In northwest China, researchers have discovered the extraordinary fossil of a frog that is around 115 million years old. What’s special: The animal’s ovaries and fallopian tubes contain numerous fossilized eggs. The female frog’s skeleton suggests that she was very young. Similar to many amphibians today, frogs apparently became sexually mature before they were fully grown. But the mating was probably fatal for the young female: according to the researchers, she probably died during the sexual act.
Many modern amphibians can reproduce before they are fully grown. Early sexual maturity can provide evolutionary advantages by launching the next generation early. However, it was still unclear when this property developed in the course of evolution. In order to determine whether an animal is sexually mature, researchers usually look at the level of development of the sexual organs. In fossils, however, no soft tissue is usually preserved, only the bones. Although these can provide information about whether an animal is still growing, they do not allow any statements to be made about the level of sexual maturity.
Eggs in the fossil’s belly
A team led by Baoxia Du from Lanzhou University in China has now found an exceptionally well-preserved fossil of an ancient frog in the Jiuquan Basin in northwest China. The fossil is dated to around 115 million years ago and, in addition to the skeleton, also includes easily recognizable parts of the soft tissue. Using computer tomographic images, the researchers reconstructed the frog’s three-dimensional body structure and assigned it to the species Gansubatrachus qilianensis. The team first described this species in 2023 using another fossil from the same site.
The researchers came across a peculiarity in the newly discovered specimen: in the animal’s stomach they discovered clusters of dots each just under a millimeter in size. “The shape, position, size and arrangement of these circular structures suggest that they are ovarian follicles and eggs,” reports the research team. “Fossilized frogs with well-preserved soft parts or eggs are extremely rare in the geological records.” The specimen described therefore represents the oldest pregnant frog in geological history discovered to date.
Sexually mature in adolescence
Analyzes of the skeleton showed that the female frog’s carpus was not yet fully ossified – a typical indication that the animal was still growing. In the specimen found in 2023, however, the carpus was ossified. Although the researchers cannot completely rule out that the skeleton of this species developed differently depending on the sex and that females only had partially ossified carpal bones even at an older age, the more likely explanation is that it was a young, not yet fully grown animal.
“This suggests that sexual maturity occurred in this frog before reaching adulthood,” the researchers write. “The discovery is direct evidence that this phenomenon occurred 115 million years ago and is likely anchored in the evolutionary history of frogs.”
Death during mating
The team also wondered how the young, pregnant female could have died. Many of the causes of death typical of frogs are eliminated in this animal: it obviously neither died of old age nor was it eaten by predators. The good conservation status and surrounding sediment also make it unlikely that the frog was crushed or died from environmental influences. Since frogs usually only produce eggs under good conditions, the animal probably did not starve or die during hibernation.
“The most likely cause of death for the female is drowning or exhaustion related to mating,” the team writes. This cause of death can also be observed in many of today’s frogs and toads. In order to fertilize the eggs as soon as they are spawned by the female, the males of many species cling to the female’s back. Sometimes they let him carry them for days, thereby ensuring that no competitor fertilizes this female’s eggs. In some cases, however, so many males cling to a female that she drowns in the water or dies of exhaustion inside a tangle of males. “The fossil we found probably represents the earliest documented death related to mating,” the researchers write.
Source: Baoxia Du (Lanzhou University, China) et al., Proceedings of the Royal Society B: Biological Sciences, doi: 10.1098/rspb.2023.2320