A tiny fossil collected about 500 kilometers from the South Pole indicates that Antarctica was once home to a type of fly that scientists long thought had never inhabited the now-icy, almost insectfree continent.
The diverse group of fly species called schizophorans includes houseflies, fruit flies, and flesh-burrowing blowflies. Previously, many researchers held that schizophorans evolved elsewhere and long after Antarctica had become geographically isolated from other major landmasses.
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The fragmentary fossil isn’t part of an adult fly but a portion of a puparium, the shell that hardens from a larva’s skin and protects the pupa as it develops into an adult insect. The puparium’s tough material–which includes chitin, a natural polymer found in insect exoskeletons and crab shells–fostered its fossilization, says Allan C. Ashworth, a paleoentomologist at North Dakota State University in Fargo.
Several of the fossil’s features, such as a single pair of round breathing holes called spiracles, mark the puparium as belonging to the schizophoran group. The puparium was probably 5 to 7.5 millimeters long, which would make the adult insect the size of today’s housefly, says Ashworth. He and F. Christian Thompson, an entomologist at the Smithsonian Institution in Washington, D.C., describe their find in the May 8 Nature.
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Ashworth excavated the relic from a 2-meter-thick outcrop of siltstone along Antarctica’s Beardmore Glacier. Other fossils unearthed there include marine microorganisms, algae, mosses, wood, leaves, freshwater mollusks, fish, and a variety of insects. This assortment of fossils, some of whose ages are known, suggests that the outcrop’s silt layers were laid down along the margins of a glacier near sea level between 3 million and 17 million years ago, says Ashworth.
The presence of some creatures, including the flies, suggests that summer temperatures in the region rose to about 5C, says Ashworth. Also, the fossil puparium indicates that the area hosted a breeding population of flies, not just individual migrants blown onto an inhospitable continent.
The distribution and diversity of schizophorans in the Northern Hemisphere suggest that the group evolved there, says Brian M. Wiegmann, an entomologist
at North Carolina State University in Raleigh. His genetic analyses suggest that schizophorans split from their sister groups of flies at least 30 million years ago.
The southern landmasses of Australia, Africa, and South America separated from a megacontinent called Gondwanaland about 80 million years ago, leaving Antarctica astride the South Pole. So, the new find challenges scientists to explain how the flies made their way across thousands of miles of ocean.
Ashworth and Thompson suggest that the species may have colonized Antarctica during an era when sea levels were low and the distance from South America smaller.
Alternatively, they note that the fossil they found may have descended from a surprisingly ancient species that evolved in Gondwanaland.
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