The living relative of the ancient bat species discovered in New Zealand. Mystacina tuberculata is known as a burrowing bat because it forages for food on the ground. Rod Morris

A 16-million-year-old fossil of a relative of New Zealand's burrowing bat reveals a species that walked on four limbs and was thrice the size of today's bats.

The finding suggests the descendants of the new species lived in the region for the last million years.

The remains were found on New Zealand's South Island in a region believed to be part of a warmer subtropical rainforest during the early Miocene era, between 16- and 19-million-years-ago.

The new species, Mystacina miocenalis, is related to the Mystacina tuberculata, or burrowing bats which still lives in New Zealand's old growth forests, the study says.

"Our discovery shows for the first time that Mystacina bats have been present in New Zealand for upwards of 16 million years, residing in habitats with very similar plant life and food sources," said lead author Suzanne Hand, an associate professor from the University of New South Wales (UNSW) in Australia.

The findings are reported in the journal PLOS ONE.

Three different species of bats constitute New Zealand's only native terrestrial mammals.

The burrowing bats forage on the ground under leaf-litter and snow, as well as in the air. They scuttle on their wrists and backward-facing feet, while keeping their wings tightly furled.

The ancient bat probably fed on nectar, pollen, fruit, insects and spiders like its present day descendants. Except for the size, most features including teeth and limb structures are the same.

While the modern bats weigh 40 grams, the ancient one was three times that size making it hard to hunt in the air.

"The size of bats is physically constrained by the demands of flight and echolocation, as you need to be small, quick and accurate to chase insects in the dark," said Hand.

She expects the finding to help evaluate biosecurity threats and conservation priorities for fragile island ecosystems.

Bats are important pollinators and seed dispersers that keep forests healthy.

The researchers also found many plant, animal and insect fossils in the region, suggesting the subtropical ecosystem million years ago closely resembled present day temperate ones.

The trees in both are similar. "Remarkably, the Miocene ecosystems associated with the fossil bat contain the kinds of trees used today by Mystacina for its colonial roosts," Hand said.

"Most of its food plants are also represented, as are terrestrial arthropods including a variety of beetles, ants and spiders, which these bats continue to hunt on the ground."