Fossils discovered on Barro Colorado Island reveal that Panama once had a giant mangrove forest with trees reaching heights of 25 to 40 meters, buried under volcanic mud approximately 23 million years ago.
A fossilized wood specimen on Barro Colorado Island. (Photo: Christian Ziegler)
An ancient mangrove forest with many trees reaching up to 40 meters was discovered over 20 million years after volcanic mud flowed over the area now known as Panama, according to research published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. Researchers first found the fossils in 2018 during a geological expedition on Barro Colorado Island (BCI). The island is located in the artificial Gatun Lake of Panama, where thousands of ships pass through each year while traversing the Panama Canal.
BCI was once part of a partially flooded hillside in 1913 when engineers built a dam on the Chagres River to create the canal, becoming a nature reserve in 1923. Today, the tropical forests of BCI are among the most studied in the world.
“We never imagined there would be fossilized wood forests on BCI, even though countless scientists have surveyed the island over the past decades; no one had ever mentioned them,” said Carlos Jaramillo, a co-author of the study and a geologist at the Smithsonian Tropical Research Institute in Panama. “The fossils are very difficult to distinguish from decaying wood in the forest because they look like rotting logs.”
Despite their appearance, the mangrove fossils are well preserved, according to Jaramillo. This is due to a volcanic eruption that buried the tree trunks about 23 million years ago during the Miocene epoch (5.3 to 23 million years ago), slowing the decomposition process and preserving the forest over time.
“The fossilized wood specimens, also known as petrified wood, contain a wealth of information,” said Camila Martínez Aguillón, the lead researcher and a paleoecologist at EAFIT University in Colombia. The cell structures were mineralized over geological periods, remaining intact and providing researchers with a rare opportunity to explore the past.
The research team examined 121 fossilized wood specimens exposed in a small creek on the island and found that 50 specimens belonged to a previously unknown species of tree, named Sonneratioxylon barrocoloradoensis. This newly discovered fossilized tree species resembles mangroves that grow in Southeast Asia, Australia, New Zealand, and various islands, as well as in several locations across Africa.
While the canopies of most living mangrove trees reach heights of 13 meters, S. barrocoloradoensis grew up to 25 to 40 meters tall. This ancient tree species likely evolved survival strategies similar to those of present-day mangroves, preferring brackish water over high salinity seawater. The forest was situated at the edge of a narrow peninsula connecting central Panama to North America before the Isthmus of Panama formed, approximately 3 to 23 million years ago.
