Plant fossils give first real picture of earliest Neotropical rainforests

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The study, which appears this week 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 nonexistent fossil record," said study co-author Fabiany Herrera, a graduate student at the Florida Museum of Natural History on the UF campus. "These specimens allow us to actually test hypotheses about their origins for the first time ever."

Herrera said the new specimens, discovered in 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.

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. "It is also intriguing that while the Cerrejón rainforest shows many of the characteristics of modern equivalents, plant diversity is lower."

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, specimens can be categorized only to the family level.

Researchers were surprised by the relative lack of diversity found in the Paleocene rainforest, Herrera said. 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.

The study's authors say the relative lack of diversity indicates either the beginning of rainforest species diversification or the recovery of existing species from the Cretaceous extinction event.

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 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 is also examining fossils from a Cretaceous site to determine differences in composition before and after the extinction event.

University of Florida