The findings were the result of a 2012 expedition to an area of rivers, mountains and rainforest in the southeastern region of the South American country, the scientists reported.
The area investigated had "virtually no human influence," the researchers said.
Particularly surprising, they said -- and encouraging -- was the number of frogs discovered.
"With many frog species rapidly disappearing around the globe, we were surprised and uplifted to discover so many frogs potentially new to science, including a stunningly sleek 'cocoa' tree frog," field biologist Trond Larsen told the BBC.
The "chocolate" frog lives on trees, using round discs on its fingers and toes to climb, the scientists said.
A ruby-colored beetle, named Canthidium cf minimum for its tiny dimensions, is possibly the second smallest dung beetle known in South America, they said.
The researchers reached the remote study area by plane, then helicopter and finally by boat and on foot, with help from local indigenous communities.
They identified 1,378 different species, saying "there are very few places left on Earth that are as pristine and untouched as this region."