A unique 46-million-year-old mosquito fossil with a belly full of dried blood has been found in a Montana riverbed, US researchers said Monday.

"It is an extremely rare fossil, the only one of its kind in the world," said Dale Greenwalt, lead author of the study in the Proceedings of the National Academy of Sciences.

Cutting-edge instruments detected the unmistakable traces of iron in her engorged abdomen, but just what creature that blood came from is a mystery since DNA cannot be extracted from a fossil that old.

Greenwalt said it might have been blood from a bird, since the ancient mosquito resembles a modern one from the genus Culicidae, which likes to feed on our feathered friends.

"But that would be pure speculation," said Greenwalt, a retired biochemist who volunteers at the Smithsonian Museum of Natural History in Washington.

While far from the oldest known mosquito fossil -- that honor goes to a 95-million-year-old mosquito in amber in Myanmar -- entomologist Lynn Kimsey of the University of California said it was "a very exciting find."

"Having an actual blood-engorged female mosquito associated with males in the same fossil formation is hugely unlikely," said Kimsey, who was not involved in the research.

"Here, the authors have been able to use mass spectrometry to elucidate the abdominal contents and thus blood-feeding in a fossil some 40 million years old," she added, describing the research as "impressive."

Greenwalt said he became fascinated with fossilized insects several years ago.

He learned about Master's student Kurt Constenius, who described his discoveries of fossilized insects along a remote Montana riverbed in an obscure geological journal more than two decades ago.

Greenwalt and Constenius discussed the fossil grounds, which lie near the Flathead River along the western boundary of Glacier National Park.

In recent years, Greenwalt has gone there every summer to collect pieces of shale from an area that is slowly eroding, exposing the sediments of an old lake.

"The rock is in very thin layers, a millimeter or two" (.04 or .08 inches), explained Greenwalt.

"With a razor blade, I can split that rock even further and expose these virgin surfaces and that is where I find the fossils."

The fossil described in PNAS came not from Greenwalt's outings, but from a collection of fossilized insects languishing in Constenius's basement since the 1980s, and which he and his family had donated to the Smithsonian museum.

"As soon as I saw it, I knew it was different," Greenwalt told AFP.

The mosquito itself is only about 0.2 inches in size. Somehow, the fragile creature ate its last meal, filling its abdomen until it was nearly ready to burst like a balloon.

Then, perhaps as the mosquito was flying over an algae-coated lake, it became caught in that mucus, enveloped in microbes that protected it from degrading, and eventually sank deep into the sediment of the lake.

Three dozen mosquito fossils have been collected from the fossil area in northwestern Montana, but no others have shown signs of blood engorgement.

Experts used a technique called non-destructive mass spectrometry to identify the source of the iron in her abdomen as heme, the molecular entity that allows hemoglobin to transport oxygen.

The method can only be used on flat surfaces, however, and would not be useful for analyzing mosquitoes preserved in amber, Greenwalt said.

The 1993 movie "Jurassic Park" showed scientists extracting dinosaur DNA from the abdomen of a mosquito trapped in amber.

Not only was the scene fictional -- no one has ever been able to extract DNA from a fossil so old -- Greenwalt said the mosquito pictured was a male, and male mosquitoes do not feast on blood.

"Like a lot of science fiction, it kind of predicted what we might be looking at in the future," he said.