Fossilized remains of a bat species that walked on four limbs and was triple the size of today's screeching mammals has been discovered in New Zealand.

The new species, Mystacina miocenalis, thrived 16 million years ago in what was a subtropical rain forest and was described in the journal PLOS ONE. It is related to another bat, Mystacina tuberculata, which still lives in New Zealand's old growth forests.

"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 Suzanne Hand, the lead author of the study, a vertebrate paleontologist at the University of New South Wales in Australia.

These are Mystacina miocenalis fossil teeth. Rod Morris

New Zealand's only native terrestrial mammals are three species of bat, including two belonging to the Mystacina genus - one of which was last sighted in the 1960s. Unlike their flying cousins, they are known as burrowing bats because they 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.

Scientists long believed the bats had an ancient history and reached as far as Australia. But until now, the oldest fossil of a Mystacina bat in New Zealand was from a cave in South Island, dating to 17,500 years ago.

"This helps us understand the capacity of bats to establish populations on islands and the climatic conditions required for this to happen," Hand said. "Bats are important pollinators and seed dispersers that keep forests healthy. Understanding the connectivity between the bat faunas of different landmasses is important for evaluating biosecurity threats and conservation priorities for fragile island ecosystems."

The new species is not so different than its modern-day relative. It has similar teeth, hinting at a diet that included nectar, pollen and fruit, as well as insects and spiders. Limb bones found in the deposit also showed similar structures specialized for walking.

The big difference is their size. Their modern bats weigh 40 grams; the ancient one was three times that size.

"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," Hand said. "The unusually large size of this bat suggests it was doing less in-flight hunting and was taking heavier prey from the ground, and larger fruit than even its living cousin."

Along with the bat, the researchers found scores of plant, animal and insect fossils, suggesting the 16-million-year-old subtropical ecosystem bore resemblance to the more temperate one that exists today.

"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."