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Hundreds of frogs died while mating in ancient swamp

Paleontologists have recently discovered that hundreds of fossil frogs died in a swamp 45 million years ago while mating. In central Germany, this watery death trap led to the death of over 50,000 ancient birds, horses, bats, fish and frogs. 

The Geisltal area was a coalfield with unique geological features housing thousands of fossils. Considered a scientific treasure trove, the area provides a unique window of how plants and animals have evolved over millions of years. 

During the middle Eocene 50 million years ago, Earth was much warmer. The Geiseltal area was a swampy, subtropical forest inhabited by ancestors of the horse, crocodiles, giant snakes, lizards, birds and plenty of frogs and toads.

Previous research determined that Geiseltal frogs could have died from oxygen depletion in the water. However, the exact cause of death has long been a mystery – until now. 

The team from University College Cork (UCC) studied the bones of the fossil frogs and were able to narrow down causes of death. 

“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.” said lead researcher Daniel Falk. 

Most of the Geiseltal fossil frogs spend their lives on land, returning to the water only to breed. “By process of elimination, the only explanation that makes sense is that they died during mating.”

This phenomenon is common in frogs today. “Female frogs are at higher risk of drowning as they are often submerged by one or more males – this often happens in species that engage in mating congregations during the short explosive breeding season,” said study senior author Professor Maria McNamara. 

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

These findings are the first of new discoveries after the reopening of the famous Geiseltal fossil collections of the “Zentralmagazin Naturwissenschaftlicher Sammlungen (ZNS)” in Germany. 

The study is published today in the journal Papers in Palaeontology.

By Katherine Bucko, Staff Writer

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