Fossilized teeth show bat family belongs to primitive lineage, had broad range
This 30-million-year-old fossilized jawbone, seen from both sides, is less than half an inch in length, but it has a lot to say about bat evolution. Photo credit: Gregg Gunnell
Durham, NC - Today, Madagascar sucker-footed bats live nowhere outside their island home, but new research shows that hasn't always been the case. The discovery of two extinct relatives in northern Egypt suggests the unusual creatures, which evolved sticky footpads to roost on slick surfaces, are primitive members of a group of bats that evolved in Africa and ultimately went on to flourish in South America.
A team of researchers described the two bat species from several sets of fossilized jawbones and teeth unearthed in the Sahara. The findings, reported Feb. 4 in the open-access journal PLOS ONE, represent the first formal description of the family in the fossil record and show the sucker-footed bat family to be at least 36 million years older than previously known.
"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," said Nancy Simmons, co-author and curator-in-charge of the American Museum of Natural History's Mammalogy Department. But until now, scientists lacked any fossil evidence to confirm it.
The discovery also shows that, like many island-dwelling, relict species, sucker-footed bats have not always been confined to their present range -- they once swooped through the African skies.
Today, the sucker-footed bats consist of two species, Myzopoda aurita and M. schliemanni, endemic to Madagascar. In contrast to almost all other bats, they don't cling upside-down to cave ceilings or branches. Sucker-footed bats roost head-up, often in the furled leaves of the traveler's palm, a plant in the bird-of-paradise family. To stick to such a smooth surface, the bats evolved cup-like pads on their wrists and ankles. Scientists previously suspected the pads held the bats up by suction, but recent research has demonstrated the bats instead rely on wet adhesion, like a tree frog.
"The fossils came from a fascinating place out in the Egyptian desert," said Gregg Gunnell, director of the Duke University Lemur Center's Division of Fossil Primates. He said the Fayum Depression in Egypt's Western Desert, where the team completed their fieldwork, is filled with the remnants of ancient and modern history: temples built for great pharaohs, Roman city ruins and even the hulls of World War II tanks. The extreme aridity helps preserve these relics as well as the famous fossil deposits where these bat teeth and jawbones were found.
The two extinct species, Phasmatonycteris phiomensis and P. butleri, date to 30 and 37 million years ago, respectively, when the environment was drastically different. Northern Africa was more tropical, said Simmons, and home to a diverse range of mammals, including primates and early members of the elephant family.
"The habitat was probably fairly forested, and there was likely a proto-Nile River, a big river that led into the ancient Tethys Ocean," said Gunnell. The fossilized teeth imply that, like their living relatives, the ancient bats fed on insects.
It's impossible to know from the fossils if the extinct species had already evolved their characteristic sucker-feet, but the teeth shed light on another aspect of bat evolution. The presence of sucker-footed bats in Africa at least 37 million years ago supports the theory that this family is one of the most primitive members of a lineage that now dominates South America.
From vampires to fruit- and nectar-eaters to carnivores, the majority of South America's bats belong to one large superfamily, known as Noctilionoidea. "We think that the superfamily originated in Africa and moved eastward as Gondwana was coming apart," Gunnell said. "These bats migrated to Australia, then actually went through Antarctica and up into South America using an ice-free corridor that connected the three continents until about 26 million years ago."
According to this hypothesis, the sucker-footed bat fossils showed up right where scientists expected to find them: at the literal and figurative base of the Noctilionoidea family tree.
"Now, we can unambiguously link them through Africa," Simmons said.
The third author on this paper, Erik Seiffert, received his Ph.D. from Duke and now works as an associate professor at Stony Brook University.
This research received support from National Science Foundation grants BCS-1231288 and DEB-0949859.