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'Sex Death Trap' Killed Hundreds of Frogs 45 Million Years Ago

It's not easy being green.

fossilized frogs
Here are some of the fossilized frogs from 45 millions ago found in modern-day Germany.
University College Cork

"Frog sex death trap" sounds like the worst heavy-metal band name ever, but it's an accurate description of what happened 45 million years ago in a swamp in Germany. 

The spot, a former coalfield, is known for fossils ranging from crocodiles to toads, as well as hundreds of frogs. A team of researchers went about trying to solve the mystery of why so many frogs died there. 

The team led by scientists at University College Cork (UCC) in Ireland published a study on the fossils in Papers in Palaeontology this week. 

Previous research pointed to oxygen depletion or the drying up of lakes as possible culprits in the frog deaths, but the amphibians' bones tell a different story. 

Fossil frog with its head turn to the side and legs visible in the rock.

Researchers found this fossil frog in a "sex death trap."

Daniel Falk

"As far as we can tell, the fossil frogs were healthy when they died, and the bones don't show any signs of predators or scavengers -- there's also no evidence that they were washed in during floods, or died because the swamp dried up," lead author Daniel Falk said Wednesday in a UCC statement.

The fossilized animals were a type of frog that spends its time on land and only heads to the water for breeding. The researchers concluded that the only sensible explanation for the frogs' demise was that it happened during mating. This is a phenomenon sometimes seen in modern frogs when females are drowned by males during the frenzied breeding period. 

The ancient frogs help connect the dots to current times, according to UCC paleontologist and study co-author Maria McNamara.

"What's really interesting is that fossil frogs from other sites also show these features," McNamara said, "suggesting that the mating behaviors of modern frogs are really quite ancient and have been in place for at least 45 million years."