Scientists in Brazil say they've discovered the first new species of river dolphin in nearly 100 years.

Researchers from the Federal University of Amazonas in Manaus, Brazil, found a small group of river dolphins, also known as botos, in the Amazon River system.

Swimming through the Araguaia basin, the dolphins were separated from other populations by a narrow canal and series of rapids.

News of the find, published in the journal Plos One, prompted members of the research community to request endangered species status for the newly-introduced mammals.

But, after more analysis and DNA testing, Tomas Hrbek and his team determined the group was actually a species -- which they have dubbed the Araguaian boto -- previously believed to have gone extinct.

The scientists said the Araguaian boto probably was separated in its development from other dolphin species over two million years ago. A series of morphological and genetic differences inidcate "strong evidence that individuals from the Araguaia River represent a distinct biological group," the study said.

With three of the four previously-known varieties categorized as "threatened" on the Red List of at-risk species issued by the International Union for Conservation of Nature, river dolphins are among the rarest animals in the world.

The Araguaian dolphin is the first new discovery of a true river dolphin since 1918, when scientists first described the species Lipotes vexillifer, also called the Chinese baiji or Yangtze River dolphin, which was declared "functionally extinct" in 2006 after a survey mission didn't turn up a single specimen.

Experts reported seeing an estimated 120 of the Araguaian animals during a 12-week study; based on those observations, they calculate as few as 600 left across the whole river basin.

Hrbek and his research colleagues fear the Araguaian dolphins could be headed toward the same fate as the Chinese group, since fishermen along the river are known to kill dolphins in an effort to protect their fish supplies.

"Since the 1960's the Araguaia River basin has been experiencing significant anthropogenic pressure via agricultural and ranching activities, and the construction of hydroelectric dams, all of which have had negative effects on many biotic and abiotic aspects of the functioning of the Araguaia River ecosystem," the report said.