Annual Review of Earth and Planetary Sciences - Current Issue

As a field geologist, I have been involved in the overwhelming excitement of three scientific revolutions—a mini revolution in structural geology, the impact-extinction revolution that freed geology from uncompromising uniformitarianism, and the plate tectonic revolution that turned the routine field of geology into one of the most exciting and essential sciences of the present time. I have also worked across several discipline boundaries, an activity I call scientific trespassing. My career has unfolded in such unexpected ways that, like anyone's life and like the history of our planet, it can only be seen as a most improbable journey. A focus on these three concepts and on the history of geology (a traditional name used here for all the Earth sciences) leads to the understanding that geology was once the crown jewel of sciences, and that after a century of necessary but routine geologic mapping, geology now needs to resume its crown jewel role because the understanding and care of our planet is becoming humanity's most urgent task.

Born on December 12, 1937, I remember the first bombing of Moscow by the Germans in 1941. My schooling began in 1944, and I soon became interested in chemistry, setting up a kind of makeshift chemical laboratory behind a high cupboard in my house. I enrolled in Moscow University in 1955 and published my first scientific paper in 1959. After entering graduate school in 1960, I produced dense silica with a rutile structure, a natural analog of which was later named stishovite. I received my doctorate in 1961 and got a job at the Institute of Crystallography in 1962, where I worked until 1993. My first visit to the West was in 1976. I was became a Fairchild Distinguished Scholar at Caltech in 1989–1990 and a member of the USSR Academy of Sciences in 1990. I was a Miller Professor at the University of California, Berkeley and an Orson Anderson Distinguished Scholar at Los Alamos National Laboratory. From 1993 to 2022, I was director of the Institute for High Pressure Physics. During that time, I was awarded the P. Bridgman Gold Medal and the Gold Medal of P. Kapitsa.

Chalcogenides (sulfides, selenides, tellurides, arsenides, antimonides) are important in natural processes, including formation of ore deposits on Earth, early stages of planetary accretion, and formation of condensates in planetary atmospheres. Their physicochemical properties render them suitable for a wide range of industrial applications. While thermodynamic data are available for many endmembers, there are significant gaps in both thermodynamic and associated structural constraints, especially for complex systems. The continuous evolution of high temperature calorimetry into oxidative drop solution calorimetry has facilitated the measurement of enthalpies of formation and mixing and surface energies involving nonoxides, including chalcogenides. These are essential for modeling processes in nature and technology and for understanding the underlying properties that define their stabilities. This article reviews the development of these calorimetric techniques and summarizes available thermochemical data for common chalcogenides.

• ▪  Over the last century, calorimetric instruments and techniques have evolved to enable accurate measurement of a wide range of materials, including chalcogenides.
• ▪  Despite the growing interest in the thermodynamic characterization of chalcogenides, a systematic review of the available data indicates that there is still a significant scope for further research.
• ▪  A systematic understanding of chalcogenides will facilitate the modeling of geological environments and enable the prediction and improvement of geo-inspired materials for industrial applications.

The response of geologic matter when subjected to large-scale impact or explosion is dependent on the time history of the encompassing shock wave. The kinetics of localized physical and chemical transitions brought about by the shock wave are responsive to this time history. Solid-state viscosity of the media is responsible for establishing the time history of a shock wave. In 2003, researcher H. Jay Melosh recognized the need for an understanding of solid viscosity spanning the petrologic and lithologic scales, and accordingly, he undertook the assessment and analysis of available nuclear ground shock measurements. This review furthers Melosh's epic efforts. In pursuing both the nuclear ground shock data and supporting laboratory test data, it undertakes methods for determining and calculations of the viscosity of solid materials on the respective scales. Further, applicability of viscoelasticity in modeling the shock response on the scales of concern is demonstrated and applied. The review closes with a discussion of universal features of the shock wave viscous time history in solid materials. Solid viscosity as an adiabatic invariant is presented, and commonalties of the solid shock wave with the nonlinear dynamics of ocean waves are noted.

• ▪  This article reviews Melosh's analysis of nuclear ground shock measurements with application to shock wave structuring viscosity.
• ▪  It discusses viscoelastic calculations with application to wave structure of nuclear ground shocks and laboratory shock waves in brittle granular solids, and universal features of the viscous shock wave structure and invariance of the dissipative action are considered.
• ▪  It also discusses wave action invariance in both the nonlinear dynamics of ocean waves and the steady wave structure of shock waves in solid matter.

Minna de Honkoku began as an online citizen science project to transcribe earthquake-related historical materials from the Earthquake Research Institute Library of the University of Tokyo. In Japan, almost all the documents are written in kuzushiji (old-style Japanese cursive script), a writing style used before ∼1900. Because the style of writing is different from modern Japanese, transcription is necessary to use the historical documents as data for earthquake research. The workspace of Minna de Honkoku consists of a viewer of a document image and a vertical (Japanese-style) editor for transcription. Users can input transcribed text while viewing its image. The ranking of characters transcribed is displayed to keep users motivated. As of October 2024, more than 9,700 people were registered for the project, with the total number of characters transcribed at about 41 million. The text generated by Minna de Honkoku can be used for various academic research fields including seismology and can be used to enhance citizens’ disaster awareness. The paired kuzushiji characters and text data generated by Minna de Honkoku are beginning to be used as training data for artificial intelligence.

• ▪  Minna de Honkoku is an online citizen science project aimed at deciphering historical documents.
• ▪  The total number of participants is 9,700, and characters transcribed by Minna de Honkoku reaches 41 million.
• ▪  Minna de Honkoku began as a project to transcribe earthquake-related historical materials.
• ▪  The text generated by Minna de Honkoku is used in seismology and various research fields and for building artificial intelligence–based kuzushiji recognition.

Ecologists rely on a wealth of data, including field observations and light stable isotopes, to infer dietary preferences and other ecological and physiological properties in living mammals. But inferring such important traits (e.g., trophic position, metabolism, pathologies) in extinct animals, including humans, can be challenging because biological processes rarely mirror morphology as preserved in the fossil record. For instance, dietary behavior does not necessarily reflect tooth morphology. As an additional challenge, some isotopic mammal tissues commonly used in modern ecology, such as collagen in bone or dentine or keratin from hair, hoof, or horn, do not generally preserve in fossil remains older than ∼200 kyr. In contrast, major constituents of bioapatite often retain their initial isotopic composition through fossilization processes. Recent analytical developments in mass spectrometry now allow, using small samples, for assessment of isotopic variability of major and trace elements such as calcium or zinc. Here, we review the application potentials of metal (nontraditional isotopes) for (paleo)ecological, (paleo)physiological, and (paleo)mobility inferences as applied to mammalian research.

• ▪  Mammals are key elements of modern ecosystems and possess a rich evolutionary history, yet inferences about their past ecologies and physiologies are challenging to retrieve using traditional geochemical toolkits.
• ▪  Metal stable isotopes provide a novel and complementary approach to unveil paleoecological and paleophysiological characteristics of extinct mammal species.
• ▪  Within a 20-year time frame, the core of metal isotopic data in mammalian research remains small compared to traditional isotopic systems (C, O, N), which is inviting for designing cost-effective instrumentation and increasing dissemination across scientific disciplines.

Critical minerals are essential for sustaining the supply chain necessary for the transition to a carbon-free energy source for society. Copper, nickel, cobalt, lithium, and rare earth elements are particularly in demand for batteries and high-performance magnets used in low-carbon technologies. Copper, predominantly sourced from porphyry deposits, is critical for electricity generation, storage, and distribution. Nickel, which comes from laterite and magmatic sulfide deposits, and cobalt, often a by-product of nickel or copper mining, are core components of batteries that power electric vehicles. Lithium, sourced from pegmatite deposits and continental brines, is another key battery component. Rare earth elements, primarily obtained from carbonatite- and regolith-hosted ion-adsorption deposits, have unique magnetic properties that are key for motor efficiency. Future demand for these elements is expected to increase significantly over the next decades, potentially outpacing expected mine production. Therefore, to ensure a successful energy transition, efforts must prioritize addressing substantial challenges in the supply of critical minerals, particularly the delays in exploring and mining new resources to meet growing demands.

• ▪  The energy transition relies on green technologies needing a secure, sustainable supply of critical minerals sourced from ore deposits worldwide.
• ▪  Copper, nickel, cobalt, lithium, and rare earth elements are geologically restricted in occurrence, posing challenges for extraction and availability.
• ▪  Future demand is expected to surge in the next decades, requiring unprecedented production rates to make the green energy transition viable.

Coccolithophores are a major group of oceanic calcifying phytoplankton, and their calcite skeletal remains, termed calcareous nannofossils, are a major component of deep-sea sediments accumulating since the Jurassic. Cocco-lithophores play a role in both the biological pump and the carbonate pump, exporting organic and inorganic carbon, respectively, out of the surface ocean. This means that they are key responders to and recorders of ocean carbon cycle and climate changes over geological and shorter timescales, and studying these responses can help elucidate the uncertain fate of calcifying phytoplankton under projected climate change scenarios. Here, we review established and emerging approaches for reconstructing (a) mixed-layer ocean temperature, (b) marine productivity, and (c) aspects of the ocean carbon cycle, using calcareous nannofossils from deep-sea sediments. For each parameter, we discuss the different proxies that have been proposed, based on abundance or species composition, inorganic geochemistry, and/or cocco-lith morphology, and explore their applications and limitations in Cenozoic paleoceanography.

• ▪  Calcareous nannofossils can be used to reconstruct upper ocean conditions and changes over centennial to million-year timescales.
• ▪  Key coccolith-based proxies for temperature, productivity, and the carbon cycle are reviewed.
• ▪  Approaches based on assemblages, geochemistry, and morphology provide novel insights into the evolution and adaptation of coccolithophores and past climate.

The depleted mantle reservoir is that part of Earth's mantle from which crust has been extracted, leaving the remaining mantle depleted in incompatible elements. Knowing how and when it formed is essential for understanding the chemical evolution of Earth, including formation of continental crust. The best-constrained Hf isotope data presented here indicate that the mantle does not become significantly depleted until as late as 700 million years after Earth's accretion. This onset of mantle depletion coincides with the first appearance of substantial volumes of continental crust in the geological record. These data compel a revision to the reference depleted mantle parameters used in Hf isotope studies of planetary evolution. This new reference line follows chondritic evolution until 3.8 Ga and then describes a linear trajectory to a present-day depleted mid-ocean ridge basalt source mantle composition (εHf = +18). We infer that stabilization of continental crust only occurred in earnest on Earth after 3.8 Ga.

• ▪  Hf isotopes show that Earth's mantle does not become significantly depleted until 700 million years after planetary accretion.
• ▪  Most of Earth's oldest rocks formed from mantle sources that had radiogenic isotope compositions similar to those of chondritic meteorites.
• ▪  Isotope evidence shows that Hadean (>4.0-billion-year-old) crust was not essential for formation of younger crust in Archean terranes.
• ▪  Growth of Earth's continents only began in earnest after 3.8 Ga.

The extensive chondrichthyan fossil record spans 400+ million years and has a global distribution. Paleontological studies provide a foundation of description and taxonomy to support deeper forays into ecology and evolution considering geographic, morphologic, and functional changes through time with nonanalog species and climate states. Although chondrichthyan teeth are most studied, analyses of dermal denticle metrics and soft tissue imprints are increasing. Recent methodological advances in morphology and geochemistry are elucidating fine-scale details, whereas large datasets and ecological modeling are broadening taxonomic, temporal, and geographic perspectives. The combination of ecological metrics and modeling with environmental reconstruction and climate simulations is opening new horizons to explore form and function, demographic dynamics, and food web structure in ancient marine ecosystems. Ultimately, the traits and taxa that endured or perished during the many catastrophic upheaval events in Earth's history contribute to conservation paleobiology, which is a much-needed perspective for extant chondrichthyans.

• ▪  The longevity and abundance of the chondrichthyan fossil record elucidates facets of ecological, evolutionary, and environmental histories.
• ▪  Though lacking postcranial, mineralized skeletons, dental enameloid and dermal denticles exquisitely preserve morphology and geochemistry.
• ▪  Technical advances in imaging, geochemistry, and modeling clarify the linkages between form and function with respect to physiology, diet, and environment.
• ▪  Conservation efforts can benefit from the temporal and spatial perspective of chondrichthyan persistence through past global change events.

The brevity of the instrumental record limits our knowledge of tropical cyclone activity on multidecadal to longer timescales and hampers our ability to diagnose climatic controls. Sedimentary archives containing event beds provide essential data on tropical cyclone activity over centuries and millennia. This review highlights the advantages and limitations of this approach and how these reconstructions have illuminated patterns of tropical cyclone activity and potential climate drivers over the last millennium. Key elements to developing high-quality reconstructions include confident attribution of event beds to tropical cyclones, assessing the potential role of other mechanisms, and evaluating the potential influence of geomorphic changes, sea-level variations, and sediment supply on a settings’ susceptibility to event bed deposition. Millennium-long histories of severe tropical cyclone occurrence are now available from many locations in the western North Atlantic and western North Pacific, revealing clear regional shifts in activity likely related to intervals of large-scale ocean-atmosphere reorganization.

• ▪  Prior to significant human influence in Earth's climate, natural climate variability dramatically altered patterns of tropical cyclone activity.
• ▪  For some regions (e.g., The Bahamas and the Marshall Islands), earlier intervals of tropical cyclone activity exceeded what humans have experienced during the recent period of instrumental measurements (∼1850 CE–present).
• ▪  Risk assessments based on the short instrumental record likely underestimate the threat posed by tropical cyclones in many regions.
• ▪  Changes in atmospheric and oceanic circulation associated with the Little Ice Age (∼1400–1800 CE) resulted in significant regional changes in tropical cyclone activity.
• ▪  Given the past sensitivity of tropical cyclone activity to climate change, we should anticipate regional shifts in tropical cyclone activity in response to ongoing anthropogenic warming of the planet.

Methane (CH4) is a simple molecule that, due to its radiative forcing, wields an outsized impact on planetary heat balance. Methane is formed by diverse abiotic pathways across a range of pressures and temperatures. Biological methanogenesis for anaerobic respiration uses a terminal nickel-containing enzyme and is limited to the archaeal domain of life. Methane can also be produced in aerobic microbes during bacterial methylphosphonate and methylamine degradation and via nonenzymatic reactions during oxidative stress. Abiotic CH4 is produced via thermogenic reactions and during serpentinization reactions in the presence of metal catalysts. Reconstructions of CH4 cycling over geologic time are largely inferential. Throughout Earth's history, CH4 has probably been the second most important climate-forcing greenhouse gas after carbon dioxide. Biological methanogenesis has likely dominated CH4 flux to Earth's atmosphere for the past ∼3.5 billion years, during which time CH4 is thought to have generally declined as atmospheric oxygen has risen. Here we review the evolution of the CH4 cycle over Earth's history, showcasing the multifunctional roles CH4 has played in Earth's climate, prebiotic chemistry, and microbial metabolisms. We also discuss the future of Earth's atmospheric CH4, the cycling of CH4 on other planetary bodies in the Solar System (with special emphasis on Titan), and the potential of CH4 as a biosignature on Earth-like extrasolar planets.

• ▪  Before life arose on Earth, abundant atmospheric CH4 in Earth's early atmosphere was likely key for establishment of habitable conditions and production of organic molecules for prebiotic chemistry.
• ▪  Biological methanogenesis for anaerobic respiration is only known to exist in some groups of anaerobic archaea, but CH4 can also be produced via enzymatic and nonenzymatic biological pathways that are not directly coupled to energy conservation. The relative importance of each of these pathways to the global CH4 cycle is a topic of active research, but archaeal methanogenesis dominates all other biological pathways for CH4 generation.
• ▪  As atmospheric O2 rose over Earth history, models suggest that atmospheric CH4 declined; in the distant deoxygenated future, atmospheric CH4 is predicted to rise again.
• ▪  Future missions to Titan will aid in understanding the complex organic chemistry on the only other planetary body in our Solar System with an active CH4 cycle.

Intrinsic magnetic fields were once commonplace across our Solar System, and many planetary bodies have sustained active dynamos to the present day. The nature and behavior of these dynamos vary widely, however, reflecting the diverse internal conditions of planets as summarized in this review. For the terrestrial planets, the existence of active dynamos and/or ancient remanent magnetization recorded in crustal rocks, or lack thereof, lead to questions about their timing and power sources. Paleomagnetic studies reveal that many small bodies in the Solar System exhibit remanent magnetization, often attributed to ancient core dynamos with little known about the fluid dynamics. For the gas giants, their dipole-dominated magnetic fields and internal structures are relatively well-characterized, with dilute cores that are not centrally concentrated and other stable layers that likely affect the dynamo in ways that are not yet understood. For the ice giants, their multipolar magnetic fields and internal structures are unusual yet poorly constrained, to the extent that even the water-to-rock ratio is not well-known. Through adoption of a broader comparative planetology approach, the study of dynamos in exoplanets and cool stars enriches our understanding of dynamo theories.

• ▪  Planetary dynamos exhibit diverse magnetic fields shaped by their distinct physical and chemical conditions.
• ▪  The study of planets and stars connects planetary science, geophysics, and astrophysics, revealing shared dynamo processes.
• ▪  While significant progress has been made in understanding planetary and stellar magnetic fields, many puzzles still persist.

Geomagnetic methods allow us to explore the behavior of Earth's geodynamo, constrain Earth's composition and structure, and locate critical minerals and other resources essential for modern technologies and the energy transition. The magnetic properties of rocks and sediments are assumed to be stable and largely attributable to inorganic processes. This conventional view overlooks mounting evidence of microorganisms as key players in rock transformations and geological processes. Iron-bearing minerals are ubiquitous in most environments and are commonly used by microorganisms as electron donors and acceptors. Microorganisms modulate rock magnetic properties by creating, altering, and dissolving Fe-bearing minerals, potentially modifying the original magnetization, complicating interpretations of the magnetic record. This review provides an overview of biogenic pathways that modulate magnetic minerals and discusses common, yet underutilized, magnetic methods for capturing such behavior. Appreciating the influence of microbial activities on magnetic properties will improve our interpretations of Earth's geologic past and its elemental cycling.

• ▪  Microorganisms modulate rock magnetic properties, challenging traditional views of a geologically stable magnetic record formed solely by inorganic processes.
• ▪  Microbial iron cycling modulates magnetic properties modifying magnetic information recorded in rocks.
• ▪  Microbial processes may have impacted Earth's magnetic history more deeply than previously understood.
• ▪  Recognizing microbial contributions is critical for accurate interpretation of paleomagnetic and environmental magnetic records and could aid in the search for life on other planetary bodies.

The notion of climate sensitivity has become synonymous with equilibrium climate sensitivity (ECS), or the equilibrium response of the Earth system to a doubling of CO2. But there is a hierarchy of measures of climate sensitivity, which can be arranged in order of increasing complexity and societal relevance and which mirror the historical development of climate modeling. Elements of this hierarchy include the well-known ECS and transient climate response and the lesser-known transient climate response to cumulative emissions and zero emissions commitment. This article describes this hierarchy of climate sensitivities and associated modeling approaches. Key concepts reviewed along the way include climate forcing and feedback, ocean heat uptake, and the airborne fraction of cumulative emissions. We employ simplified theoretical models throughout to encapsulate well-understood aspects of these quantities and to highlight gaps in our understanding and areas for future progress.

• ▪  There is a hierarchy of measures of climate sensitivity, which exhibit a range of complexity and societal relevance.
• ▪  Equilibrium climate sensitivity is only one of these measures, and our understanding of it may have reached a plateau.
• ▪  The more complex measures introduce new quantities, such as ocean heat uptake coefficient and airborne fraction, which deserve increased attention.

More than 70% of Earth's magmatic output occurs in the ocean. This volcanism shapes major features of the seafloor, directly impacts the chemical composition of the oceans through water/rock interactions, and drives hydrothermal circulation of seawater. The formation of seafloor mineral deposits and chemosynthetic habitats that encircle the globe along mid-ocean ridges, volcanic arcs, and hotspots is driven by volcanism. The style, magnitude, depth, and frequency of seafloor eruptions create a wide range of physical, chemical, and biological impacts on the seafloor. Research and exploration over the past 30 years have revealed some of the diversity of seafloor eruptions and their impact on the undersea environment.

• ▪  Submarine eruptions are simultaneously the most common and the least observed form of volcanism on Earth.
• ▪  Hydrostatic pressure at the vent depth modulates explosive versus effusive eruption and the form of eruptive behavior.
• ▪  Submarine eruptions have significant impacts on marine biological communities and chemical fluxes to the ocean.
• ▪  Resilience of fauna to eruption events is also variable, and recovery dynamics can be slow with many years or decades required for communities to reform.

Field experiences are highly valued in geoscience education. However, logistical, financial, and accessibility challenges associated with fieldwork and rapid advancements in technology have all prompted geoscience educators to explore virtual field experiences (VFEs) as alternatives. Rigorous assessment of the effectiveness of VFEs has not kept pace with their implementation, but recent studies offer meaningful and actionable findings that can inform ongoing and future use of VFEs in geoscience education. We present a review of selected studies that address three significant aspects of this still-evolving modality. First, we examine current characterization and classification of VFEs. Second, we examine studies that evaluate the effectiveness of teaching with VFEs. Third, we extend this review to studies that compare VFEs with in-person field experiences (IPFEs). The studies we review demonstrate that VFEs are a valuable approach to teaching introductory geoscience content, even compared to IPFEs.

• ▪  Challenges associated with field geoscience education and improvements in technology have led geoscience educators to develop and implement virtual field experiences (VFEs) as teaching tools.
• ▪  VFEs are tested, practical, and effective alternatives to in-person field experiences in introductory geoscience education.

The emergence of continental crust above sea level influences Earth's surface environments and climate patterns, and it creates diverse habitats that promote biodiversity. Earth exhibits bimodal hypsometry with elevated continents and a submerged seafloor. However, it remains elusive when and how this unique feature was first established. The geological record suggests the presence of subaerial landmasses between ca. 3.8 and 2.4 billion years ago (Ga), but their spatial extent and longevity remain unclear. Further, the tectonic processes governing the proportion of continental land to ocean basins and topography during this period are poorly understood. Here, we synthesize a variety of geological and geochemical proxies to suggest that crustal emergence did occur in the early-to-mid Archean, primarily exposing precratonized volcanic crust for brief time periods. Stable continental crust on a regional scale (as cratons) began emerging around ca. 3.2–3.0 Ga, facilitated by the development of thick, stable cratonic lithospheres. Over hundreds of millions of years, voluminous magmatism within a plateau-type setting led to the formation of thick, felsic crust and depleted mantle keels, allowing cratons to rise above sea level via isostatic adjustment. The areal extent of emergent land increased from ca. 3.0 to 2.5 Ga owing to the formation of more cratons, likely coinciding with the onset of plate tectonics, and culminated around ca. 2.5–2.2 Ga when land surface area and freeboard conditions resembled those observed today. These newly emerged landmasses possibly played a critical role in oxygenating the atmosphere and oceans, cooling the climate, and promoting biodiversity during the late Archean to early Paleoproterozoic.

• ▪  Continental emergence marks a pivotal moment in Earth's history, impacting the planet's atmosphere, oceans, climate, and life evolution.
• ▪  We review the rock record to infer the timing, nature, and tectonic drivers of continental emergence on early Earth.
• ▪  Emergence on early Archean Earth was mostly transient, exposing primarily volcanic crust.
• ▪  The first stable continental land formed at ca. 3.2–3.0 Ga due to the development of thick cratons and their isostatic adjustment.
• ▪  Emergent land area increased from ca. 3.0 to 2.5 Ga as more felsic crust formed and plate tectonics began.

The continental fossil record has exceptional, long sequences of fossiliferous strata that are the basis for evaluating ecosystem dynamics and their formative influences. The Siwalik sequence of South Asia is one example. It occurs in the Potwar Plateau (Punjab Province, Pakistan) and spans 18–1 Ma. The sequence consists of alluvial sediments deposited in a foreland basin created by the collision of the Indian and Eurasian tectonic plates. Sediments representing large and small river channels and their associated floodplain deposits correspond to mountain-sourced large rivers and foothill-sourced smaller rivers. Vegetation attributes are recorded in stable carbon isotopes and biomarkers in paleosols. Molluscs, fishes, crocodilians, turtles, lizards, snakes, birds, and mammals are preserved throughout the sequence. Mammalian faunas had exceptionally high species richness (116 species) at their peak and included up to 18 species of co-occurring megaherbivores (>800 kg). Significant changes over time in species richness, taxonomic composition, and ecological structure of mammalian faunas coincided with major changes in climate and vegetation.

• ▪  Siwalik strata and fossils document a long continuous sequence of South Asian continental sediments and ecosystems south of the Himalaya Mountains.
• ▪  A multidisciplinary analysis of tectonics, fluvial systems, climate history, and vertebrate diversity documents ecosystem dynamics from 18 to 6 million years ago.
• ▪  A sparse portion of the Siwalik record coinciding with the Miocene Climatic Optimum raises the possibility that humid heat stress limited occupancy of the floodplain by most mammals for much of this time.
• ▪  The timing and magnitude of change in mammalian species richness and ecological structure are consistent with environmental forcing as a significant influence on these features.