The monkeys started it. About 3 million years ago, one of the most epic ecological struggles of all time – between the animals of North and South America – was at its peak.

Now a fossil from Panama suggests the opening salvo actually came some 18 million years earlier, courtesy of a monkey invader.

The appearance of the Panama isthmus, generally dated to about 3 million years ago, provided a land connection between North and South America for the first time. Once it formed, animals from the north – including sabre-toothed cats, deer and horses – surged south, as southern species – such as terror birds, ground sloths and armadillos – pushed north.

The ecological battles, dubbed the Great American Interchange, raged for generations. Many species were lost.

Now it seems that the first animal to cross continents did so long before the isthmus emerged.

Jonathan Bloch at the University of Florida, Gainesville, and his colleagues have been studying 21-million-year-old sediments in Panama that were deposited on the North American side before the isthmus formed. They found seven teeth belonging to a South American monkey (see photo, below).

Aldo Rincon

Bloch says the researchers were initially stumped by the teeth, which clearly belong to the 21-million-year-old deposits and weren’t a later addition. “How could they possibly be those of a monkey given what almost seems like a law at this point: there are no monkeys in North America prior to the Great American Interchange?”

But a careful examination confirmed the fossils’ affinities. “Unambiguously, these are the teeth of a South American monkey that somehow managed to do what no other mammal could do at that time – get across the Central American Seaway,” says Bloch.

At the time the seaway separating North and South America was at least 160 kilometres wide, he says.

The monkey – Panamacebus – is an ancient member of one of the five families of monkey found in the Americas, a family that also includes capuchins and squirrel monkeys. It wasn’t the only animal that crossed between North and South America before the Panama isthmus emerged, but it is by far the oldest.

“Prior to this discovery, the oldest fossil evidence for mammalian dispersal from South to North America is 9-million-year-old sloths,” he says.

“The discovery is absolutely astounding,” says Richard Kay at Duke University in Durham, North Carolina. “A 21-million-year-old fossil monkey appearing in Central America is extraordinary and a bolt from the blue.”

Monkey wanderlust

Just as surprising is that monkeys, having made it to North America so early, then apparently expanded no further into the continent.

Monkeys have a tremendous wanderlust. They first appeared in Africa, and some of them somehow crossed the Atlantic to South America about 26 million years ago, where they spread rapidly in all directions. “By 20 million years ago we see them all the way down to the southern tip of Argentina,” says Bloch.

It would make sense for those first North American monkeys to repeat that history by spreading far and wide – but they didn’t. “What strange animals these are,” says Bloch.

The researchers speculate that it might have been the lack of forests that kept the monkeys at bay. Panama was covered in South America-derived rainforests 21 million years ago, but those forests weren’t found further north.

Kay isn’t convinced – he says the idea needs to be developed and fleshed out to be persuasive. “They could be right, but it’s early days.”

Source: New Scientist