The world's tiniest known vertebrate – a species of frog that fits in the centre of a dime – has been discovered in New Guinea.

At 7.7 millimetres, Paedophryne amauensis replaces Paedocypris progenetica, an Indonesian fish averaging more than eight millimetres.

"It was particularly difficult to locate [the frog], Paedophryne amauensis, due to its diminutive size and the males' high pitched insect-like mating call," said Chris Austin of Louisiana State University who led a team of scientists on a three-month expedition to New Guinea, the world's largest and highest tropical island.

"The size limit of vertebrates, or creatures with backbones, is of considerable interest to biologists because little is understood about the functional constraints that come with extreme body size, whether large or small."

The team discovered two new species of frogs; their findings were published Wednesday in the journal PloS ONE.

There are currently 60,000 identified vertebrates, the largest being the blue whale with an average size of more than 25 metres, and the previous smallest being the Indonesian fish.

Scientists had believed that extreme size in vertebrates was associated with aquatic species because the buoyancy of water offered support and facilitated the development of very large or very small creatures.

However, since both new species of frogs are terrestrial, that theory no longer holds.

"The ecosystems these extremely small frogs occupy are very similar, primarily inhabiting leaf litter on the floor of the tropical rainforest environments," Austin said.

"We now believe that these creatures aren't just biological oddities, but instead represent a previously undocumented ecological guild. They occupy a habitat niche that no other vertebrate does."