Pedogenic carbonate stable isotopic evidence for wooded habitat preference of early Pleistocene tool makers in the Turkana Basin

The origin and evolution of early Pleistocene hominin lithic technologies in Africa occurred within the context of savanna grassland ecosystems. The Nachukui Formation of the Turkana Basin in northern Kenya, containing Oldowan and Acheulean tool assemblages and fossil evidence for early members of Homo and Paranthropus, provides an extensive spatial and temporal paleosol record of early Pleistocene savanna flora. Here we present new carbon isotopic (δ¹³C_VPDB) values of pedogenic carbonates (68 nodules, 193 analyses) from the Nachukui Formation in order to characterize past vegetation structure and change through time. We compared three members (Kalochoro, Kaitio, and Natoo) at five locations spanning 2.4–1.4 Ma and sampled in proximity to hominin archaeological and paleontological sites. Our results indicate diverse habitats showing a mosaic pattern of vegetation cover at each location yet demonstrate grassland expansion through time influenced by paleogeography. Kalochoro floodplains occurred adjacent to large river systems, and paleosols show evidence of C₃ woodlands averaging 46–50% woody cover. Kaitio habitats were located along smaller rivers and lake margins. Paleosols yielded evidence for reduced portions of woody vegetation averaging 34–37% woody cover. Natoo environments had the highest percentage of grasslands averaging 21% woody cover near a diminishing Lake Turkana precursor. We also compared paleosol δ¹³C_VPDB values of lithic archaeological sites with paleosol δ¹³C_VPDB values of all environments available to hominins at 2.4–1.4 Ma in the Nachukui and Koobi Fora Formations. Grassy environments became more widespread during this interval; woody canopy cover mean percentages steadily decreased by 12%. However, significantly more wooded savanna habitats were present in the vicinity of lithic archaeological sites and did not mirror the basin-wide trend of grassland spread. Hominin lithic archaeological sites consistently demonstrated woody cover circa 40% throughout our study interval and were 4–12% more woody than coeval basin environs. We propose that Turkana Basin early tool makers may have preferred a more wooded portion of the savanna ecosystem to reduce heat stress and to gain differential access to potable water, raw materials, animal carcasses, and edible plants.     

Improved age control on early Homo fossils from the upper Burgi Member at Koobi Fora,Kenya

To address questions regarding the evolutionary origin, radiation and dispersal of the genus Homo, it is crucial to be able to place the occurrence of hominin fossils in a high-resolution chronological framework. The period around 2 Ma (millions of years ago) in eastern Africa is of particular interest as it is at this time that a more substantial fossil record of the genus Homo is first found. Here we combine magnetostratigraphy and strontium (Sr) isotope stratigraphy to improve age control on hominin-bearing upper Burgi (UBU) deposits in Areas 105 and 131 on the Karari Ridge in the eastern Turkana Basin (Kenya). We identify the base of the Olduvai subchron (bC2n) plus a short isolated interval of consistently normal polarity that we interpret to be the Pre-Olduvai event. Combined with precession-forced (∼20 kyr [thousands of years]) wet–dry climate cycles resolved by Sr isotope ratios, the magnetostratigraphic data allow us to construct an age model for the UBU deposits. We provide detailed age constraints for 15 hominin fossils from Area 131, showing that key specimens such as cranium KNM-ER 1470, partial face KNM-ER 62000 and mandibles KNM-ER 1482, KNM-ER 1801, and KNM-ER 1802 can be constrained between 1.945 ± 0.004 and 2.058 ± 0.034 Ma, and thus older than previously estimated. The new ages are consistent with a temporal overlap of two species of early Homo that can be distinguished by their facial morphology. Further, our results show that in this time interval, hominins occurred throughout the wet–dry climate cycles, supporting the hypothesis that the lacustrine Turkana Basin was a refugium during regionally dry periods. By establishing the observed first appearance datum of a marine-derived stingray in UBU deposits at 2.058 ± 0.034 Ma, we show that at this time the Turkana Basin was hydrographically connected to the Indian Ocean, facilitating dispersal of fauna between these areas. From a biogeographical perspective, we propose that the Indian Ocean coastal strip should be considered as a possible source area for one or more of the multiple Homo species in the Turkana Basin from over 2 Ma onwards.     

Early Human Speciation,Brain Expansion and Dispersal Influenced by African Climate Pulses

Early human evolution is characterised by pulsed speciation and dispersal events that cannot be explained fully by global or continental paleoclimate records. We propose that the collated record of ephemeral East African Rift System (EARS) lakes could be a proxy for the regional paleoclimate conditions experienced by early hominins. Here we show that the presence of these lakes is associated with low levels of dust deposition in both West African and Mediterranean records, but is not associated with long-term global cooling and aridification of East Africa. Hominin expansion and diversification seem to be associated with climate pulses characterized by the precession-forced appearance and disappearance of deep EARS lakes. The most profound period for hominin evolution occurs at about 1.9 Ma; with the highest recorded diversity of hominin species, the appearance of Homo (sensu stricto) and major dispersal events out of East Africa into Eurasia. During this period, ephemeral deep-freshwater lakes appeared along the whole length of the EARS, fundamentally changing the local environment. The relationship between the local environment and hominin brain expansion is less clear. The major step-wise expansion in brain size around 1.9 Ma when Homo appeared was coeval with the occurrence of ephemeral deep lakes. Subsequent incremental increases in brain size are associated with dry periods with few if any lakes. Plio-Pleistocene East African climate pulses as evinced by the paleo-lake records seem, therefore, fundamental to hominin speciation, encephalisation and migration.     

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.     

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.     

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).     

Faunal change, environmental variability and late Pliocene hominin evolution

Global change during the late Pliocene was manifested in declining temperatures, increased amplitude of climate cycles, and shifts in the periodicity of orbital climate forcing. Linking these changes to the evolution of African continental faunas and to hominin evolution requires well-documented fossil evidence that can be examined through substantial periods of time. The Omo sequence of southern Ethiopia provides such a database, and we use it to analyze change in the abundances of mammal taxa at different levels of temporal and taxonomic resolution between 4 and 2 Ma. This study provides new evidence for shifts through time in the ecological dominance of suids, cercopithecids, and bovids, and for a trend from more forested to more open woodland habitats. Superimposed on these long-term trends are two episodes of faunal change, one involving a marked shift in the abundances of different taxa at about 2.8+/-0.1 Ma, and the second the transition at 2.5 Ma from a 200-ka interval of faunal stability to marked variability over intervals of about 100 ka. The first appearance of Homo, the earliest artefacts, and the extinction of non-robust Australopithecus in the Omo sequence coincide in time with the beginning of this period of high variability. We conclude that climate change caused significant shifts in vegetation in the Omo paleo-ecosystem and is a plausible explanation for the gradual ecological change from forest to open woodland between 3.4 and 2.0 Ma, the faunal shift at 2.8 +/-0.1 Ma, and the change in the tempo of faunal variability of 2.5 Ma. Climate forcing in the late Pliocene is more clearly indicated by population shifts within the Omo mammal community than by marked turnover at the species level.     

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.     

Faunal change in the Turkana Basin during the late Oligocene and Miocene

Faunal evolution over the last 65 million years of earth's history was dominated by mammalian radiations, but much of this era is poorly represented in Africa. Mammals first appeared early in the Mesozoic, living alongside dinosaurs for millions of years, but it was not until the extinction of dinosaurs 65 myr ago that the first major explosion of mammalian taxa took place. The Cenozoic (65 Ma to Recent) witnessed repeated and dynamic events involving the radiation, evolution, and extinction of mammalian faunas. Two of these events, each marking the extinction of one diverse fauna and subsequent establishment of another equally diverse fauna, both involving advanced catarrhine primates, are recorded in sites in the Turkana Basin, despite the poorly represented record of Cenozoic faunas elsewhere in sub-Saharan Africa. The first of these events occurred at the Oligocene-Miocene transition and the other at the Miocene-Pliocene transition.     

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.     

Morphology and locomotor adaptations of the bovid femur in relation to habitat

Extant bovids inhabit a wide diversity of environments that range from forest to savanna and display locomotor patterns that are habitat specific. I report here on an investigation of the linkage between these locomotor patterns and habitat based on a study of the morphology of the bovid femur. Femoral head shape, shaft dimensions, and knee structure are examined and support a statistically significant separation of the different morphological complexes present in bovids from forest, broken cover, and savanna habitats. Morphological differences are primarily related to locomotor patterns as reflected in the degree of cursoriality displayed by bovids in different habitats. Cursorial bovids from savanna environments have laterally expanded femoral heads that act to limit the degree of abduction and axial rotation at the hip, and elliptically shaped distal femora that increase the moment arm of the extensor muscles that cross the knee. Forest bovids have spherically shaped femoral heads. This morphology permits a much higher degree of abduction and axial rotation at the hip and appears to provide greater maneuverability in a vegetationally complex habitat. Bovids living in broken cover environments that fall between the extremes of closed canopy forest and savanna display an intermediate set of femoral characters. This approach to the relationship between habitat and locomotion offers a potentially powerful means with which to examine the interplay between structural form and function in bovid evolution.     

Turkana Basin-Middle Awash Valley correlations and the age of the Sagantole and Hadar Formations

The Moiti, Lomogol and Wargolo Tuffs of the Turkana Basin correlate with similar layers in the Sagantole Formation exposed in the Middle Awash Valley, Ethiopia. These correlations demonstrate that the tephrostratigraphy of the Turkana Basin can be applied to the Middle Awash Valley. Consideration of our analyses together with previous analyses shows that more volcanic ash layers are present in the Sagantole Formation than have been reported and also that the proposed correlation of VT-2 between Bodo and Belohdelie is incorrect. In addition, the correlations suggest that the age of the Cindery Tuff of the Sagntole Formation is about 3·8 Ma, at the young end of the range proposed earlier (3·8–4·0 Ma). This implies that the hominid skull fragments from the Sagantole Formation are 3·8–3·9 Ma. The upper part of the Sagantole Formation contains the Lomogol Tuff (ca. 3·6 Ma). Because the Sagantole Formation underlies the Hadar Formation, the base of the Hadar Formation can be no older than 3·8 Ma and is probably younger than 3·6 Ma, in agreement with correlation of the Tulu Bor (ca. 3·35 Ma) and Sidi Hakoma Tuffs. As all hominid fossils presently known from the Hadar Formation are younger than the Sidi Hakoma Tuff, they must be younger than 3·35 Ma. This estimate agrees with faunal evidence from the lower part of the Hadar Formation, and allows placement of hominids from Laetoli, the Middle Awash Valley and the Turkana Basin in temporal order.     

The evolution of diversity in ancient ecosystems: a review

On a perfect planet, such as might be acceptable to a physicist, one might predict that from its origin the diversity of life would grow exponentially until the carrying capacity, however defined, was reached. The fossil record of the Earth, however, tells a very different story. One of the most striking aspects of this record is the apparent evolutionary longueur, marked by the Precambrian record of prokaryotes and primitive eukaryotes, although our estimates of microbial diversity may be seriously incomplete. Subsequently there were various dramatic increases in diversity, including the Cambrian ''explosion'' and the radiation of Palaeozoic-style faunas in the Ordovician. The causes of these events are far from resolved. It has also long been appreciated that the history of diversity has been punctuated by important extinctions. The subtleties and nuances of extinction as well as the survival of particular clades have to date, however, received rather too little attention, and there is still a tendency towards blanket assertions rather than a dissection of these extraordinary events. In addition, some but perhaps not all mass extinctions are characterized by long lag-times of recovery, which may reflect the slowing waning of extrinsic forcing factors or alternatively the incoherence associated with biological reassembly of stable ecosystems. The intervening periods between the identified mass extinctions may be less stable and benign than popularly thought, and in particular the frequency of extraterrestrial impacts leads to predictions of recurrent disturbance on timescales significantly shorter than the intervals separating the largest extinction events. Even at times of quietude it is far from clear whether biological communities enjoy stability and interlocked stasis or are dynamically reconstituted at regular intervals. Finally, can we yet rely on the present depictions of the rise and falls in the levels of ancient diversity? Existing data is almost entirely based on Linnean taxa, and the application of phylogenetic systematics to this problem is still in its infancy. Not only that, but even more intriguingly the pronounced divergence in estimates of origination times of groups as diverse as angiosperms, diatoms and mammals in terms of the fossil record as against molecular data point to the possibilities of protracted intervals of geological time with a cryptic diversity. If this is correct, and there are alternative explanations, then some of the mystery of adaptive radiations may be dispelled, in as much as the assembly of key features in the stem groups could be placed in a gradualistic framework of local adaptive response punctuated by intervals of opportunity.     

Strengthening of the East Asian summer monsoon revealed by a shift in seasonal patterns in diet and climate after 2-3 Ma in northwest China

To examine climate variability in northwest China in the late Cenozoic and to test hypotheses regarding the development of C₄ ecosystems and the dynamics of the Asian monsoons, the carbon and oxygen isotopic compositions of 32 bulk and 368 serial tooth enamel samples from herbivores in the Linxia Basin (Gansu Province), ranging in age from 25 Ma to the present, were determined. The results corroborate and improve the record previously obtained from the area, showing that all mammals in the Linxia Basin lived in habitats consisting primarily of C₃ vegetation prior to 2-3 Ma and that C₄ grasses did not become a significant component of local ecosystems until the Quaternary. The data also show that shifts in climate to drier and/or warmer conditions after about 14, 9.5, 7, and 2.5 Ma, as indicated by positive δ¹⁸O excursions in the bulk enamel-δ¹⁸O record, were accompanied by increased seasonality; whereas negative δ¹⁸O shifts in the bulk data after about 11, 6, and 1.2 Ma, which indicate shifts to wetter and/or cooler climate, were associated with decreases in seasonality. Intra-tooth δ¹³C and δ¹⁸O profiles reveal significant changes in the seasonal patterns of diet and climate after ~ 2-3 Ma. Prior to ~ 2-3 Ma, there was little or no seasonal variation in herbivores' diets and all herbivores fed on C₃ vegetation year around. After that time, the data show a significant seasonal variation in the diets of horses and bovids, ranging from a pure C₃ to a mixed C₃/C₄ diet (with C₄ plants accounting for up to ~ 60% of the diet). An inverse relationship (or negative correlation) between δ¹³C and δ¹⁸O values within individual teeth — a pattern characteristic of the summer monsoon regime — is observed in younger (< 2-3 Ma) horses and bovids but not in older fossils. These changes in intra-tooth isotopic patterns provide strong evidence for an enhanced monsoon climate since about 2-3 Ma.     

Diversity and extinction in the Cenozoic history of Caribbean reefs

Occurrences of reef corals are examined at Caribbean fossil localities to determine how biodiversity has changed within the region over the past 50 million years. Analyses of 294 species (66 genera) at 58 fossil localities show that Caribbean generic diversity rose to 44 between 50–22 Ma, ranged from 32–39 between 22–2 Ma, and dropped to 25 afterwards. Regional species diversity was high at 40–36 Ma, 28–22 Ma, and 5–2 Ma. Origination rates were elevated throughout each high diversity interval, but extinction was concentrated near the end of each interval. Regional highs of origination and extinction, therefore, differed in timing and duration, causing the observed regional diversity increases during the three remarkably long intervals of turnover. Highs of generic origination decreased in magnitude as immigration from the Mediterranean ceased, but speciation highs increased in association with emergence of the Central American isthmus. Peaks of extinction coincided with regional changes in climate and oceanic circulation. Maximum species diversities within assemblages increased to 40–60 between 50–36 Ma, and have remained relatively constant ever since. Assemblage compositions differed among localities having similar ages and environments, suggesting that the timing and pattern of turnover varied across the region. Stable diversities but variable compositions within assemblages suggest that dispersal and recruitment influenced the pattern of faunal change during turnover. Accepted: 22 August 1999     

An analytical approach for estimating fossil record and diversification events in sharks, skates and rays

Background

Modern selachians and their supposed sister group (hybodont sharks) have a long and successful evolutionary history. Yet, although selachian remains are considered relatively common in the fossil record in comparison with other marine vertebrates, little is known about the quality of their fossil record. Similarly, only a few works based on specific time intervals have attempted to identify major events that marked the evolutionary history of this group.

Methodology/Principal Findings

Phylogenetic hypotheses concerning modern selachians’ interrelationships are numerous but differ significantly and no consensus has been found. The aim of the present study is to take advantage of the range of recent phylogenetic hypotheses in order to assess the fit of the selachian fossil record to phylogenies, according to two different branching methods. Compilation of these data allowed the inference of an estimated range of diversity through time and evolutionary events that marked this group over the past 300 Ma are identified. Results indicate that with the exception of high taxonomic ranks (orders), the selachian fossil record is by far imperfect, particularly for generic and post-Triassic data. Timing and amplitude of the various identified events that marked the selachian evolutionary history are discussed.

Conclusion/Significance

Some identified diversity events were mentioned in previous works using alternative methods (Early Jurassic, mid-Cretaceous, K/T boundary and late Paleogene diversity drops), thus reinforcing the efficiency of the methodology presented here in inferring evolutionary events. Other events (Permian/Triassic, Early and Late Cretaceous diversifications; Triassic/Jurassic extinction) are newly identified. Relationships between these events and paleoenvironmental characteristics and other groups’ evolutionary history are proposed.     

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.     

Palaeoceanographical changes recorded by Cenozoic deep-sea ostracod assemblages from the South Atlantic and the Southern Ocean (ODP Sites 1087 and 1088)

Cenozoic palaeoceanography of the SE Atlantic and Southern Oceans has been investigated using Late Eocene/Early Oligocene to Quaternary ostracod assemblages from 49 samples of ODP Sites 1087 and 1088. Although the overall abundance of ostracods is relatively low (means of 17 and 49 specimens per sample at Sites 1087 and 1088, respectively) and there is an apparently high level of endemism (ranging from 50% to 80% at Sites 1087 and 1088), three major changes in the faunal assemblages are identified at Site 1087 (denoted A, B and C) and two at Site 1088 (denoted B' and C'). The assemblage boundaries, detected on the basis of stepwise changes in the abundance, diversity, dominance, endemism, faunal turnover and relative abundance of common taxa, coincide broadly with previously identified, ostracod-based palaeoceanographical 'events' discussed by Benson and co-workers over the last two decades. The data do not extend sufficiently far back to record the initiation of Assemblage A, but the faunal change between Assemblages A and B, marked by a decline in abundance, species diversity and faunal turnover, occurs within the Middle Miocene (NN5-6). It coincides with a previously documented palaeoceanographical 'event' at 16-14 Ma which, we suggest, may be related to the initiation of North Atlantic Deep Water (NADW) production and/or an expansion of the East Antarctic ice sheet. Assemblage B' is subdivided into the two Sub-assemblages B'¹ and B'² mainly on the basis of an increase in diversity, a peak in faunal turnover and a drop in the relative abundance of the genus Krithe in early Late Miocene time (NN9, c. 10.5 Ma). The B'¹/B'² Sub-assemblage boundary cannot be related to any previously documented faunal change in deep-sea ostracods. Changes associated with the boundaries between Assemblages B and C, and B' and C', which we believe to be synchronous, both include a decrease in diversity and abundance. In addition, two strong turnover peaks occur near the B'/C' boundary at Site 1088. The B/C and B'/C' boundaries coincide with a previously documented midPliocene 'event' (3.5 Ma) (NN15-16) which may be linked to putative closure of the Straits of Panama and increased production of NADW, the latter in turn leading to increased production of Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW). Alternatively, fiuctuations in size of the Antarctic ice sheet during possible Pliocene warm periods could indirectly be responsible for the observed midPliocene faunal changes.     

Pregnancy outcome for nomadic Turkana pastoralists of Kenya

The outcome of an individual pregnancy is a key component of overall reproductive success and is strongly influenced by environmental, behavioral, and biological variation within populations. This prospective study examined the outcomes of 68 pregnancies among nomadic Turkana of Kenya from July 1993 to July 1994. A total of 12 women experienced pregnancy losses, and 3 women experienced live births with a subsequent perinatal death. The following characteristics are associated with increased risk for experiencing a fetal or perinatal death: severe morbidity episodes, shorter nonpregnant intervals, lower third-trimester weight, higher third-trimester summed skinfolds but limited third-trimester reduction in summed skinfolds, and higher activity levels late in gestation. Turkana newborns measured within 48 hr of birth (n = 19) weighed an average of 2,860 +/- 640 g and had a mean ponderal index of 2.72 +/- 0.46. Variation in birth weight was predicted by preterm delivery, delivery during the wet season, and maternal morbidity levels. Birth length averaged 50.8 +/- 6.8 cm and was positively influenced by full term delivery, better maternal health, and birth during the dry season. These results suggest complicated pathways linking the physical environment, subsistence requirements, heterogeneity in maternal fitness, behavior, and pregnancy outcome. The results augment the larger picture of reproductive success for Turkana nomads who live in a highly variable savanna environment.     

Parasitoid fig‐wasp evolutionary diversification and variation in ecological opportunity

Ecological processes are manifest in the evolution and form of phenotype diversity. The great abundance of parasitoid species has led to speculation whether rates of speciation and extinction are dependent on parasitoid diversity. If these factors are mutually exclusive, species diversity should fluctuate instead of remaining relatively constant over time. It is not known whether radiations constrained by coevolutionary interactions conform to density‐dependent diversification processes. Here we test the prediction that parasitoid fig wasp diversification responds to changes in ecological opportunity and density‐independent processes. A phylogenetic approach is used to estimate relative divergence times and infer diversification rate changes using γ‐statistics. Monte Carlo constant rates tests that accommodate incomplete sampling could not reject constant rates diversification. Parasitoid fig wasp diversification is consistent with a more complex explanation than density‐dependent cladogenesis. The results suggest contemporary African parasitoid fig wasp diversity remains a legacy of an ancient ecological opportunity facilitated by fig tree diversification following the breakup of Pan‐African forests and evolution of the savanna biome over the last 55 Ma and the more recent aridification of the African continent in the last 5 Ma. These results imply that amplified phenotypic differentiation of specialist insects coevolving with plants is coupled to evolutionarily infrequent changes in ecological opportunity.