The carcasses were moved by light currents in the swampy lakes and sank to the bottom in the cold, deep, and undisturbed regions of the lake. The cold temperatures (probably about 8℃) prevented decay and kept many skeletons in good condition. In some skeletons, even small bones such as finger bones or toe bones are still well defined.
Some frogs may have frozen to death, died from diseases, or from old age. That’s information the frogs took to the grave with them, as these three causes of death are difficult to verify. But after months of studying these fossils and analyzing what we know about their lifestyles, my team came to an astonishing conclusion.
The most likely explanation for why there are several groups of frogs, each numbering in the hundreds, that died almost at the same time in different ponds is that their enthusiastic mating killed them. It explains why similar mass graves have been found in different parts of the world.
The German Geiseltal fossil collection was closed for decades but recently reopened to the public and scientists. It is an incredible time capsule of over 50,000 fossils from the former lignite (brown coal) opencast mine in the Geiseltal.
The fossils include crocodiles, huge snakes, giant flightless birds, and dog-sized primeval horses. Many of the Geiseltal fossils are so well preserved they show remarkable details, including bones, scales, skin, internal organs, and gut contents.
The mine was flooded to create a recreational area in the early 2000s and is a giant lake now.
Don’t take frogs for granted
While these mating deaths sound extreme, a far more common cause of frog and toad mortality is humans destroying their homes, polluting water sources, and spreading disease.