Freshwater stingrays from the Plio-Pleistocene of the Turkana Basin, Kenya and Ethiopia

The fossil freshwater stingray from the Turkana Basin of Kenya and Ethiopia is redescribed and reassigned to Dasyatis africana (Arambourg) on the basis of extensive new collections. The ray apparently evolved into an endemic freshwater species derived from a stock which entered the Turkana Basin from the Indian Ocean at about 1.9 Ma. At that time, the ancestral Omo River system flowed through a major lake and exited to the southeast. A fluvial corridor, termed the Turkana River, connected the Turkana Basin with the Indian Ocean. Once established in the basin, the rays flourished and persisted for over half a million years. Their extinction has been placed subsequent to 1.3 Ma, and likely reflects the changing environmental and tectonic conditions recorded in subsequent strata. The fluvial corridor which formed the route of migration into the Turkana Basin has important implications for modern African biogeography as well as that of the past. □ Turkana Basin, Kenya, Ethiopia, Pliocene, Pleistocene, Stingrays, Dasyatidae.     

Geochronology of the Turkana depression of northern Kenya and southern Ethiopia

Mesozoic and Cenozoic sedimentary rocks in the Turkana Depression of northern Kenya and southern Ethiopia rest on basement rocks that yield K/Ar cooling ages between 433 and 522 Ma. Proven Cretaceous strata are exposed in Lokitaung Gorge in northwest Kenya. Eocene basalts and rhyolites in Lokitaung Gorge, the Nabwal Hills, and at Kangamajoj, date between 34 and 36 Ma, recording the earliest volcanism in the region. Oligocene volcanic rocks, with associated fossiliferous sedimentary strata at Eragaleit, Nakwai, and Lokone, all west of Lake Turkana, are 23 to 28 Ma old, as is the Langaria Formation east of Lake Turkana. Lower and Middle Miocene volcanic and sedimentary sequences are present both east and west of Lake Turkana, where ages from 17.9 to 9.1 Ma have been measured at many levels. Upper Miocene strata are presently known only at Lothagam, with ages ranging from 7.4 to 6.5 Ma. Deposition of Pliocene strata of the Omo Group begins in the Omo-Turkana, Kerio, and South Turkana basins -4.3 Ma ago and continues in parts of those basins until nearly the present time, but with some gaps. These strata are linked through volcanic ash correlations at many levels, as are Pleistocene strata of the Omo Group (principally the Shungura, Koobi Fora, and Nachukui formations). (40) Ar/(39) Ar dates on many volcanic ash layers within the Omo Group, supplemented by K/Ar ages on intercalated basalts and paleomagnetic polarity stratigraphy, provide excellent age control from 4.2 to 0.75 Ma, although there is a gap in the record between -1 Ma and 0.8 Ma. Members I to III of the Kibish Formation in the lower Omo Valley record deposition between 0.2 and 0.1 Ma ago; Member IV, correlative with the Galana Boi Formation, was deposited principally between 12 and 7 ka BP.     

Paleogeographic variations of pedogenic carbonate delta13C values from Koobi Fora, Kenya: implications for floral compositions of Plio-Pleistocene hominin environments

Plio-Pleistocene East African grassland expansion and faunal macroevolution, including that of our own lineage, are attributed to global climate change. To further understand environmental factors of early hominin evolution, we reconstruct the paleogeographic distribution of vegetation (C(3)-C(4) pathways) by stable carbon isotope (delta(13)C) analysis of pedogenic carbonates from the Plio-Pleistocene Koobi Fora region, northeast Lake Turkana Basin, Kenya. We analyzed 202 nodules (530 measurements) from ten paleontological/archaeological collecting areas spanning environments over a 50-km(2) area. We compared results across subregions in evolving fluviolacustrine depositional environments in the Koobi Fora Formation from 2.0-1.5 Ma, a stratigraphic interval that temporally brackets grassland ascendancy in East Africa. Significant differences in delta(13)C values between subregions are explained by paleogeographic controls on floral composition and distribution. Our results indicate grassland expansion between 2.0 and 1.75 Ma, coincident with major shifts in basin-wide sedimentation and hydrology. Hypotheses may be correct in linking Plio-Pleistocene hominin evolution to environmental changes from global climate; however, based on our results, we interpret complexity from proximate forces that mitigated basin evolution. An approximately 2.5 Ma tectonic event in southern Ethiopia and northern Kenya exerted strong effects on paleography in the Turkana Basin from 2.0-1.5 Ma, contributing to the shift from a closed, lacustrine basin to one dominated by open, fluvial conditions. We propose basin transformation decreased residence time for Omo River water and expanded subaerial floodplain landscapes, ultimately leading to reduced proportions of wooded floras and the establishment of habitats suitable for grassland communities.     

New Pliocene remains of Camelus grattardi (Mammalia,Camelidae) from the Shungura Formation,Lower Omo Valley,Ethiopia, and the evolution of African camels

Abstract

Camels are exceptionally rare in the Plio-Pleistocene fossil record of Africa, hindering attempts to understand the evolution of this family on the continent. Here we describe recently collected camel specimens from the Shungura Formation, Lower Omo Valley, Ethiopia, and attribute these remains to Camelus grattardi. The new specimens date to the late Pliocene (~3 to 2.6 Ma) and consist of three upper molars, one upper premolar, and two proximal metatarsals. The dental specimens confirm this species’ small P4 relative to its molars, a trait that differs significantly from all extant and fossil Old World camels. The metatarsals indicate that C. grattardi was similar in size to the living Bactrian camel, C. bactrianus. Phylogenetically, we find no suitable ancestor, sister, or descendant of the eastern African fossil camel, which suggests greater lineage diversity in Plio-Pleistocene Camelus than previously recognised. Microwear analyses suggest that C. grattardi was likely a mixed-feeder preferring browse, which is consistent with carbon isotopes of enamel from the Turkana Basin. A review of the fossil record of African camels suggests no clear paleoenvironmental association, as fossil camels occur in a range of environments from dry savannas with no permanent water bodies to closed woodlands along the paleo-Omo River.     

Patterns of vegetation along the Omo River in southwest Ethiopia

This study investigates the riverine vegetation along the perennial Omo River, which flows from the Ethiopian highlands to its terminus, in the southwestern lowlands, at Lake Turkana (formerly Lake Rudolf). Broadly defined to include the levee backslopes and adjacent mudflats (or ancient floodplains), the riverine zone in the lower Omo basin supports a relatively luxuriant vegetation compared with the dry grasslands in the surrounding plains environments. Habitat conditions along the lower Omo have changed significantly during the past century, primarily due to a period of reduced rainfall and river flow, a drop in lake level (L. Turkana) at the river's terminus and increased exposure of natural levees near the Omo's terminus. The floristic and physiognomic character of riverine vegetation were studied at selected sites, ranging from the relatively straight channel section in the modern delta to a strongly meandering (upstream) section of the river. A combination of open canopy woodland, shrub thicket and grassland (with scattered tree emergents) prevailed on the more recently exposed levees, in and near the modern delta; closed canopy woodland and forest predominated on fore levees in the meandering segment. Floristic diversity was relatively low at all study sites. Ficus sycomorus, Tapura fischeri, Melanodiscus oblongus, Celtis integrifolia and Trichilia roka were most significant in upstream forest sites, whereas Cordia sinensis, Acacia mellifera, Ziziphus mauritiana and Ficus sycomorus were more common in communities nearer the lake. Older forests generally contained greater diversity of more woody species, greater tree height and truck diameter than downstream levee vegetation, but they did not exhibit clear stratification. Species patchiness was broadly characteristic of the riverine zone, particularly the mudflats (ancient floodplains) extending away from the fore levees. Fires are frequently set by local inhabitants throughout the grasslands of the lower Omo Basin; within the riverine zone, they are particularly common in the mudflats but have only limited impact on woodland and forest vegetation. It is suggested that fire is a highly significant factor in the establishment of sharp boundaries between the closed canopy woodland/forest along the river and the more xerophytic vegetation of the mudflats.     

New palaeoanthropological research in the Plio-Pleistocene Omo Group,Lower Omo Valley,SNNPR (Southern Nations,Nationalities and People Regions), Ethiopia

Through previous works in the early 1930s by C. Arambourg and in the 1960–1970s by the International Omo Research Expedition (IORE) initiated by F. C. Howell, the Omo Group deposits of the Lower Omo Valley provided decisive data on Plio-Pleistocene environmental change and hominid evolution in eastern Africa. Y. Coppens directed the IORE French component with Arambourg, then alone from 1970 to 1976. After 30-year hiatus, the Omo Group Research Expedition reinitiated field work on Shungura Formation deposits aged between 3 Ma and 2 Ma. In 2006 and 2007, renewed methods led to the collection of more than 600 vertebrate specimens with a particularly precise record of contextual data. These specimens include significant hominid remains dated to 2.5 Ma and slightly older. Changes in faunal distributions were also recorded. Additionally, the Shungura Formation archaeological record is reconsidered. These first results are indicative of future advances in the study of biodiversity evolution and its relationship with global and regional environmental changes.     

Extinct large colobines from the Plio-Pleistocene of Africa

Three genera and six species of extinct African Plio-Pleistocene Colobinae are discussed. One new genus, Rhinocolobus and three new species, R. turkanaensis, Cercopithecoides kimeui and Paracolobus mutiwa are named. These colobines show a diversity in postcranial and dental morphology not seen among extant species. Rhinocolobus was most similar to extant Colobus in postcranial morphology and had similar high-cusped shearing teeth. Cercopithecoides shows a number of postcranial skeletal features typical of terrestrial cercopithecid species and has lower cusped teeth. Paracolobus, while generally more similar to Rhinocolobus than to Cercopithecoides, is intermediate in some features of its postcranial morphology. The distribution of the various taxa among East and South African sites with different palaeoenvironments is generally consistent with the morphological interpretation. With the exception of Cercopithecoides kimeui, which persisted a little longer, these large colobines disappear from the fossil record about 1.8 million years ago. Their disappearance coincides with an interval of increasing aridity documented at Olduvai Gorge, the Omo Valley, and East Turkana.     

First occurrence of early Homo in the Nachukui Formation (West Turkana, Kenya) at 2.3-2.4 Myr

Cognitive abilities and techno-economic behaviours of hominids in the time period between 2.6-2.3 Myr have become increasingly well-documented. This time period corresponds to the oldest evidence for stone tools at Gona (Kada Gona, West Gona, EG 10-12, OGS 6-7), Hadar (AL 666), lower Omo valley (Ftji1, 2 & 5, Omo 57, Omo 123) in Ethiopia, and West Turkana (Lokalalei sites -LA1 & LA2C-) in Kenya. In 2002 a new palaeoanthropological site (LA1alpha), 100 meters south of the LA1 archaeological site, produced a first right lower molar of a juvenile hominid (KNM-WT 42718). The relative small size of the crown, its marked MD elongation and BL reduction, the relative position of the cusps, the lack of a C6 and the mild expression of a protostylid, reinforced by metrical analyses, demonstrate the distinctiveness of this tooth compared with Australopithecus afarensis, A. anamensis, A. africanus and Paranthropus boisei, and its similarity to early Homo. The LA1alpha site lies 2.2 m above the Ekalalei Tuff which is slightly younger than Tuff F dated to 2.34+/-0.04 Myr. This juvenile specimen represents the oldest occurrence of the genus Homo in West Turkana.     

New insights into the molecular phylogeny and taxonomy of mormyrids (Osteoglossiformes,Actinopterygii) in northern East Africa

Based on morphological data and analysis of mitochondrial cytochrome b gene and nuclear (S7 intron 1) DNA sequences, the phylogenetic relationships of all Pollimyrus species known from the Omo‐Turkana enclosed basin and Nile system below the Murchison Falls were solved. A mormyrid “Pollimyrus” petherici is distantly related to all other studied Pollimyrus species and clusters together with Cyphomyrus species forming with the later a monophyletic group. Moreover, the West African (but not the Congo River) populations of Cyphomyrus psittacus, the type species of the genus, seem to be conspecific to C. petherici. That is, the range of the genus Cyphomyrus is extended toward the Nile and Omo‐Turkana basins. This genus belongs to the large clade widely distributed in sub‐Saharian Africa and characterized by the presence of a chin appendage. Significance of this character for mormyrid phylogeny is discussed. Two distinct lineages of Pollimyrus occurring sympatrically in the White Nile tributaries and previously reported as the light and dark forms of Pollimyrus isidori together with five other congeneric species studied form a monophyletic group. The light form apparently represents P. isidori distributed in the Nile system downstream of the Murchison Falls and West Africa; the dark‐colored form (designated as Pollimyrus “D”) represents a distinct phylogenetic lineage inhabiting both the Omo‐Turkana and the White Nile basin. Morphological and ecological data suggest that this form may be conspecific to East African Pollimyrus nigricans or most probably represents a new species.     

The Omo-Turkana Basin fossil hominins and their contribution to our understanding of human evolution in Africa

The Omo-Turkana Basin, including the hominin fossil sites around Lake Turkana and the sites along the lower reaches of the Omo River, has made and continues to make an important contribution to improving our murky understanding of human evolution. This review highlights the various ways the Omo-Turkana Basin fossil record has contributed to, and continues to challenge, interpretations of human evolution. Despite many diagrams that look suspiciously like comprehensive hypotheses about human evolutionary history, any sensible paleoanthropologist knows that the early hominin fossil record is too meager to do anything other than offer very provisional statements about hominin taxonomy and phylogeny. If history tells us anything, it is that we still have much to learn about the hominin clade. Thus, we summarize the current state of knowledge of the hominin species represented at the Omo-Turkana Basin sites. We then focus on three specific topics for which the fossil evidence is especially relevant: the origin and nature of Paranthropus; the origin and nature of early Homo; and the ongoing debate about whether the pattern of human evolution is more consistent with speciation by cladogenesis, with greater taxonomic diversity or with speciation by anagenetic transformation, resulting in less taxonomic diversity and a more linear interpretation of human evolutionary history.     

Quaternary fossil fish from the Kibish Formation, Omo Valley, Ethiopia

The late Quaternary Kibish Formation of the Omo Valley, southwestern Ethiopia, preserves environments reflecting a history of fluctuations in the level of nearby Lake Turkana over the past 200,000 years. The Kibish Formation has yielded a diverse mammalian fauna (as well as birds and crocodiles), stone tools, and the oldest anatomically modern Homo sapiens. Fish, the most common vertebrate fossils in this unit, are reported in this study. Catfish (especially clariids and Synodontis) and Nile perch (Lates niloticus) predominate, but the gymnarchid Gymnarchus, a cyprinid (Barbus), tigerfish (Hydrocynus), pufferfish (Tetraodon), and other catfish are also present. In total, nine teleost genera are found in the Kibish Formation, representing a subset of the 37 genera that constitute the modern Omo-Turkana ichthyofauna. Several taxa present in the modern fauna, including Polypterus and members of the family Cichlidae, are not found in the Kibish deposits. Most specimens are preserved as disarticulated or broken skeletal elements, but some preservation of articulated elements (e.g., sets of vertebrae, crania with lower jaws or cleithra) also occurs. Many of the catfish and Nile perch specimens are larger than the largest reported from the modern river or lake. Faunas of Kibish Members I and III closely resemble one another; the fauna from Member IV contains only the three most common taxa (Clarias, Synodontis, Lates), though this may result from insufficient sampling. Barbed bone points have been collected from the upper part of the formation, indicating a long association between the human inhabitants and the fish fauna of the Omo Valley.     

The femur of extinct bunodont otters in Africa (Carnivora,Mustelidae, Lutrinae)

This study compares fossil femora attributed to extinct African bunodont lutrines with extant mustelids and ursids to reconstruct locomotor behavior. Due to the immense size differences among taxa, shape data were used to compare morphology. Based on morphological differences, the fossil femora are suggested to belong to different taxa with different locomotor abilities and habitat preferences. The Langebaanweg femur is the oldest and has a typical mustelid morphology suggesting that it was a locomotor generalist like most mustelids. The West Turkana form is more like extant nonbunodont otters, but much larger, and may have belonged to a semiaquatic taxon. The enormous Omo femur shares some features with truly aquatic taxa (e.g., Enhydra) and is the most likely to have been fully aquatic. The same may hold true for the Hadar species as it is most similar to that from the Omo. If these femora truly belong to bunodont lutrines, then they are more diverse in postcranial morphology than in dental morphology.     

Influence of Plio-Pleistocene aridification on human evolution: evidence from paleosols of the Turkana Basin, Kenya

New stable carbon isotope measurements, coupled with paleoprecipitation estimates, both from Plio-Pleistocene paleosols of the Turkana Basin, Kenya, provide a high-resolution record of aridification and increasing C4 biomass during the past 4.3 Ma. This aridification trend is marked by several punctuations at 3.58-3.35, 2.52-2, and 1.81-1.58 Ma, during which the running mean and variance of delta13C and paleoaridity estimates increase, suggesting that the proportion of C4 biomass increases in savanna mosaics during periods of heightened aridity. Increase in C4 biomass during these aridification events not only increases the proportion of open habitats, but increases the spatial neg-entropy, or heterogeneity of the ecosystem. The aridification events identified correspond to intervals of increased turnover, but more importantly, increased diversity of bovids. Although the record of hominins from the Turkana Basin lacks the temporal resolution and diversity of the bovid record, the aridification intervals identified are marked by similar increases in the diversity and turnover of hominins. These results support the hypothesis that hominins evolved in savanna mosaics that changed through time, and suggest that the evolution of bovids and hominins was driven by shifts in climatic instability and habitat variability, both diachronic and synchronic.     

Plio-Pleistocene facies environments from the KBS Member, Koobi Fora Formation: implications for climate controls on the development of lake-margin hominin habitats in the northeast Turkana Basin (northwest Kenya)

Climate change is hypothesized as a cause of major events of Plio-Pleistocene East African hominin evolution, but the vertically discontinuous and laterally confined nature of the relevant geological records has led to difficulties with assessing probable links between the two. High-resolution sedimentary sequences from lacustrine settings can provide comprehensive data of environmental changes and detailed correlations with well-established orbital and marine records of climate. Hominin-bearing deposits from Koobi Fora Ridge localities in the northeast Turkana Basin of Kenya are an archive of Plio-Pleistocene lake-margin sedimentation though significant developmental junctures of northern African climates, East African environments, and hominin evolution. This study examines alluvial channel and floodplain, nearshore lacustrine, and offshore lacustrine facies environments for the approximately 136-m-thick KBS Member (Koobi Fora Formation) exposed at the Koobi Fora Ridge. Aspects of the facies environments record information on the changing hydrosedimentary dynamics of the lake margin and give insights into potential climatic controls. Seasonal/yearly climate changes are represented by the varve-like laminations in offshore mudstones and the slickensides, dish-shaped fractures, and other paleosol features overprinted on floodplain strata. Vertical shifts between facies environments, however, are interpreted to indicate lake-level fluctuations deriving from longer-term, dry-wet periods in monsoonal rainfall. Recurrence periods for the inferred lake-level changes range from about 10,000 to 50,000 years, and several are consistent with the average estimated timescales of orbital precession ( approximately 20,000 years) and obliquity ( approximately 40,000 years). KBS Member facies environments from the Koobi Fora Ridge document the development of lake-margin hominin habitats in the northeast Turkana Basin. Environmental changes in these habitats may be a result of monsoonal rainfall variations that derive from orbital insolation and/or glacial forcing.     

Ecological change in the lower Omo Valley around 2.8 Ma

Late Pliocene climate changes have long been implicated in environmental changes and mammalian evolution in Africa, but high-resolution examinations of the fossil and climatic records have been hampered by poor sampling. By using fossils from the well-dated Shungura Formation (lower Omo Valley, northern Turkana Basin, southern Ethiopia), we investigate palaeodietary changes in one bovid and in one suid lineage from 3 to 2 Ma using stable isotope analysis of tooth enamel. Results show unexpectedly large increases in C₄ dietary intake around 2.8 Ma in both the bovid and suid, and possibly in a previously reported hippopotamid species. Enamel δ¹³C values after 2.8 Ma in the bovid (Tragelaphus nakuae) are higher than recorded for any living tragelaphin, and are not expected given its conservative dental morphology. A shift towards increased C₄ feeding at 2.8 Ma in the suid (Kolpochoerus limnetes) appears similarly decoupled from a well-documented record of dental evolution indicating gradual and progressive dietary change. The fact that two, perhaps three, disparate Pliocene herbivore lineages exhibit similar, and contemporaneous changes in dietary behaviour suggests a common environmental driver. Local and regional pollen, palaeosol and faunal records indicate increased aridity but no corresponding large and rapid expansion of grasslands in the Turkana Basin at 2.8 Ma. Our results provide new evidence supporting ecological change in the eastern African record around 2.8 Ma, but raise questions about the resolution at which different ecological proxies may be comparable, the correlation of vegetation and faunal change, and the interpretation of low δ¹³C values in the African Pliocene.     

Stratigraphic context of fossil hominids from the Omo group deposits: northern Turkana Basin, Kenya and Ethiopia

The chronometric framework developed for Plio-Pleistocene deposits of the northern Turkana Basin is reviewed in light of recent advances in lithostratigraphy, geochemical correlation, paleomagnetic stratigraphy, and isotopic dating. The sequence is tightly controlled by 20 precise ages on volcanic materials. These ages are internally consistent but are at variance with estimates for the boundaries of the magnetic polarity time scale by about 0.07 my. This discrepancy can be only partially resolved at present. Based on the established chronometric framework and stratigraphic sequences, depositional ages can be estimated for significant marker beds. These ages can in turn be used to constrain the 449 hominid specimens thus far reported from the basin. Ages for most hominid specimens can be estimated with a precision of +/- 0.05 my. In addition, the chronometric framework will be applicable to other paleontological collections, archeological excavations, and future discoveries in the basin.     

A reappraisal of the locomotion and habitat preference of Theropithecus oswaldi

The one modern member of the genus Theropithecus, T. gelada (Primates, Cercopithecidae), inhabits grassland and is highly terrestrial. It is often supposed that Theropithecus oswaldi, one of the most common primates of the Plio-Pleistocene of East and southern Africa, was also a highly terrestrial open habitat species. Ecomorphic analysis was used to assess the locomotor strategy and habitat preference of T. oswaldi, and it was found that this species was unlikely to have had a locomotor strategy and habitat preference identical to that of T. gelada, with T. oswaldi possibly using arboreal substrates in a manner similar to some modern baboon groups. Thus, it appears that there has been considerable ecological diversity in the genus Theropithecus over the course of its evolution, mirroring the diversity evident in the hominin fossil record.     

Cranium of a juvenile Australopithecus boisei from the lower Omo Basin,Ethiopia

A partial cranium of a juvenile Australopithecus boisei, recovered from the Shungura Formation in the lower Omo basin, southern Ethiopia, and dated at 2.1 m.y. B.P. , is described anatomically and compared to young and adult australopithecines, modern Homo sapiens, chimpanzees, and gorillas. A resemblance to the gracile Australopithecus is observed but is attributed mainly to the generalized appearance of the Omo specimen resulting from its young individual age. An attempt is made to reconstruct part of the ontogenetic process of A. boisei. This process is compared to the developmental changes exhibited by the African great apes and modern man and is found to combine characteristics of both.     

Plio-Pleistocene climatic change in the Turkana Basin (East Africa): evidence from large mammal faunas

We investigated palaeoclimatic change in the Turkana Basin during the Pliocene climatic shift toward increased aridity in Africa. We analyzed the palaeoecology of this area using mammal faunas as environmental indicators. Twenty Plio-Pleistocene fossil assemblages and a comparative dataset of 16 modern localities covering a wide range of climatic and ecological conditions across Africa were analyzed. We constructed community profiles using taxonomic variables which reflect ecological information. Principal component analysis and bivariate correlation were used to study changes in the community structure of these mammalian faunas and to draw palaeoenvironmental inferences. Subsequently, least-squares regressions yielded climatic estimates (annual rainfall and drought length) for the studied period. An additional set of 8 modern faunas was used to validate these regression models. The climatic estimates showed a drying trend throughout the sequence. The biomes in the Turkana Basin changed from semi-evergreen rain forest to deciduous woodland and savanna during the middle-late Pliocene. This was the most important climatic shift detected in our study. Evidence suggests a continuous presence of savannas from 2.5 million years ago onwards. This pattern of climatic change is consistent with isotopic evidence on global climate, and with independently derived regional palaeoenvironmental evidence (i.e., micromammals, palaeovegetation, soil carbonates and palaeosols).