The metamorphosis of frogs from tadpoles to adults represents one of nature's extraordinary transformations. Tadpoles initially exhibit oversized heads and long tails that later transform into more typical frog forms, incorporating tiny legs.
Despite the marvel of this transformation, tracing the evolutionary history of frog metamorphosis has posed significant challenges. Paleontologist Mariana Chuliver from the Universidad Maimónides in Buenos Aires noted that while adult frog fossils date back to the Triassic Period, approximately 217 million years ago, tadpole fossils were only known from the Cretaceous, around 140 million years ago, raising questions about the origins of tadpoles.
On Wednesday, Dr. Chuliver and her team revealed the discovery of a 161 million-year-old fossilized tadpole, marking it as the oldest known specimen. The tadpole, comparable in length to a standard hot dog bun, is related to a close ancestor of modern frogs and was documented in the journal Nature.
Finding adult frog fossils is already challenging due to their generally small and fragile nature, but tadpoles are even harder to locate. Dr. Chuliver explained that tadpoles consist mainly of soft tissues and cartilage, making their fossilization a daunting task.
In 2020, a team of Argentine and Chinese paleontologists, initially searching for dinosaur fossils in Santa Cruz Province, Argentina, unexpectedly unearthed hundreds of adult specimens of Notobatrachus, a prehistoric bullfrog relative from the Jurassic Period. While examining sedimentary layers, the team discovered a remarkably preserved tadpole measuring over six inches long, showcasing not only its head, body, and tail but also specific anatomical details, including eye and mouth structures and nerve outlines. The presence of developing limbs indicated that the tadpole was close to metamorphosis.
Dr. Chuliver noted that the tadpole was in an optimal developmental stage, combining features from various frog life cycle stages. The team confirmed its classification as Notobatrachus based on shared vertebral characteristics with adult fossils found at the site, which contained no other frog species.
The size of this tadpole was unexpected, as most species develop into smaller adults. Dr. Chuliver suggested that the lack of food competitors in the seasonal ponds allowed for significant growth, leading to the tadpole's unusual size.
One of the most significant findings was the tadpole's gill system, used for filtering food particles from the water, a characteristic still evident in over 6,000 modern tadpole species. Dr. Chuliver stated that this filtering system was present since the early stages of frog evolution.
A notable gap existed between the earliest known frogs from the Triassic and the previously oldest tadpole fossils from the Cretaceous, leading to speculation among researchers about the presence of metamorphosis in frog ancestors. Some frog species today skip the tadpole stage entirely, raising further questions.
Notobatrachus belongs to a group of frog-like amphibians that coexisted with true frogs during the Mesozoic Era and shared a common ancestor with contemporary frogs. The identification of a tadpole confirms that the metamorphosis process we see in modern frogs likely originated much earlier than previously believed.
The new findings serve as significant evidence supporting the hypothesis that metamorphosis has been a characteristic of frogs since their early evolution, according to Kim Roelants, a frog biology expert at Vrije Universiteit Brussel in Belgium, who did not participate in the study.