David L. Fox

The assembly of Africa’s iconic C ₄ grassland ecosystems is central to evolutionary interpretations of many mammal lineages, including hominins. C ₄ grasses are thought to have become ecologically dominant in Africa only after 10 million years ago (Ma). However, paleobotanical records older than 10 Ma are sparse, limiting assessment of the timing and nature of C ₄ biomass expansion. This study uses a multiproxy design to document vegetation structure from nine Early Miocene mammal site complexes across eastern Africa. Results demonstrate that between ~21 and 16 Ma, C ₄ grasses were locally abundant, contributing to heterogeneous habitats ranging from forests to wooded grasslands. These data push back the oldest evidence of C ₄ grass–dominated habitats in Africa—and globally—by more than 10 million years, calling for revised paleoecological interpretations of mammalian evolution.

Living hominoids are distinguished by upright torsos and versatile locomotion. It is hypothesized that these features evolved for feeding on fruit from terminal branches in forests. To investigate the evolutionary context of hominoid adaptive origins, we analyzed multiple paleoenvironmental proxies in conjunction with hominoid fossils from the Moroto II site in Uganda. The data indicate seasonally dry woodlands with the earliest evidence of abundant C ₄ grasses in Africa based on a confirmed age of 21 million years ago (Ma). We demonstrate that the leaf-eating hominoid Morotopithecus consumed water-stressed vegetation, and postcrania from the site indicate ape-like locomotor adaptations. These findings suggest that the origin of hominoid locomotor versatility is associated with foraging on leaves in heterogeneous, open woodlands rather than forests.

The Pliocene Epoch is a high priority for understanding climatic and geomorphic responses to rising CO2 levels, yet the majority of paleoclimate records from this interval are from marine basins. Paleosols (fossil soils) preserve in situ archives of terrestrial paleoclimate, paleovegetation, and surface processes. This presentation showcases ongoing efforts to leverage paleosols from Pliocene deposits in North America and eastern Africa to develop quantitative estimates of mean annual precipitation (MAP), temperature (MAT) and paleovegetation (C3 vs. C4 biomass). We use a previously published random-forest recursive partitioning model (RF-MAP) that reconstructs MAP values using 10 major and minor elemental oxides from paleosol B horizons as input variables and is applicable on most soil types in most climate regimes. Our work in the Meade Basin of Kansas, USA, shows evidence of expanding and contracting wetlands during the Pliocene, with paleosols consisting of Vertisols to Aridisols depending on geomorphic position. Average paleoprecipitation totals appear to be near the semi-arid to subhumid boundary (~500 mm, similar to modern values) and remain steady across the Pliocene through early Pleistocene. Pliocene deposits of the Baringo Basin of central Kenya consists of orbitally-controlled lacustrine-alluvial cycles. Paleosols tend to be Vertisols in lowland marsh to alluvial plain settings, however localized Andisols and Inceptisols are also present. Our continuing efforts include analyses of pedogenic carbonate formation temperatures from clumped isotopes (Δ47) development of paleovegetation records from δ13C analyses on pedogenic carbonates and organic matter, and application of the RF-MAP model on Baringo Basin paleosols.  

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Terrestrial paleoenvironmental reconstructions from the Pliocene Epoch (5.3–2.6 Ma) of the Neogene Period are rare from the North American continental interior, but are important because they provide insight into the evolutionary context of modern landscapes and ecological systems. Pliocene marine records indicate that global climate was warmer and atmospheric pCO₂ was higher than pre-industrial conditions, spurring efforts to understand regional climate and environmental variability under conditions potentially analogous to future warming scenarios. In this study, we investigate sedimentary environments and paleoclimate conditions from the Meade Basin of southwest Kansas, a moderately sized basin formed from dissolution and withdrawal of deep evaporites. Pliocene intervals of the Meade Basin have yielded classic faunal assemblages representing the early to middle Blancan North American Land Mammal Age (∼ 4.5–3.2 Ma). We reconstruct the paleoenvironments using a multidisciplinary approach of lithofacies analysis, paleopedology, and ichnology. The stratigraphic interval we examined is bounded by large-scale, fluvial trunk channels that show paleocurrent trends to the south-southwest—tangential to modern drainages—likely due to local halotectonic subsidence during the Neogene. The stratigraphic interval between these fluvially dominated phases consists of palustrine landscapes with temporally and laterally variable subaqueous and subaerial facies. Paleosols are abundant; however, most pedotypes are poorly to variably drained, and so their elemental compositions do not reflect local climate state. The few mature, oxidized, and relatively well-drained paleosols observed contain elemental signatures consistent with subhumid climate conditions. Frequent and recursive ponding events are discerned through the tiering of burrows (Camborygma isp.) similar to those produced by modern freshwater decapod crustaceans (i.e., crayfish). The drivers of these flooding events are most likely episodic halotectonic subsidence and groundwater discharge, though influence from intervals of relatively wetter climate cannot be ruled out. By the late middle Pliocene, landscapes returned to fluvially dominated environments as sedimentation began to outpace accommodation. Our results collectively indicate that climate was likely wetter than modern conditions in the early to middle Pliocene in the western Great Plains, contrary to forecasts for the region under current pCO₂-driven warming.

Grasslands dominated by taxa using the C₄ photosynthetic pathway first developed on several continents during the Neogene and Quaternary, long after C₄ photosynthesis first evolved among grasses. The histories of these ecosystems are relatively well-documented in the geological record from stable carbon isotope measurements (of fossil vertebrate herbivores and paleosols) and the plant microfossil record (pollen and/or phytolith assemblages). The distinct biogeography and ecophysiology of modern C₃ and C₄ grasses have led to hypotheses explaining the origins of C₄ grasslands in terms of long-term changes in the Earth system, such as increased aridity and decreasing atmospheric pCO₂. However, quantitative proxies for key abiotic drivers of these hypotheses (e.g., temperature, precipitation, pCO₂) are still in development, not yet widely applied at the continental or global scale or throughout the late Cenozoic, and/or remain contentious. Testing these hypotheses globally therefore remains difficult. To understand better the potential links between changes in the Earth system and the origin of C₄ grasslands, we undertook a global-scale comparison between observational records of C₄ plant abundances in Miocene and Pliocene localities compiled from the literature and three increasingly complex models of C₄ physiology, dominance, and abundance. The literature compilation comprises >2,600 δ¹³C-values each of fossil terrestrial vertebrates and of paleosol carbonates, which we interpret as primarily proxies for the abundance of C₄ grasses, based on the modern contribution of C₄ grasses to terrestrial net primary productivity. We forced the vegetation models with simulated monthly climates from the HadCM3 family of coupled ocean-atmosphere general circulation models (OAGCMs) over a range of pCO₂-values for each epoch to model C₄ dominance or abundance in grid cells as: (1) months per year exceeding the temperature at which net carbon assimilation is greater for C₄ than C₃ photosynthesis (crossover temperature model); (2) the number of months per year exceeding the crossover temperature and having sufficient precipitation for growth (≥25 mm/month; Collatz model); and (3) the Sheffield Dynamic Global Vegetation Model (SDGVM), which models multiple plant functional types (PFTs) (C₃ and C₄ grasses, evergreen, and deciduous trees). Model-data agreement is generally weak, although statistically significant for many comparisons, suggesting that regional to local ecological interactions, continent-specific plant evolutionary histories, and/or regional to local climatic conditions not represented in global scale OAGCMs may have been equally strong or stronger in driving the evolution of C₄ grasslands as global changes in the Earth system such as decreases in atmospheric pCO₂ and late Cenozoic global cooling and/or aridification.

Pedogenesis produces fine-grained magnetic minerals that record important information about the ambient climatic conditions present during soil formation. Yet, differentiating the compounding effects of non-climate soil forming factors is a nontrivial challenge that must be overcome to establish soil magnetism as a trusted paleoenvironmental tool. Here, we isolate the influence of vegetation by investigating magnetic properties of soils developing under uniform climate, topography, and parent material but changing vegetation along the forest-prairie ecotone in NW Minnesota. Greater absolute magnetic enhancement in prairie soils is related to some combination of increased production of pedogenic magnetite in prairie soils, increased deposition of detrital magnetite in prairies from eolian processes, or increased dissolution of fine-grained magnetite in forest soils due to increased soil moisture and lower pH. Yet, grain-size specific magnetic properties associated with pedogenesis, for example relative frequency dependence of susceptibility and the ratio of anhysteretic to isothermal remanent magnetization, are insensitive to changing vegetation. Further, quantitative unmixing methods support a fraction of fine-grained pedogenic magnetite that is highly consistent. Together, our findings support climate as a primary control on magnetite production in soils, while demonstrating how careful decomposition of bulk magnetic properties is necessary for proper interpretation of environmental magnetic data.

Early Miocene outcrops near Karungu, Western Kenya, preserve a range of fluvio-lacustrine, lowland landscapes that contain abundant fossils of terrestrial and aquatic vertebrates. Primates are notably rare among these remains, although nearby early Miocene strata on Rusinga Island contain a rich assemblage of fossilized catarrhines and strepsirrhines. To explore possible environmental controls on the occurrence of early Miocene primates, we performed a deep-time Critical Zone (DTCZ) reconstruction focused on floodplain paleosols at the Ngira locality in Karungu. We specifically focused on a single stratigraphic unit (NG15), which preserves moderately developed paleosols that contain a microvertebrate fossil assemblage. Although similarities between deposits at Karungu and Rusinga Island are commonly assumed, physical sedimentary processes, vegetative cover, soil hydrology, and some aspects of climate state are notably different between the two areas. Estimates of paleoclimate parameters using paleosol B horizon elemental chemistry and morphologic properties are consistent with seasonal, dry subhumid conditions, occasional waterlogging, and herbaceous vegetation. The reconstructed small mammal community indicates periodic waterlogging and open-canopy conditions. Based on the presence of herbaceous root traces, abundant microcharcoal, and pedogenic carbonates with high stable carbon isotope ratios, we interpret NG15 to have formed under a warm, seasonally dry, open riparian woodland to wooded grassland, in which at least a subset of the vegetation was likely C₄ biomass. Our results, coupled with previous paleoenvironmental interpretations for deposits on Rusinga Island, demonstrate that there was considerable environmental heterogeneity ranging from open to closed habitats in the early Miocene. We hypothesize that the relative paucity of primates at Karungu was driven by their environmental preference for locally abundant closed canopy vegetation, which was likely absent at Karungu, at least during the NG15 interval if not also earlier and later intervals that have not yet been studied in as much detail.