Illinois 61801, USA Received 17 September 2002; accepted 24 March 2003 Available online 17 June 2004 Abstract The Middle Awash paleontological study area, located in the Afar Rift of Ethiopia, has yielded fossils spanning the last six million years. The geology and geochronology of the Mio-Pliocene sites of the study area have been refined and a reliable chronostratigraphy has been established by 40Ar/39Ar radiometric dating. The latest Miocene Adu-Asa Formation is divided into four members distinguished from each other by silicic and basaltic tuff marker horizons, most of which are dated basaltic tuffs. Radiometric dating has constrained the age of the Adu-Asa Formation to between 5.2-5.8 Ma. These dates are also supported by paleomagnetic results and biochronology. More than 2,000 fossil specimens were collected from the Adu-Asa Formation between 1992 and 2000. These fossils document 64 mammalian species belonging to 32 genera, 23 families, and 8 orders. This assemblage includes a number of new taxa. Included in the assemblage are First and Last Appearance Datums (FADs and LADs) of some groups, including the earliest record of the hominid genus Ardipithecus. Most of the taxa indicate a predominance of mesic and wooded habitat during the deposition of the Adu-Asa Formation. In these deposits, colobines, viverrids, mustelids, bovines, boselaphines, and tragelaphines are abundant, whereas alcelaphines are absent. Quantitative analyses of biogeographic relationships of the Middle Awash Late Miocene (MALM) mammalian fauna indicate stronger relationships with other African sites than with faunas from Eurasian sites. The MALM deposits have generated a critical dataset for analytic work on past environments, biogeographic relationships, and African vertebrate evolution. Moreover, the geographic position of the Middle Awash, coupled with its precise calibration and chronological span, make it a key section for interpreting latest Miocene faunal interchanges between Africa and Eurasia. © 2004 Elsevier SAS. All rights reserved. Résumé Les gisements fossilifères de la moyenne vallée de l’Aouache, situés dans le rift afar en Ethiopie, ont livré des fossiles chronologiquement répartis sur l’ensemble des six derniers millions d’années. La géologie et la géochronologie des sites Mio-Pliocènes de la moyenne vallée de l’Aouache ont été affinées et une chronostratigraphie fiable a été établie au moyen de datations radiométriques 40Ar/39Ar. La Formation d’Adu-Asa, du Miocène terminal, est divisée en quatre membres identifiés par des horizons repères d’origine volcanique (tufs cinéritiques ou * Corresponding author. E-mail address: [email protected] (Y. Haile-Selassie). © 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.geobios.2003.03.012 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 537 basaltiques). La plupart de ces horizons sont des tufs basaltiques d’âge connu. L’ensemble de cette formation est radiométriquement daté entre 5,2 et 5,8 Ma. Cet intervalle est confirmé par les données paléomagnétiques et biochronologiques. Entre 1992 et 2000, plus de 2000 fossiles ont été mis au jour dans la Formation d’Adu-Asa. Soixante-quatre espèces de mammifères, représentant 32 genres, 23 familles et 8 ordres ont ainsi été recensées. Certains de ces taxons sont nouveaux. Cet enregistrement fossile comprend aussi les dates de première apparition (FAD) ou de dernière apparition (LAD) de quelques groupes, notamment les plus anciens restes du genre hominidé Ardipithecus. La plupart des taxons indiquent que les dépôts de la Formation d’Adu-Asa ont eu lieu dans des environnements essentiellement mésiques et boisés. Ces dépôts sont marqués par l’abondance des Colobinae, Viverridae, Mustelidae, Bovini, Boselaphini et Tragelaphini, tandis que les Alcelaphini sont absents. Sur le plan biogéographique, des analyses quantitatives révèlent qu’au Miocène récent, la faune de mammifères de la moyenne vallée de l’Aouache montre des affinités plus fortes avec les autres faunes africaines qu’avec celles d’Eurasie. Les dépôts Miocènes de la moyenne vallée de l’Aouache ont fourni un ensemble de données capitales pour l’étude des paléoenvironnements, des affinités biogéographiques et de l’évolution des vertébrés africains. La position géographique de ces dépôts et l’intervalle de temps précisément daté qu’ils recouvrent font de la moyenne vallée de l’Aouache une zone-clé pour l’interprétation des échanges fauniques intervenus à la fin du Miocène entre l’Afrique et l’Eurasie. © 2004 Elsevier SAS. All rights reserved. Keywords: Paleontology; Geology; Mammals; Miocene; Middle Awash; Ethiopia Mots clés : Paléontologie ; Géologie ; Mammifères ; Miocène ; Moyenne Vallée de l’Aouache ; Éthiopie 1. Introduction The diversity and distribution of the African Late Miocene mammalian fauna is not well understood, largely due to a long-standing gap in the fossil record between 14 Ma and 4 Ma (Maglio, 1973a ; Hill, 1987,1995 ; Harrison and Mbago, 1997). However, recent fossil discoveries in East and Central Africa have provided some new data for the ca. 5-7 Ma period (Vignaud et al., 2002; Pickford and Senut, 2001; WoldeGabriel et al., 1994, 2001; Leakey et al., 1995). There are currently at least 18 African sites sampling the time between 12 Ma and 4 Ma (Fig. 1), most of them dated radiometrically (Fig. 2). The Middle Awash area, located in the Afar Rift of Ethiopia (Fig. 3), includes some radiometrically dated sites that have yielded fossil remains from the 5.2-5.8 Ma period. The first geological work in the area now known as the Middle Awash Valley was conducted between 1936 and 1938 by an Italian Geological Mission, though no fossils were reported (Gortani and Bianchi, 1973). In 1974, Maurice Taieb, a French geologist who was gathering data for his Ph.D. thesis on the Quaternary geology of the Awash Basin, mentioned, for the first time, the presence of vertebrate fossils in the Middle Awash area (Taieb, 1974). Taieb’s stratigraphic study laid the background for later geological and paleontological work in the area. The Rift Valley Research Mission in Ethiopia (RVRME), led by Jon Kalb, documented a preliminary geology of the Neogene and Quaternary deposits east and west of the modern Awash River between Gewane town in the south and Hadar in the north. The RVRME also collected fossil specimens from the Middle Awash area between 1975 and 1978. The Middle Awash Fig. 1. Distribution and approximate location of the late Miocene/basal Pliocene paleontological sites of Africa. Fig. 1. Distribution et emplacement approximatif des sites paléontologiques du Miocène supérieur/Pliocène basal d’Afrique. 538 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 Fig. 2. Temporal distribution of basal Pliocene and late Miocene sites in Africa. Fig. 2. Distribution temporelle des sites Pliocène basal et Miocène dernier d’Afrique. paleoanthropological research project, led by Tim White and the late Desmond Clark of the University of California, initiated large-scale paleontological work in the Middle Awash area in 1981 (Clark et al., 1984). This is an ongoing research project that has to date collected more than 10,000 fossil specimens sampling a diverse fauna through time. 2. Geography The Middle Awash study area (Fig. 3) encompasses 5,000 km2 with more than 80% of this area being covered by recent alluvium and/or volcanics. The modern Awash River, which centrally transects the study area, flows south to north and discharges into Lake Abbé along the Ethiopia-Djibouti border. Perennial and seasonal rivers from the highlands along the western shoulder and escarpment feed the Awash River. The elevation of the Middle Awash study area ranges from ca. 550 m adjacent to the Awash River to ca. 850 m along the foothills of the western margin. 3. Geology Tertiary/Quaternary fluviatile and lacustrine deposits, intercalated with volcanogenic and biogenic horizons, domi- Fig. 3. Location map of major fossil localities along the western margin of the Middle Awash, Ethiopia (after WoldeGabriel et al., 2001). Fig. 3. Localisation des gisements fossilifères majeurs le long de la Marge Ouest de la moyenne vallée de l’Aouache, Ethiopie (d’après WoldeGabriel et al., 2001). nate the foothills of the western margin of the Middle Awash study area. The western margin is represented by multiple faulted blocks that are antithetically tilted toward the rift floor. In most cases, the faulted blocks are terminated along strike by transverse fault systems. Deep canyons created by perennial and seasonal streams cut across the fault blocks, thereby exposing thick sections of sediments and intercalated lava flows, silicic and basaltic tephra layers, and diatomite deposits. In most cases, these canyons developed along fault lines. Normal faults, erosional gaps, and facies changes are common, impeding lithological correlation (Kalb and Jolly, 1982; Clark et al., 1984; Renne et al., 1999; WoldeGabriel et al., 2001). The sediments in the terminal Miocene/basal Pliocene section are mostly lacustrine, fresh water deltaic, fluvial and terrestrial facies, intercalated with basaltic and silicic ash fallout, thick diatomites, and lava flows. Kalb et al. (1982) initially named four geological formations in the 539 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 SAITUNE DORA VP-1 DIGIBA DORA ADU-ASA FORMATION HOMINID FOSSIL BASALTIC LAVA SAITUNE DORA VP-2 ALTERED BASALTIC TEPHRA BASALTIC TEPHRA MA95-7 5.54 NORTH ALAYLA RAWA MBR. R MA98-56 CONGLOMERATE MA98-45 (HABT) SILTY CLAY MA98-11 R R CARBONATE ASA KOMA R R R SANDS MA98-57 DIATOMITE MA99-99 MA98-46 (DOBT) MA95-5 SILICIC TEPHRA ADU DORA N R R 5.57 5.63 R MA96-30 (WMMT) 5.68 MA98-47 POLARITY Ar/Ar DATE MA95-4 SIMA98-70 CORRELATION SIMA98-76 R R MA96-25 ASA KOMA MBR. MA98-48 MA97-15 (LABT) 5.75 5.77 5.85 ADU DORA MBR. MA99-97 SARAITU MBR. MA98-10 10 MA99-98 (BABT) MA99-96 5 0m MA00-22(ANBT) MA95-1 6.16 MA95-22 6.33 Fig. 4. A composite section of the Adu-Asa Formation (after WoldeGabriel et al., 2001) Fig. 4. Section composite de la Formation d’Adu-Asa (d’après WoldeGabriel et al., 2001). Middle Awash region: the Adu-Asa, Sagantole, Matabaietu, and Wehaietu. Kalb assigned these formations to the “Awash Group,” along with two other units previously described as the Chorora Formation (Sickenberg and Schönfeld, 1975) and the Hadar Formation (Taieb et al., 1976). The Adu-Asa Formation was originally divided into the Adu and Asa Members. However, detailed geological work has resulted in the recognition of four members on the basis of associated tephra markers and distinctive lithofacies (WoldeGabriel et al., 2001; Fig. 4). The former Adu Member is now divided into the fluvial Saraitu and overlying lacustrine Adu Dora Members. The Saraitu Member is capped by the Ankarara Basaltic Tuff (ANBT). This basaltic tuff was not identified in Kalb’s (1993) section. The overlying member is the Adu Dora Member. This member is composed of lacustrine sediments, mostly diatomites, ranging in thickness from 5 m to ca. 20 m. Additional sediments within this Member are silicified diatomites, diatomaceous silty clays, and basaltic tephra layers. This member is capped by the Bakella Basaltic Tuff [BABT = Kalb’s (1993) bottom of “AAT-1”], which is exposed at numerous places along the western margin. The Asa Koma Member overlies the Adu Dora Member. This member is ca. 40 m thick and consists of bentonitic and sandy silty clays intercalated with numerous tephra layers. The Asa Koma Member is the most fossiliferous, yielding all the hominid remains from the western margin (Haile-Selassie, 2001a,b). This member is capped by the Dobaado Basaltic Tuff [DOBT = Kalb’s (1993) “AAT-3”], which also forms the base of the younger Rawa Member. Unlike the Asa Koma Member, the Rawa Member lacks phreatomagmatic basaltic eruptions and primarily consists of conglomerate, reddish brown silty clays, and paleosols. The Rawa Member is ca. 75 m thick at its type section at Asa Koma and it is capped by the Hantuuta Basaltic Tuff (HABT). No fossils have been collected from this member yet. The Kuseralee Member was originally described by Kalb et al. (1982) as the uppermost member of the Adu-Asa Formation. Renne et al. (1999) formally transferred the Kuseralee Member from the Adu-Asa Formation to the Sagantole Formation and described the member as the lowermost member of the Sagantole Formation. The fossiliferous deposits of the Kuseralee and Amba paleontological localities are within this member. These deposits are composed of lacustrine and fluvial sediments, mainly gypsiferous siltstone and claystone, with interbedded bentonite layers and sandstone. They are overlain at both locality areas by thick 5.2 Myr basaltic flows of the Gawto Member (Renne et al., 1999). 540 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 4. Geochronology All 40Ar/39Ar dates reported here were first published by WoldeGabriel et al. (2001). MA95-1 is a basaltic lava sampled from the bottom of the Saraitu Member. This sample was radiometrically dated and yielded an incremental heating plateau age of 6.33±0.07 Myr. Another basaltic lava (MA95-22), also sampled from the bottom of the Saraitu Member, yielded a similar age of 6.16±0.06 Myr. These two radiometric dates yield a maximum age for the newly defined Adu-Asa Formation of the Middle Awash. MA96-30 (Witti Mixed Magmatic Tuff; WMMT) is a mixed magmatic tuff sampled from one of the Asa Koma vertebrate localities. Splits of basaltic glass from this sample yielded radiometric ages of 5.63±0.12 and 5.57±0.08 Myr. MA97-15 (Ladina Basaltic Tuff; LABT) was sampled from Alayla vertebrate locality. This tuff underlies all the vertebrate fauna recovered from Alayla and thus yield a maximum age for the Alayla vertebrate fossils. LABT is radiometrically dated to 5.77±0.08 Myr. Paleomagnetic, radiometric and biostratigraphic correlations have yielded a precise geochronology of the latest Miocene/earliest Pliocene of the Middle Awash (Clark et al., 1984; Hall et al., 1984; Walter et al., 1985; White et al., 1993; Clark et al., 1994; WoldeGabriel et al., 1994; White et al., 1994; De Heinzelin et al., 1999; Renne et al., 1999; WoldeGabriel et al., 2001). Plagioclase phenocrysts from MA95-4, a basaltic tuff overlying fossils at Digiba Dora locality, yielded a weighted mean radiometric age of 5.68±0.07 Myr. MA95-7 is a basaltic lava flow sampled from Saitune Dora locality and yielded a radiometric plateau age of 5.54±0.17 Myr. This age also yielded a minimum age for the vertebrate fossils collected from the western margin of the Middle Awash study area. All analyzed paleomagnetic samples are consistent with the 40Ar/ 39Ar results and the paleomagnetic polarity timescale (Cande and Kent, 1995; WoldeGabriel et al., 2001). Chemical correlations among the various stratigraphic markers coupled with the age determinations of selected units have provided firm temporal constraints on the Late Miocene fossil assemblage described here. 5. Mammalian fauna The Middle Awash Late Miocene (MALM) deposits sample mammalian faunas that are now securely dated to between 5.2 Ma and 5.8 Ma. Sixty-four mammalian species have been identified, belonging to 23 families, and 8 orders, suggesting a much higher mammalian diversity than previously known during the Late Miocene of Africa (Fig. 5; Boaz et al., 1987; Pickford and Senut, 2001; Leakey et al., 1996). These deposits also record a number of first and last appearances for various mammalian taxa among rodents, lagomorphs, carnivores, and artiodactyls. Among the rodents, Paraxerus, Xenohystrix, Atherurus, and Tachyoryctes make their first appearance in the African fossil record in the Middle Rodentia Muridae M urinae cf. Lemniscomys sp. Gerbillinae Gen. et sp. indet. Rhizom yidae Tachyoryctes sp. nov. Thryonom yidae Thryonom ys sp. nov. Sciuridae Paraxerus sp. Hystricidae Hystrix sp. Xenohystrix sp. indet. Atherurus sp. indet Lagomo rpha Leporidae ? Alilepus sp. Lepus sp. Prima tes Hom inidae Homininae Ardipithecus ramidus kadabba Cercopithecidae Colobinae gen. et sp. indet Cercopithecinae Parapapio cf. P. lothagame nsis Macaca sp. Carnivora Viverridae Viverrinae Genetta sp. Viverra cf. V . leakeyi Herpestinae Herpestes sp. indet. Helogale aff. H . kitafe Mustelidae Lutrinae Gen. et sp. indet Enhydriodon sp. M ellivorinae Mellivora aff. M. benfieldi Guloninae Plesiogulo a ff. P. monspessulanus Hyaenidae Hyaenictitherium sp. Hyaenictis sp. Felidae M achairodintinae Machairodus sp. Dinofelis sp. Felidae g en et sp. indet Ursidae Agriotherium s p. Artiodactyla Bovidae Boselaphini Tragoportax sp. nov. Bovini Gen. et sp. indet Sima therium aff. S. de missum Hippotragini Gen. et sp. indet Tragelaphini Tragelaphus sp. A Tragelaphus sp. B Reduncini Gen. et sp. indet.. Kobus sp. nov. Kobus aff. K . porrecticornis Kobus subdolus Antilopini Gazella sp. cf. Prostrepsiceros aff. P. liybicus Madoqua sp. Neotragini Raphiceras sp. Suidae Tetraconodontinae Nyanzachoerus syrticus Ny. cf. devauxi Ny. cf. waylandi Ny. australis Nyanzachoerus sp. indet. Subfamily indet. Cainochoerus aff. C . africanus Giraffidae Sivatheriinae Sivatherium sp. Giraffinae Palaeotragus s p. Giraffa sp. Hippopotamidae Hexaprotodon cf. H. harvardi Perissodactyla Equidae Gen. et sp. indet. Eurygnathohippus turkanense Eurygnathohippus sp. (small) Rhinocerotidae cf. Brachypotherium lewisi Ceratotherium cf. C . praecox Diceros sp. Proboscidea Gom photheriidae Anancus cf. A. kenyensis Anancus sp. indet. Elephantidae Primelephas gomphotheroides Mam muthus subplanifrons Stegodibelodon s chneideri Deinotheriidae Deinotherium s p. Tubulidentata Orycteropodidae Orycteropus sp. Fig. 5. Composite faunal list of terminal Miocene mammals from the AduAsa and lower Sagantole Formations, Middle Awash, Ethiopia. Fig. 5. Liste composite des mammifères des formations miocènes terminales de l’Adu-Asa et Sagantole inférieure, moyenne vallée de l’Aouache, Éthiopie. Awash deposits. These genera were previously known only from sites younger than 3.5 Ma. Among the lagomorphs, the earliest record of the genus Lepus is documented. This genus was previously known only from the Pliocene to recent. At least six genera of carnivores appear in sub-Saharan Africa for the first time in the MALM deposits (Herpestes, Helogale, Mellivora, Plesiogulo, Machairodus, and Agriotherium). Most of these genera were previously reported from the Vaarswater Formation of Langebaanweg, South Africa, from strata possibly slightly younger than 5 Ma (Hendey, 1974). None of these genera have been reported from contemporaneous deposits at Lothagam and Lukeino, Kenya. Most of the artiodactyl genera and species identified from the MALM deposits were also present at other contemporaneous African sites. However, the bovid remains from the Middle Awash suggest the presence of a much higher diversity in some bovid tribes. The nyanzachoere suids were also highly diversified. The large number of specimens within Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 different individual nyanzachoere species has also clarified some of the dental evolutionary trends in the genus. The 5.2 Ma Amba locality in the Central Awash Complex (CAC) also documents the last appearance of Ny. syrticus and possibly the best evidence for the first appearance of Ny. australis. 5.1. Primates Primates are generally rare in the Late Miocene fossil record of Africa. This low abundance and diversity characterizes almost all Late Miocene African sites (Leakey et al., 1996). However, recent discoveries have increased the number of known genera and species of hominids and cercopithecids from this time period (Brunet et al., 2002; Pickford and Senut, 2001; Haile-Selassie, 2001b; White et al., 1994,1995; Leakey et al., 2003). Primates comprise 7.4% of the total mammalian genera and 6.2% of the total mammalian species identified from the Late Miocene deposits of the Middle Awash (Fig. 6). Hominids are represented by one species of Ardipithecus (HaileSelassie, 2004). Although there are a plethora of definitions for the family Hominidae (Simpson, 1945; Goodman, 1963; Koop et al., 1986; Groves, 1986; Miyamoto et al., 1988; Schwartz et al., 1978; Goodman et al., 1998; Ruvolo, 1997; Cela-Conde, 1998; Wood and Richmond, 2000; Strait and Wood, 1999; Strait, 2001), it is defined here as a family comprising humans and their relatives subsequent to the split from their common ancestor with African great apes. Even A) Percentage of genera represented in each Order Tubulidentata (2%) Prodoscidea (9%) Perissodactyla (7%) Rodentia (15%) Lagomorpha (4%) Primates (7%) Artiodactyla (30%) Carnivora (26%) B) Percentage of species represented in each Order Tubulidentata (2%) Prodoscidea (9%) Perissodactyla (9%) 541 though there is little or no information on the morphology or environment of Late Miocene African hominoids (Andrews and Humphrey, 1999), recent discoveries indicate that the earliest hominids such as Orrorin tugenensis, Ardipithecus kadabba, and A. ramidus may have lived in closed wooded habitats (Pickford and Senut, 2001; WoldeGabriel et al., 1994, 2001). Despite their abundance in the Plio-Pleistocene, cercopithecids are relatively rare elements in the Late Miocene fossil record of the Middle Awash. Their sample is mostly composed of isolated teeth and fragmentary postcranial elements. However, there are at least four distinct species in the Adu-Asa and lower Sagantole Formations. Two cercopithecine species are assigned to Parapapio sp. indet. and Papionini gen. et sp. indet. The Parapapio from the Middle Awash shows some affinities with P. lothagamensis (Leakey et al., 1996) from the Nawata Formation of Lothagam. However, it differs from P. lothagamensis by a number of mandibular characters. The second cercopithecine species, referred to Papionini gen. et sp. indet, has affinities with Macaca. However, its dental morphology resembles colobines rather than later cercopithecines. The molars are relatively high-crowned compared to terrestrial cercopithecines, who have lower-crowned molars (Fleagle and MacGrew, 1999). This may be a result of adaptation to a habitat different from that of later terrestrial cercopithecines. The Middle Awash Papionini might have been an arboreal species feeding on seeds and leaves like extant arboreal colobines. Two colobine species are identified in this assemblage. The larger colobine is comparable in dental morphology and size to Rhinocolobus. This colobine seems to be a highly sexually dimorphic species based on canine size, though it is possible that two species might be represented in the sample. The smaller colobine, with its thin supraorbital torus and relatively thin interorbital crest, resembles the new Cercopithecoides species from Lothagam (Leakey et al., 2003). Most of the postcranial elements recovered are fragmentary and are tentatively assigned only to the subfamily level. Moreover, there appears to be considerable morphological overlap between the two tribes due to adaptation to similar habitats, rendering taxonomic identification of fragmentary postcranial remains difficult. Rodentia (12%) Lagomorpha (3%) 5.2. Lagomorpha Primates (6%) Carnivora (22%) Artiodactyla (37%) Fig. 6. Histograms showing the percentage of mammalian genera (A) and species (B) recovered from the Middle Awash late Miocene deposits. Fig. 6. Histogrammes montrant le pourcentage de genres (A) et d’espèces (B) de mammifères récoltés des dépôts miocènes supérieurs de la moyenne vallée de l’Aouache. African fossil leporids have only been known from the Middle Pliocene and Pleistocene deposits of Omo, Ethiopia (Wesselman, 1984), Koobi Fora, Kenya (Harris, 1978), South African cave sites (Cooke, 1963), and Olduvai Bed I, Tanzania (Leakey, 1965). Their presence in the Late Miocene has not been well documented, suggesting that they were not then as diversified as they became in the Plio-Pleistocene. Leporids are represented in the Adu-Asa Formation of the Middle Awash by isolated teeth tentatively referred to Lepus sp. The material assigned here to the genus Lepus and a 542 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 second genus, Alilepus, from the Nawata Formation of Lothagam constitute the earliest African records of the family. A single tooth from Lukeino has also been assigned to Alilepus. There seems to be dental plesiomorphy in the family Leporidae, making it difficult to securely assign isolated fossil lagomorph teeth to a lower taxonomic level. Given the age of the fossils, the referral of the Adu-Asa specimens to the extant Lepus capensis is unwarranted despite their general morphological similarity. Therefore, the dental remains are tentatively referred to Lepus sp. indet. until more complete cranial remains are recovered. Most extant leporids inhabit open grassy areas. However, there are also some leporid species that are found in evergreen forests (Nowak, 1991). The leporid from the Adu-Asa Formation could very well be an indicator of the presence of open grassy areas. 5.3. Rodentia Eight rodent species belonging to eight genera are recognized from the MALM deposits, comprising 14.8% of the total identified mammalian genera and 12.3% of the total identified mammalian species. At least seven genera and seven species in five families are known from the Adu-Asa and lower Sagantole Formations. This collection includes the earliest record of Thryonomys, Tachyoryctes, Xenohystrix, and Atherurus in Africa. Rodents were equally diverse in the MALM deposits compared to their record in the contemporaneous Nawata Formation of Lothagam, where at least 7 species have been reported (Winkler, 2003). However, the species composition at the two sites seems different. Fossil rhizomyids are best known by multiple genera from the Siwalik sequence of Southwest Asia (Jacobs, 1978; Black, 1972). Their fossil record in Africa is largely limited to the Plio-Pleistocene. However, the discovery of an early Tachyoryctes species in the Adu-Asa and Sagantole Formations of the Middle Awash pushes the genus’ African fossil record to the Late Miocene. A number of rhizomyid dentognathic remains from the MALM are here tentatively referred to Tachyoryctes sp. Preliminary analysis shows that the recovered specimens may represent a new species of Tachyoryctes closely related to the Asian Kanisamys lineage. Tachyoryctes sp. from the MALM deposits is more similar to Kanisamys than to the other tachyoryctine genera. Sabatier (1978) also reported that the Pliocene species, Tachyoryctes pliocaeneus from Hadar resembles the Kanisamys lineage of the Siwaliks. The Middle Awash Tachyoryctes establishes a close relationship between the Asian and African tachyoryctines. It would be the best candidate for the ancestry of African fossil and extant tachyoryctines with a descent from Kanisamys or a younger Kanisamys-like Protachyoryctes. The available evidence, therefore, suggests that extant and fossil tachyoryctines form a monophyletic group. Extant Tachyoryctes live in highland settings (Kingdon, 1974). The genus is limited to Africa and inhabits wet uplands at elevations of up to 4,500 meters (Kingdon, 1974; Nowak, 1991). The presence of Tachyoryctes in the Adu-Asa Formation possibly suggests that the formation was deposited at a much higher elevation than today. This suggestion is also consistent with stable isotope data collected from the Formation (WoldeGabriel et al., 2001). Murids are represented in the Adu-Asa Formation by at least two genera. These are represented by an as yet undescribed gerbilline genus and species, and a species similar to Lemniscomys. The sample size of the gerbil is limited to a single specimen and its formal naming and description has to await recovery of additional specimens. Lemniscomys is also identified from a single specimen. Murids are better known from contemporaneous deposits of the Nawata Formation of Lothagam (Winkler, 2003). Their rarity in the MALM deposits is either due to habitat differences between the two areas or taphonomic factors and sampling biases. Sciurids are one of the rare elements in the MALM deposits. The family is represented by a single genus, Paraxerus. The earliest squirrels in Africa are documented in the Late Miocene deposits of Africa (Kingdon, 1974) and they may not have been diverse during the deposition of the Adu-Asa Formation. Extant Paraxerus (African bush squirrels) are known to live in forests, plantations, and other areas where trees are available (Kingdon, 1974). Thryonomyids are represented in the Adu-Asa Formation by one genus and species, Thryonomys sp. The two extant Thryonomys, T. swinderianus and T. gregorianus, have different habitat niches. T. swinderianus is semi aquatic and lives in marshes and reed beds whereas T. gregorianus lives in dry ground in moist savanna (Kingdon, 1974). The fossil Thryonomys from the Adu-Asa Formation is comparable in size to T. swinderianus and may also have been semi aquatic. Three hystricid genera are represented in the MALM deposits: Xenohystrix, Atherurus, and Hystrix. The large Xenohystrix (previously known only from younger deposits at Langebaanweg, Omo, Laetoli, and Hadar) and Atherurus document the earliest record of the genera. Hystrix has also been documented from other contemporaneous sites such as Lothagam (Leakey et al., 1996; Winkler, 2003) and Lukeino (Hill et al., 1986; Pickford and Senut, 2001). This genus is ubiquitous in most African Plio-Pleistocene sites. The three hystricid genera from the Middle Awash are mostly represented by isolated teeth or fragmentary mandibles. 5.4. Carnivora Carnivores are relatively diverse in the Adu-Asa Formation and Kuseralee Member of the Sagantole Formation. At least 13 species belonging to 12 genera and five families have been recognized, representing 26% of the identified mammalian genera and 22% of the identified mammalian species. Amphicyonids, despite their presence in the contemporaneous Nawata Formation of Lothagam (Werdelin, 2003), are absent from the Middle Awash. Felids are the dominant carnivores in the MALM deposits, constituting 37% of the total number of carnivore specimens identified to the lowest taxonomic level. Large felids include Machairodus, Dinofe- Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 lis, and Homotherium. A caracal-sized felid is also present even though it has not been identified at the genus or species level because of the fragmentary nature of the remains. Dinofelis is represented by limited dentognathic and postcranial remains. The dental remains are similar in size and morphology to those found from the Upper Nawata Member of the Lothagam Formation and assigned to Dinefelis sp. A. (Werdelin and Lewis, 2001; Werdelin, 2003). A large Machairodus-like felid from the Lothagam Nawata Formation has been assigned to a new genus and species, Lokotunjairulus emageritus (Werdelin, 2003). However, the Middle Awash Machairodus is different from the Lothagam form by being larger in size (the size of M. giganteus), and having a shorter and wider M1 and might very well represent a different genus and species. Two hyaenid genera, Hyaenictitherum and Hyaenictis, are present. These genera are also known from other terminal Miocene sites of Africa (Leakey et al., 1996; Pickford and Senut, 2001). The size and morphology of the mandible and teeth of Hyaenictis sp. are comparable to Hyaenictitherium. However, the retention of P1 in the latter distinguishes the two taxa. The Hyaenictitherium recovered from the Middle Awash is also much larger than Hyaenictis based on M1 size. The small number of Middle Awash specimens referred to Hyaenictitherium sp. does not allow a detailed phylogenetic analysis. However, the size and morphology of the M1 indicate close relationship with Hyaenictitherium namaquensis from Langebaanweg, South Africa (Hendey, 1978). A similar, but slightly smaller Hyaenictitherium has been described from the Nawata Formation of Lothagam and assigned to Hyenictitherium cf. H. parvum (Werdelin, 2003). However, Werdelin (2003) noted dental morphological differences between H. parvum and the Lothagam form and concluded that the Lothagam form might represent a new Hyaenictitherium species. Dental and mandibular specimens referred to Hyaenictis sp. from the Middle Awash are morphologically and metrically similar to the Lothagam material referred to Hyaenictis sp. Both hypodigms lack P1 and retain the metaconid on the M1. The Middle Awash Hyaenictis sp. differs from Hyaenictis gracea (Gaudry) by its retention of the metaconid on the M1 and lack of P1. Hyaenictis sp. and H. hendeyi (sensu Werdelin et al., 1994) are similar in the lack of P1 and the absence of a hypoconulid on the M1 talonid. However, the two are different because the latter lacks a metaconid on its M1. It can be inferred that the presence of metaconid on the hyaenid M1 is a primitive retention for Hyaenictis. Its absence in H. hendeyi makes that species more derived than the Middle Awash and Lothagam Hyaenictis. Therefore, Hyaenictis sp. could be a possible ancestor of H. hendeyi. Moreover, these relationships would suggest that the Middle Awash Hyaenictis is much older than the Langebaanweg H. hendeyi. Ursids are represented by one genus, Agriotherium, and the Middle Awash form is probably larger than Agriotherium africanum from Langebaanweg. Agriotherium africanum is 543 the oldest ursid species recognized in Africa. It was first documented from Langebaanweg (Hendey, 1974) and is known by numerous cranial, dental, and postcranial remains from that site (Hendey, 1980). Another ursid, Indarctos atticus, is known only by a lower second molar and a left proximal femur, and possibly a left distal humerus, recovered from Sahabi (Libya; Howell, 1987). Agriotherium has also been reported from Lothagam (Howell, 1982; Leakey et al., 1996) and more recently from the Basal Member of the Nkondo Formation of Uganda (Petter, 1994). However, Werdelin (2003) contends that there is no Agriotherium at Lothagam and assigns all the specimens previously referred to Agriotherium, into genera in the family Amphycionidae. The last appearance of the genus Agriotherium is perhaps in the early Pliocene Aramis Member (Renne et al., 1999) of the Middle Awash (White et al., 1994). Mustelids are represented in the Adu-Asa Formation by four species belonging to four genera. They constitute 24% of the total number of carnivore specimens. The Mustelids from the Adu-Asa Formation include the earliest African record of Plesiogulo, a genus previously known only from younger deposits at Langebaanweg (Hendey, 1978). Enhydriodon sp. is represented by isolated teeth, whereas Mellivora aff. M. benfieldi is represented by a number of dental and mandibular specimens. A large mustelid, yet unidentified at the genus or species levels, is also represented by a complete humerus. Mustelid fossil remains have been recovered from Ngorora, Lukeino, Lothagam, Nkondo (Uganda), and Langebaanweg. However, they are not documented from Sahabi (Howell, 1987). The mustelids from Ngorora and Lukeino are tentatively assigned to Sivaonyx sp. and Lutrinae sp. (Pickford and Senut, 2001), respectively. The mustelid remains from Lothagam include four species of musteline, mellivorine, and lutrine (Leakey et al., 1996; Werdelin, 2003). Werdelin (2003) described two new genera and three new species of the family Mustelidae. Ekorus ekakeran is a new musteline genus and species of quite large size. Erokomellivora lothagamensis is a new small-sized mellivorine also named from the Nawata Formation. A second mellivorine is also present but its genus and species are unknown. Werdelin (2003) also has identified a new lutrine species of the genus Vishnuonyx. The mustelid genera and species represented in the Middle Awash appear to be substantially different from those at Lothagam probably indicating age and/or habitat difference between the two areas. Viverrids are represented by four species belonging to four genera in two subfamilies. This sample includes Viverra, Genetta, Herpestes, and Helogale. The latter two genera were previously known only from Pliocene deposits of East Africa. The Genetta and Viverra species from the Middle Awash are similar to those from Lothagam. Numerous cranial, dental, and postcranial remains of Viverra leakeyi and two species of Herpestes, indeterminate at a specific level, and possibly Genetta sp. have been reported from the “E” Quarry of Langebaanweg (Hendey, 1974). The viverrid hypodigm of Sahabi is composed of a right lower carnassial and 544 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 a left humeral diaphysis, which probably represent two distinct Viverra species (large and small; Howell, 1987). Werdelin (2003) recognizes at least five viverrid species from the Nawata Formation. Two of them are assigned to cf. Genetta sp. A and cf. Genetta sp. B. A larger species is assigned to Viverridae gen. et sp. indet. The other two species are assigned to Viverrinae sp. indet. and Viverra cf. V. leakeyi. 5.5. Proboscidea Gomphothere remains are quite common elements in African Late Miocene-Early Pliocene deposits. However, most of these remains are assigned to a single genus, Anancus. The MALM deposits exhibit the presence of two contemporaneous and sympatric species of the genus Anancus at the end of the Miocene. The larger Anancus is tetralophodont and is recognized here as Anancus cf. A. kenyensis (MacInnes, 1942). The second species, here referred to as Anancus sp. indet., is a small form with the size and morphology of the M3 being different from Anancus cf. A. kenyensis. A total of 776 elephantid fossil specimens were collected from the Late Miocene deposits of the Middle Awash. These specimens, added to what has been previously collected by the RVRME, represent three species: Primelephas gomphotheroides, cf. Stegodibelodon schneideri, and Mammuthus sp. Kalb and Mebrate (1993) reported the presence of Stegodon cf. S. kaisensis and Stegotetrabelodon orbus, in addition to the former three species. This was largely based on a small sample size and misidentification of specimens. The current craniodental material from a large sample size clearly shows that S. orbus and S. cf. S. kaisensis are not present in the MALM localities. Detailed study of the MALM elephantids is underway. Deinotherium is a rare taxon in the terminal Miocene African fossil record compared to its Miocene record in Eurasia. The deinothere fossil remains from the Adu-Asa and Kuseralee Formations of the Middle Awash are limited to fragmentary dental remains and a detailed systematic analysis is therefore impossible. They are here tentatively referred to Deinotherium sp. until more complete specimens are recovered. The deinothere remains from the Lothagam Nawata Formation are assigned to D. bozasi (Leakey et al., 1996). Its presence at Lukeino has also been reported and referred to Deinotherium sp. (Pickford and Senut, 2001). This species is otherwise present in almost all Pliocene African localities (Bernor and Pavlakis, 1987). 5.6. Perissodactyla Equids are known from the MALM by at least two species of the genus Eurygnathohippus. The equid fossil remains are principally isolated teeth, though there are some mandibular and maxillary fragments in addition to a few postcranial elements. The referral of most of the isolated teeth to the genus Eurygnathohippus is based on the presence of ectostylid on the permanent lower cheek teeth (Bernor and Armour-Chelu, 1999; Bernor and Harris, 2003). Eurygnathohippus turkanense is the most common equid in the MALM deposits, as in other contemporaneous African localities such as Lothagam (Leakey et al., 1996). Bernor and Harris (2003) have modified Hooijer and Maglio’s (1973) diagnosis of Eurygnathohippus turkanense (Leakey et al., 1996). While retaining the Eurygnathohippus dental morphology, the second equid species is much smaller than E. turkanense and it is assignable to Eurygnathohippus feibli (Bernor and Harris, 2003). There are additional isolated teeth which lack an ectostylid on the lower permanent molars and may represent a different genus and hence, is tentatively referred to Equidae gen. et sp. indet. Rhinocerotid fossil remains are relatively rare in the AduAsa Formation of the Middle Awash. However, the small number of recovered rhinocerotid specimens show the presence of a large rhinocerotid similar to Brachypotherium lewisi from Lothagam, one species of Diceros similar to D. bicornis, and a third species, Ceratotherium sp., from the slightly younger Sagantole Formation. The latter two genera have extant relatives that are dedicated grazers and browsers, respectively. However, the habitat preference and dietary adaptation of Brachypotherium is currently unknown. The terminal Miocene fossil record of rhinocerotids in Africa is scanty. There are only four species documented from the terminal Miocene sites of Sahabi, Lothagam, and Langebaanweg: Brachypotherium lewisi, Diceros bicornis, Ceratotherium neumayri, and Ceratotherium praecox. All of these species, except C. neumayri are known from Lothagam. Brachypotherium lewisi is known only from the lower Nawata, which is older than most of the MALM localities. 5.7. Artiodactyla Artiodactyls are the most abundant elements in the MALM, as in all other East African Late Miocene sites, comprising 32% of all identified mammalian fossil specimens. There are at least six Nyanzachoerus species identified from the Adu-Asa Formation and the Kuseralee Member of the Sagantole Formation: Nyanzachoerus syrticus, N. devauxi, N. kanamensis, N. australis, N. waylandi, and a new nyanzachoere species currently under study (HaileSelassie and White, in prep.). Most of these taxa are also known from other contemporaneous East African sites. In terms of their distribution in the MALM deposits, Ny. syrticus and Ny. devauxi are limited to the western margin localities dated to older than 5.5 Myr, except one specimen referable to Ny. syrticus from the Central Awash Complex (CAC), whereas Nyanzachoerus kanamensis australis (= Ny. australis), and Ny. waylandi, are limited to the terminal Miocene CAC localities. In addition, the small tayassuid-like suid, Cainochoerus, is also present in the Middle Awash. This species has been previously documented from Langebaanweg, and contemporaneous deposits at Lothagam. The MALM fossil record shows that the tetraconodont suids Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 (nyanzachoeres) were highly diversified around the Miocene/Pliocene boundary, as were the reduncines and boselaphines. Bovids constitute 19% of all mammalian genera and 21.5% of all mammalian species identified from the MALM deposits. Out of this, 43% are bovines and 57% are antilopines. At least eleven species in seven tribes are identified, six of which also occur in sub-Saharan Africa today. Boselaphines were abundant during the Eurasian and African Late Miocene, and are present in the MALM. Today, they are restricted to Asia. They probably became extinct in Africa at the end of the Miocene. Boselaphine remains from the MALM are tentatively referred to Tragoportax sp. indet. until detailed study is undertaken. The Lothagam boselaphines have been referred to three different species (Harris, 2003a). Tragoportax sp. indet. from the Middle Awash is absolutely larger than Tragoportax sp. A and Tragoportax sp. B from Lothagam. The size and morphology of Tragoportax sp. indet. from the Middle Awash overlaps with what has been referred to as T. aff. T. cyrenaeicus from Lothagam However, the Middle Awash Tragoportax has more bilaterally compressed horn cores and stronger anterior keel. Other cranial and dental features seem to also differentiate Tragoportax sp. indet. from the Middle Awash from all other known species of the genus. Two bovine groups are identified from the MALM: Simatherium aff. S. demissum and Bovini gen. et sp. indet. The assignment of horn core and dental remains from the Middle Awash to the former group is largely based on their similarity to Simatherium demissum from Langbaanweg. The horn cores referred to Bovini gen. et sp. indet. have primitive horn core features that are otherwise documented for early Pliocene European and Indian buffaloes such as Parabos and Proamphibos, and Late Pliocene taxa such as Hemibos acuticornis. The Boselaphini and Bovini fossil remains from the MALM deposits are of interest in terms of the phylogenetic relationships between the boselaphines, the extant Bos/Bison, and Bubalus/Syncerus groups. Tragoportax and Miotragocerus appeared in Africa during the Middle Miocene and diversified during the Late Miocene, before going extinct at the end of the Miocene (Gentry, 1999b). Hence, the presence of many boselaphine species in terminal Miocene deposits of East Africa may not be a surprise. However, this also raises a question of how they suddenly became extinct toward the end of the Miocene in Africa. It is also at this time that tragelaphines become more diverse suggesting that the boselaphine niche was probably filled by the tragelaphines. Two tragelaphine species are identified from the MALM based on size, horn core morphology, and hypsodonty. The larger species, Tragelaphus sp. “A”, has horn cores that are anteroposteriorly less compressed, the posterolateral keel very strong all the way to the tip, and large supraorbital pits. The horn cores are inserted upright and they slightly curve medially toward the distal end. The molars are relatively less hypsodont. The horn cores assigned to this species are mor- 545 phologically and metrically similar to KNM-LU 852 from Lukeino, a specimen assigned to T. cf. spekei by Thomas (1980). The second species, Tragelaphus sp. “B”, is the size of the modern Nyala (T. angasi) based on the size of its horn cores. Tragelaphus sp. “B” is absolutely larger in size than Tragelaphus sp. “A”. The spiralling of the horn cores is comparable to the modern T. angasi. Moreover, the horn cores do not converge distally as in T. kyaloae, or become parallel as in T. nakuae. Reduncines are represented in the MALM by at least four species. A number of specimens are assigned to Kobus sp. aff. K. porrecticornis based on their similarity with K. porrecticornis from the Dhok Pathan Formation of the Siwaliks sequence. Some horn core specimens are assigned to Kobus sp. indet. These specimens are different in their morphology from all other known reduncines and apparently represent a new species. K. subdolus, a species previously known from Langebaanweg, South Africa, is also present in the MALM, in addition to another reduncine group currently unidentified at a genus or species level. Alcelaphines are absent in the Adu-Asa Formation, whereas reduncines and tragelaphines are abundant. The absence of alcelaphines may be an indication of the absence of a vast expanse of large open grasslands. The abundance of tragelaphines and reduncines indicates a mixture of woodland and grassland biomes. Bovines, antilopines, hippotragines, and neotragines were also relatively abundant in the Adu-Asa Formation. Giraffids are rare in the MALM fossil record. However, the recovered specimens indicate the presence of one sivathere, Sivatherium sp., and two giraffines, Palaeotragus sp. and Giraffa sp. These species are also known from other contemporaneous sites in Africa and elsewhere. Sivatherium is the most common giraffid at Langebaanweg and younger East African sites dated to between the Early Pliocene and Middle Pleistocene (Churcher, 1978). However, it is rare both at Lothagam and in the MALM deposits. A species of Palaeotragus, better known from other contemporary sites such as Lothagam (Harris, 2003b), is known from the Middle Awash by a limited number of dental remains. The MALM remains here referred to the genus Giraffa possibly represent the earliest record of the genus in East Africa since all other Giraffa species are documented from Plio-Pleistocene deposits. Giraffa is not described from the Nawata Formation, Lothagam, although Giraffa stillei is documented from younger deposits. Most of the hippopotamid specimens from East African Late Miocene sites are usually referred to a single species, Hexaprotodon harvardi (Coryndon, 1977). The hippopotamid sample from the Late Miocene deposits of the Middle Awash is limited to isolated dental and postcranial materials and referred to Hexaprotodon harvardi. However, some isolated teeth indicate that there might have been a second species larger than H. harvardi. This species is better known from the Nawata Formation, Lothagam. Its presence during the Late Miocene is also documented from other sites such as Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 the Sahabi (Gaziry, 1987), Abu Dhabi (Gentry, 1999a), and Spain (Lacomba et al., 1986). Fossil specimens of the genus are also known from the Plio-Pleistocene of East Africa (Harris, 1991) and Southeast Asian (Colbert, 1943) localities. Weston (2000) has recently recognized a new small Hexaprotodon, H. lothagamensis, from the lower part of the Nawata Formation of Lothagam. This new species has not been identified from the Middle Awash yet. 6. Paleoenvironment Reconstruction of past environments is largely dependent on the availability of extant counterparts to the fossil taxa recovered. Most of the identified genera from the MALM deposits have extant counterparts that may yield clues about their extinct relatives. However, Solounias and DawsonSaunders (1988) caution that extant taxa do not necessarily indicate similar habitats for their extinct relatives. Therefore, past environments reconstructed on the basis of fossil fauna and comparisons with extant relatives may not be entirely accurate, unless supported by other lines of evidence such as functional anatomy or carbon isotope analysis. Kalb and Jolly (1982: 124) have previously stated that the paleofaunas from the Middle Awash Adu-Asa and Sagantole Formations indicate “non-closed-forest biotas, presumably including a mosaic of woodland, bush, grassland and waterside habitats.” This inference was based on a small number of taxa across a time span of more than 2.5 Ma. As a result, its accuracy is questionable given the inadequacy of the sample. The mammalian fossil assemblage from the localities in the Adu-Asa Formation and Kuseralee Member has now increased to 64 mammalian species in 23 families, and 8 orders. Although this may still be inadequate for high precision local or regional paleoenvironmental reconstruction, the MALM faunal assemblage is more diverse than most of the known Late Miocene African faunal assemblages. The Adu-Asa Formation is characterized by fluvial and lacustrine deposits intercalated by basaltic and silicic tephra, and basaltic lavas. The thick fluvial, lacustrine, and phraetomagmatic units of the formation indicate the presence of lakes and river systems during the deposition of the Adu-Asa Formation. Additional data derived from carbon isotope analyses suggests the presence of woodland to grassy woodland habitat at some of the vertebrate localities in the AduAsa Formation (WoldeGabriel et al., 2001). Stable isotope analysis of oxygen and carbon in pedogenic carbonates provides an independent line of evidence for reconstructing past floral habitats and environments (Cerling, 1984; Cerling and Quade, 1993). Low d13C values (-10‰ to -12‰) reflect C3 plant biomes (mainly trees, shrubs and other dicotyledenous plants), and high values (>0‰) reflect open C4 (tropical grassland) floras. Oxygen isotope ratios are influenced by a complex combination of water source composition, elevation, temperature, relative humidity, and evaporation. Higher d18O values reflect higher tem- 10 8 Carbonate δ18O PDB ‰ 546 6 0.3 2.5 Open warm dry 4 1.0 2 0 Asa Koma Hilltop Digiba Dora Upper Amba 2.5 -2 2.5 Age ( Myr) -4 4.4 -6 Closed cool -8 humid -10 -10 -8 -6 -4 -2 0 2 Carbonate δ 13C PDB ‰ Fig. 7. Stable carbon and oxygen isotopic composition of Western Margin pedogenic carbonates, expressed in parts per thousand (‰) difference (d) from a standard (PDB), calculated as follows: d‰ = (Rsample/Rstandard) – 1 × 1000, where R is the ratio of the heavier to the lighter isotope, and the standard is the Pee Dee Belemnite fossil carbonate. Ellipses indicating ranges of variation in isotopic composition in selected time horizons in the Middle Awash (Ambrose, in preparation) are included for comparison. The late Miocene sites have isotopic compositions that reflect the most closed, humid, cool habitats of any period in the Middle Awash Valley. Fig. 7. Composition isotopique du carbone et de l’oxygène des carbonates pédogéniques de la Marge Ouest, exprimé en partie pour mille (‰) d’une différence (d) par rapport à un standard (PDB), calculé comme suit : d ‰ = (Rsample/Rstandard) – 1 × 1000, où R est le rapport de l’isotope lourd par rapport au léger et le standard est le carbonate fossile de Pee Dee Belemnite. Les ellipses représentant les variations des compositions isotopiques observées dans quelques niveaux choisis (Ambrose, en préparation) sont données pour comparaison. Les sites du Miocène supérieur ont des compositions isotopiques qui indiquent les habitats les plus fermés, humides et froids de toutes les périodes rencontrées dans la moyenne vallée de l’Aouache. peratures and evaporation rates, and lower humidity. The isotopic composition of paleosol carbon and oxygen of carbonate were analyzed in three of seven stratigraphic sections excavated in the western margin, and from carbonates adhering to two fossils at Amba, near the central complex (WoldeGabriel et al., 1994). The western margin sites have weakly developed paleosols, and predominantly coarse silty and sandy textures. This suggests rapid sedimentation, perhaps due to proximity to the tectonically active, eroding rift margin. Pedogenic carbonates form mainly in arid to semi-arid environments, so their absence from four of the seven excavated sections suggests mesic climates predominated. Low carbonate carbon isotope ratios indicate woodland to grassy woodland (20-45% grass biomass) at all Late Miocene sites. Low carbonate oxygen isotope ratios in west margin sites suggest relatively humid and/or cool habitats with low evaporation rates, and or formation at high altitudes. Amba has the highest oxygen isotope ratios, suggesting a lower elevation, drier habitat on the rift floor (Fig. 7). These data are comparable to those from Mio-Pliocene sites at Gona, Kanapoi and Tugen Hills (Kingston, 1992; Levin, 2002; Wynn, 2000), which all show that floral habitats in east African Rift Valley sites were predominantly C3 woodlands to open woodlands, with small proportions of C4 grasses. In comparison to later times in the Middle Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 Awash Valley, Late Miocene sites have the lowest carbon and oxygen isotope ratios (Fig. 7), indicating predominantly closed, humid, cool, and possibly high elevation habitats. Boselaphines are relatively abundant in the Lower Nawata compared to other bovids, just as in the Middle Awash. They seem to be poorly represented in the Upper Nawata. Boselaphines of the Middle Awash persist as late as ca. 4.8 Ma. The absence of alcelaphines from the MALM deposits and their relative abundance in the Upper Nawata at Lothagam (Harris, 2003a) clearly reflects ecological differences. Extant alcelaphines are known to live in arid plains, deserts, savanna, and grassland. These habitats may have been readily available for the alcelaphines to flourish in the Upper Nawata, whereas these habitats were not possibly available for the alcelaphines to flourish in the Middle Awash. Seven alcelaphine specimens have also been documented from the Lower Nawata. A closed habitat has been inferred for Aramis, an early Pliocene Middle Awash site in Ethiopia (WoldeGabriel et al., 1994). At this site, alcelaphines constitute less than 2% of the total number of collected bovid specimens. No alcelaphines have yet been documented from the Middle Awash deposits older than 4.4 Ma. Some taxa from the Adu-Asa Formation (such as Tachyoryctes) indicate that the Middle Awash region was at a much higher elevation during the deposition of the Adu-Asa Formation, than it is today, possibly by about 1,500 m (WoldeGabriel et al., 2001). In addition, extant herpestines are primarily terrestrial and some of the species cover a wide range of altitudes up to 2,500 m in montane forest (Kingdon, 1997). It is possible that their fossil relatives from the Middle Awash may also have lived in similar habitats. However, other extant viverrids are also known to live in a variety of habitats in Africa today. Genets inhabit variously drier bush and woodlands into moister valleys and forests. What can be inferred from this observation is that despite the synchronicity of the MALM and Upper Nawata deposits, they were possibly deposited at different elevations and the latter had more open grassland habitat due to its lower elevation. However, there seems to be no evidence, whatsoever, to suggest a transition from more closed habitat in the Late Miocene to a more open habitat in the Pliocene. What the evidence suggests is that there were spatial and temporal variations in the distribution of every sampled habitat due to factors such as latitudinal position, elevation, and overall climatic regime. The functional morphology of faunal remains can be used to infer paleoenvironments, an approach often labeled “ecomorphology” (Kappelman et al., 1997). The theoretical basis of this approach as applied to postcranial remains is that an organism’s locomotor anatomy should be adapted for movement across the substrates it frequents. This is particularly true for animals, like bovids, that are under selection by predation and typically employ locomotion to escape. “Ecomorphological” techniques have some advantages over taxon-based methods of inferring paleoenvironments, since they do not assume stasis in a lineage’s habitat preference 547 over evolutionary time. Furthermore, they do not require lower-level taxonomic identifications or phylogenetic reconstruction. As such, they are a good complement to taxonbased habitat reconstructions. However, sometimes there could be inconsistencies in the results derived from such analyses. For example, Bishop’s (1999) and Bishop et al.’s (1999) study on “ecomorphology” of suid nyanzachoeres inferred that nyanzachoeres preferred a mixed C3/C4 habitat. McCrossin (1987) concluded that suids from the Sahabi (Libya) were living in an open habitat. On the other hand, analysis of stable carbon isotopes of tooth enamel of Miocene and Early Pliocene suids by Harris and Cerling (2002) shows they were mixed feeders, with progressively more C4 grass through time in Pliocene species. Isotopic analysis of most species usually provides data mainly on their dietary preferences for browse (C3) versus grass (C4), rather than proportions of tree and grass biomass in the habitats they occupied (Ambrose and DeNiro, 1986). Habitat and dietary generalists such as bushpig (Potamochoerus porcus) are a notable exception (Ambrose and DeNiro, 1989). MioPliocene brachydont suids that resemble the bushpig (Nyanzachoerus and Notochoerus), analyzed by Harris and Cerling (2002), show a range of variation, which suggests that they may be useful monitors of tree/grass ratios in ancient African floral habitats. Two of the authors (DD and EV) applied several new methods of ecomorphological analysis to the bovid astragali and phalanges recovered from the MALM (n=60). These methods use measurements and a discriminant function, based on a large modern African bovid sample, to predict general habitat preference. Of the 60 analyzed MALM specimens, 29 (48%) were classified as “heavy cover” (bush, woodland, swamp, and near-water habitats), 17 (28%) were classified as “forest,” 8 (13%) were classified as “open” (edge/ecotone, arid, and open country) and 6 (10%) were classified as “light cover” (light bush, tall grass, and hilly areas). Of these 60 classifications, 37 have a confidence level of 95% or greater. Of those 37 “highly confident” classifications, 25 are heavy cover and 10 are forest. The bovid postcranial “ecomorphology” suggests that, on a broad level, the MALM assemblages were drawn from a mix of heavy cover and, to a somewhat lesser extent, forested environments. The presence in the area of some more open habitats cannot be ruled out, especially given the spatial area covered by the collection zones, but they do not appear to have contributed significantly to at least the bovid fauna. 7. Biogeographic relationships Numerous methods have been developed through the years to compare various faunal assemblages and quantify similarities and differences (Simpson, 1945, 1960; Cheetham and Hazel, 1969; Shuey et al., 1978; Pickford, 1986, among others). However, most workers choose Simpson’s (1945, 1960) Faunal Resemblance Index (FRI) because 548 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 A B MAL M MAL M L OT L UK LAN KB S AH W A NT S IW 100 L OT L UK LAN 66 68 100 60 100 KB S AH WN AT S IW 72 60 44 44 31 37 50 47 41 44 19 24 48 47 28 33 31 28 100 35 33 33 19 31 100 24 33 19 41 100 56 25 22 0 22 100 100 S PN S PN 0 100 M A L M L A N L U K L O T C H A D W N A T S A H S P N S I W Tree Length = 191 CI = 0.63 Fig. 8. A. Simpson’s Faunal Similarity Index (FRI) at the generic level. MALM = Middle Awash; LOT = Lothagam; LUK = Lukeino; LAN = Langebaanweg; KB = Chad; SAH = Sahabi; WANT = Wadi Natron (Egypt); SIW = Siwaliks (Indo-Pakistan); SPN = Spain. B. Cluster analysis on the taxonomy of some late Miocene African and Eurasian localities based on the results in Fig. 8(a). Fig. 8. A. Index de Similarité Faunique du Simpson (FRI) au niveau générique. MALM = moyenne vallée de l’Aouache ; LOT = Lothagam ; LUK = Lukeino ; LAN = Langebaanweg ; KB = Tchad ; SAH = Sahabi ; WANT = Wadi Natron (l’Egypte) ; SIW = Siwaliks (Indo-Pakistan) ; SPN = l’Espagne. B. Classification hiérarchique basée sur la taxonomie de quelques sites miocènes supérieurs d’Afrique et des régions eurasiennes à partir des résultats de la Fig. 8(a). of its greater consistency in values when dealing with a larger and more complete dataset (De Bonis et al., 1992; Bernor and Pavlakis, 1987, for example). In addition to the similarities and differences of the actual faunal communities, such comparisons can also be influenced by inter-site variation in time, space, environment, taphonomy, and taxonomic methods. A total of 121 genera were included in the quantitative analysis. PAUP* 4.0 (Sinauer Associates) program was used for the cluster analysis of the sites under consideration. Here, the sites were considered as taxa and the genera were considered as characters. The most obvious result of applying FRI to the Late Miocene African and Eurasian sites is the higher similarity among the various East African sites to the exclusion of the other sites (Fig. 8(a)). However, the Langebaanweg faunal seems to be more similar to the Middle Awash fauna, than the latter is to the geographically closer Lothagam fauna. This could be a result of the recent naming of numerous taxa from the Nawata Formation of Lothagam (Leakey and Harris, 2003). What is more interesting is the position of the Chadian site, Toros Menalla, relative to the North African and subSaharan African sites. Even though the size of the assemblage is not as large as the East African contemporaries, most of the genera present at Toros Menalla are present as well in North and East African sites. This might possibly be a result of its geographic position between North and East Africa, thereby serving as a bridge between the two biogeographic sub-regions. The North African sites are more similar to each other than they are to any of the East and South African sites at the generic level (Fig. 8(a, b)). One of the two North African sites, Sahabi, shows the second highest similarity (44%) with the Middle Awash assemblage. Kalb and Jolly (1982) previously reported an estimated 40-50% similarity between these two faunal sets. The North African sites share more genera with East African sites than with southern Europe. The Spanish and Siwaliks assemblages have no genera in common, while the Siwaliks assemblage shares more taxa with East African sites. Again, this could be a result of geography. The African assemblages in general, show more similarities with the Spanish assemblage than with the Siwaliks assemblage, although there are some genera shared exclusively between the African and Siwaliks assemblages. The sub-Saharan Africa localities are grouped together as a cluster, relative to the North African, southern European, and southern Asian localities (Fig. 8(b)). The Siwaliks faunal clearly separates itself from the rest of the localities under comparison and this is more likely to be a consequence of either geographic remoteness or presence of barriers for possible faunal interchange during the time in question. In general, the quantitative results indicate a higher faunal interchange within Africa during the Late Miocene than between Africa and either Europe or Asia. However, it should also be noted that the presence of taxa shared exclusively between eastern Africa and southern Asia suggests the presence of faunal interchange between these regions. The diversity of a given faunal assemblage is primarily related to immigration, speciation, and extinction. Major faunal changes usually take place as a result of climatedriven extinction or diversification, the introduction of new fauna via immigration, and/or evolutionary changes within lineages (Vrba, 1999; Barry et al., 1991). However, immigration is one of the most powerful processes that affect diversity of a given faunal assemblage (Barry et al., 1991). For example, immigration of large and small mammal species from western Eurasia into Africa during the Late Miocene may have played a major role in the composition of the Early Pliocene African faunal assemblage (Bernor and Tobien, 1990) and later periods. There are numerous questions that could possibly challenge the results generated from quantitative analyses of faunal assemblages. One major question is the accuracy of species recognition. This is always a consideration in such analyses, but what is usually ignored in this regard is the Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 influence of geography over the systematics of the fauna from newly discovered sites. In the African Neogene, the trend has always been to compare newly discovered faunal sets to fauna from the geographically closest analog sites, faunas which may be geographically close, but may not even be contemporaneous. For example, North African faunas tend to be assessed with more regard for each other than for eastern and southern African faunas. As a result, the taxonomies that are used in biogeographic assessments are biased. Secondly, a fossil species from one locality is more likely to be assigned to a new species, than to be subsumed with a conspecific species geographically farther away. One example would be the recognition of Nyanzachoerus syrticus in North Africa and Ny. “tulotos” in East Africa. It is now known that these two “species” represent a single species, Ny. syrticus. Earlier workers on biogeographic relationships between North Africa and East Africa were more likely to treat this single species as two different species. Therefore, biogeographic relationships can be wrongly inferred or underestimated depending on how the initial taxa identifications were made, and not only precise identification of a taxon, which usually means not misidentifying, for example, a murid to a cricetid. It is clear that more work is required to correct such biases. Moreover, the new Chadian faunas from Central Africa, when calibrated by contemporaneous analogs in the Middle Awash, will play a pivotal role in reaching an accurate understanding of faunal relationships across Africa and beyond. 8. Summary The Late Miocene history of African mammals is known from only a handful of sites. The inadequacy of the record between 4 Ma and 14 Ma has been long recognized as one of the major hurdles in our understanding of the evolutionary history of Late Miocene African mammal communities and their relationships with Pliocene mammals (Maglio, 1973b; Hill, 1987, 1995; Harrison and Mbago, 1997). In addition to the lack of the sites themselves, however, there is a recurrent problem associated with determination of precise ages for the available faunal assemblages. The age estimates for most of the Late Miocene sites in Africa are based on biochronology (Langebaanweg, Kossom Bougoudi, and all North African Late Miocene sites). The eastern African sites of Lothagam (MacDougall and Feibel, 1999), Lukeino (Pickford and Senut, 2001), and the Middle Awash (Renne et al., 1999; WoldeGabriel et al., 2001) have only recently been more or less firmly dated by 40Ar/39Ar radiometric dating. The Middle Awash paleontological study area presents a particularly important dataset critical in analytical works on past environments, biogeographic relationships, and African vertebrate evolution during and after the Late Miocene. Its tight calibration, chronological span, and geographic placement are unparalleled in Africa for the time span it represents. Recent field studies conducted by the Middle Awash 549 paleontological research project have now established a refined stratigraphy for the geological sequences of the MALM deposits, coupled with multiple 40Ar/39Ar dates, paleomagnetic data, and tephrachemistry (WoldeGabriel et al., 2001; Renne et al., 1999). Ongoing stratigraphic and geochronological work has allowed the Adu-Asa Formation to be divided into four members: Saraitu, Adu Dora, Asa Koma, and Rawa. This refined stratigraphic framework has now replaced previous schemes (Kalb et al., 1982; Kalb, 1993), which were based on isolated sections and temporally controlled only by biochronology. The MALM deposits sample mammalian faunas that are now firmly dated to between 5.2 Ma and 5.8 Ma. This time period was poorly represented even within the known Late Miocene African sites. The MALM mammalian fauna suggests a much higher diversity during the Late Miocene of Africa than previously known. Several previously unknown taxa, first appearances of some taxa, and last appearances of others, are documented. The MALM fauna not only substantially increases the number of taxa present during the terminal Miocene of Africa, but also clarifies the evolutionary history and phylogenetic relationships of some groups. The fauna from the MALM deposits largely suggests a mosaic of habitats ranging from grassy woodland and woodland, to riverine and forest settings. This is what one would expect in light of the diversity of the animal community and availability of diverse habitat to accommodate it. A similar mosaic palaeoenvironment is inferred for the Nawata Formation of Lothagam (Leakey et al., 1996). The MALM fauna is largely contemporaneous with the fauna from the Upper Nawata Formation of Lothagam. This is also supported by the radiometric ages obtained from both areas (Renne, et al., 1999; WoldeGabriel et al., 2001; MacDougall and Feibel, 1999). However, the inferred habitat of the MALM fauna seems to be more similar to that of the Lower Nawata fauna, which is chronologically older. Acknowledgements The fieldwork conducted along the western margin of the Middle Awash for the last five years involved a number of individuals. We thank A. Ademassu, A. Amzaye, B. Asfaw, M. Asnake, T. Assebework, M. Black, R. Bonnefille, G. Curtis, A. Defleur, A. Elema, S. Eshete, K. Geleta, A. Getty, G. Heiken, H. Gilbert, E. Güleç, L. Hlusko, B. Latimer, Ç. Pehlevan, P. Renne, H. Saegusa, S.S. Simpson, G. Suwa, and S. Yosef for their participation in field survey and excavations. We also thank the Afar people who participated in the survey and excavations. The Authority for Research and Conservation of Cultural Heritage of the Ministry of Youth, Sports and Culture granted permits to fieldwork and access to the collections in the Paleoanthropology Laboratory of the National Museum of Ethiopia. The Cleveland Museum of Natural History, the South African Museum of Cape Town, the British Museum of Natural History, and the National 550 Y. Haile-Selassie et al. / Geobios 37 (2004) 536–552 Museums of Kenya gave us access to the various faunal collections in their care. We thank Drs. M.E. Leakey, M. Avery, J. Harris, B. Latimer, C. Stringer, and P. Andrews in this regard. M.E. Leakey and J. 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