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The assembly of Africa’s iconic C4 grassland ecosystems is central to evolutionary interpretations of many mammal lineages, including hominins. C4 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 C4 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, C4 grasses were locally abundant, contributing to heterogeneous habitats ranging from forests to wooded grasslands. These data push back the oldest evidence of C4 grass–dominated habitats in Africa—and globally—by more than 10 million years, calling for revised paleoecological interpretations of mammalian evolution. The hominoid lineage underwent a major morphological change in the Miocene, acquiring strong hind legs and a more upright posture. The prevailing hypothesis pertaining to these changes has been that they were adaptive for foraging on fruit in the terminal branches of tropical forest trees. A pair of papers now argue that, instead, such changes may have been driven by adaptation to feeding on leaves in seasonally dry and open forests. Peppe et al. used new data from fossil mammal study sites and found that the expansion of grassy biomes dominated by grasses with the C4 photosynthetic pathway in eastern Africa likely occurred more than 10 million years earlier than prior estimates. MacLatchy et al. looked at fossils of the earliest ape in this region at this time, Morotopithecus, and found isotope evidence of the consumption of water-stressed vegetation and postcranial morphology indicative of strong hind limbs similar to modern apes. Together, these papers suggest that early hominoids emerged in a dryer and more irregular environment than was previously believed. —BEL and SNV A combination of data sources from fossil mammal sites pushes back the date of C4 grassland expansion in eastern Africa by over 10 million years. - Early Miocene habitat heterogeneity, open habitats, and locally abundant C4 vegetation - Implications for Neogene ecosystems of eastern Africa

INTRODUCTIONInherent in traditional views of ape origins is the idea that, like living apes, early large-bodied apes lived in tropical forests. In response to constraints related to locomoting in forest canopies, it has been proposed that early apes evolved their quintessential upright torsos and acrobatic climbing and suspensory abilities, enhancing their locomotor versatility, to distribute their weight among small supports and thus reach ripe fruit in the terminal branches. This feeding and locomotor transition from a quadruped with a horizontal torso is thought to have occurred in the Middle Miocene due to an increasingly seasonal climate and feeding competition from evolving monkeys. Although ecological and behavioral comparisons among living apes and monkeys provide evidence for versions of terminal branch forest frugivory hypotheses, corroboration from the early ape fossil record has been lacking, as have detailed reconstructions of the habitats where the first apes evolved.RATIONALEThe Early Miocene fossil site of Moroto II in Uganda provides a unique opportunity to test the predictions of terminal branch forest frugivory hypotheses. Moroto II documents the oldest [21 million years ago (Ma)] well-established paleontological record of ape teeth and postcranial bones from a single locality and preserves paleoecological proxies to reconstruct the environment. The following lines of evidence from Moroto II were analyzed: (i) the functional anatomy of femora and a vertebra attributed to the ape Morotopithecus; (ii) dental traits, including molar shape and isotopic profiles of Morotopithecus enamel; (iii) isotopic dietary paleoecology of associated fossil mammals; (iv) biogeochemical signals from paleosols (ancient soils) that reflect local relative proportions of C3 (trees and shrubs) and C4 (tropical grasses and sedges that can endure water stress) vegetation as well as rainfall; and (v) assemblages of phytoliths, microscopic plant-derived silica bodies that reflect past plant communities.RESULTSA short, strong femur biomechanically favorable to vertical climbing and a vertebra indicating a dorsostable lower back confirm that ape fossils from Moroto II shared locomotor traits with living apes. Both Morotopithecus and a smaller ape from the site have elongated molars with well-developed crests for shearing leaves. Carbon isotopic signatures of the enamel of these apes and of other fossil mammals indicate that some mammals consistently fed on water-stressed C3 plants, and possibly also C4 vegetation, in a woodland setting. Carbon isotope values of pedogenic carbonates, paleosol organic matter, and plant waxes all point to substantial C4 grass biomass on the landscape. Analysis of paleosols also indicates subhumid, strongly seasonal rainfall, and phytolith assemblages include forms from both arid-adapted C4 grasses and forest-indicator plants.CONCLUSIONThe ancient co-occurrence of dental specializations for leaf eating, rather than ripe fruit consumption, along with ape-like locomotor abilities counters the predictions of the terminal branch forest frugivory hypotheses. The combined paleoecological evidence situates Morotopithecus in a woodland with a broken canopy and substantial grass understory including C4 species. These findings call for a new paradigm for the evolutionary origins of early apes. We propose that seasonal, wooded environments may have exerted previously unrecognized selective pressures in the evolution of arboreal apes. For example, some apes may have needed to access leaves in the higher canopy in times of low fruit availability and to be adept at ascending and descending from trees that lacked a continuous canopy. Background - Terminal branch forest frugivory hypotheses - The Moroto II locality - The hominoids of Moroto II Results - Geologic and taphonomic setting - Chronology - Hominoid and other catarrhine fossils -- Femur -- Vertebra -- Dental specimens -- Stable isotopes of ename - Paleoecology -- Stable carbon isotopes of fossil enamel -- Paleosol carbon isotopes -- Paleosol n-alkanes -- Paleosols and hydroclimate -- Phytoliths Discussion - Context of the hominoid remains from Moroto - Hominoid adaptive evolution in eastern Africa Materials and methods summary

The bulk elemental composition of soil subsurface (B) horizons is influenced by environmental, biological, geological, and climatic factors. Because fossil soils (paleosols) are common in the geologic record, quantitative models that link climate to paleosol geochemistry are highly desirable in the paleoclimate community. Error associated with these models is typically reported as the root mean square error (RMSE) of a regression analysis and reflects the variability imparted by non-climatic influences on soil formation and the uncertainty associated with model fitting. However, for prediction purposes, the RMSE is well known to underestimate model uncertainty. In this work we re-evaluate a widely used transfer function for mean annual precipitation (MAP) based on the chemical index of alteration minus potassium (CIA-K) using data science best practices on two continental-scale soil data sets. Data set inter-comparisons and cross-validation of exponential regression models indicate that the root mean square prediction error (RMSPE) between CIA-K and MAP for soils representative of climates across the continental United States is around 299 mm, significantly higher than the currently accepted 182 mm RMSE. Further, CIA-K is unable to predict perhumid (>2000 mm MAP) climate zones. We show that transitioning from a simple regression framework to one of recursive partitioning via random forests can significantly increase prediction accuracy while automating variable selection. We introduce two new, widely applicable random forest models for MAP (RF-MAP) that use 10 elemental oxides as input variables and were calibrated on the Baylor University Soil Informatics (BU-SI) data set. RF-MAP version 1.0 (RF-MAP1.0) was generated using the entire BU-SI data set (n = 685) and can predict MAP values up to 6865 mm with a RMSPE of 395 mm. RF-MAP version 2.0 (RF-MAP2.0) was generated using a modification of the BU-SI data set (n = 642) and can predict MAP values up to ∼1600 mm with a RMSPE of 209 mm. Pruned regression trees provide insight into the mechanisms driving the random forest models and demonstrate the first empirical confirmation of the sensitivity of soil elemental responses to global climate zones. The RF-MAP1.0 and RF-MAP2.0 models predict MAP values comparable to independent proxy estimates for a range of deep-time paleosols. We advocate for application of RF-MAP1.0 in settings where no a priori information on paleoclimate is available, and encourage the use of either RF-MAP1.0 or RF-MAP2.0 if users have independent constraints that paleo-MAP was below 1600 mm.

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 (∼ 4.5-3.2Ma).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.

Paleosols (fossil soils) are abundant in the sedimentary record and reflect, at least in part, regional paleoclimate. Paleopedology thus offers a great potential for elucidating high resolution, deep-time paleoclimate records. However, many fossil soils did not equilibrate with climate prior to burial and instead dominantly express physical and chemical features reflective of other soil forming factors. Current models that use elemental oxides for climate reconstruction bypass the issue of soil-climate equilibration by restricting datasets to narrow ranges of soil properties, soil-forming environments and mean annual precipitation (MAP) and mean annual temperature (MAT). Here we evaluate a data-driven paleosol-paleoclimate model (PPM1.0) that uses subsoil geochemistry to test the ability of soils from wide-ranging environments to predict MAP and MAT as a joint response with few initial assumptions. The PPM1.0 was developed using a combined partial least squares regression (PLSR) and a nonlinear spline on 685 mineral soil B horizons currently forming under MAP ranging from 130 to 6900 mm and MAT ranging from 0 to 27 °C. The PLSR results on 11 major and minor oxides show that four linear combinations of these oxides (Regressors 1-4), akin to classic oxide ratios, have potential for predicting climate. Regressor 1 correlates with increasing MAP and MAT through Fe oxidation, desilication, base loss and residual enrichment. Regressor 2 correlates with MAT through temperature-dependent dissolution of Na- and K-bearing minerals. Regressor 3 correlates with increasing MAP through decalcification and retention of Si. Regressor 4 correlates with increasing MAP through Mg retention in mafic-rich parent material. The nonlinear spline model fit on Regressors 1 to 4 results in a Root Mean Squared Error (RMSEMAP) of 228 mm and RMSEMAT of 2.46 °C. PPM1.0 model simulations result in Root Mean Squared Predictive Error (RMSPEMAP) of 512 mm and RMSPEMAT of 3.98 °C. The RMSE values are lower than some preexisting MAT models and show that subsoil weathering processes operating under a wide range of soil forming factors possess climate prediction potential, which agrees with the state-factor model of soil formation. The nonlinear, multivariate model space of PPM1.0 more accurately reflects the complex and nonlinear nature of many weathering processes as climate varies. This approach is still limited as it was built using data primarily from the conterminous USA and does not account for effects of diagenesis. Yet, because it is calibrated over a broader range of climatic variable space than previous work, it should have the widest array of potential applications. Furthermore, because it is not dependent on properties that may be poorly preserved in buried paleosols, the PPM1.0 model is preferable for reconstructing deep time climate transitions. In fact, previous studies may have grossly underestimated paleo-MAP for some paleosols.

Recent research has demonstrated that the Lilliput effect (reduction of body size in response to a mass extinction) affected all trophic levels in the marine realm following the Cretaceous-Paleogene (K-Pg) event. However, it is unclear if this size change was strictly a marine signal, or a global phenomenon that also affected continental ecosystems. Herein we present the results of an ichnological proxy for body size of soil-dwelling insects across the K-Pg boundary in Big Bend National Park, Texas, U.S.A. Quantitative efforts focused on Naktodemasis isp., which are characterized as unbranching burrows composed of ellipsoidal packets of backfill menisci. These traces were likely produced by beetle larvae or cicada nymphs based on previous comparison with structures generated in modern soils and laboratory experiments. As an approximation for the body size of the subterranean insects, this dataset indicates that a smaller Naktodemasis diameter (DN) is statistically correlated (α < 0.05) with several edaphic factors including poor drainage and weak development (Entisols). Additionally, the DN in strata immediately superjacent to the highest Cretaceous-specific taxa is smaller by 23% (5.6 ± 1.8 mm) in comparison to DN within the subjacent Cretaceous interval (7.3 ± 2.7 mm). This abrupt shift occurs in a well-drained Inceptisol, and cannot be attributed to facies changes, drainage, or paleosol maturity. Furthermore, a reduced DN (6.6 ± 2.3 mm) persists above this anomalous shift for at least 20 stratigraphic meters within chron 29r. The cause for this negative response in body size within soil-dwelling biota may be attributed to plant-community shifts in taxonomic composition and ecological strategies, which would have caused fundamental alterations to the diet of the herbivorous, subterranean insects. This study provides empirical evidence that the Lilliput effect was not restricted to marine environments during the aftermath of the K-Pg event.

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.