The site of Geiseltalsouth of Halle (Germany) is a window into the past: paleontologists have found remains of plants and animals that lived between 48 and 41 million years. More than 50,000 fossils, including small mammals, bats, birds, crocodiles, reptiles, and fish. Many even show well-preserved soft tissues and organs. But what most caught the attention of scientists was the number of frog skeletons: hundreds of them died there, in what was once a swampy subtropical forest. What happened?
“The fossil frogs were healthy when they died, and the bones show no signs of predators or scavengers; there is also no evidence that they were swept away during hypothetical floods or that they died because the swamp dried up,” he explains. Daniel Falkpaleontologist at University College Cork in Ireland and author of the study published in the journal ‘Papers in Paleontology’, where a new hypothesis is pointed out: that ancient bed became a ‘deadly sexual trap’ for frogs.
“By process of elimination, the only explanation that makes sense is that they died during mating,” says Falk. Because although many frogs live in humid environments, many others do not. And most of the fossils found in the Geiseltal rock seem to belong to species that preferred drier environments. Specifically, they are amphibians belonging to the family Pelobatidaeof which only one species survives today, and it is not aquatic.
In fact, he only comes close to the water to mate. And this process can be quite dangerous for female frogs, as the males, in the frenzy of the moment, can literally drown them. “Females are at greater risk, as they are often submerged by one or more males underwater; this happens frequently in species that participate in mating congregations during the short and explosive breeding season, “he explains. Maria McNamaraalso a paleontologist at University College Cork and co-author of the study.
Although the fact of drowning future mothers may seem like an ineffective natural system for the perpetuation of the species, the truth is that it is not a big problem in the world of frogs. There are species, such as Amazonian Rhinella proboscidea which is capable of extracting the eggs of a deceased female by squeezing her, then successfully fertilizing them. Although it is true that this is not a practice observed in the Pelobatidae modern, could be a plausible explanation.
In any case, the new analysis can help us understand the evolution of frogs over the centuries and put ancient fossil frog beds into context. “What’s really interesting is that fossil frogs from other sites also show these features, suggesting that the mating behaviors of modern frogs are quite ancient and have been around for at least 45 million years,” says McNamara.