For fossil frogs, an ancient swamp is a sex death trap.
Paleontologists at University College Cork (UCC) have determined why hundreds of fossil frogs in a prehistoric swamp perished 45 million years ago while mating.
Over 50,000 prehistoric animals perished in the watery death trap in the Geiseltal region of central Germany, including birds, horses, bats, fish, and hundreds of frogs. The former coalfield of Geiseltal in Saxony-Anhalt is regarded as a scientific treasure trove because of its distinctive geological characteristics and numbers of fossils, offering a unique view into how the Earth’s vegetation and animals developed over millions of years.
Nearly 50 million years ago, during the middle Eocene, when Earth was considerably warmer, the area of Geiseltal was a swampy subtropical forest home to anurans, or frogs and toads, as well as large crocodiles, gigantic snakes, lizards, ground-dwelling birds, and progenitors of the horse.
According to earlier research, the Geiseltal frogs perished when lakes dried up or the water’s oxygen levels dropped. But until today, it was unclear precisely what killed these creatures.
By studying the bones of the fossil frogs, the UCC team was able to narrow down the options. “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 UCC researcher and study leader Daniel Falk. What’s more, most of the Geiseltal fossil frogs are species that 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 senior author Professor Maria McNamara. “What’s really interesting is that fossil frogs from other sites also show these features, suggesting that the mating behaviors of modern frogs are really quite ancient and have been in place for at least 45 million years”.
Reference: “The skeletal taphonomy of anurans from the Eocene Geiseltal Konservat-Lagerstätte, Germany: insights into the controls on fossil anuran preservation” by Daniel Falk, Oliver Wings and Maria E. McNamara, 5 July 2022, Papers in Palaeontology.
These findings are among the first new discoveries to come to light following the reopening of the famous Geiseltal fossil collections of the “Zentralmagazin Naturwissenschaftlicher Sammlungen (ZNS)” in Halle (Saale), Germany to the public. The study is part of a research cooperation between UCC and Martin-Luther-University Halle-Wittenberg with funding from the Irish Research Council.