Caroline A E Strömberg

• Doctor of Philosophy
• Professor at University of Washington

Research on silicon (Si) biogeochemistry and its beneficial effects for plants has received significant attention over several decades, but the reasons for the emergence of high-Si plants remain unclear. Here, we combine experimentation, field studies and analysis of existing databases to test the role of temperature on the expression and emergence...

The Cretaceous/Paleogene (K/Pg) mass extinction was a pivotal event in Earth history, the latest among five mass extinctions that devastated marine and terrestrial life. Whereas much research has focused on the global demise of dominant vertebrate groups, less is known about changes among plant communities during the K/Pg mass extinction. This stud...

Background and Aims The absence of a modern plant-based ‘dicotyledon’ phytolith reference baseline impedes the accurate interpretation of fossil phytolith records in archaeological and palaeoecological research within North-western Europe. This study aims to fill this gap by documenting and analysing the phytolith record from modern dicotyledon tax...

Miocene ecosystem change in the Central Andes is not well understood because of a dearth of well-dated fossil sites from the region. The late Middle Miocene (~13–12 Ma) Quebrada Honda Basin (QHB) in southern Bolivia (22◦ S) helps fill this gap and provide vital insights into Neotropical paleoenvironments. The site is among the best-characterized Mi...

The fossil record of the U.S. Pacific Northwest preserves many Middle Miocene floras with potential for revealing long-term climate-vegetation dynamics during the Miocene Climatic Optimum. However, the possibility of strong, eccentricity-paced climate oscillations and concurrent, intense volcanism may obscure the signature of prevailing, long-term...

Premise Seed dispersal is a critical process impacting individual plants and their communities. Plants have evolved numerous strategies and structures to disperse their seeds, but the evolutionary drivers of this diversity remain poorly understood in most lineages. We tested the hypothesis that the evolution of wind dispersal traits within the meli...

Tribute to Estella B. Leopold

Background and aims Palm fossils are often used as evidence for warm and wet palaeoenvironments, reflecting the affinities of most modern palms. However, several extant palm lineages tolerate cool and/or arid climates, making a clear understanding of the taxonomic composition of ancient palm communities important for reliable palaeoenvironmental in...

Analyzing the leaf morphology (leaf physiognomy) of paleofloras can provide crucial data for reconstructing terrestrial paleoclimates across periods of major climatic change, such as those occurring in the Miocene epoch. The Miocene Climatic Optimum (MCO; ~17-14 Ma) was Earth's most recent major warming event and the Middle Miocene Climatic Transit...

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...

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...

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

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...

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 been establ...

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...

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...

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...

Response by Caroline Strömberg for the presentation of the 2017 Schuchert Award of the Paleontological Society - Volume 92 Issue 5 - Caroline Strömberg

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...

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...

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...

https://deepblue.lib.umich.edu/bitstream/2027.42/148576/1/Harris_et_al_2017_GSA_Bulletin-revised_chronology_for_Railroad_Canyon.pdf

Solid biosilica (phytoliths) deposited in plant tissues is thought to function as structural support, as a cost‐effective alternative to lignin, and in herbivore defence, by limiting nutrient access/extraction and abrading herbivore mouthparts. It has been assumed that active phytolith accumulation evolved for these purposes, but these hypotheses r...

Animal dung is evaluated here as a tool to reconstruct recent societal and environmental changes. Studies completed on the macro- and micro-contents from dung deposited in a mountain cave in Catalonia during the 1970–1980s, preceding the socio-economic changes in the area, was supplemented by the testimony of the last shepherd alive in the area. Th...

The cycads (Division: Cycadophyta) are sometimes referred to as “living fossils.” These ancient plants have their evolutionary origins in the late Paleozoic, and have been persistent fixtures of understory flora since their first appearance in the fossil record. Here we attempt to exploit their long fossil record in order to investigate the antiqui...

Premise of research. Phytoliths (bio-opal) of grasses are preserved in the fossil record and constitute the best method to study grasses and grassland evolution in deep time. However, methods currently used to reconstruct habitat openness based on grass phytolith abundance are potentially biased because of the prolific production of phytoliths in g...

A new paleovegetation reconstruction based on both phytolith and paleosol carbon isotopic data is presented for the middle–late Miocene (11.2–9.5Ma) of southwestern Montana. Both sources of data indicate an open-habitat, C3 grassland ecosystem with a small C4 component (b10%). However, while a penecontemporaneous record from further south within th...

Aim C 4 photosynthesis is an adaptation that maintains efficient carbon assimilation in high‐light, high‐temperature conditions. Due to the importance of C 4 grasses for carbon and surface energy fluxes, numerous models have been proposed to describe their spatial distribution and forecast their responses to climate change. These models often rely...

Vegetation structure is a key determinant of ecosystems and ecosystem function, but paleoecological techniques to quantify it are lacking. We present a method for reconstructing leaf area index (LAI) based on light-dependent morphology of leaf epidermal cells and phytoliths derived from them. Using this proxy, we reconstruct LAI for the Cenozoic (4...

https://deepblue.lib.umich.edu/bitstream/2027.42/148589/1/Chen_et_al_2015_Palaeo-3-MBJ_paleoveg.pdf

Early land plant fossils can be challenging to interpret due to their morphological simplicity and often fragmentary nature. Morphometric techniques using commonly preserved characters might increase diagnostic value of such material. To evaluate the utility of morphometrics in assessing morphospecies boundaries in the Devonian, we compared degrees...

Many questions in evolutionary biology require an estimate of divergence times but, for groups with a sparse fossil record, such estimates rely heavily on molecular dating methods. The accuracy of these methods depends on both an adequate underlying model and the appropriate implementation of fossil evidence as calibration points. We explore the ef...

Biogeographic patterns of survival help constrain the causal factors responsible for mass extinction. To test whether biogeography influenced end-Cretaceous (K-Pg) extinction patterns, we used a network approach to delimit biogeographic units (BUs) above the species level in a global Maastrichtian database of 329 bivalve genera. Geographic range is...

Supporting information

The evolution of high-crowned cheek teeth (hypsodonty) in herbivorous mammals during the late Cenozoic is classically regarded as an adaptive response to the near-global spread of grass-dominated habitats. Precocious hypsodonty in middle Eocene (∼38 million years (Myr) ago) faunas from Patagonia, South America, is therefore thought to signal Earth'...

The origin and spread of grasslands is one of the key evolutionary events in the Cenozoic, which is characterized by long-term cooling and drying climatic conditions. One way to examine the relationship between vegetation and climate is to study the isotopic composition of organic carbon preserved in paleosols. Paleosols that preserve organic matte...

We describe an in situ fossil flora of Late Cretaceous age (∼73 Ma [mega-annum or million years]) from Big Cedar Ridge in central Wyoming, USA, which we sampled using a modified line-intercept method to quantify the relative abundances of 122 taxa at 100 sites across 4 km of exposed sedimentary deposits. We also measured three physical variables at...

http://deepblue.lib.umich.edu/bitstream/2027.42/116378/1/ecm201282123.pdf

—C 4 grasses form the foundation of warm-climate grasslands and savannas and provide important food crops such as corn, but their Neogene rise to dominance is still not fully understood. Carbon isotope ratios of tooth enamel, soil carbonate, carbonate cements, and plant lipids indicate a late Miocene—Pliocene (8-2 Ma) transition from C 3 vegetation...

The rapid ecological expansion of grasses with C4 photosynthesis at the end of the Neogene (8-2 Ma) is well documented in the fossil record of stable carbon isotopes. As one of the most profound vegetation changes to occur in recent geologic time, it paved the way for modern tropical grassland ecosystems. Changes in CO 2 levels, seasonality, aridit...

Rice and its relatives are a focal point in agricultural and evolutionary science, but a paucity of fossils has obscured their deep-time history. Previously described cuticles with silica bodies (phytoliths) from the Late Cretaceous period (67-65 Ma) of India indicate that, by the latest Cretaceous, the grass family (Poaceae) consisted of members o...

The evolution and subsequent ecological expansion of grasses (Poaceae) since the Late Cretaceous have resulted in the establishment of one of Earth's dominant biomes, the temperate and tropical grasslands, at the expense of forests. In the past decades, several new approaches have been applied to the fossil record of grasses to elucidate the patter...

The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C4 grassland expansion were considerably more complex than originally assumed. Previo...

The evolution of grasses using C₄ photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C₄ grassland expansion were considerably more complex than originally assumed. Previo...

et al. 1 Supplementary Figure S1. Cladogram for the Poaceae. Based on the combined Bayesian phylogenetic analysis of molecular and morphological data using MrBayes. Posterior probability values are shown on nodes. Also illustrated are the placements of the seven fossil calibrations used in the BEAST analyses (letters a – g), the three alternative p...

et al. 1 Supplementary Figure S1. Cladogram for the Poaceae. Based on the combined Bayesian phylogenetic analysis of molecular and morphological data using MrBayes. Posterior probability values are shown on nodes. Also illustrated are the placements of the seven fossil calibrations used in the BEAST analyses (letters a – g), the three alternative p...

Constraining the ages and duration of Cenozoic South American Land Mammal Ages (SALMAs) has been based on 40Ar/39Ar dating and magnetic polarity stratigraphy. At Gran Barranca (68.7°W, 45.7°S) - South America’s most important site and sequence for constraining SALMAs -uncertainties of ~ 1 Myr persist. To better constrain the ages of mammalian and p...

The John Day Basin of central Oregon contains a remarkably detailed and well-dated Early Eocene–Late Miocene sedimentary sequence, known for its superb fos-sils. This fi eld trip examines plant fossil assemblages from throughout the sequence in the context of their geological and taphonomic setting and regional and global signifi cance. The Early t...

In quantitative phytolith analysis, chance error associated with insufficient counts can affect the robustness of the interpretation, whether it is vegetation reconstruction or taxonomic differentiation. It is therefore vital to choose a count size that will ensure statistically reliable results, while minimizing the time expended. Numerical statis...