Trends, rhythms, and aberrations in global climate 65 Ma to present

Ichnotaxonomy and ichnostratigraphy of chambered trace fossils in palaeosols attributed to coleopterans, ants and termites

Most recorded trace fossils in palaeosols are burrows and chambers attributed to bees, ants, termites and coleopterans. Ichnogenera attributed to bees are grouped in the ichnofamily Celliformidae, whereas those attributed to ants, termites and coleopterans are included herein in the new ichnofamilies Pallichnidae, Krausichnidae and Coprinisphaeridae respectively. Shape, type of wall, fillings and associated burrows of chambers are the main morphological ichnotaxobases used for this classification; they are weighed with regard to the behaviour and architecture of the supposed trace-makers. Coprinisphaeridae are spherical, pear-shaped or ovoid structures, having active or passive fillings and constructed walls. The ichnogenera included are: Fontanai, Coprinisphaera, Eatonichnus, Monesichnus, Teisseirei and Rebuffoichnus, attributed to coleopterans. The similar Pallichnidae show lined or structureless walls, and include Pallichnus, Fictovichnus and Scaphichnium, also attributed to coleopterans. The Krausichnidae constitutes trace fossils composed of chambers of different shapes interconnected by burrow systems of inconsistent diameter or isolated chambers associated with burrow systems of different diameters. The Krausichnidae include Attaichnus, Parowanichnus, Krausichnus, Archeoentomichnus, Tacuruichnus, Vondrichnus, Fleaglellius, Termitichnus and Syntermesichnus, attributed to ants and termites. The stratigraphic ranges of insect ichnotaxa in palaeosols are reviewed and compared with the body fossil record of potential tracemakers, revealing that in most cases insect trace and body fossils show similar ranges. As stated by earlier authors, the Cretaceous was a critical period during which the oldest body fossils of dung-beetles, bees, termites and ants are recorded, whereas the trace fossils of these groups are recorded from this period or shortly after, during Cenozoic times.

A shrew-sized origin for primates

The origin of primates has had a long history of discussion and debate, with few ever considering the impact of the original body weight on subsequent primate adaptive radiations. Here, I attempt to reconstruct early primate evolution by considering the initial size of primates as well as the critical functional-adaptive events that had to occur prior to the early Eocene. Microcebus is often viewed as a living model, and thus 40-65 g might represent a practical ancestral weight for the origin of primates. I consider a smaller original body weight, likely 10-15 g in actual size, and I address the biological implications for shrew-sized primates by comparing the behavioral ecology of mouse lemurs, our smallest living primates, to another tiny-sized mammalian group, the shrews (Family Soricidae). Several behavioral and ecological characteristics are shared by shrews and mouse lemurs, and several mammalian trends are evident with decreased size. I suggest that a shrew-sized ancestral primate would have had high metabolic, reproductive, and predation rates, relatively low population densities, and a dispersed and solitary existence with a promiscuous mating system. Although small mammals like shrews provide insights concerning the ancestral size of primates, primate origins have always been tied to arboreality. I assess other potential arboreal models such as Ptilocercus and Caluromys. By combining all of this information, I try to sequence the events in a functional-adaptive series that had to occur before the early Eocene primate radiations. I suggest that all of these important adaptive events had to occur at a small body size below 50 g.

Recent ice ages on Mars

A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters. Recent Mars exploration has revealed dusty, water-ice-rich mantling deposits that are layered, metres thick and latitude dependent, occurring in both hemispheres from mid-latitudes to the poles. Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 2.1 to 0.4 Myr ago. The deposits were emplaced symmetrically down to latitudes of approximately 30 degrees--equivalent to Saudi Arabia and the southern United States on the Earth--in response to the changing stability of water ice and dust during variations in obliquity (the angle between Mars' pole of rotation and the ecliptic plane) reaching 30-35 degrees. Mars is at present in an 'interglacial' period, and the ice-rich deposits are undergoing reworking, degradation and retreat in response to the current instability of near-surface ice. Unlike the Earth, martian ice ages are characterized by warmer polar climates and enhanced equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to mid-latitudes.

Cenozoic climate change as a possible cause for the rise of the Andes

Causal links between the rise of a large mountain range and climate have often been considered to work in one direction, with significant uplift provoking climate change. Here we propose a mechanism by which Cenozoic climate change could have caused the rise of the Andes. Based on considerations of the force balance in the South American lithosphere, we suggest that the height of, and tectonics in, the Andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands, and shear stresses in the subduction zone depend on the amount of subducted sediments. We propose that the dynamics of subduction and mountain-building in this region are controlled by the processes of erosion and sediment deposition, and ultimately climate. In central South America, climate-controlled sediment starvation would then cause high shear stress, focusing the plate boundary stresses that support the high Andes.

The radiation of the Cape flora, southern Africa

The flora of the south-western tip of southern Africa, the Cape flora, with some 9000 species in an area of 90,000 km2 is much more speciose than can be expected from its area or latitude, and is comparable to that expected from the most diverse equatorial areas. The endemism of almost 70%, on the other hand, is comparable to that found on islands. This high endemism is accounted for by the ecological and geographical isolation of the Cape Floristic Region, but explanations for the high species richness are not so easily found. The high species richness is accentuated when its taxonomic distribution is investigated: almost half of the total species richness of the area is accounted for by 33 'Cape floral clades'. These are clades which may have initially diversified in the region, and of which at least half the species are still found in the Cape Floristic Region. Such a high contribution by a very small number of clades is typical of island floras, not of mainland floras. The start of the radiation of these clades has been dated by molecular clock techniques to between 18 million years ago (Mya) (Pelargonium) and 8 Mya (Phylica), but only six radiations have been dated to date. The fossil evidence for the dating of the radiation is shown to be largely speculative. The Cenozoic environmental history of southern Africa is reviewed in search of possible triggers for the radiations, climatic changes emerge as the most likely candidate. Due to a very poor fossil record, the climatic history has to be inferred from larger scale patterns, these suggest large-scale fluctuations between summer wet (Palaeocene, Early Miocene) and summer dry climates (Oligocene, Middle Miocene to present). The massive speciation in the Cape flora might be accounted for by the diverse limitations to gene flow (dissected landscapes, pollinator specialisation, long flowering times allowing much phenological specialisation), as well as a richly complex environment providing a diversity of selective forces (geographically variable climate, much altitude variation, different soil types, rocky terrain providing many micro-niches, and regular fires providing both intermediate disturbances, as well as different ways of surviving the fires). However, much of this is based on correlation, and there is a great need for (a) experimental testing of the proposed speciation mechanisms, (b) more molecular clock estimates of the age and pattern of the radiations, and (c) more fossil evidence bearing on the past climates.

A transient rise in tropical sea surface temperature during the Paleocene-Eocene thermal maximum

The Paleocene-Eocene Thermal Maximum (PETM) has been attributed to a rapid rise in greenhouse gas levels. If so, warming should have occurred at all latitudes, although amplified toward the poles. Existing records reveal an increase in high-latitude sea surface temperatures (SSTs) (8 degrees to 10 degrees C) and in bottom water temperatures (4 degrees to 5 degrees C). To date, however, the character of the tropical SST response during this event remains unconstrained. Here we address this deficiency by using paired oxygen isotope and minor element (magnesium/calcium) ratios of planktonic foraminifera from a tropical Pacific core to estimate changes in SST. Using mixed-layer foraminifera, we found that the combined proxies imply a 4 degrees to 5 degrees C rise in Pacific SST during the PETM. These results would necessitate a rise in atmospheric pCO2 to levels three to four times as high as those estimated for the late Paleocene.

The penalty of a long, hot summer. Photosynthetic acclimation to high CO2 and continuous light in "living fossil" conifers

Deciduous forests covered the ice-free polar regions 280 to 40 million years ago under warm "greenhouse" climates and high atmospheric pCO2. Their deciduous habit is frequently interpreted as an adaptation for minimizing carbon losses during winter, but experiments with "living fossils" in a simulated warm polar environment refute this explanation. Measured carbon losses through leaf abscission of deciduous trees are significantly greater than losses through winter respiration in evergreens, yet annual rates of primary productivity are similar in all species. Here, we investigate mechanisms underlying this apparent paradox by measuring the seasonal patterns of leaf photosynthesis (A) under pCO2 enrichment in the same trees. During spring, A increased significantly in coastal redwood (Sequoia sempervirens), dawn redwood (Metasequoia glyptostroboides), and swamp cypress (Taxodium distichum) at an elevated pCO2 of 80 Pa compared with controls at 40 Pa. However, strong acclimation in Rubisco carboxylation capacity (Vc,max) completely offset the CO2 response of A in all species by the end of 6 weeks of continuous illumination in the simulated polar summer. Further measurements demonstrated the temporary nature of acclimation, with increases in Vc,max during autumn restoring the CO2 sensitivity of A. Contrary to expectations, the acclimation of Vc,max was not always accompanied by accumulation of leaf carbohydrates, but was associated with a decline in leaf nitrogen in summer, suggesting an alteration of the balance in plant sources and sinks for carbon and nitrogen. Preliminary calculations using A indicated that winter carbon losses through deciduous leaf abscission and respiration were recovered by 10 to 25 d of canopy carbon fixation during summer, thereby explaining the productivity paradox.

Evidence for rapid climate change in the Mesozoic-Palaeogene greenhouse world

The best-documented example of rapid climate change that characterized the so-called 'greenhouse world' took place at the time of the Palaeocene-Eocene boundary: introduction of isotopically light carbon into the ocean-atmosphere system, accompanied by global warming of 5-8 degrees C across a range of latitudes, took place over a few thousand years. Dissociation, release and oxidation of gas hydrates from continental-margin sites and the consequent rapid global warming from the input of greenhouses gases are generally credited with causing the abrupt negative excursions in carbon- and oxygen-isotope ratios. The isotopic anomalies, as recorded in foraminifera, propagated downwards from the shallowest levels of the ocean, implying that considerable quantities of methane survived upward transit through the water column to oxidize in the atmosphere. In the Mesozoic Era, a number of similar events have been recognized, of which those at the Triassic-Jurassic boundary, in the early Toarcian (Jurassic) and in the early Aptian (Cretaceous) currently carry the best documentation for dramatic rises in temperature. In these three examples, and in other less well-documented cases, the lack of a definitive time-scale for the intervals in question hinders calculation of the rate of environmental change. However, comparison with the Palaeocene-Eocene thermal maximum (PETM) suggests that these older examples could have been similarly rapid. In both the early Toarcian and early Aptian cases, the negative carbon-isotope excursion precedes global excess carbon burial across a range of marine environments, a phenomenon that defines these intervals as oceanic anoxic events (OAEs). Osmium-isotope ratios ((187)Os/(188)Os) for both the early Toarcian OAE and the PETM show an excursion to more radiogenic values, demonstrating an increase in weathering and erosion of continental crust consonant with elevated temperatures. The more highly buffered strontium-isotope system ((87)Sr/(86)Sr) also shows relatively more radiogenic signatures during the early Toarcian OAE, but the early Aptian and Cenomanian-Turonian OAEs show the reverse effect, implying that increased rates of sea-floor spreading and hydrothermal activity dominated over continental weathering in governing sea-water chemistry. The Cretaceous climatic optimum (late Cenomanian to mid Turonian) also shows evidence for abrupt cooling episodes characterized by episodic invasion of boreal faunas into temperate and subtropical regions and changes in terrestrial vegetation; drawdown of CO(2) related to massive marine carbon burial (OAE) may be implicated here. The absence of a pronounced negative carbon-isotope excursion preceding the Cenomanian-Turonian OAE indicates that methane release is not necessarily connected to global deposition of marine organic carbon, but relative thermal maxima are common to all OAEs. 'Cold snaps' have also been identified from the Mesozoic record but their duration, causes and effects are poorly documented.

Palaeoclimatic insights into future climate challenges

Palaeoclimatic data document a sensitive climate system subject to large and perhaps difficult-to-predict abrupt changes. These data suggest that neither the sensitivity nor the variability of the climate are fully captured in some climate-change projections, such as the Intergovernmental Panel on Climate Change (IPCC) Summary for Policymakers. Because larger, faster and less-expected climate changes can cause more problems for economies and ecosystems, the palaeoclimatic data suggest the hypothesis that the future may be more challenging than anticipated in ongoing policy making. Large changes have occurred repeatedly with little net forcing. Increasing carbon dioxide concentration appears to have globalized deglacial warming, with climate sensitivity near the upper end of values from general circulation models (GCMs) used to project human-enhanced greenhouse warming; data from the warm Cretaceous period suggest a similarly high climate sensitivity to CO(2). Abrupt climate changes of the most recent glacial-interglacial cycle occurred during warm as well as cold times, linked especially to changing North Atlantic freshwater fluxes. GCMs typically project greenhouse-gas-induced North Atlantic freshening and circulation changes with notable but not extreme consequences; however, such models often underestimate the magnitude, speed or extent of past changes. Targeted research to assess model uncertainties would help to test these hypotheses.

Molecular estimation of eulipotyphlan divergence times and the evolution of "Insectivora"

"Insectivores" are one of the key groups in understanding mammalian origins. For years, systematics of "Lipotyphla" taxa remained extremely unstable and challenged. Today, with the application of molecular techniques, "Lipotyphla" appears to be a paraphyletic assemblage that encompasses hedgehogs, shrews, and moles (i.e., Eulipotyphla-a member of Laurasiatheria), and golden moles and tenrecs (i.e., Afrosoricida-a member of Afrotheria). Based on nuclear genes and on this well-established phylogenetic framework, we estimated Bayesian relaxed molecular clock divergence times among major lineages of "Lipotyphla." Crown placental mammals are shown to diversify 102+/-6 million years ago (Mya; mean+/-one standard-deviation), followed by Boreoeutheria (94+/-6 Mya), Laurasiatheria (85+/-5 Mya), and Eulipotyphla (73+/-5), with moles separating from hedgehogs+shrews just at the K/T boundary (65+/-5 Mya). During the Early and Middle Eocene, all extant eulipotyphlan subfamilies originated: Uropsilinae (52+/-5 Mya), and Desmaninae, Talpinae, Erinaceinae, Hylomyinae, Soricinae, and Crocidurinae (38-42+/-5 Mya). Afrosoricida separated from Macroscelidae 69+/-5 Mya, golden moles from tenrecs 63+/-5 Mya, and the diversification within tenrecs occurred 43+/-5 Mya. Divergence times are shown to be in reasonably good agreement with the fossil record of eulipotyphlans, but not with the one of afrosoricid "insectivores." Eulipotyphlans diversification might have been sculpted by variations in paleoclimates of the cenozoic era.

The Sahara as a vicariant agent, and the role of Miocene climatic events, in the diversification of the mammalian order Macroscelidea (elephant shrews)

Although the Sahara is a major geographical feature of the African continent, its role in the diversification of animal species is not well understood. We present here a molecular phylogeny for members of the endemic African mammalian order Macroscelidea (elephant shrews) with molecular-clock calculations; this molecular phylogeny provides convincing evidence that the genus Elephantulus is diphyletic. Elephantulus rozeti, the only elephant shrew species that resides north of the Sahara, is the sister group of a species from a different genus (Petrodromus tetradactylus), which resides just south of the Sahara. The split between these taxa coincided with major Miocene climatic events, which triggered the cooling and aridification of midlatitude continental regions, and a shift in the Sahara from a tropical to an arid environment. Thus, the North African distribution of E. rozeti is not the result of dispersion from an eastern species of the genus, but instead the result of a vicariant event involving the formation of the Sahara. The splitting events involved with most Elephantulus species in our analysis appear to coincide with these climatic events. This coincidence suggests that the environmental consequences associated with this period played an important role in the radiation of this order of mammals. The strongly supported phylogeny provides compelling evidence for a complex history of mosaic evolution, including pronounced bradytelic morphological evolution in some lineages, accelerated morphological evolution in others, and a remarkably slow rate of evolution of the male reproductive structure.

High plant diversity in Eocene South America: evidence from Patagonia

Tropical South America has the highest plant diversity of any region today, but this richness is usually characterized as a geologically recent development (Neogene or Pleistocene). From caldera-lake beds exposed at Laguna del Hunco in Patagonia, Argentina, paleolatitude approximately 47 degrees S, we report 102 leaf species. Radioisotopic and paleomagnetic analyses indicate that the flora was deposited 52 million years ago, the time of the early Eocene climatic optimum, when tropical plant taxa and warm, equable climates reached middle latitudes of both hemispheres. Adjusted for sample size, observed richness exceeds that of any other Eocene leaf flora, supporting an ancient history of high plant diversity in warm areas of South America.

Eocene El Niño: evidence for robust tropical dynamics in the "hothouse"

Much uncertainty surrounds the interactions between the El Niño-Southern Oscillation (ENSO) and long-term global change. Past periods of extreme global warmth, exemplified by the Eocene (55 to 35 million years ago), provide a good testing ground for theories for this interaction. Here, we compare Eocene coupled climate model simulations with annually resolved variability records preserved in lake sediments. The simulations show Pacific deep-ocean and high-latitude surface warming of approximately 10 degrees C but little change in the tropical thermocline structure, atmosphere-ocean dynamics, and ENSO, in agreement with proxies. This result contrasts with theories linking past and future "hothouse" climates with a shift toward a permanent El Niño-like state.

Xantusiid "night" lizards: a puzzling phylogenetic problem revisited using likelihood-based Bayesian methods on mtDNA sequences

Contentious issues in Night Lizard (Xantusiidae) evolution are revisited using Maximum Likelihood-based Bayesian methods and compared with results from Neighbor-Joining and Maximum Parsimony analyses. Fragments of three mitochondrial genes, the 12S and 16S ribosomal genes, and the cytochrome b gene, are sampled across an ingroup composed of seven xantusiid species and a 12-species outgroup chosen to bracket ancestral states for six additional clades of scleroglossan lizards. Our phylogenetic analyses afford robust support for the following conclusions: Xantusiidae is part of Scincomorpha, rather than being allied with Gekkota; Lepidophyma is sister to Xantusia, rather than to Cricosaura; Xantusia riversiana is imbedded within, rather than being sister to, other Xantusia species; and rock-morph Xantusia are not closely related to one another. Convergence related to retarded rates of growth and development, or to physical constraints imposed by living in rock crevices, may be responsible for much of the character discordance underlying conflicts in xantusiid phylogeny. Fossil-calibrated Maximum Likelihood-based divergence time estimates suggest that although the xantusiid stem may have originated in the Mesozoic, the crown clade is exclusively Tertiary in age. Thus, the clade including extant Cricosaura does not appear to have been extant during the K-T boundary bolide impact, as has been suggested. Moreover, our divergence-time estimates indicate that the xantusiid island endemics, Cricosaura typica on Cuba and Xantusia riversiana on the California Channel Islands, arrived via dispersal rather than vicariance, as previously proposed.

History and evolution of the arctic flora: in the footsteps of Eric Hultén

A major contribution to our initial understanding of the origin, history and biogeography of the present-day arctic flora was made by Eric Hultén in his landmark book Outline of the History of Arctic and Boreal Biota during the Quarternary Period, published in 1937. Here we review recent molecular and fossil evidence that has tested some of Hultén's proposals. There is now excellent fossil, molecular and phytogeographical evidence to support Hultén's proposal that Beringia was a major northern refugium for arctic plants throughout the Quaternary. In contrast, most molecular evidence fails to support his proposal that contemporary east and west Atlantic populations of circumarctic and amphi-Atlantic species have been separated throughout the Quaternary. In fact, populations of these species from opposite sides of the Atlantic are normally genetically very similar, thus the North Atlantic does not appear to have been a strong barrier to their dispersal during the Quaternary. Hultén made no detailed proposals on mechanisms of speciation in the Arctic; however, molecular studies have confirmed that many arctic plants are allopolyploid, and some of them most probably originated during the Holocene. Recurrent formation of polyploids from differentiated diploid or more low-ploid populations provides one explanation for the intriguing taxonomic complexity of the arctic flora, also noted by Hultén. In addition, population fragmentation during glacial periods may have lead to the formation of new sibling species at the diploid level. Despite the progress made since Hultén wrote his book, there remain large gaps in our knowledge of the history of the arctic flora, especially about the origins of the founding stocks of this flora which first appeared in the Arctic at the end of the Pliocene (approximately 3 Ma). Comprehensive analyses of the molecular phylogeography of arctic taxa and their relatives together with detailed fossil studies are required to fill these gaps.

Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2

The sudden, widespread glaciation of Antarctica and the associated shift towards colder temperatures at the Eocene/Oligocene boundary (approximately 34 million years ago) (refs 1-4) is one of the most fundamental reorganizations of global climate known in the geologic record. The glaciation of Antarctica has hitherto been thought to result from the tectonic opening of Southern Ocean gateways, which enabled the formation of the Antarctic Circumpolar Current and the subsequent thermal isolation of the Antarctic continent. Here we simulate the glacial inception and early growth of the East Antarctic Ice Sheet using a general circulation model with coupled components for atmosphere, ocean, ice sheet and sediment, and which incorporates palaeogeography, greenhouse gas, changing orbital parameters, and varying ocean heat transport. In our model, declining Cenozoic CO2 first leads to the formation of small, highly dynamic ice caps on high Antarctic plateaux. At a later time, a CO2 threshold is crossed, initiating ice-sheet height/mass-balance feedbacks that cause the ice caps to expand rapidly with large orbital variations, eventually coalescing into a continental-scale East Antarctic Ice Sheet. According to our simulation the opening of Southern Ocean gateways plays a secondary role in this transition, relative to CO2 concentration.

Paleosols and paleoenvironments of the middle Miocene, Maboko Formation, Kenya

The middle Miocene (15 Ma) Maboko Formation of Maboko Island and Majiwa Bluffs, southwestern Kenya, has yielded abundant fossils of the earliest known cercopithecoid monkey (Victoriapithecus macinnesi), and of a kenyapithecine hominoid (Kenyapithecus africanus), as well as rare proconsuline (Simiolus leakeyorum, cf. Limnopithecus evansi) and oreopithecine apes (Mabokopithecus clarki, M. pickfordi), and galagids (Komba winamensis). Specific habitat preferences can be interpreted from large collections of primate fossils in different kinds of paleosols (pedotypes). Fossiliferous drab-colored paleosols with iron-manganese nodules (Yom pedotype) are like modern soils of seasonally waterlogged depressions (dambo). Their crumb structure and abundant fine root-traces, as well as scattered large calcareous rhizoconcretions indicate former vegetation of seasonally wet, wooded grassland. Other fossiliferous paleosols are evidence of nyika bushland (Ratong), and early-successional riparian woodland (Dhero). No fossils were found in Mogo paleosols interpreted as saline scrub soils. Very shallow calcic horizons (in Yom, Ratong, and Mogo paleosols) and Na-montmorillonite (in Mogo) are evidence of dry paleoclimate (300-500 mm MAP=mean annual precipitation). This is the driest paleoclimate and most open vegetation yet inferred as a habitat for any Kenyan Miocene apes or monkeys. Victoriapithecus was abundant in dambo wooded grassland (Yom) and riparian woodland (Dhero), a distribution like that of modern vervet monkeys. Kenyapithecus ranged through all these paleosols, but was the most common primate in nyika bushland paleosols (Ratong), comparable to baboons and macaques today. Mabokopithecus was virtually restricted to riparian woodland paleosols (Dhero), and Simiolus had a similar, but marginally wider, distribution. Habitat preferences of Mabokopithecus and Simiolus were like those of modern colobus monkeys and mangabeys. A single specimen of Komba was found in dambo wooded grassland paleosol (Yom), a habitat more like that of the living Senegal bushbaby than of rainforest galagids. A shift to non-forest habitats may explain the terrestrial adaptations of Victoriapithecus, basal to the cercopithecid radiation, and of Kenyapithecus, basal to the hominoid radiation. Both taxa are distinct from earlier Miocene arboreal proconsulines, oreopithecines and galagids.

Planetary biology--paleontological, geological, and molecular histories of life

The history of life on Earth is chronicled in the geological strata, the fossil record, and the genomes of contemporary organisms. When examined together, these records help identify metabolic and regulatory pathways, annotate protein sequences, and identify animal models to develop new drugs, among other features of scientific and biomedical interest. Together, planetary analysis of genome and proteome databases is providing an enhanced understanding of how life interacts with the biosphere and adapts to global change.

Carbon dioxide and climate over the past 300 Myr

The link between atmospheric CO(2) levels and global warming is an axiom of current public policy, and is well supported by physicochemical experiments, by comparative planetary climatology and by geochemical modelling. Geological tests of this idea seek to compare proxies of past atmospheric CO(2) with other proxies of palaeotemperature. For at least the past 300 Myr, there is a remarkably high temporal correlation between peaks of atmospheric CO(2), revealed by study of stomatal indices of fossil leaves of Ginkgo, Lepidopteris, Tatarina and Rhachiphyllum, and palaeotemperature maxima, revealed by oxygen isotopic (delta(18)O) composition of marine biogenic carbonate. Large and growing databases on these proxy indicators support the idea that atmospheric CO(2) and temperature are coupled. In contrast, CO(2)-temperature uncoupling has been proposed from geological time-series of carbon isotopic composition of palaeosols and of marine phytoplankton compared with foraminifera, which fail to indicate high CO(2) at known times of high palaeotemperature. Failure of carbon isotopic palaeobarometers may be due to episodic release of CH(4), which has an unusually light isotopic value (down to -110 per thousand, and typically -60 per thousand delta(13)C) and which oxidizes rapidly (within 7-24 yr) to isotopically light CO(2). Past CO(2) highs (above 2000 ppmv) were not only times of catastrophic release of CH(4) from clathrates, but of asteroid and comet impacts, flood basalt eruptions and mass extinctions. The primary reason for iterative return to low CO(2) was carbon consumption by hydrolytic weathering and photosynthesis, perhaps stimulated by mountain uplift and changing patterns of oceanic thermohaline circulation. Sequestration of carbon was promoted in the long term by such evolutionary innovations as the lignin of forests and the sod of grasslands, which accelerated physicochemical weathering and delivery of nutrients to fuel oceanic productivity and carbon burial.

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.

Mammalian dispersal at the Paleocene/Eocene boundary

A profound faunal reorganization occurred near the Paleocene/Eocene boundary, when several groups of mammals abruptly appeared on the Holarctic continents. To test the hypothesis that this event featured the dispersal of groups from Asia to North America and Europe, we used isotope stratigraphy, magnetostratigraphy, and quantitative biochronology to constrain the relative age of important Asian faunas. The extinct family Hyaenodontidae appeared in Asia before it did so in North America, and the modern orders Primates, Artiodactyla, and Perissodactyla first appeared in Asia at or before the Paleocene/Eocene boundary. These results are consistent with Asia being a center for early mammalian origination.

Reading a CO2 signal from fossil stomata

The inverse relationship between atmospheric CO₂ and the stomatal index (proportion of epidermal cells that are stomata) of vascular land plant leaves has led to the use of fossil plant cuticles for determining ancient levels of CO₂ . In contemporary plants the stomatal index repeatedly shows a lower sensitivity atmospheric CO₂ levels above 340 ppm in the short term. These observations demonstrate that the phenotypic response is nonlinear and may place constraints on estimating higher-than-present palaeo-CO₂ levels in this way. We review a range of evidence to investigate the nature of this nonlinearity. Our new data, from fossil Ginkgo cuticles, suggest that the genotypic response of fossil Ginkgo closely tracks the phenotypic response seen in CO₂ enrichment experiments. Reconstructed atmospheric CO₂ values from fossil Ginkgo cuticles compare well with the stomatal ratio method of obtaining a quantitative CO₂ signal from extinct fossil plants, and independent geochemical modelling studies of the long-term carbon cycle. Although there is self-consistency between palaeobiological and geochemical CO₂ estimates, it should be recognized that the nonlinear response is a limitation of the stomatal approach to estimating high palaeo-CO₂ levels.

Cenozoic palaeogeography and the rise of modern biodiversity patterns

The steepest latitudinal and longitudinal gradients in taxonomic diversity at the present day are those associated with tropical high diversity foci. Although there has been a tendency in the past to regard these features as either evolutionary ‘cradles’ or ‘museums’ of considerable antiquity, this may not be the case. Within the marine realm, a uniform, pan-tropical fauna was progressively disrupted by a series of plate tectonic events, the most important of which were the Early Miocene (c. 20 Ma) collisions of Africa/Arabia with Europe and Australia/New Guinea with Indonesia, and the Middle Miocene-latest Pliocene rise of the Central American Isthmus. This had the net effect of establishing two main tropical high diversity foci: the Indo-West Pacific and the Atlantic-Caribbean-East Pacific. Similar foci were also established at the same time in the terrestrial realm.

Together with the physical isolation of Antarctica, these same tectonic events contributed significantly to global cooling throughout the Cenozoic Era. This in turn led to the imposition of a series of thermally defined provinces, and thus a considerable degree of biotic differentiation on a regional scale. However, something else seems to have been involved in the creation of very steep tropical diversity peaks. This could in part be a coincidental radiation of a series of unrelated taxa, or some sort of evolutionary feedback mechanism between interacting clades. Alternatively, Late Cenozoic rates of origination may have been enhanced by an external forcing mechanism such as changes in Orbital Range Dynamics.

Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs

Climate models with increased levels of carbon dioxide predict that global warming causes heating in the tropics, but investigations of ancient climates based on palaeodata have generally indicated cool tropical temperatures during supposed greenhouse episodes. For example, in the Late Cretaceous and Eocene epochs there is abundant geological evidence for warm, mostly ice-free poles, but tropical sea surface temperatures are generally estimated to be only 15-23 degrees C, based on oxygen isotope palaeothermometry of surface-dwelling planktonic foraminifer shells. Here we question the validity of most such data on the grounds of poor preservation and diagenetic alteration. We present new data from exceptionally well preserved foraminifer shells extracted from impermeable clay-rich sediments, which indicate that for the intervals studied, tropical sea surface temperatures were at least 28-32 degrees C. These warm temperatures are more in line with our understanding of the geographical distributions of temperature-sensitive fossil organisms and the results of climate models with increased CO2 levels.