A fossil of a frog who lived beside dinosaurs around 100 million years ago has been found with an egg-filled belly. Scientists have announced that this is the oldest such frog that has ever been found.

Gravid Frog

Scientists think that the frog could have been killed during mating. A male frog gripping the female could have drowned the latter.

Frogs are incapable of getting pregnant. Rather, female ones grow a batch of eggs for laying. This is known as a "gravid" state. Eventually, the eggs are laid. Males then fertilize these eggs.

The frog belongs to the Gansubatrachus qilianensis species. It was discovered in a fossil bed within northwest China. It dates back to the Cretaceous period that happened around 66 to 145 million years ago. Its discovery was documented in the "A cretaceous frog with eggs from northwestern China provides fossil evidence for sexual maturity preceding skeletal maturity in anurans" study.

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Reproduction Discovery

Scientists then compared the gravid frog fossil to other specimens that were previously found. Using fossil CT scans, the researchers made a high-resolution model of the species' skeleton. They also used X-rays to analyze the eggs' composition.

The analysis reveals that the frog was immature in a skeletal sense. This means that she had the capacity to sexually reproduce before being fully developed physically. While this is prevalent among modern animals, there has not been direct fossil evidence that shows that this was the case for ancient toads and frogs.

Paleontologist Baoxia Du from Lanzhou University explains that reproduction's evolution is crucial in biological evolution. This is especially the case for strategies for reproduction. He adds that knowing more about early frogs' reproductive strategies could shed light on their full reproductive history.

Mysterious Cause of Death

Given the skeletal immaturity of the frog, the scientists ruled out age as a potential death cause. Du explains that environmental factors, such as abrupt weather condition changes or algae blooms, were also not likely. If this were the case, they should have been apparent in the studied deposits.

There were also a few more frog fossils that were discovered within the deposits. These suggest that a disastrous event could not have led to the species' death, as this would have supposedly resulted in a mass death and several fossils of frogs close by.

Du explains that they think that the most likely death cause is weakness or possibly even suffocation following amplexus behavior. This is quite prevalent among frogs that exist today. Amplexus refers to the behavior wherein males grip and mount onto females using their front legs. They do so for hours or even days at a time until the eggs of these females are fertilized. The study notes that while this was taking place, the female could have died due to exhaustion or drowning.

Though more fossil records are necessary to strengthen the findings regarding the sexual maturity of frogs before adulthood, the findings offer a great glimpse into ancient frog development. Du explains that the fact that early frogs thrived in the dinosaur era and were able to endure various mass extinctions makes looking into their strategies for survival extremely valuable.

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