As Earth faces unprecedented climate change, a look into the planet’s deep past may provide vital insights into what may lie ahead. But knowledge of the natural world millions of years ago is fragmented.

A 15-year study of a site in Bolivia by a joint U.S.-Bolivia team has provided a comprehensive view of an ancient ecosystem when Earth was much warmer than it is today. The researchers’ findings online Nov. 1 by the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

Located in the Andes Mountains of southern Bolivia, the site, known as the — or QHB — was deposited 13 million years ago during the Miocene Epoch, when Earth’s climate was rebounding from a prior period of warming. Globally, temperatures were 3-4 degrees Celsius warmer than today, and mammal biodiversity was increasing markedly.

Today, the site is 11,500 feet above sea level. Back in the Miocene, the site was lower, but exactly how much was a matter of debate. Previous studies using geochemical methods estimated that the Miocene QHB was relatively high, close to 10,000 feet. But the team’s new findings, based on careful analysis of plant and animal fossils and other features at the site, favor an alternative theory: That the Miocene QHB was at a much lower elevation, likely less than 3,000 feet.

“Our new data indicates that this area was once covered by mosaic vegetation with a mix of trees, including palms, bamboos and other grasses,” said lead author , a 91̽�� professor of biology. “Although this vegetation lacks a good comparison in today’s South America, it was likely most similar to modern neotropical dry forest or wooded savanna growing at low elevation.”

A lower-elevation Miocene QHB site has potentially global consequences.

“When put together with previous work at QHB, our study — including looking at fossil soils, turtles and other ectothermic vertebrates, and mammal ecologies — suggests that the Central Andes still had not undergone substantial uplift by 12 million years ago,” said Strömberg. “This is important because it helps us understand when this major mountain chain formed. The rise of the Andes is thought to have contributed to making tropical South America the most biodiverse area on Earth.”

Understanding ecosystems of the past can help predict what might happen in the future due to human-related climate change.

“Sites like this one in Bolivia are essential for helping us calibrate climate models,” said co-author and project leader , professor of anatomy at Case Western Reserve University. “Our understanding of climate change is based on models, and those models are based on information from the past.”

Between 2007 and 2017, Croft and co-author Frederico Anaya, a professor of geology at Universidad Autonóma Tomás Frías in Bolivia, led six international teams to the QHB to collect fossils. Despite its warmer, forested past, the site today is a high-altitude desert grassland.

During those trips, the team found many different types of fossils: bones and teeth of mammals and other vertebrates, microscopic plant remains, ancient soils, and tracks and traces of insects and other invertebrates. Analyzing these fossils contributed to the researchers’ conclusion that the Miocene QHB was at a lower elevation. For example, fossils from “cold-blooded” animals found at the site — a giant tortoise, a side-necked turtle and a very large snake — suggest the site’s elevation when these animals lived was less than 3,000 feet, based on modern-day distributions of closely related species.

Strömberg studied fossilized phytoliths from QHB. These are microscopic pieces of silica found in the cells and cell walls of plants, and the shapes of phytoliths differ depending on the type of plant they came from. She compared the fossilized phytoliths with those found in contemporary vegetation to identify the assortment of plants at the site during the Miocene.

Layers of volcanic ash and magnetic signatures in rocks at QHB allowed the fossils to be accurately dated. The diversity of preserved material allowed the team to make detailed reconstructions of the plants and animals and their living conditions. The team named 13 new species of fossil mammals based on remains from the site, including marsupials, hoofed mammals, rodents and armadillos. Most of the species have not been found anywhere else in South America and have no modern descendants.

“Nature has a wide variety of body plans, often much greater than the limited variety we see today,” said co-author Russell Engelman, a Case Western Reserve University graduate student who worked on the mammal fossils.

Moving forward, Croft is hoping to study another Bolivian Miocene site of a similar age, but over a longer time period.

“We are getting into uncharted territory in terms of climate, and you have to go deeper in time to get conditions that are similar,” said Croft.

Other co-authors are Beverly Saylor, Case Western Reserve University professor of Earth, environmental and planetary sciences; Angeline Catena, geology professor at Diablo Valley Community College in California; and Daniel Hembree, professor of Earth and planetary sciences at the University of Tennessee. The research was funded by the National Science Foundation.

For more information, contact Strömberg at caestrom@uw.edu.

Adapted from a by Case Western Reserve University.

published April 14 in the journal Science paint a new picture about apes, ancient Africa and the origins of humans. Many scientists had once hypothesized that the first apes to evolve in Africa more than 20 million years ago ate primarily fruit and lived within the thick, closed canopy of a nearly continent-wide forest ecosystem. Instead, the new research indicates that early apes ate a leafy diet in a more arid ecosystem of varyingly open woodlands with abundant grasses.

The findings by an international team of paleontologists, primate experts and plant scientists — including paleobotanists at the 91̽�� — push the origin of tropical ecosystems dominated by C₄ grasses back by more than 10 million years. In doing so, they link the emergence of C₄ grasses — named for the type of photosynthesis they employ to make food from the sun’s energy — to the emergence of the forerunners to all apes living today. That includes the most abundant ape in history: humans.

Previously, many researchers argued that during the early Miocene, between about 15 and 20 million years ago, equatorial Africa was covered by a semi-continuous forest. Under that hypothesis, more open habitats with C₄ grasses didn’t proliferate until about 8 to 10 million years ago. Yet one study showed some evidence of C₄ grasses in East Africa around 15 million years ago. The research team wanted to understand if that study was an anomaly or a clue to the true diversity of ecosystems at that time.

“The history of grassland ecosystems in Africa prior to 10 million years had remained a mystery, in part because there were so few plant fossils,” said co-author , the Estella B. Leopold Professor of Biology at the UW.

The international collaboration — funded largely by the National Science Foundation — drew together multiple different lines of evidence to try to reconstruct the species that dominated East Africa in the early Miocene. Researchers incorporated analyses of fossil soils, animal fossils, stable isotopes and phytoliths, which are plant silica microfossils.

Strömberg, who is an expert in phytoliths, worked with co-authors Alice Novello, a former 91̽��postdoctoral scientist who is currently working at Aix-Marseille University in France, and of the National Museums of Kenya and the Max Planck Institute of Geoanthropology in Germany to reconstruct what types of plants were present at several sites in East Africa during the early Miocene.

“Phytoliths are particularly informative for revealing the history of grassland ecosystems. They can tell us not just that there were grasses, but which grasses were there and how abundant they were on the landscape,” said Strömberg, who is also curator of paleobotany at the UW’s .

Their data, combined with other lines of evidence, essentially disproved the theory that equatorial Africa in the early Miocene was heavily forested. The findings have important implications for understanding the features and adaptations of early apes.

“Multiple lines of evidence show that C₄ grasses and open habitats were important parts of the early Miocene landscape and that early apes lived in a wide variety of habitats, ranging from closed canopy forests to open habitats like scrublands and wooded grasslands with C₄ grasses,” said co-author , an associate professor of geosciences at Baylor University. “It really changes our understanding of what ecosystems looked like when the modern African plant and animal community was evolving.”

“What we found was thrilling, and very different from what was the accepted story,” said Strömberg. “We used to think tropical, C₄ dominated grasslands only appeared in the last 8 million years or so, depending on the continent. Instead, both phytolith data and isotopic data showed that C₄ dominated grassy environments appeared over 10 million years earlier, in the early Miocene in eastern Africa.”

In addition to its findings about C₄ grassy habitats, the team is also reporting discoveries about a 21-million-year-old fossil ape, Morotopithecus. Anthropologists long thought that our ape ancestors evolved an upright torso in order to pick fruit in forests. With an upright posture, an ape can more easily grab onto different branches with its hands and feet. Morotopithecus definitely had an upright stature. Paleontologists on the team performed careful analyses of the shape of its molars, as well as the chemical composition of its dental enamel, to determine its diet.

“The expectation was: We have this ape with an upright back. It must be living in forests and it must be eating fruit,” said co-author , a professor of anthropology at the University of Michigan. “But as more and more bits of information became available, the first surprising thing we found was that the ape was eating leaves. The second surprise was that it was living in woodlands.”

Together, the evidence showed that Morotopithecus lived in a seasonal woodland with a broken canopy composed of trees and shrubs and open, grassy areas. In addition, the team’s plant and climate reconstruction efforts determined that, for at least part of the year, Morotopithecus had to rely on leaves and other plant material — instead of fruit — for food.

The fact that abundant C₄ grass and woodland ecosystems arose much earlier than once thought also upends another view of human origins: That our bipedalism evolved as a response to the emergence of grassland environments in Africa between 10 and 7 million years ago.

“Now that we’ve shown that such environments were present at least 10 million years before bipedalism evolved, we need to really rethink human origins, too,” said MacLatchy.

In addition to MacLatchy, Strömberg, Kinyanjui and Peppe, other lead researchers on the collaboration behind these discoveries include co-authors , associate professor of anthropology and archaeology at the University of Calgary; , a professor of anthropology at the University of Minnesota; , professor of Earth and environmental sciences at the University of Minnesota; and , an associate professor of anthropology at the University of Michigan. Many members of the team participated through the Research on East African Catarrhine and Hominoid Evolution — or — Project. Strömberg and Novello initially participated through separate funding from the European Union to study the evolution of grasslands in East Africa.

For more information, contact Strömberg at caestrom@uw.edu.

Adapted from press releases by the and .

A new published April 30 in the identified three factors critical in the rise of mammal communities since they first emerged during the Age of Dinosaurs: the rise of flowering plants, also known as angiosperms; the evolution of tribosphenic molars in mammals; and the extinction of non-avian dinosaurs, which reduced competition between mammals and other vertebrates in terrestrial ecosystems.

Previously, mammals in the Age of Dinosaurs were thought to be a relatively small part of their ecosystems and considered to be small-bodied, nocturnal, ground-dwelling insectivores. According to this long-standing theory, it wasn’t until the about 66 million years ago, which wiped out all non-avian dinosaurs, that mammals were then able to flourish and diversify. An astounding number of fossil discoveries over the past 30 years has challenged this theory, but most studies looked only at individual species and none has quantified community-scale patterns of the rise of mammals in the Mesozoic Era.

Co-authors are Meng Chen, a 91̽�� alumnus and current postdoctoral researcher at Nanjing University; , a 91̽�� biology professor and curator of paleobotany at the UW’s ; and , a 91̽��associate professor of biology and Burke Museum curator of vertebrate paleontology. The team created a Rubik’s Cube-like structure identifying 240 “eco-cells” representing possible ecological roles of mammals in a given ecospace. These 240 eco-cells cover a broad range of body size, dietary preferences, and ways of moving of small-bodied mammals. When a given mammal filled a certain type of role or eco-cell, it filled a spot in the ‘Rubik’s Cube.’ This method provides the first comprehensive analysis of evolutionary and ecological changes of fossil mammal communities before and after K-Pg mass extinction.

“We cannot directly observe the ecology of extinct species, but body size, dietary preferences and locomotion are three aspects of their ecology that can be relatively easily inferred from well-preserved fossils,” said Chen. “By constructing the ecospace using these three ecological aspects, we can visually identify the spots filled by species and calculate the distance among them. This allows us to compare the ecological structure of extinct and extant communities even though they don’t share any of the same species.”

The team analyzed living mammals to infer how fossil mammals filled roles in their ecosystems. They examined 98 small-bodied mammal communities from diverse biomes around the world, an approach that has not been attempted at this scale. They then used this modern-day reference dataset to analyze five exceptionally preserved mammal paleocommunities ― two Jurassic Period and two Cretaceous Period communities from northeastern China, and one Eocene Epoch community from Germany. Usually Mesozoic Era mammal fossils are incomplete and consist of fragmentary bones or teeth. Using these remarkably preserved fossils enabled the team to infer ecology of these extinct mammal species, and look at changes in mammal community structure during the last 165 million years.

The team found that, in current communities of present-day mammals, ecological richness is primarily driven by vegetation type, with 41 percent of small mammals filling eco-cells compared to 16 percent in the paleocommunities. The five mammal paleocommunities were also ecologically distinct from modern communities and pointed to important changes through evolutionary time. Locomotor diversification occurred first during the Mesozoic, possibly due to the diversity of microhabitats, such as trees, soils, lakes and other substrates to occupy in local environments. It wasn’t until the Eocene that mammals grew larger and expanded their diets from mostly carnivory, insectivory and omnivory to include more species with diets dominated by plants, including fruit. The team determined that the rise of flowering plants, new types of teeth and the extinction of dinosaurs likely drove these changes.

Before the rise of flowering plants, mammals likely relied on conifers and other seed plants for habitat, and their leaves and possibly seeds for food. By the Eocene, flowering plants were both diverse and dominant across forest ecosystems. Flowering plants provide more readily available nutrients through their fast-growing leaves, fleshy fruits, seeds and tubers. When becoming dominant in forests, they fundamentally changed terrestrial ecosystems by allowing for new modes of life for a diversity of mammals and other forest-dwelling animals, such as birds.

“Flowering plants really revolutionized terrestrial ecosystems,” said Strömberg. “They have a broader range of growth forms than all other plant groups ― from giant trees to tiny annual herbs ― and can produce nutrient-rich tissues at a faster rate than other plants. So when they started dominating ecosystems, they allowed for a wider variety of life modes and also for much higher ‘packing’ of species with similar ecological roles, especially in tropical forests.”

Tribosphenic molars ― complex multi-functional cheek teeth ― became prevalent in mammals in the late Cretaceous Period. Mutations and natural selection drastically changed the shapes of these molars, allowing them to do new things like grinding. In turn, this allowed small mammals with these types of teeth to eat new kinds of foods and diversify their diets.

Lastly, the K-Pg mass extinction event that wiped out all dinosaurs except birds 66 million years ago provided an evolutionary and ecological opportunity for mammals. Small body size is a way to avoid being eaten by dinosaurs and other large vertebrates. The mass extinction event not only removed the main predators of mammals, but also removed small dinosaurs that competed with mammals for resources. This ecological release allowed mammals to grow into larger sizes and fill the roles the dinosaurs once had.

“The old theory that early mammals were held in check by dinosaurs has some truth to it,” said Wilson. “But our study also shows that the rise of modern mammal communities was multifaceted and depended on dental evolution and the rise of flowering plants.”

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For more information contact Andrea Godinez with the 91̽��Burke Museum at burkepr@uw.edu.

Burke Museum story .

An international team led by the 91̽�� has discovered a way to determine the tree cover and density of trees, shrubs and bushes in locations over time based on clues in the cells of plant fossils preserved in rocks and soil. Tree density directly affects precipitation, erosion, animal behavior and a host of other factors in the natural world. Quantifying vegetation structure throughout time could shed light on how the Earth’s ecosystems changed over millions of years.

“Knowing an area’s vegetation structure and the arrangement of leaves on the Earth’s surface is key for understanding the terrestrial ecosystem. It’s the context in which all land-based organisms live, but we didn’t have a way to measure it until now,” said lead author Regan Dunn, a paleontologist at the UW’s . Dunn completed this work as a 91̽��doctoral student in the lab of , the Estella B. Leopold associate professor in biology and curator of paleobotany at the Burke Museum.

The Jan. 16 in the journal Science.

The team focused its fieldwork on several sites in Patagonia, Argentina, which have some of the best-preserved fossils in the world and together represent 38 million years of ecosystem history (49-11 million years ago). Paleontologists have for years painstakingly collected fossils from these sites, and worked to precisely determine their ages using radiometric dating. The new study builds on this growing body of knowledge.

In Patagonia and other places, scientists have some idea based on ancient plant remains such as fossilized pollen and leaves what species of plants were alive at given periods in Earth’s history. For example, the team’s documented vegetation composition for this area of Patagonia. But there hasn’t been a way to precisely quantify vegetation openness, aside from general speculations of open or bare habitats, as opposed to closed or tree-covered habitats.

“Now we have a tool to go and look at a lot of different important intervals in our history where we don’t know what happened to the structure of vegetation,” said Dunn, citing the period just after the mass extinction that killed off the dinosaurs.

“The significance of this work cannot be understated,” said co-author Strömberg. “Vegetation structure links all aspects of modern ecosystems, from soil moisture to primary productivity to global climate. Using this method, we can finally quantify in detail how Earth’s plant and animal communities have responded to climate change over millions of years, which is vital for forecasting how ecosystems will change under predicted future climate scenarios.”

Work by other scientists has shown that the cells found in a plant’s outermost layer, called the epidermis, change in size and shape depending on how much sun the plant is exposed to while its leaves develop. For example, the cells of a leaf that grow in deeper shade will be larger and curvier than the cells of leaves that develop in less covered areas.

Dunn and collaborators found that these cell patterns, indicating growth in shade or sun, similarly show up in some plant fossils. When a plant’s leaves fall to the ground and decompose, tiny silica particles inside the plants called phytoliths remain as part of the soil layer. The phytoliths were found to perfectly mimic the cell shapes and sizes that indicate whether or not the plant grew in a shady or open area.

The researchers decided to check their hypothesis that fossilized cells could tell a more complete story of vegetation structure by testing it in a modern setting: Costa Rica.

Dunn took soil samples from sites in Costa Rica that varied from covered rainforests to grassy savannahs to woody shrub lands. She also took photos looking directly up at the tree canopy (or lack thereof) at each site, noting the total vegetation coverage.

Back in the lab, she extracted the phytoliths from each soil sample and measured them under the microscope. When compared with tree coverage estimated from the corresponding photos, Dunn and co-authors found that the curves and sizes of the cells directly related to the amount of shade in their environments. The researchers characterized the amount of shade as “,” which is a standard way of measuring vegetation over a specific area.

Testing this relationship between leaf area index and plant cell structures in modern environments allowed the team to develop an equation that can be used to predict vegetation openness at any time in the past, provided there are preserved plant fossils.

“Leaf area index is a well-known variable for ecologists, climate scientists and modelers, but no one’s ever been able to imagine how you could reconstruct tree coverage in the past — and now we can,” said co-author Richard Madden of the University of Chicago. “We should be able to reconstruct leaf area index by using all kinds of fossil plant preservation, not just phytoliths. Once that is demonstrated, then the places in the world where we can reconstruct this will increase.”

When Dunn and co-authors applied their method to 40-million-year-old phytoliths from Patagonia, they found something surprising — habitats lost dense tree cover and opened up much earlier than previously thought based on other paleobotanic studies. This is significant because the decline in vegetation cover occurred during the same period as cooling ocean temperatures and the evolution of animals with the type of teeth that feed in open, dusty habitats.

The research team plans to test the relationship between vegetation coverage and plant cell structure in other regions around the world. They also hope to find other types of plant fossils that hold the same information at the cellular level as do phytoliths.

Other co-authors are Matthew Kohn of Boise State University and Alfredo Carlini of Universidad Nacional de La Plata in Argentina.

The research was funded by the National Science Foundation, the Geological Society of America, 91̽��Biology and the Burke Museum.

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For more information, contact Dunn at dunnr@uw.edu or 206-685-0374.

New research led by the 91̽�� challenges the 140-year-old assumption that finding fossilized remains of prehistoric animals with such teeth meant the animals were living in grasslands and savannas. Instead it appears certain South American mammals evolved the teeth in response to the gritty dust and volcanic ash they encountered while feeding in an ancient tropical forest.

The new work was conducted in Argentina where scientists had thought Earth’s first grasslands emerged 38 million years ago, an assumption based on fossils of these specialized teeth. But the grasslands didn’t exist. Instead there were tropical forests rich with palms, bamboos and gingers, according to , 91̽��assistant professor of and lead author of an in .

“The assumption about grasslands and the evolution of these teeth was based on animal fossils,” Strömberg said. “No one had looked in detail at evidence from the plant record before. Our findings show that you shouldn’t assume adaptations always came about in the same way, that the trigger is the same environment every time.”

To handle a lifetime of rough abrasion, the specialized teeth – called high-crowned cheek teeth – are especially long and mostly up in the animals’ gums when they are young. As chewing surfaces of the teeth wear away, more of the tooth emerges from the gums until the crowns are used up. In each tooth, bone-like dentin and tough enamel are complexly folded and layered to create strong ridged surfaces for chewing. Human teeth have short crowns and enamel only on the outside of each tooth.

In Argentina, mammals apparently developed specialized teeth 20 million years or more before grasslands appeared, Strömberg said. This was different from her previous work in North America and western Eurasia where she found the emergence of grasslands coincided with the early ancestors of horses and other animals evolving specialized teeth. The cause and effect, however, took 4 million years, considerably more lag time than previously thought.

The idea that specialized teeth could have evolved in response to eating dust and grit on plants and the ground is not new. In the case of Argentine mammals, Strömberg and her co-authors hypothesize that the teeth adapted to handle volcanic ash because so much is present at the study site. For example, some layers of volcanic ash are as thick as 20 feet (six meters). In other layers, soils and roots were just starting to develop when they were smothered with more ash.

Chewing grasses is abrasive because grasses take up more silica from soils than most other plants. Silica forms minute particles inside many plants called phytoliths that, among other things, help some plants stand upright and form part of the protective coating on seeds.

Phytoliths vary in appearance under a microscope depending on the kind of plant. When plants die and decay, the phytoliths remain as part of the soil layer. In work funded by the National Science Foundation, Strömberg and her colleagues collected samples from Argentina’s Gran Barranca, literally “Great Cliff,” that offers access to layers of soil, ash and sand going back millions of years.

The phytoliths they found in 38-million-year-old layers – when ancient mammals in that part of the world developed specialized teeth – were overwhelmingly from tropical forests, Strömberg said.

“In modern grasslands and savannas you’d expect at least 35 to 40 percent – more likely well over 50 percent – of grass phytoliths. The fact we have so little evidence of grasses is very diagnostic of a forested habitat,” she said.

The emergence of grasslands and the evolution of specialized teeth in mammals are regarded as a classic example of co-evolution, one that has occurred in various places around the world. However, as the new work shows, “caution is required when using this functional trait for habitat reconstruction,” the co-authors write.

Other co-authors are , a 91̽��doctoral candidate; , University of Chicago; Boise State University; and Alfredo Carlini, National University of La Plata, Argentina.

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For more information:
Strömberg is on parental leave, email her at caestrom@uw.edu to arrange interviews