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The Origin of Modern Tropical Rainforest
Tropical rainforests are famous for their great diversity. Many hypotheses have been proposed to explain the profusion of species, but most are difficult to test because tropical rainforests have a very poor fossil record. Rock exposures are rare in areas that are still mostly covered by forest.
Since 2003 I have been working with Carlos Jaramillo of the Smithsonian Tropical Research Institute and Fabiany Herrera of the University of Florida, studying fossils from the Cerrejón coal mine in northeastern Colombia. Plant fossils from this mine provide a unique window into rainforests that lived ~60 million years ago, during the Paleocene epoch.
The Cerrejón coal mine provides huge areas of exposed rock that we can explore for fossils. This pit is several km long and hundreds of meters deep.
Paleontological work in the mine has to be carefully supervised for safety reasons. These trucks hold hundreds of tons of rock and are so large that the driver can hardly see smaller vehicles close by.
The crew collecting fossils in the shale beneath a coal bed that has already been mined. Fabiany Herrera at top.
Part of Fabiany’s collection of fossil plants in 2004, here on the floor at the mine office. Note how large the leaves are – typical of living and Paleocene tropical rainforests. (My boot is ~10 cm wide.)
During our studies at the Cerrejón we have found that not only do the fossil leaves have similar shapes and sizes as those in modern day tropical rainforest, but also that the same plant families are common and diverse in modern and Paleocene rainforests. Many of these plant lineages have a long evolutionary history in this biome, which may account for their high species diversity. Among the most important plant groups are beans (family Fabaceae), palms (family Arecaceae), and laurels (family Lauraceae). The hibiscus family (Malvaceae) is also common among Cerrejón fossils.
A fossil palm leaf (Arecaceae).
A fossil leaf of the informal taxon CJ1 (family Fabaceae).
A fossil leaf of the informal taxon CJ22 (family Lauraceae). Under a microscope you can see small droplets of amber-colored resin on this leaf, a common feature in living laurels.
A fossil leaf of the informal taxon CJ26 (family Malvaceae).
Plants are not the only fossils found at the Cerrejón mine. In 2009 our colleagues reported on fossil vertebrae of a giant snake they called Titanoboa. They estimated the snake was about 13 meters long and weighed more than a ton. Apparently it looked something like a living anaconda, and probably ate crocodiles.
The vertebra on the right is from a recently living 4 m long anaconda, the vertebra on the left is from Titanoboa. (photo courtesy of J. Bloch, U. Florida)
An artist’s rendition of Titanoboa and some of the crocodile-like creatures it lived with, as well as the giant freshwater turtles also represented by fossils from the Cerrejón mine. Note the abundant palm trees! (reconstruction by Jason Bourke)
One irony about Titanoboa fossils from Cerrejón – they show that Hollywood doesn’t always make its monsters big enough. In the B movie Anaconda (with Jennifer Lopez and Ice T), the fake monster snake was about the same size as the fossil Titanoboa.
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