by Nathan Jud, Florida Museum of Natural History
The ongoing expansion of the Panama Canal provides an exceptional opportunity to collect fossils in the tropics where abundant vegetation often obscures outcrop. Since 2010, researchers from the University of Florida, the Smithsonian Tropical Research Institute, the New Mexico Museum of Natural History and beyond have collected, prepared, cataloged, and studied thousands of fossils from exposures along the Canal.
Over the course of the last several years, some of the most exciting finds have been the variety of large-bodied mammals with North American affinities such as horses, camels, rhinos, and beardogs. However, as Panama emerged during the early Miocene it was something of a foreign land to the animals of North America – fossils tell us that the tropical forest was composed almost entirely of South American plants that dispersed across the Central American Seaway long before North and South America were connected.
Fossil wood and fruits are common along the Panama Canal today, and one of the most common fruits in the Cucaracha Formation are those of Parinari (coco-plum). The fossils are technically endocarps (the inner-most layer of the fruit) and they are somewhat like a peach-pit with two seeds. These fossil endocarps are among the oldest records of the genus, and the family Chrysobalanaceae as a whole. Chrysobalanaceae comprise one of the 20 most abundant families of tropical trees and they are found throughout the tropics. Parinari is one of the most common members of the family, and roughly half of all Parinari species are native to tropical South America today. Their fleshy fruits are consumed and dispersed by a wide variety of mammals and birds, including parrots, bats, elephants, tapirs, people, and many others.
In a recent paper, postdoc Nathan Jud, along with PIRE research assistant Chris Nelson and former graduate student Fabiany Herrera formally described the Parinari fossils from the Canal and named the new species Parinari panamensis. With this foundation, they also reviewed the fossil record of the family and the biogeographic history of these important trees. To study the fossils they sliced the permineralized fruits and then made thin sections in order to examine the anatomy under the microscope with transmitted light.
Several lines of evidence suggest that Parinari originated in Africa, but about half of the species alive today are native to tropical South America. Until now, it was not clear whether arrival and diversification of Parinari in the Neotropics was a recent phenomenon, or whether there has been a long time for this diversity to accumulate. The oldest evidence of Chrysobalanace worldwide are late Oligocene pollen fossils (~23-25 million years old), but it is difficult to distinguish the different genera. Now the presence of the genus Parinari in the Cucaracha Formation along the Panama Canal provides a minimum date of 19 million years ago for the arrival of Parinari in the Neotropics. Parinari has probably been diversifying in South America over the last 19 million years.
This story provides an interesting contrast with an earlier study by Fabiany Herrera and colleagues who showed that another common fossil fruit, Sacoglottis (Humiriaceae), originated and diversified in the Neotropics, but it also dispersed across the Atlantic Ocean, and one species is native to West Africa today. In both cases, the dispersal across the Atlantic was recent enough that it must have occurred via oceanic dispersal long after the breakup of South America and Africa during the Cretaceous Period. Jud, Nelson and Herrera also compiled all reports of megafossils from the literature that have been attributed to Parinari or other members of Chrysobalanaceae and evaluated each one for age and reliability. These data now serve as a framework for dating major events in the evolutionary history and dispersal history of not only Parinari but the whole family Chrysobalanaceae.
Although the tropical forests of Panama were quite different from the forests further North in what is now Mexico, the U.S., and Canada, the abundance of fruit and browse appears to have suited the North American mammals well, and they may have been pleased to consume and help disperse plants like Parinari.