Washington: You can call them the contrarians of the bat world.

While nearly all bats roost upside down from tree limbs or cave ceilings, two species of tiny “sucker-footed” bats currently found only in Madagascar roost head-up, typically in the furled leaves of a tree known as the traveller’s palm.

But these oddballs of the bat world once were much more common than they are today. Scientists reported on Tuesday the discovery in a desert in Egypt of the fossilised remains of two earlier extinct species of these bats — one that lived 37 million years ago and the other 30 million years ago.

The discovery indicates that “sucker-footed” bats swooped out of trees on the African continent for millions of years before being consigned to Madagascar, an island off Africa’s coast known for its unique collection of animals, they said.

These types of small, insect-hunting bats — about the size of a human thumb, with a wingspan measuring up to 6 inches (15cm) — have two unusual characteristics. They not only do not hang upside down but they possess unusual sticky footpads that help them roost on slick surfaces.

“They have these little adhesive pads on their thumbs and ankles that they use to cling to leaves,” paleontologist Gregg Gunnell of Duke University in North Carolina, one of the researchers, said in a telephone interview.

“So they don’t really hang upside down like most other bats — they hang upright with these pads,” he added. “The leaves curl up, so they’re hidden inside the leaf.” Scientists once thought the pads held the bats to the leaves by suction, but recent research has shown that the animals instead rely on wet adhesion, like a tree frog does.

Gunnell and his colleagues studied fossilised jawbones and teeth of the extinct bats that were unearthed in Egypt’s Western Desert. He said the remains were virtually identical to the jaws and teeth of the two existing Madagascar species.

Bats, in the history of Earth, are one of the three types of vertebrates to develop the ability to fly, joining birds and the extinct flying reptiles known as pterosaurs. The oldest remains of bats date back to about 52 million years ago.

The desert where the fossils were found probably was a forested region alongside a big river at the time the extinct bats lived, the researchers said. They lived alongside ancient primates, primitive elephants and superficially wolf-like predators known as creodonts, Gunnell said.

“We’ve assumed for a long time that they were an ancient lineage based on DNA sequence studies that have placed them close to very old groups in the bat family tree,” Nancy Simmons of the American Museum of Natural History in New York, another of the researchers, said in a statement.

But the fossil evidence of that had been lacking until now.

Previous genetic research had connected the “sucker-footed” bats to a large “superfamily” of bats that today resides mostly in South America, the scientists said.

The discovery suggests that the ancestors of the South American bats first arose in Africa, migrated to Australia, then Antarctica and into South America via an land bridge that linked the continents until 26 million years ago, they added.

The research was published in the journal PLOS ONE.