As our Museum writers, scientists, and exhibition developers produce the script for the new Fossil Hall, they’re discovering lots of fascinating stories about how life evolved. We’ll be sharing some of our favorites here.
In March 2014, I visited a fossil excavation project along the Panama Canal run by the Smithsonian Tropical Research Institute. We don’t normally search for fossils in the lush tropics, but the current expansion project to widen the channels and build new locks is exposing more rocks. This gives scientists a rare opportunity to find otherwise inaccessible fossils. It was both strange and wonderful to stand on the 100-year-old canal and look at fossils that were far older. Panama’s story is essentially the tale of multiple waves of invasive species. The formation of this narrow strip of land millions of years ago allowed organisms from North and South America to cross into new environments. Today, humans bring animals and plants to Panama from distant ports. Along the banks of the canal, invasive species separated by millions of years meet—elephant grass native to South Asia slowly encroaches on fossils that reveal new information about our planet’s past.
Lay of the Land
Panama’s geology is crucial to this story of species migration. Before it was an isthmus, Panama was a peninsula jutting off of North America. Between 21 and 18 million years ago, the distance between North and South America was about 124 miles (200 km) and covered in deep seas. The sediments along the canal reveal a series of drastic transformations during this time: underneath volcanic basalt, paleontologists have uncovered a shallow marine environment sandwiched between two terrestrial layers. These layers tell scientists that within the span of a few million years this region was first above sea level, then underwater, then above sea level again, and later covered in lava—all due to tectonic activity. Fast-forward to 1914, when the landscape again changed dramatically (this time thanks to humans): where a land bridge once separated two oceans, the United States government divided the continents by completing a series of canal locks connected by an enormous artificial lake.
Around three million years ago, land animals migrated in earnest across the Isthmus of Panama. Scientists call this the Great American Biotic Interchange (or GABI). Camels, elephant-like gomphotheres, tapirs, deer, foxes, rabbits, bears, peccaries, and cougars moved into South America while large flightless birds, giant ground sloths, capybaras, armadillos, porcupines, and opossums came north. This migration totally changed the face of the fauna on both continents—in fact, many animals that we think of as stereotypically South American (such as llamas) actually have North American origins.
The rocks along the canal are at least 15 million years older than GABI, so scientists expected to find North American animals like horses, camels, bear-dogs, and raccoons. They didn’t expect South American species, but that is what they are now unearthing. Take the 19.3 million year old boa fossil they found. Boas can swim, but crossing a 124-mile (200 km) seaway is an impressive feat. How did it get to Panama? Did it “island-hop,” or raft across on storm-swept debris? Or was there an older, more solid connection between North and South America? Fossils like this one are pushing back the timing of the formation of the isthmus, and making scientists re-evaluate past assumptions about when and how species migrated.
Understanding how ancient animals fared in new environments is relevant to our world today because so many plants and animals follow on the heels of humans. These species change ecosystems wherever they’re introduced. For example, elephant grass, or canal grass, grows everywhere along the canal. No one seems to know when it was introduced, though canal workers might have planted it to control erosion along the banks of the original canal cuts. Today it has become a pest: elephant grass is the first plant to appear in new clearings, driving out the native pioneer communities. One scientist is actually doing genetic research to determine if all the grass came from a single introduction or represents multiple arrivals. But that’s a story for another day!
Here on the banks of the Panama Canal the present tangles with the past. Elephant grass—a recent invasive—grows so quickly that it covers the fossil dig sites on the banks of the expanded canal. Paleontologists have only a narrow window of time to recover fossils before the plants take over again and cover the past for good.
By Juliana Olsson, Exhibits Writer/Editor, National Museum of Natural History