A Florida Museum of Natural History paleobotanist and other researchers have used a rich cache of plant fossils discovered in Colombia to provide the first reliable evidence of how Neotropical rainforests looked 58 million years ago.
Scientists from the Smithsonian Institution, University of Florida and two other universities found that many of the dominant plant families existing in today’s Neotropical rainforests – including legumes, palms, avocado and banana – have maintained their ecological dominance despite major changes in South America’s climate and geological structure.
The study, which appeared Oct. 13 in the online edition of the Proceedings of the National Academy of Sciences, examined more than 2,000 megafossil specimens, some nearly 10 feet long, from the Cerrejón Formation in northern Colombia. The fossils are from the Paleocene epoch, which occurred in the 5- to 7-million-year period following the massive extinction event responsible for the demise of the dinosaurs.
“Neotropical rainforests have an almost non-existent fossil record,” said study co-author Fabiany Herrera, a graduate student at the Florida Museum. “These specimens allow us to actually test hypotheses about their origins for the first time ever.”
Herrera said the new specimens, discovered since 2003, also provide information for future studies that promise to provide an even stronger understanding of the plants that formed the earliest Neotropical communities.
Many previous assumptions and hypotheses on the earliest rainforests are based on studies of pollen fossils, which did not provide information about climate, forest structure, leaf morphology or insect herbivory.
Home to Titanoboa
The new study provides evidence Neotropical rainforests were warmer and wetter in the late Paleocene than today but were composed of the same plant families that now thrive in rainforests. “We have the fossils to prove this,” Herrera said.
The site, one of the world’s largest open-pit coal mines, also yielded the fossil for the giant snake known as Titanoboa, described by UF scientists earlier this year.
“These new plant fossils show us that the forest during the time of Titanoboa, 58 million years ago, was similar in many ways to that of today,” said Florida Museum vertebrate paleontologist Jonathan Bloch, who described Titanoboa but was not part of the rainforest study. “Like Titanoboa, which is clearly related to living boas and anacondas, the ancient forest of northern Colombia had similar families of plants as we see today in that ecosystem. The foundations of the Neotropical rainforests were there 58 million years ago.”
Megafossils found at the Cerrejón site made it possible to use leaf structure to identify specimens down to the genus level. This resolution allowed the identification of plant genera that still exist in Neotropical rainforests. With pollen fossils of a similar age, most specimens can be categorized only to the family level.
This resolution has been important because it has identified plant genera that have existed in Neotropical rainforests up until today, Herrera said. The fossil specimens have the same leaf morphology found today in Neotropical rainforests: big leaves with smooth edges.
But researchers were surprised by the relative lack of diversity found in the Paleocene rainforest, Herrera said. “It’s intriguing that while the Cerrejón rainforest shows many of the characteristics of modern equivalents, plant diversity is lower.”
Statistical analyses showed that the plant communities found in the Cerrejón Formation were 60 percent to 80 percent less diverse than those of modern Neotropical rainforests. Evidence of herbivory also showed a low diversity level among insects, which appear to have lacked highly specialized groups.
The study’s authors say the lack of diversity indicates either the very beginning of rainforest species diversification or the recovery of existing species from the Cretaceous extinction event. It could also be a result of both, they said.
The researchers estimate the Paleocene rainforest received about 126 inches of rainfall annually and had an average annual temperature greater than 86 degrees. The Titanoboa study, which used different methods, estimated an average temperature between 89 and 91 degrees. Today the region’s temperatures average about 81 degrees.
Herrera found the fossil deposit at Cerrejón in 2003 as an undergraduate. No one had seen Paleocene plant fossils from the Neotropics before, he said. The site yielded not only leaves but flowers, seeds and fruits. Some of the specimens even have their cell structure intact.
Once Herrera arrived at UF a year later, he and other researchers who contributed to the PNAS article alerted Bloch to the site’s potential for holding vertebrate specimens.
Herrera is now comparing fossils from the Cerrejón site to specimens from other Paleocene sites in Colombia to see how far the early rainforest extended geographically. He and other researchers are also examining fossils from a Cretaceous site to determine differences in plant community composition before and after the extinction event.
Preliminary findings suggest that the Paleocene rainforest was indeed extensive and that species composition in Cretaceous rainforests were significantly different. In particular, legumes are absent from the Cretaceous sites but are the most diverse family found at Cerrejón. It appears legumes found a niche following the Cretaceous extinction and diversified rapidly, Herrera said.
By Bill Kanapaux | More articles by Bill Kanapaux
• Learn more about the Paleobotany & Palynology Collection at the Florida Museum.