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About 66 million years ago, a huge asteroid crashed into what is now Yucatán, plunging Earth into darkness. The impact transformed the rainforests, giving rise to the kingdom of flowers.
Today’s rainforests are biodiversity hotspots and play an important role in the world’s climate systems. A new study published today in Science sheds light on the origins of modern rainforests and can help scientists understand how rainforests will respond to a rapidly changing climate in the future.
The study led by researchers from the Smithsonian Tropical Research Institute (STRI) shows that the impact of an asteroid that ended the reign of the dinosaurs 66 million years ago also caused the extinction of 45% of the plants in what is now Colombia , and ushered in the reign of flowering plants in modern rainforests.
From fern-filled forests to flower-filled forests: Plants began to produce attractive flowers that contain sugary rewards for insects that carry pollen grains (basically male sperm from plants) to other flowers, helping plants to breed. This strategy was so successful that flowering plants took over tropical forests and the world. Credit: A long time ago. An illustrated paleontological journey through Colombia. Alexander von Humboldt Institute and Smithsonian Tropical Research Institute. Image Bank (BIA), Alexander von Humboldt Institute.
“We wondered how the rainforests changed after a drastic ecological disturbance like the Chicxulub impact, so we looked for tropical plant fossils,” said Mónica Carvalho, first author and joint postdoctoral fellow at STRI and the Universidad del Rosario in Colombia. “Our team examined more than 50,000 fossil pollen records and more than 6,000 leaf fossils before and after the impact.”
125 to 100 million years ago, during the reign of the dinosaurs, much of what is now Colombia was covered by forests dominated by conifers and ferns. Credit: A long time ago. An illustrated paleontological journey through Colombia. Alexander von Humboldt Institute and Smithsonian Tropical Research Institute. Image Bank (BIA), Alexander von Humboldt Institute.
In Central and South America, geologists scramble to find fossils exposed by roadblocks and mines before heavy rains wash them away and the jungle hides them again. Before this study, little was known about the effect of this extinction on the evolution of the flowering plants that now dominate the American tropics.
Carlos Jaramillo, STRI paleontologist and his team, mostly STRI fellows, many of them from Colombia, studied pollen grains from 39 sites including rock outcrops and cores drilled for oil exploration in Colombia, to paint a regional picture. great of the forests before. and after impact. Pollen and spores obtained from rocks older than the impact show that the rainforests were equally dominated by ferns and flowering plants. Conifers, such as the relatives of the Kauri pine and the Norfolk Island pine, sold in supermarkets at Christmas (Araucariaceae), were common and cast their shadows on dinosaur trails. After the impact, conifers almost completely disappeared from the New World tropics and flowering plants took over. Plant diversity did not recover for about 10 million years after the impact.
After the impact of an asteroid in Mexico, almost half of the existing plants before the impact were extinguished. After the impact, flowering plants came to dominate modern tropical forests. Credit: A long time ago. An illustrated paleontological journey through Colombia. Alexander von Humboldt Institute and Smithsonian Tropical Research Institute. Image Bank (BIA), Alexander von Humboldt Institute.
The leaf fossils told the team a lot about the past climate and the local environment. Carvalho and Fabiany Herrera, a postdoctoral research associate at the Negaunee Institute for Conservation Science and Action at the Chicago Botanical Garden, led the study of more than 6,000 specimens. Working with Scott Wing at the Smithsonian’s National Museum of Natural History and others, the team found evidence that rainforest trees before impact were widely spaced, allowing light to reach the forest floor. Within 10 million years after impact, some tropical forests were dense, such as today, where the leaves of trees and vines cast a deep shadow over the smaller trees, shrubs, and herbaceous plants below. The more scattered crowns of the pre-impact forests, with fewer flowering plants, would have moved less water from the ground into the atmosphere than those that grew in the millions of years after.
“It was just as rainy in the CretaceousBut the forests functioned differently, ”Carvalho said.
Modern Bogotá is an Andean city almost 3,000 m (9,000 ft) above sea level. But in the Paleocene (the 10-million-year period that followed the asteroid’s impact) it was covered by tropical forests. Insect damage to fossil leaves collected near Bogotá tells researchers that after impact, picky eaters (insects that only ate a certain species) became less common, replaced by insects with a broader flavor. that they could eat many different plants. Credit: A long time ago. An illustrated paleontological journey through Colombia. Alexander von Humboldt Institute and Smithsonian Tropical Research Institute. Image Bank (BIA), Alexander von Humboldt Institute.
The team found no evidence of legume trees prior to the extinction event, but there was great diversity and abundance of legume leaves and pods afterward. Today legumes are a dominant family in rainforests and through associations with bacteria they take nitrogen from the air and turn it into fertilizer for the soil. The increase in legumes would have drastically affected the nitrogen cycle.
Carvalho also worked with Conrad Labandeira at the Smithsonian’s National Museum of Natural History to study insect damage to leaf fossils.
From 66 to 100 million years ago, flowering plants began to diversify in sea level swamps and lowland forests, where the Andes mountains are today. Credit: A long time ago. An illustrated paleontological journey through Colombia. Alexander von Humboldt Institute and Smithsonian Tropical Research Institute. Image Bank (BIA), Alexander von Humboldt Institute.
“Insect damage to plants can reveal in the microcosm of a single leaf or in the extension of a plant community, the basis of the trophic structure in a tropical forest,” Labandeira said. “The energy that resides in the mass of plant tissues that is transmitted along the food chain, ultimately to boas, eagles and jaguars, begins with the insects that skeletonize, chew, pierce and suck, extract, crack and pierce plant tissues. The evidence for this consumer food chain begins with all the diverse, intensive and fascinating ways insects consume plants. “
“Before the impact, we see that different types of plants have different damages: the feeding was specific to the host,” said Carvalho. “After the impact, we found the same type of damage in almost all the plants, which means that the feeding was much more generalist.”
Fossil leaves from the collection used for this study. Spurge leaves heavily fed by insects from the 58-60 Myr neotropical rainforests of the Bogotá Formation in Colombia. Today, the spur family is one of the most abundant and diverse in the lowland rainforests. Credit: by Monica Carvalho.
How did the aftermath of the impact transform the sparse, coniferous-rich rainforests of the dinosaur era into the rainforests of today, towering trees dotted with yellow, purple, and pink flowers dotted with orchids? Based on the evidence from both pollen and leaves, the team comes up with three explanations for the change, all of which may be correct. One idea is that dinosaurs kept pre-impact forests open by feeding and moving across the landscape. A second explanation is that the ashfall from the impact enriched the soils of the tropics, giving faster-growing flowering plants an advantage. The third explanation is that the preferential extinction of coniferous species created an opportunity for flowering plants to take over the tropics.
“Our study follows a simple question: How do rainforests evolve?” Carvalho said. “The lesson learned here is that under rapid disturbances, geologically speaking, tropical ecosystems don’t just bounce back; they are replaced and the process takes a long time. “
Reference: “Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests” by Mónica R. Carvalho, Carlos Jaramillo, Felipe de la Parra, Dayenari Caballero-Rodríguez, Fabiany Herrera, Scott Wing, Benjamin L. Turner, Carlos D ‘ Apolito, Millerlandy Romero-Báez, Paula Narváez, Camila Martínez, Mauricio Gutierrez, Conrad Labandeira, German Bayona, Milton Rueda, Manuel Paez-Reyes, Dairon Cárdenas, Álvaro Duque, James L. Crowley, Carlos Santos and Daniele Silvestro, April 2, 2021 , Science.
DOI: 10.1126 / science.abf1969
The authors of this article are affiliated with STRI in Panama, Universidad del Rosario Bogotá, Colombia; The University of Montpellier, CNRS, EPHE, IRD, France; University of Salamanca, Spain; the Colombian Petroleum Institute, Bucaramanga, Colombia; the Chicago Botanical Garden; National Museum of Natural History, Washington, DC; University of Florida, US; Federal University of Mato Grosso, Cuiabá, Brazil; ExxonMobil Corporation, Spring, Texas, US; Technological Scientific Center-CONICET, Mendoza, Argentina; University of Chile, Santiago; University of Maryland, College Park, US; Capital Normal University, Beijing, China; Corporación Geológica Ares, Bogota, Colombia; Paleoflora Ltda., Zapatoca, Colombia; University of Houston, Texas, US; Amazon Institute for Scientific Research SINCHI, Leticia, Colombia; National University of Colombia, Medellín, Colombia; Boise State University, Boise, Idaho, US; BP Exploration Co. Ltd., UK; and University of Friborg, Switzerland.
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