"With these discoveries we will be able to get more information about the process by which the continual land bridge was formed," Smithsonian geologist Camilo Montes told Reuters.

Since February, the geologists accompanied engineers on excavations to expand the canal, having been invited by the government to make sure nothing of value was destroyed.

Scientists believe the South American and Caribbean tectonic plates collided around 15 million years ago, causing volcanic activity that eventually formed a thin strip of land linking the Americas and separating the Pacific and Atlantic oceans.

The bridge was probably fully formed, in a way that mammals could walk over it, some 3 million years ago.

By comparing the Panama discoveries to fossil records from each continent, paleontologists hope to determine where individual species came from. Volcanic debris embedded in the same layer of rock as the fossils will help pinpoint the time when the animal was found on either side of the land bridge.

"We will be able to get a much more precise date for when the continents started to close together," said Montes.

The forging of the Americas resulted in a mass migration of animals, while the separation of the two oceans transformed the world's climate and prompted the development of new species.

Montes said determining exactly when this closure happened could be key to understanding the link between major changes in ocean currents and our climate, providing some insight into the impact of global warming.

"The closure could be linked to an ice age which affected North America around the same time, perhaps by altering ocean currents," Montes said.

"Some have argued the timing of the ice age is a coincidence. A more accurate timeline for the closure could tell us whether those two things were separate or linked."

The excavations are part of an archeological project to explore an area that will soon become part of the $US5.25 billion project to expand the overcrowded Panama Canal. (Editing by John O'Callaghan)