Methods Used to Reconstruct Ancient Rain Forests May Need Revision, Says Study

One way in which scientists use carbon isotopes found in fossils to identify the sites of ancient rain forests may not work as expected.

Earth Institute
October 05, 2020

Closed-canopy rain forests are vital parts of the earth’s modern ecosystems, but because tropical plants are not well preserved in the fossil record, it is difficult to tell how long these habitats have existed and where they might have once grown. Instead, scientists look to the diets of extinct animals, whose remains hold evidence of the vegetation they ate. But a new study finds that the paradigm used to identify closed-canopy rain forests through dietary signatures may need to be reassessed. The findings appear this week in the journal Proceedings of the National Academy of Sciences.

“The Amazon is the world’s most diverse rain forest, home to one in ten known species,” said Julia Tejada-Lara, who led the study as a graduate student at Columbia University’s Lamont-Doherty Earth Observatory and the American Museum of Natural History. “Closed-canopy rain forests have been proposed to occur in this area since at least the Eocene, some 50 million years ago, but we know very little about their extent and evolution through time.”

rain forest
Rain forest in the Peruvian Amazon. (Courtesy Rodolfo Salas-Gismondi, Universidad Cayetano Heredia in Lima, Peru)

To reconstruct ancient ecosystems, including rain forests, researchers often use analyses of stable carbon isotopes preserved in the remains of herbivores. Stable carbon isotopes, which form in specific proportions inside different types of plants, are preserved in the body tissues of the animals that eat those plants. Samples from animals’ bones, teeth, toenails or other material can help scientists determine the kinds of plants they consumed, and thus the kind of ecosystem they inhabited.

In the new study, Tejada and her colleagues analyzed specimens held by the American Museum of Natural History and the Museum of Natural History in Lima, Peru. The samples represented 45 modern herbivores and 12 species of “secondary consumers” (meat-, insect-, and fish-eaters) that live in western Amazonia. The authors then compared their results with a landmark analysis of modern mammals in equatorial Africa, a generally accepted proxy used to identify past closed-canopy rain forests on all continents.

“Up to this point, there had only been one other broad isotopic sampling—and inferences of food sources—of a tropical closed-canopy rain-forest mammal community, and that was in central Africa,” said study coauthor John Flynn, who curates fossil mammals at the American Museum. “So we knew that if we wanted to learn more about both modern and ancient Amazon ecosystems, we had to test whether we should expect the tropical forest conditions to be roughly the same in these two continents that separated more than 90 million years ago, and have a 1,600-mile-wide ocean between them today.”

The comparison revealed that Amazonian and African closed-canopy rain forests have a very similar mean dietary isotopic value, which may be representative of mammalian herbivores in any closed canopy rain forest. But beyond this newly discovered way of recognizing ancient rain forests, Amazonian mammals in this study lacked highly negative dietary values found in a few of the African animals. These negative values are often used outright to infer closed-canopy rain forests in fossil records.

“We have found that these negative isotopic values can no longer be used as an indispensable indicator of a rain forest,” Tejada said. “And further, that many of the long-time assumptions about ecological niches, feeding habits and isotopic signatures characterizing tropical communities likely need to be reassessed.”

The study was also coauthored by researchers at the Museum of Natural History in Lima; France’s University of Montpellier; and the University of Utah.

Adapted from a press release by the American Museum of Natural History.