Caroline Stromberg

University of Washington Seattle | UW · Department of Biology & Burke Museum of Natural History and Culture

Cycads are ancient seed plants (gymnosperms) that emerged by the early Permian. Although they were common understory flora and food for dinosaurs in the Mesozoic, their abundance declined markedly in the Cenozoic. Extant cycads persist in restricted populations in tropical and subtropical habitats and, with their conserved morphology, are often cal...

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 Moro...

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...

Some of the most extensive terrestrial biomes today consist of open vegetation, including temperate grasslands and tropical savannas. These biomes originated relatively recently in Earth’s history, likely replacing forested habitats in the second half of the Cenozoic. However, the timing of their origination and expansion remains disputed. Here, we...

Grassy biomes are >20 million years old but are undervalued and under threat today.

The shift from denser forests to open, grass‐dominated vegetation in west‐central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat transition (OHT) is considered by some to be evidence for drier summers, more seasonal precipitation, or a cooler climate, but others have...

Open vegetation today constitutes one of the most extensive biomes on earth, including temperate grasslands and tropical savannas. Yet these biomes originated relatively recently in earth history, likely replacing forested habitats as recently as the second half of the Cenozoic, although the timing of their origination and the dynamics of their exp...

The Miocene epoch (23.03-5.33 Ma) was a time interval of global warmth, relative to today. Continental configurations and mountain topography transitioned toward modern conditions, and many flora and fauna evolved into the same taxa that exist today. Miocene climate was dynamic: long periods of early and late glaciation bracketed a ∼2 Myr greenhous...

The Miocene epoch (23.03–5.33 Ma) was a time interval of global warmth, relative to today. Continental configurations and mountain topography transitioned toward modern conditions, and many flora and fauna evolved into the same taxa that exist today. Miocene climate was dynamic: long periods of early and late glaciation bracketed a ∼2 Myr greenhous...

To improve paleoenvironmental reconstruction based on fossil phytoliths, it is vital to establish modern calibrations that explicitly consider important functional aspects of vegetation, such as structure, composition, diversity, and spatial heterogeneity. No such studies currently exist for Central-and South America. To begin to solve this problem...

The assembly of Africa’s iconic C4 grassland and savanna ecosystems is central to evolutionary interpretations of many mammals, including hominins. Based largely on pollen, biomarkers, and isotopic data, C4 grasses are thought to have become ecologically dominant in Africa only after 10 Ma. However, paleobotanical records older than 10 Ma are spars...

This article comments on: Kristýna Hošková, Adéla Pokorná, Jiří Neustupa and Petr Pokorný, Inter- and intraspecific variation in grass phytolith shape and size: a geometric morphometrics perspective, Annals of Botany, Volume 127, Issue 2, 01 February 2021, Pages 191–201, https://doi.org/10.1093/aob/mcaa102

Quebrada Honda (QH) is among the best-characterized middle Miocene terrestrial vertebrate sites of South America, with >40 spp. (mainly mammals) documented. Our work clarifies its paleoenvironmental and geochronological context. The studied section (~180 m) can be divided into: (1) a lower unit of reddish mudstone and minor sandstone with basal all...

The uptake and deposition of silicon (Si) as silica phytoliths is common among land plants and is associated with a variety of functions. Among these, herbivore defense has received significant attention, particularly with regards to grasses and grasslands. Grasses are well known for their high silica content, a trait which has important implicatio...

Fossil-rich sediments of the Santa Cruz Formation, Patagonia, Argentina, span the initiation of the Miocene Climatic Optimum (MCO), the most recent period of warm and wet conditions in the Cenozoic. These conditions drove the expansion of tropical and subtropical ecosystems to much higher latitudes, with the fossiliferous Santa Cruz Formation recor...

Process‐based vegetation models attempt to represent the wide range of trait variation in biomes by grouping ecologically similar species into plant functional types (PFTs). This approach has been successful in representing many aspects of plant physiology and biophysics but struggles to capture biogeographic history and ecological dynamics that de...

Modern grasses in the family Poaceae exhibit exceptional taxonomic and ecological diversity, with grassland-dominated habitats covering about 40% of Earth’s land surface. Grasslands today occur in a range of temperate to tropical ecosystems, spanning low to high altitudes and wet to arid climates. However, because of the scant fossil record of gras...

The Middle Miocene Climatic Optimum (MMCO; ca. 17-14.5 Ma) is the most recent Cenozoic global warming event. It affected ecosystems that were broadly comparable to today's, making it one of our best analogues for ongoing, anthropogenic climate change. Whereas vegetation responses to the MMCO in the northern hemisphere are well documented, southern...

Rationale: Fossil grass silica short cell phytoliths (GSSCP) have been used to reconstruct the biogeography of Poaceae, untangle crop domestication history, and detect past vegetation shifts. These inferences depend on accurately identifying the clade to which the fossils belong. Patterns of GSSCP shape and size variation across the family have not...

The effects of global climate change are manifested at the regional level; consequently, evaluation of links between palaeoenvironmental change and turnover of past biotas must use regional-scale climate data. Here we test whether climate change influenced faunal and floral patterns leading up to, and during, the mid-Miocene Climatic Optimum (MMCO)...

Covering ancient geomorphological landscapes, and surrounded by some of the most diverse forests on Earth, the Neotropical savannas were once perceived by naturalists as ancient environments. However, current evidence suggests that tropical forests have existed in the Neotropics since the Paleocene, whereas most plant lineages present in South Amer...

Rationale: Fossil grass silica short cell phytoliths (GSSCP) have been used to reconstruct the biogeography of Poaceae, untangle crop domestication history, and detect past vegetation shifts. These inferences depend on accurately identifying the clade to which the fossils belong. Patterns of GSSCP shape and size variation across the family have not...

Phytolith analysis, a well-established tool in archaeology and Quaternary paleoecology, has become a source of data for deep-time paleoecological studies only in the last 15 years. Recent years have also witnessed the publication of numerous soil phytolith inventories from extant vegetation types, representing modern analogues. However, this work s...

Process-based vegetation models attempt to represent the wide range of trait variation in biomes by grouping ecologically similar species into plant functional types (PFTs). This approach has been successful in representing many aspects of plant physiology and biophysics, but struggles to capture biogeographic history and ecological dynamics that d...

Miocene paleoecology of East Africa has implications for human origins and understanding the vicariant legacy forests found today on either side of the East African Rift. Fossil leaves preserved in 21.73 million year old lacustrine sediments from the Mush Valley, Ethiopia, provide a unique opportunity to investigate forest composition and dominance...

Molecular nitrogen (N2) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN2). However, fluctuations in pN2 may have occurred on 107–109 year timescales in Earth's past, perhaps altering the isotopic composition of atmospheric nitrogen. Here, we explore an archive that may record the isotopic compos...

Bastin et al .’s estimate (Reports, 5 July 2019, p. 76) that tree planting for climate change mitigation could sequester 205 gigatonnes of carbon is approximately five times too large. Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of s...

Background: Opal phytoliths (microscopic silica bodies produced in and between the cells of many plants) are a very resilient, often preserved type of plant microfossil. With the exponentially growing number of phytolith studies, standardization of phytolith morphotype names and description is essential. As a first effort in standardization, the I...

Accurate age-depth models for proxy records are crucial for inferring changes to the environment through space and time, yet traditional methods of constructing these models assume unrealistically small age uncertainties and do not account for many geologic complexities. Here we modify an existing Bayesian age-depth model to foster its application...

Significance Amazing fossil discoveries over the last 30 years have led to the paleontological consensus that some Mesozoic mammaliaforms underwent ecomorphological diversification in the midst of dinosaurs. However, the ecological structure of Mesozoic mammaliaform communities remains unclear. Here, we quantify the ecological structure of extinct...

Global change impacts on the Earth System are typically evaluated using biome classifications based on trees and forests. However, during the Cenozoic, many terrestrial biomes were transformed through the displacement of trees and shrubs by grasses. While grasses comprise 3% of vascular plant species, they are responsible for more than 25% of terre...

Global change impacts on the Earth System are typically evaluated using biome classifications based on trees and forests. However, during the Cenozoic, many terrestrial biomes were transformed through the displacement of trees and shrubs by grasses. While grasses comprise 3% of vascular plant species, they are responsible for more than 25% of terre...

Phytoliths, microscopic plant silica bodies, are often preserved in modern and fossil soils and sediment, as well as in archaeological contexts. They record unique characteristics of past vegetation and, unlike palynomorphs and macrofossils, are commonly found in direct association with fossil vertebrates, providing vital paleoecological data. With...

Anecdotal evidence indicating substantial silica accumulation in tissues of bryophytes suggests that silica (phytolith) deposition evolved early on in embryophytes. To test this hypothesis, we conducted the first survey of phytolith content representing the major liverwort, moss and hornwort clades. We also assessed the diagnostic value of bryophyt...

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 he...

Phytolith analysis has been used in Archaeobotany and Quaternary paleoecology for over 40 years, but it is only in the last 15 years that the plant silica record has emerged as a source of data also for deep-time paleoecological studies. As the use of phytoliths in Quaternary paleoecology grows, it is crucial to refine this tool by establishing mor...

C4 grasslands are a major global ecosystem with an important role as the primary source of food resources and agricultural land for the planet. Despite this, the causes and timeline of their expansion are still not fully known and appear to be variable in different parts of the world. By combining phytolith and stable isotope methods, we produce ro...

High global temperature and pCO2 characterized the Middle Miocene Climatic Optimum (MMCO; ca. 17-14.5 Ma) and are thought to have promoted highly diverse ecosystems in warm and wet climates at high latitudes. However, only sparse observations inform interpretations of regional climate and biotas outside the northern hemisphere. The Santa Cruz Forma...

Phytoliths, microscopic plant silica bodies, are often preserved in modern and fossil soils and sediment, as well as in archaeological contexts. They record unique characteristics of past vegetation and, unlike palynomorphs and macrofossils, are commonly found in direct association with fossil vertebrates, providing vital paleoecological data. With...

Terrestrial biogeochemical carbon (C) sequestration is coupled with the biogeochemical silicon (Si) cycle through mechanisms such as phytolith C sequestration, but the size and distribution of the phytolith C sink remain unclear. Here, we estimate phytolith C sequestration in global terrestrial biomes. We used biome data including productivity, phy...

Bastin et al. (Reports, 12 May 2017, p. 635) infer forest as more globally extensive than previously estimated using tree cover data. However, their forest definition does not reflect ecosystem function or biotic composition. These structural and climatic definitions inflate forest estimates across the tropics and undermine conservation goals, lead...

Today mammalian communities show drastic ecological responses to ongoing environmental change on both temporal and spatial scales. Similarly, environmental alteration has played a vital role in shaping ecological dynamics in the past, resulting in today’s faunal associations. Mesozoic terrestrial environments, which had a great impact on the earlie...

During silicate weathering, atmospheric carbon dioxide (CO2) is consumed and base cations are released from silicate minerals to form carbonate and bicarbonate ions, which are finally deposited as carbonate complexes. Continental silicate weathering constitutes a stable carbon sink that is an important influence on long-term climate change, as it s...

ABSTRACT. The Piedra Clavada Formation (= Kachaike Fm.) is a very significant lithostratigraphic unit (306 m thick) in the Cretaceous sedimentary record of the Austral Basin in southern Patagonia, Argentina, because it represents the final part of a rift stage forming a large passive-margin delta system, before the onset of a foreland basin stage....

The mid-Miocene climatic optimum (MMCO; initial warming beginning ca. 18 Ma and with peak warming ca. 17–14.75 Ma) constitutes the Earth's most recent greenhouse episode, characterized by a transient shift to higher global atmospheric CO2 levels and warmer, possibly wetter, climatic conditions. Combined with the spread of grass-dominated habitats,...

The early–middle Miocene was an important transitional period in the evolution of Earth's biota and climate that has been poorly understood in North America due to a paucity of continuous, fossil-bearing rock records in this interval for which the ages have been robustly constrained. In the northern Rocky Mountains, United States, one site in parti...

Topographically complex regions on land and in the oceans feature hotspots of biodiversity that reflect geological influences on ecological and evolutionary processes. Over geologic time, topographic diversity gradients wax and wane over millions of years, tracking tectonic or climatic history. Topographic diversity gradients from the present day a...