The metamorphosis of a frog from baby to adult is one of nature’s most remarkable transformations. Tadpoles are born with oversized heads and long tails. Soon the tails vanish, swapped out for tiny legs and a more frog-like shape.
This incredible anatomical rejiggering is easy to take for granted. But tracking the evolution of frog metamorphosis has been tricky.
The fossil record of adult frogs is known dating back to the Triassic Period, 217 million years ago, said Mariana Chuliver, a paleontologist at the Universidad Maimónides in Buenos Aires. “But tadpoles are known only from the Cretaceous, 140 million years ago. So there was kind of a question regarding the origin of tadpoles.”
On Wednesday, Dr. Chuliver and her colleagues announced the discovery of a 161 million-year-old fossil tadpole, the oldest yet found. More than that, this tadpole was long enough to fit in your typical hot dog bun. The find, which relates to a close relative of our contemporary frogs, was described Wednesday in the journal Nature.
Adult frog fossils can be difficult to find, because the animals tend to be small and fragile. But tadpoles pose an even greater challenge.
“They have these really soft bodies with a lot of soft tissue, and most of its body is made of cartilage, so it’s really hard to find preserved tadpoles because of these characteristics,” Dr. Chuliver said.
In 2020, a group of Argentine and Chinese paleontologists visited a quarry in Santa Cruz Province, Argentina. While they’d come in search of dinosaurs, their excavations turned up hundreds of adult specimens of Notobatrachus, a bullfrog-size relative of the modern frog that had lived in Jurassic Period Patagonia.
As they cracked open sedimentary slabs, one peeled away to reveal an exquisitely preserved tadpole, over six inches in length. The find included not just the head, body and tail, but also details of the eye, mouth and nerves. The presence of a tiny forelimb, hind limb, and a bony vertebral column showed that the tadpole was in an “advanced state of development,” Dr. Chuliver said, likely about to begin its metamorphosis.
“It’s in the perfect stage of its life,” Dr. Chuliver said. “It has a lot of information relating not only to the larval period, but also the adult period. It’s a mix of the three different stages of the frog life cycle.”
The team identified the tadpole as a Notobatrachus, as it shared specific vertebrae features with the adult specimens, and the site contained no other frog species to which it might have belonged.
Its size came as a surprise: most tadpoles are far smaller than adult frogs, Dr. Chuliver said, while a few species — like South America’s paradoxical frog — begin as large tadpoles before developing into small adults. The Notobatrachus tadpole, however, was the same length as one of the adult frogs.
“That feature is really, really hard to find in nature today,” Dr. Chuliver said. “Such a big size was probably because the tadpoles had no food competitors in the seasonal ponds, so food resources were available for these tadpoles to grow indefinitely.”
The most notable feature of the find, however, was the tadpole’s gill system, which it used to filter food particles out of the water, as is the case with modern tadpoles. “We now know that this filter feeding system was present since the beginning of the group,” Dr. Chuliver said, adding that the gill system is still found in over 6,000 species of tadpole frogs today.
The gap between the first known frogs in the Triassic Period and the previous oldest-known tadpoles in the Cretaceous Period led some researchers to wonder whether the ancestors of frogs had a metamorphosis at all, Dr. Chuliver said. Some researchers pointed to the old lineage of living frog species, such as the cliff chirping frogs of Texas, whose life cycle skips the tadpole stage entirely.
Notobatrachus, however, is part of a group of frog-like amphibians that lived alongside contemporary frogs in the Mesozoic Era, and shared with them a common ancestor. The appearance of a tadpole suggests that metamorphosis was present in frogs as we know them today from the beginning, and had probably emerged much earlier.
The new paper offers “the first time we have such solid and beautiful evidence” of that hypothesis, said Kim Roelants, a specialist in frog biology at Vrije Universiteit Brussel in Belgium, who did not participate in the research.
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