Stratigraphy, U-Th chronology, and paleoenvironments at Gladysvale Cave: insights into the climatic control of South African hominin-bearing cave deposits

Gladysvale Cave is one of the few Plio-Pleistocene hominin-bearing cave sites in South Africa that contains a well-stratified cave fill with clastic sediments interspersed with flowstones. The clastic sediments can be divided into units based on the presence of intercalated flowstones, forming flowstone bounded units (FBU). Ten MC-ICP-MS uranium-series dates on several flowstone horizons in the Gladysvale Internal Deposit fan indicate deposition from the late mid-Pleistocene ( approximately 570 ka) to Holocene ( approximately 7 ka) during limited periods of higher effective moisture. Clastic sedimentation occurred during the interceding, presumably more arid, periods. This sequence is not consistent with earlier models for South African caves that simply assumed interglacial sedimentation and glacial erosion. (13)C/(12)C data suggest that flowstone tended to form during periods with higher proportions of C(3) plants in the local vegetation, while clastic sediments reflect higher proportions of C(4) grasses, although this is not always the case. We argue that flowstones are precipitated during periods of higher effective precipitation and restricted cave entrances, while clastic sediments accumulated during periods with more open vegetation. The sedimentary fill of the fossiliferous deposits are, therefore, highly episodic in nature, with large periods of time unlikely to be represented. This has serious implications for the other hominin-bearing caves close by, as these deposits are likely to be similarly episodic. This is especially pertinent when addressing extinction events and reconstructions of paleoenvironments, as large periods of time may be unrecorded. The Gladysvale Cave fill sediments may serve as a climatically forced chronostratigraphic model for these less well-stratified and well-dated Plio-Pleistocene sites.     

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.     

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.     

Fire and fire-adapted vegetation promoted C4 expansion in the late Miocene

Large proportions of the Earth's land surface are covered by biomes dominated by C(4) grasses. These C(4)-dominated biomes originated during the late Miocene, 3-8 million years ago (Ma), but there is evidence that C(4) grasses evolved some 20 Ma earlier during the early Miocene/Oligocene. Explanations for this lag between evolution and expansion invoke changes in atmospheric CO(2), seasonality of climate and fire. However, there is still no consensus about which of these factors triggered C(4) grassland expansion. We use a vegetation model, the adaptive dynamic global vegetation model (aDGVM), to test how CO(2), temperature, precipitation, fire and the tolerance of vegetation to fire influence C(4) grassland expansion. Simulations are forced with late Miocene climates generated with the Hadley Centre coupled ocean-atmosphere-vegetation general circulation model. We show that physiological differences between the C(3) and C(4) photosynthetic pathways cannot explain C(4) grass invasion into forests, but that fire is a crucial driver. Fire-promoting plant traits serve to expand the climate space in which C(4)-dominated biomes can persist. We propose that three mechanisms were involved in C(4) expansion: the physiological advantage of C(4) grasses under low atmospheric CO(2) allowed them to invade C(3) grasslands; fire allowed grasses to invade forests; and the evolution of fire-resistant savanna trees expanded the climate space that savannas can invade.     

Do "savanna" chimpanzees consume C4 resources?

Several stable carbon isotopic studies have shown that South African australopiths consumed significant quantities of C(4) resources (tropical grasses, sedges, or animals that eat those foods), but relatively little is known about the consumption of such resources by chimpanzees. Here, we present stable carbon isotopic data for 36 chimpanzee hair samples from Fongoli, one of the driest and most open areas inhabited by chimpanzees. These data suggest that the Fongoli chimpanzees consume little in the way of C(4) vegetation or animals that eat such vegetation, even though these resources are locally abundant and preferred fruits are more widely scattered than at most chimpanzee study sites. The homogeneity of the Fongoli results is especially striking and recalls the narrow isotopic distribution of stenotopic savanna mammals. This is in stark contrast to what has been observed for australopiths, which had highly variable diets and consumed about 35% C(4) vegetation on average. Carbon isotope data for modern and fossil Papio depict a dietarily variable genus with a tendency to consume C(4) vegetation. This trophic flexibility, or willingness to consume C(4) savanna resources, may make Papio a more profitable ecological analog for australopiths than chimpanzees.     

Early pleistocene habitat in member 1 Olorgesailie based on paleosol stable isotopes.

Stable carbon and oxygen isotope values from soil carbonates were used to determine the vegetation context of archaeological sites and local climatic conditions represented in a approximately 0.99 Ma paleosol that is exposed laterally in the Olorgesailie basin, southern Kenya rift valley. As part of this landscape-scale project, samples of an upper Member 1 paleosol were analyzed along nearly 4 km of outcrop in three adjacent parts of the basin. Modern East African soil and plant community analogs are used to interpret the isotope ratios. The carbon isotopic composition of the paleosol carbonates indicates that the area supported a local biomass of about 75-100% C(4)plants during the period of soil formation. After averaging the data for each trench, an open C(4)grassland is represented by half of the carbon values, with wooded grassland more abundant across the paleolandscape than it is in the area now. This vegetational reconstruction is supported by the mammalian faunal assemblage, which has a high percentage of grazers. Although the relatively small sample size outside the main excavation area precludes firm characterization of vegetational diversity across the basin in upper Member 1 times, eastern and western localities in the study area may have had more woody C(3)plants than the widely sampled zone in between. Oxygen isotopes indicate that the lowland basin was slightly cooler and moister than today's semi-arid climate, with greater annual rainfall. Archaeological traces have a virtually continuous distribution across the paleolandscape, but vary in density of occurrence. With the strong evidence for C(4)grassland as the primary vegetation context across most of the study area, no habitat preference by the Acheulean toolmakers at Olorgesailie is shown in our initial comparison between carbon isotope values and stone/bone densities.     

Speleology and magnetobiostratigraphic chronology of the GD 2 locality of the Gondolin hominin-bearing paleocave deposits, North West Province, South Africa

Speleological, paleomagnetic, mineral magnetic, and biochronological analyses have been undertaken at the Gondolin hominin-bearing paleocave, North West Province, South Africa. Two fossiliferous but stratigraphically separate sequences, GD2 and GD1/3, which were once part of a large cavern system, have been identified. Although some comparative paleomagnetic samples were taken from the GD 1, 3, and 4 localities that are currently under investigation, the research presented here focuses on the fossil-rich, in situ deposits at locality GD 2, excavated by E.S. Vrba in 1979. The GD 2 deposits are dominated by normal-polarity calcified clastic deposits that are sandwiched between clastic-free flowstone speleothems. The lower flowstone has a sharp contact with the red siltstone deposits and is of reversed polarity. The capping flowstone shows a change from normal to reversed polarity, thereby preserving a polarity reversal. While the paleomagnetic work indicates that the GD 2 fossil material was deposited during a normal-polarity period, the shortness of the sequence made matching of the magnetostratigraphy to the geomagnetic polarity time scale (GPTS) impossible without the aid of biochronology. While lacking multiple time-sensitive taxa, the recovery of specimens attributable to Stage III Metridiochoerus andrewsi is consistent with a deposition date between 1.9 and 1.5 Ma. A comparison of the magnetostratigraphy with the GPTS therefore suggests that the fauna-bearing siltstone of GD 2 date to the Olduvai normal-polarity event, which occurred between 1.95 and 1.78 Ma, and that the reversal from normal to reversed polarity identified in the capping flowstone dates to 1.78 Ma. The main faunal layers therefore date to slightly older than 1.78 Ma. Deposits from the GD 1 locality are dominated by reversed directions of magnetization, which show that this deposit is not of the same age as the faunal layers from the GD 2 locality.     

Timing of C4 grass expansion across sub-Saharan Africa

The emergence of C(4) grass biomes is believed to have first taken place in the upper Miocene, when a series of events modified global climate with long-lasting impacts on continental biotas. Changes included major shifts in floral composition-characterized in Africa by shrinking of forests and emergence of C(4) grasses and more open landscapes-followed by large-scale evolutionary shifts in faunal communities. The timing of the emergence of C(4) grasses, and the subsequent global expansion of C(4) grass-dominated biomes, however, is disputed, leading to contrasting views of the patterns of environmental changes and their links to faunal shifts, including those of early hominins. Here we evaluate the existing isotopic evidence available for central, eastern, and southern Africa, and review interpretations in light of these data. Pedogenic and biomineral carbonate delta(13)C data suggest that clear evidence for C(4) biomass in low latitudes exists only from 7-8 Ma. This likely postdates the emergence of C(4) plants, whose physiology is adapted to low atmospheric carbon dioxide concentrations. Biomes with C(4) grasses appeared later in mid-latitude sites. Moreover, C(4) grasses apparently remained a relatively minor component of most environments until the late Pliocene and early Pleistocene. Hence establishment of C(4) grasses, even as minor components of African biomes, precedes the very earliest evidence for bipedalism by two million years, and the more abundant and secure evidence by some three to four million years. This may suggest a protracted process of hominin adaptation to these emerging, more open landscapes.     

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.     

Molar microwear and dietary reconstructions of fossil cercopithecoidea from the Plio-Pleistocene deposits of South Africa

The South African Plio-Pleistocene cave deposits have yielded a diverse cercopithecoid fauna. In this study, the possible dietary proclivities of these extinct species are examined using details of molar microwear. Although sample sizes are often small, wear patterns suggest possible temporal changes in the diets of Parapapio jonesi from Makapansgat to Sterkfontein, of Papio robinsoni from Sterkfontein to Swartkrans, and Cercopithecoides williamsi from Makapansgat to Sterkfontein to Swartkrans. However, there does not appear to have been a significant change in the dietary habits of Parapapio broomi over time. The microwear patterns of the two temporally successive congeners, Theropithecus darti and T. oswaldi show no significant differences from one another. The sympatric congeners, Parapapio broomi and Pp. jonesi, have microwear signatures that differ significantly at Makapansgat (Members 3 and 4) but not at Sterkfontein (Member 4). Finally, the microwear analyses suggest that the extinct cercopithecoid species did not necessarily have diets similar to those of their closest living relatives.     

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.     

Microwear evidence for Plio-Pleistocene bovid diets from Makapansgat Limeworks Cave, South Africa

Makapansgat Limeworks Cave is a well-known Australopithecus africanus bearing locality that has spawned a considerable amount of paleoecological research because of its hominin component. Most recently, the paleoecology of this Plio-Pleistocene site has been studied by determining the diet and habitat of other extinct taxa, particularly the bovids. The diets of seven bovids (Aepyceros sp., Gazella vanhoepeni, Makapania broomi, Parmularius braini, Redunca darti, Tragelaphus sp. aff. T. angasii, and Tragelaphus pricei) have now been classified using taxonomic uniformitarianism, ecomorphology, stable carbon isotopes, and mesowear analysis. Here, dental microwear is applied to the same bovids for additional comparison and to further elucidate the strengths and weaknesses of each method. The different dietary proxy methods noted provide a temporal continuum, with genetic signals such as ecomorphology and taxonomic uniformitarianism indicating behavioral adaptations over geologic time, while nongenetic data such as stable carbon isotopes and mesowear reflect different aspects of average diet over extended portions of an animal's life, and dental microwear provides dietary snapshots.Microwear separated an extant baseline of ten bovid species into expected dietary categories and the Makapansgat bovids clearly fell into two groups with the same degree of separation as between extant grazers and browsers. The results indicate that a multidisciplinary approach produces a more accurate and robust reconstruction of past diets. In sum, the microwear analysis is in-line with the isotope and mesowear results, which suggest a stronger browsing component than either taxonomic uniformitarianism or ecomorphology imply.     

Hominins, sedges, and termites: new carbon isotope data from the Sterkfontein valley and Kruger National Park

Stable carbon isotope analyses have shown that South African australopiths did not have exclusively frugivorous diets, but also consumed significant quantities of C4 foods such as grasses, sedges, or animals that ate these foods. Yet, these studies have had significant limitations. For example, hominin sample sizes were relatively small, leading some to question the veracity of the claim for australopith C4 consumption. In addition, it has been difficult to determine which C4 resources were actually utilized, which is at least partially due to a lack of stable isotope data on some purported australopith foods. Here we begin to address these lacunae by presenting carbon isotope data for 14 new hominin specimens, as well as for two potential C4 foods (termites and sedges). The new data confirm that non-C3 foods were heavily utilized by australopiths, making up about 40% and 35% of Australopithecus and Paranthropus diets respectively. Most termites in the savanna-woodland biome of the Kruger National Park, South Africa, have intermediate carbon isotope compositions indicating mixed C3/C4 diets. Only 28% of the sedges in Kruger were C4, and few if any had well-developed rhizomes and tubers that make some sedges attractive foods. We conclude that although termites and sedges might have contributed to the C4 signal in South African australopiths, other C4 foods were also important. Lastly, we suggest that the consumption of C4 foods is a fundamental hominin trait that, along with bipedalism, allowed australopiths to pioneer increasingly open and seasonal environments.     

Re-appraisal of the stratigraphy and determination of new U-Pb dates for the Sterkfontein hominin site, South Africa

Sterkfontein Caves is the single richest early hominin site in the world with deposits yielding one or more species of Australopithecus and possible early Homo, as well as an extensive faunal collection. The inability to date the southern African cave sites accurately or precisely has hindered attempts to integrate the hominin fossil evidence into pan-African scenarios about human evolutionary history, and especially hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium carbonate flowstone inter-bedded between the fossiliferous sediments. For the first time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2 is between 2.8 ± 0.28 and 2.6 ± 0.30 Ma and Member 4 between 2.65 ± 0.30 and 2.01 ± 0.05 Ma. The age of 2.01 ± 0.05 Ma for the top of Member 4 constrains the last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we have reproduced the U-Pb age of ∼2.2 Ma of for the flowstones associated with StW573. We believe that these deposits, including the fossil and the flowstones, accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments are exposed both in the underground chambers and at surface. We present a new interpretation of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every effort was made to retain the Member system, however, only Members 2 and 4 are recognized in the boreholes. We propose that the deposits formally known as Member 3 are in fact the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of cone-like deposits and probably never filled up the cave. The U-Th ages show that there are substantial deposits younger than 400 ka in the underground cave, underlying the older deposits, highlighting again that these cave fills are not simple layer-cakes.     

Carbon isotopic record of terrestrial ecosystems spanning the Late Miocene extinction of Oreopithecus bambolii, Baccinello Basin (Tuscany, Italy)

Oreopithecus bambolii is a Late Miocene hominoid with an extensive fossil record in the Baccinello Basin (Tuscany, Italy), and was the only western European hominoid to survive a major extinction event ca. 9.6 Ma (millions of years ago). Oreopithecus lived in the insular Tusco-Sardinian paleobioprovince, where it evolved many unique anatomical specializations that make it important for understanding the mechanisms and history of Late Miocene hominoid evolution. The eventual extinction of Oreopithecus and its associated fauna ca. 6.5 Ma has generally been attributed to interaction with species that arrived from continental Europe following tectonic collision of the Tusco-Sardinian province with mainland Italy, but palynological, paleontological, and sedimentological records indicate an environmental shift toward more variable climate across the extinction event.To explore the possibility of environmental change as a contributing factor in the extinction of Oreopithecus, we developed a stable carbon and oxygen isotope record from organic matter in paleosols from the Baccinello Basin. These data show very low temporal and spatial variability (indicating plant ecosystem stability through time and space) and provide no evidence for ecologically significant changes in floral composition spanning the extinction event, suggesting that environmental change was not an underlying cause for the extinction of Oreopithecus and its associated fauna. The carbon isotope values fall entirely within the range of isotopic variability for modern plants following the C₃ photosynthetic pathway (trees, shrubs, cool-season grasses), indicating that C₄ vegetation (warm-season grasses) was not an important component of biomass. When corrected for temporal variation in the carbon isotopic composition of atmospheric carbon dioxide, the paleosol carbon isotope values are consistent with predicted values based on modern plants and the Baccinello palynoflora, supporting the reliability of paleosol isotopic records as paleoecological proxies.     

Ecosystem consequences of plant life form changes at three sites in the semiarid United States

Many semiarid rangelands have recently experienced changes in dominant plant life form. Both woody plant expansion into grasslands and the invasion of annual grasses into shrublands have potential influence on regional carbon cycling. Soil carbon content, chemistry, and distribution may change following shifts in dominant plant life form because plant life forms differ in litter chemistry and patterns of detrital input. This study assesses the amount, quality, and distribution of soil C below woody vegetation and grasses at three rangelands in Texas, New Mexico, and Utah. At each of these sites there has been a well-documented shift in dominant plant life form. In Texas and New Mexico, woody plants have increased in grasslands, while grasses have invaded into former shrublands in Utah. We measured total soil carbon, particulate organic matter (POM) C, and the carbon isotopic composition of soil carbon beneath woody plants and grasses at each of these three sites. At the La Copita Research Area in south-central Texas there was significantly more soil C found beneath Prosopis glandulosa, the dominant woody plant, than was found beneath grasses. Mean soil C content to 1 m was 7.2 kg C m^–2 beneath P. glandulosa and 6.0 kg C m^–2 beneath grasses. There was also significantly more POM C beneath P. glandulosa than beneath grasses. Stable carbon isotopic composition indicated that the expansion of P. glandulosa in savannas in Texas first influences carbon cycling in surface soils, then deep soil C, and finally throughout the soil profile. At the Sevilleta National Wildlife Refuge in central New Mexico, we found that there was significantly more soil C in the upper 10 cm of the soil profile beneath Larrea tridentata than was found beneath Bouteloua spp. Stable carbon isotopic composition indicated that the expansion of L. tridentata influenced C cycling throughout the soil profile. At Curlew Valley in northern Utah, we found no significant differences in total profile soil C beneath different plant life forms. However, there was significantly more soil C found at the soil surface beneath woody plants than was observed beneath annual grasses. There was significantly less POM C beneath annual grasses than was found beneath woody plants or perennial grasses. Based on stable carbon isotopic analyses, we concluded that the invasion of grasses into shrublands influenced only the upper 30 cm of the soil profile. We determined that following changes in plant life form dominance, the most consistent change in soil C was an alteration in content and distribution of POM C, a slowly cycling pool of soil C. While we failed to find a consistent change in total profile soil C with plant life form across our sites, the change in soil C chemistry may have important implications for long-term soil C storage in semiarid systems where there have been shifts in plant life form. Received: 30 March 1999 / Accepted: 11 August 1999     

Recent diversification in African greenbuls (Pycnonotidae: Andropadus) supports a montane speciation model

It is generally accepted that accentuated global climatic cycles since the Plio-Pleistocene (2.8 Ma ago) have caused the intermittent fragmentation of forest regions into isolated refugia thereby providing a mechanism for speciation of tropical forest biota contained within them. However, it has been assumed that this mechanism had its greatest effect in the species rich lowland regions. Contrary evidence from molecular studies of African and South American forest birds suggests that areas of recent intensive speciation, where mostly new lineages are clustered, occur in discrete tropical montane regions, while lowland regions contain mostly old species. Two predictions arise from this finding. First, a species phylogeny of an avian group, represented in both lowland and montane habitats, should be ordered such that montane forms are represented by the most derived characters. Second, montane speciation events should predominate within the past 2.8 Ma. In order to test this model I have investigated the evolutionary history of the recently radiated African greenbuls (genus Andropadus), using a molecular approach. This analysis finds that montane species are a derived monophyletic group when compared to lowland species of the same genus and recent speciation events (within the Plio-Pleistocene) have exclusively occurred in montane regions. These data support the view that montane regions have acted as centres of speciation during recent climatic instability.     

Astronomically forced climate change in the Kenyan Rift Valley 2.7-2.55 Ma: implications for the evolution of early hominin ecosystems

Global climate change, linked to astronomical forcing factors, has been implicated in faunal evolutionary change in equatorial Africa, including the origin and diversification of hominin lineages. Empirical terrestrial data demonstrating that orbital forcing has a significant effect, or is detectable, at early hominin sites in equatorial continental interiors during the Pliocene, however, remain limited. Sedimentation patterns in the Baringo Basin within the Central Kenyan Rift Valley between ca. 2.7 and 2.55 Ma, controlled by climatic factors, provide a detailed paleoenvironmental record spanning 35 fossil vertebrate localities, including three hominin sites. The succession includes a sequence of diatomites that record rhythmic cycling of major freshwater lake systems consistent with approximately 23-kyr Milankovitch precessional periodicity. The temporal framework of shifting precipitation patterns, relative to Pliocene insolation curves, implicate African monsoonal climatic control and indicate that climatic fluctuations in Rift Valley ecosystems were paced by global climatic change documented in marine cores. These data provide direct evidence of orbitally mediated environmental change at Pliocene Rift Valley hominin fossil localities, providing a unique opportunity to assess the evolutionary effect of short-term climatic flux on late Pliocene East African terrestrial communities.     

Not all forests are expanding over central Brazilian savannas

Recently we reported on the expansion of riparian forests into savannas in central Brazil. To enlarge the scope of the earlier study we investigated whether upland deciduous and xeromorphic forests behaved similarly. We investigated past vegetation changes that occurred in forest/savanna transitions using carbon isotope ratios (δ¹³C) measured in the soil organic matter as a tracer. We analyzed the ¹⁴C activity where δ¹³C showed major shifts in vegetation. The role of soil chemical and physical attributes in defining vegetation distribution is discussed. Structural changes in vegetation were found to be associated with shifts in the isotope composition (δ¹³C) of soil organic matter. This was attributed to intrinsic differences in the biomass of trees and grasses and allowed for the determination of past shifts in vegetation by evaluating δ¹³C at different depths. The deciduous forest decreased in area approximately 980 years ago. Tree cover increased in the xeromorphic forest, but the border stayed stable through time. The deciduous forest and adjacent savanna have eutrophic soils while the xeromorphic forest and adjacent savanna have dystrophic soils. However, greater organic carbon, nitrogen and phosphorus concentrations are observed in the forests. We provide concrete evidence of deciduous forest retreat unlike the stability observed in the xeromorphic forest/savanna boundary. These results contrast with the expansion of riparian forests recently reported in the same region.     

The spread of grass-dominated habitats in Turkey and surrounding areas during the Cenozoic: Phytolith evidence

The arrival of hipparionine horses in the eastern Mediterranean region around 11 Ma was traditionally thought to mark the simultaneous westward expansion of savanna vegetation across Eurasia. However, recent paleoecological reconstructions based on tooth wear, carbon isotopes, and functional morphology indicate that grasses played a minor role in Late Miocene ecosystems of the eastern Mediterranean, which were more likely dry woodlands or forests. The scarcity of grass macrofossils and pollen in Miocene floras of Europe and Asia Minor has been used to support this interpretation. Based on the combined evidence, it has therefore been suggested that Late Miocene ungulate faunal change in the eastern Mediterranean signals increased aridity and landscape openness, but not necessarily the development of grass-dominated habitats.

To shed new light on the Miocene evolution of eastern Mediterranean ecosystems, we used phytolith assemblages preserved in direct association with faunas as a proxy for paleovegetation structure (grassland vs. forest). We extracted phytoliths and other biogenic silica from sediment samples from well-known Early to Late Miocene ( 20–7 Ma) faunal localities in Greece, Turkey, and Iran. In addition, a Middle Eocene sample from Turkey yielded phytoliths and served as a baseline comparison for vegetation inference.

Phytolith analysis showed that the Middle Eocene assemblage consists of abundant grass phytoliths (grass silica short cells) interpreted as deriving from bambusoid grasses, as well as diverse forest indicator phytoliths from dicotyledonous angiosperms and palms, pointing to the presence of a woodland or forest with abundant bamboos. In contrast, the Miocene assemblages are dominated by diverse silica short cells typical of pooid open-habitat grasses. Forest indicator phytoliths are also present, but are rare in the Late Miocene (9–7 Ma) assemblages. Our analysis of the Miocene grass community composition is consistent with evidence from stable carbon isotopes from paleosols and ungulate tooth enamel, showing that C₄ grasses were rare in the Mediterranean throughout the Miocene. These data indicate that relatively open habitats had become common in Turkey and surrounding areas by at least the Early Miocene ( 20 Ma), > 7 million years before hipparionine horses reached Europe and arid conditions ensued, as judged by faunal data.