"You simply don't find them very often," Gunnell said, noting that the 33 specimens took decades to accumulate.
The rare samples were collected by Duke University paleontologist Elwyn L. Simons, who has been digging for animal fossils in el Faiyum since the 1960s.
Simons said Egypt, particularly el Faiyum, holds the best fossil records in Africa from the period between 37 million and 27 million years ago.
(See related news photos: "Egypt's Earliest Farm Settlement Discovered" [February 12, 2008].)
"[This ten-million-year span] is a big slice of Earth history as far as Africa is concerned," Simons said.
"[Africa] is a high-standing continent—that means it doesn't have a lot of sedimentary basins [with deposits that yield] ancient mammals—but Egypt does."
The bat fossils were set aside for many years until the recent study, which looked into the details of the teeth to determine the similarities and differences with other bat species.
From the 33 fragments, experts determined that at least six distinct species could be parsed out.
The names of the new species will be revealed in an article in an upcoming issue of the Journal of Vertebrate Paleontology.
Experts were able to link an upper and lower jawbone and several teeth from different individual bats to a previously unknown "giant" species of microbat.
"It is probably one of [the largest], if not the largest, microbat that has ever been found," said Erik R. Seiffert, a paleontologist at New York's Stony Brook University who has been digging with Simons since 1999.
"It is quite surprising actually."
Microbats are smaller than megabats, or fruit bats, which only are found in the tropical climates and rely on their sense of smell rather than echolocation.
(See related photo: "Bizarre Horseshoe Bat Photographed for First Time" [June 22, 2007].)
Gunnell said the larger microbat could have had a wingspan close to 2 feet (0.6 meter). Modern-day megabats, by comparison, have wingspans as wide as three feet (about a meter).
"It would have been loud—it would have been obnoxious," Gunnell said.
"Just going by the large echolocating bats that I know that live today many are very loud and very pushy and very boisterous. I am assuming these bats would have been, too."
The paleontologists say the diversity of species discovered in El Faiyum is the strong evidence that bat species evolved in Africa.
The prevailing theory had held that primitive bats came to resemble modern bats over a long period of evolution that occurred in the northern continents, according to Seiffert.
The only fossil bats found in the Northern Hemisphere, however, have been extremely primitive species with much less resemblance to modern bats.
"This [new study] definitely provides some really compelling fossil evidence that modern [microbats] may actually have had an African origin," Seiffert said.
Gunnell said he thinks a primitive bat species made its way to Africa some 50 million years ago, "then differentiated into these more modern forms."
"Africa may have played an important role as kind of an incubator for the evolution of new animals," he said.
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