A boy who found a fossil fragment in a creek stumbled upon an entirely new species, researchers say.
The Costa Rican boy who made the find 17 years ago uncovered a species of giant sloth bear that lived 5.8 million years ago.
Researchers in the country say the landmark discovery of the previously unknown giant sloth bear, could rewrite the history of the American continent.
The creature would have stood at an incredible three metres (9.84 feet) tall and weighed about four tonnes, researchers from National Museum of Costa Rica said.
The fossil fragments were found near San Gerardo de Limoncito, a town in southern Costa Rica, and constitute an entirely new species, Ana Lucia Valerio, a geologist and paleontologist at the museum confirmed.
The new species has been named Sibortherium ka, which literally means "beast of Sibo" in the local Bribri language.
Sibo is the name of the area where the fossil fragments were initially discovered.
"It is a new species for science and for the world", Ms Valerio said.
The Costa Rican giant sloth bear belongs to the family of Megatheriidae, according to Ascanio Rincon, palaeontologist at Venezuela's Institute of Scientific Investigations, which also took part in the research project.
"It weighed around four tonnes and was herbivorous. It is believed that it would stand on its hind legs and use its paws to reach the tree canopies to feed", said Ms Valerio.
Despite its impressive size, the Sibortherium ka was still somewhat smaller than the American giant sloth bear, Megatherium americanum Eremotherium.
The species reached a height of four metres and a weight of seven tonnes.
Remains of the American giant sloth bear have been found in the US, but they are millions of years younger than the newly-discovered Costa Rican species.
The Costa Rican giant sloth bear is the first of its family to have crossed from South America to Central America, according to the researchers.
Other fossilised remains of giant sloth bears have previously been discovered in Costa Rica, but not of the same age.
The researchers could identify the fossil as a new species through comparison with similar fossils from the Megatheriidae family tree.
"What makes this fossil distinct from other members of the Megatheriidae family is the ankle bones, which are wider, and the first mandibular tooth, which means the first inferior tooth," their statement said.
The new species probably came from Costa Rica, in Central America, from South America as part of the larger migration of prehistoric animals to North America.
The movement is known as the great American biotic interchange, which took place an estimated three million years ago.
But the new discovery raises the possibility the interchange began much earlier than previously thought.
If the giant sloth bear was present in Costa Rica 5.8 million years ago, there must have been a land bridge between north and south already back then, the research team has concluded.
Up until now, it was believed that the Panama Isthmus was established during the Pliocene period some 3 million years ago/
But if it was possible for the Sibortherium ka to migrate from South America to Costa Rica at least 5.8 million years ago, the researchers believe there must have been a connection much earlier that allowed animals to move between the land masses.
The research project was initiated back in 2003 after reports a Costa Rican boy had found some highly interesting rocks in a local creek and brought them to his house.
National Museum researchers visited the boy's home several times to recover a number of fossils, then started a systematic search in the place where the rocks had been found, according to Ms Valerio.
"A visit to the boy's house revealed that the strange rocks were actually fossils of vertebrates," she said.
The fossils were transferred to the museum for examination and the area where the rocks had been found was carefully examined over a seven-year period.
The team could only work during the dry season, as the area is located in an area with much rain, high humidity and unstable ground, making it prone to mudslides.
The researchers concluded the work in 2010, Valerio said.
"5.8 million years ago this area was a great estuary with a wealth of animal life," Ms Valerio said.
In addition to the giant sloth bear, fossils from three different species of horses were found, as were a single camel species, a mastodon species, and two species of peccary, as well as fossils from dolphins, crocodiles, turtles, birds, fish and a giant armadillo.