Frogs once lived alongside the dinosaurs. About 45 million years ago, the North Sea covered half of Germany. It’s amazing to think that these little creatures survived the extinction of the dinosaurs. But a lower level of mass mortality occurred in what is now called the Geiseltal region in central Germany, and the cause has long remained a mystery.
Hundreds of frog fossils have been found in a mass grave on the swampy coasts of Geiseltal, 45 million years old, and their reason for being there has baffled scientists for decades. But my team’s study found an explanation: they died of exhaustion while mating.
We also found evidence that the mating behavior of modern frogs and toads dates back at least 45 million years, as frog fossils from mass graves from other sites show similar characteristics on the skeletons of specimens from Geisel Valley.
What we found
My German-Irish research team studied the fossil skeletons of frogs. We paleontologists took pictures, made drawings and analyzed the fossils. We checked how many bones were still in place and which bones and joints were still attached.
With this data, we were able to reveal what happened to the frog skeletons after they died and interpret the reason for their death. Our research also found many skeletons in a layer of sediment, which showed that most of the fossilized frogs died during mass mortality events (recurring events in which several hundred frogs died in a short time) .
Other scientists thought the frogs and toads in Geiseltal died when the lakes dried up and oxygen levels dropped rapidly. But our research showed that this was unlikely as the frogs could have easily headed for nearby bodies of water. We also found evidence that the frog carcasses floated in the water for some time before sinking to the bottom of the lake. The lake has therefore not dried up.
Our comparisons of Geiseltal skeletons with modern frogs revealed that most Geiseltal frogs were in fact toads. Toads follow a terrestrial lifestyle except when they return to ponds to mate. They mated with many other toads during the very short breeding season, which in some modern tropical species lasts only a few hours.
Sex can be a death trap for modern species of toads and frogs. Individuals are regularly overcome by exhaustion and drown. Female frogs and toads are at a higher risk of drowning because they are often submerged underwater by one or more males. Even today, mass toad graves are found on migration routes and near or in breeding ponds. It was probably the same situation for the Geiseltal specimens.
The carcasses were moved by gentle currents in the marshy lakes and sank to the bottom in the cold, deep, undisturbed regions of the lake. Cold temperatures (probably around 8 degrees Celsius) prevented decay and kept many skeletons in good condition. In some skeletons, even small bones such as finger or toe bones are still well defined.
Some frogs may have died of cold, disease or old age. This is information that the frogs took with them to the grave because these three causes of death are difficult to verify. But after months of studying these fossils and analyzing what we know about their way of life, my team has come to a startling conclusion.
The most likely explanation for why there are multiple groups of frogs, each numbering in the hundreds, that died almost simultaneously in different ponds, is that their enthusiastic mating killed them. This explains why similar mass graves have been discovered in different parts of the world.
Germany’s Geiseltal fossil collection was closed for decades but has recently reopened to the public and scientists. This is an incredible time capsule of more than 50,000 fossils from the former open pit lignite (lignite) mine in the Geiseltal.
Fossils include crocodiles, huge snakes, giant flightless birds and primitive dog-sized horses. Many Geiseltal fossils are so well preserved that they show remarkable detail, including bones, scales, skin, internal organs, and the contents of intestines.
The mine was flooded to create a recreation area in the early 2000s and is now one giant lake.
Frogs in danger
While these mating deaths seem extreme, a far more common cause of frog and toad mortality is humans destroying their homes, polluting water sources, and spreading disease.
Frogs and toads have survived several climate changes and extinction events on earth. However, some species have disappeared. In 2021, one of the few remaining frog species from an ancient amphibian lineage was declared likely extinct, having not been seen for 60 years.
A 2019 UN report showed that amphibians, especially frogs, are among the hardest hit by nature’s crisis. Frogs can migrate short distances if the environmental conditions of their pond change. But they are vulnerable to disease, which can be caused by human impacts on nature.
Frogs and toads live almost everywhere, including on trees, in flowers, in the jungle, and in the desert. Some look almost as colorful as a rainbow and some can even fly. Imagine these creatures feeding next to a T-Rex. It would be a tragedy if we lost other species.
This article was originally published on The conversation by Daniel Falk at University College Cork. Read the original article here.