Present day tropical plant families survived in warmer, wetter tropics 58 million years ago

Jeremy Hance
October 18, 2009

Fifty eight million years ago the tropical rainforests of South America shared many similarities with today's Neotropical forests, according to research published in the Proceedings of the National Academy of Sciences. Looking at over 2,000 fossils in Colombia from one of the world's largest open pit coal mines, scientists were able to recreate for the first time the structure of a long vanished rainforest. One inhabited by a titanic snake, giant turtles, and crocodile-like reptiles.

Despite large changes in the climate and geology since—including a cooling in the tropics—scientists found that many flora species dominating the landscape 58 million years ago, known as the Paleocene, still dominate today, including legumes, palms, avocado, and banana. Their discoveries could have important implications for researchers working to predict how the tropics will react to climate change. Many biologists have feared that a warmer earth will devastate rainforests assuming the flora will simply be unable to withstand hotter temperatures. This study may bring everyone back to the drawing board.

Leaf in the tropical rainforests of Colombia. Photo by: Rhett A. Butler.
"Neotropical rainforests have an almost nonexistent fossil record," said study co-author Fabiany Herrera, a graduate student at the Florida Museum of Natural History. "These specimens allow us to actually test hypotheses about their origins for the first time ever."

The fact that the scientists were able to examine fossils made a big difference. Researchers have had to depend on pollen analysis for information about past tropical forests, but pollen only allows identification of species to the family level, while the fossils revealed lineage all the way down to the plant's genus by examining the structure of preserved leafs. Moreover, fossils provide important information about insect types, climate, and forest structure.

Studying the many and varied fossils, researchers found that the Paleocene in Colombia was warmer and wetter than today with an estimated annual temperature of 86 degrees Fahrenheit and 126 inches of rainfall, whereas the average temperature of today's Colombian rainforest is 81 degrees Fahrenheit.

"It is also intriguing that while the Cerrejón rainforest shows many of the characteristics of modern equivalents, plant diversity is lower," Herrera said. Statistical analysis found that the diversity in the ancient tropical forests was 60 to 80 percent less than diversity today. Two different hypotheses have been put forward to explain this: one is that the period represents the epoch in which tropical forest diversification was just beginning; the other is that the forest ecosystem was in the midst of species-recovery after the extinction event that wiped out the dinosaurs and many others.

A boa constrictor in Colombia--an ancestor of Titanoboa. Photo by: Rhett A. Butler.
The open pit coal mine in the Cerrejon Formation has yielded more than plant fossils. Last year, researchers announced the discovery of the largest snake ever from the same pit. Named the Titanoboa, the snake made headlines worldwide and boggled the imagination of armchair paleontologists: longer than a city bus, the massive serpent is though to have replaced dinosaurs as the region's resident super-predator.

"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," said Florida Museum vertebrate paleontologist Jonathan Bloch, who described Titanoboa but was not part of the rainforest study.

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Jeremy Hance
mongabay.com (October 18, 2009).

Present day tropical plant families survived in warmer, wetter tropics 58 million years ago.