Scientists Find 2 Mating Flies Trapped in Prehistoric Amber

Amber, or fossil tree resin, exists in rock layers all over the world. But for some reason, paleontologists never found much of it in Australia or New Zealand.

That’s why it was exciting when Jeffrey Stilwell, a paleontologist at Monash University in Melbourne, and a group of students, found a stash in southern Australia in 2011. The find led Dr. Stilwell to mount a search for more amber at sites across Australia and New Zealand, with one of the specimens more unusual than all the rest: It contained two insects frozen in the act of mating.

“I looked at the piece under the microscope, and when I looked at it, I said, ‘This looks really important, because it looks like they’re almost attached or something,’” he said. “I couldn’t believe it — it looks like they’re mating.”

Dr. Stilwell and his colleagues detailed their findings in a paper published Thursday in Scientific Reports.

Dr. Stilwell calls this “frozen behavior.” Such a thing is exceptionally rare in the fossil record, because it means absolutely nothing happened in the moments between when the flies were living and when they died and became entombed. It’s comparable to what happened to humans in 79 A.D. at Pompeii, when volcanic ash from the eruption of Mount Vesuvius smothered and froze some Romans in a flash.

It’s possible that the flies are not in the precise position they were when they died — but they’re close. “It’s true and valid information,” said Victoria McCoy, a paleontologist at the University of Wisconsin, Milwaukee, who was not involved in the discovery. “It’s possible one fly was trapped in the amber and the other was a little excited and tried to mate.”

The first amber samples the team found came from Australia’s Otway Basin, a site where Dr. Stilwell led students for research in May 2011. He did a double-take when they first stumbled upon the amber.

“I started to see bits of gold, ‘clear gold,’ if you want to call it that,” said Dr. Stilwell. “I knew exactly what it was. I knew it was amber. I could not believe my eyes when I saw that.”

He applied for funding from the Australian government to excavate the amber, and he and his research team went on to unearth a parade of amber-entombed creatures ranging in age from 230 million to 40 million years old.

They discovered ants that are between 42 million and 40 million years old, which are the first fossil ants ever found in Australia, and which are part of a group of ants that’s still alive today. They discovered a cluster of spiders — though they don’t yet know what kinds of spiders they are — and they discovered mites as well as springtails that seem to preserve their original colors.

The rock layers from which the in flagrante flies and the other creatures come from is made of coal, which, as the pressurized remains of ancient plant life, tells Dr. Stilwell and his team that the creatures lived somewhere that once teemed with plants — including the trees that smothered the animals in resin that then turned to amber.

The oldest amber in the team’s haul comes from the Triassic Period, which stretched from 252 million to 201 million years ago. During that time, Earth’s separate land masses formed one supercontinent, Pangea. Australia and Antarctica, stitched together, comprised part of Pangea’s southern reaches, Gondwana. It eventually split from the supercontinent. That means the amber-entombed animals lived in what are today polar and subpolar latitudes, hinting at the environment that once prevailed on what was then a unified landmass.

Why fossil-rich amber seems to be so rare from this part of our planet — discovered only now after more than a century of paleontologists working in places like Australia — remains to be seen.

“It’s not clear if this is due to a lack of fieldwork in the Southern Hemisphere, or a lack of amber-producing forests,” Dr. McCoy said.

More fieldwork could help resolve this, but for now, Dr. McCoy says, the next stage of the team’s discovery can begin: describing each of the animals in the amber specimens, which might include species from never-before-seen groups of animals. “We might see lineages that we never knew existed,” she said.

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