Scientist have recently uncovered a remarkable find. Not only is this particular species, Notobatrachus degiustoi, remarkable for being the oldest known tadpole fossil — dating back around 160 million years to the Middle Jurassic — but it’s also incredibly large, measuring approximately 159 millimeters (6.3 inches) in length. It also offers some key answers regarding how frogs evolved during their early days.

A “rare gem”

Finding a fossilized tadpole is kind of like discovering a rare gem because soft-bodied animals are rarely preserved as fossils. In this case, volcanic ash and silt acted as natural preservatives, encasing the tadpole shortly after its death in a shallow pond. This environment shielded the fossil from scavengers and preserved soft tissues, including the tadpole’s intricate feeding apparatus.

“Our tadpole probably died naturally, and after its death was covered by volcanic ash and silt, which is a fine grain sediment. The deposition of these sediments promoted an exquisite preservation of the specimen, which lasted more than 160 million years,” says Mariana Chuliver Pereyra, from Fundación de Historia Natural “Félix de Azara”, Argentina, to ZME Science.

But this discovery is more than just a pretty sight. It marks a milestone in our understanding of amphibian evolution. Particularly it adds to the origins and endurance of the biphasic life cycle seen in frogs today.

Frog metamorphosis is ancient

Frogs and toads belong to an order called Anura — so technically speaking, frogs and toads are anurans. Anurans begin life as aquatic, gilled larvae, known as tadpoles, and undergo a dramatic metamorphosis to become air-breathing, typically terrestrial adults. This transition from a larval to an adult stage involves significant morphological changes, including the development of limbs, lungs, and the loss of the tail, which are adaptations for life outside water.

Anurans are highly diverse, with more than 7,000 species occupying a wide range of habitats across the globe, from rainforests to deserts — but they all undergo this metamorphosis.

Despite being separated by millions of years, the Jurassic tadpole bears a striking resemblance to its modern counterparts, showing that this metamorphosis had already emerged at that time.

“It provides the first evidence for the presence of a tadpole followed by a drastic metamorphosis in the life cycle of anurans from the very beginning of the evolutionary history of the group. Moreover, we proved that the presence of a larval stage with filter-feeding habits was a key trait that was already present in the anuran life cycle more than 160 million years ago,” adds co-author Federico Agnolín from Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” in Argentina, to ZME Science.

A Jurassic habitat

The fossil of the Notobatrachus degiustoi tadpole was discovered in the La Matilde Formation in southern Patagonia. The Middle Jurassic period, where this tadpole thrived, was marked by a warm, tropical climate interspersed with seasonal dry and wet phases.

The fossil’s exceptional preservation is owed due to the area’s unique sedimentary conditions. This depositional environment, characterized by shallow ponds that periodically dried out, created ideal conditions for fossilization — but also shaped this creature’s evolution as it was a habitat that demands adaptability and resilience.

“The habitat wherein this tadpole lived was probably a shallow pond, which dried out periodically. These microhabitat features were one of the most relevant aspects influencing the features of the tadpole of Notobatrachus degiustoi, more than particular environmental conditions that may have characterized the Jurassic period,” co-author Agustín Scanferla explained for ZME Science.

The researchers hypothesize that gigantism in Notobatrachus degiustoi was an adaptation to the unique conditions of its environment.

Why was it so big?

Perhaps the most surprising revelation of this study is just how big the Jurassic tadpole was. Although it’s not larger than today’s largest tadpoles, it’s still large enough to be considered “tadpole gigantism.” This phenomenon, though uncommon, has appeared independently at several points along the frog evolutionary tree.

“Gigantism of tadpoles was probably related to a long development and the absence of predators. Moreover, the high availability of food resources, and the absence of competitors such as fish or other anuran species were also key factors for the tadpoles to grow giant. These giant tadpoles probably metamorphosed and gave rise to large froglets, which in turn under the proper environmental conditions will also grow into a giant adult,” mentions Pereyra.

“Gigantism in Notobatrachus degiustoi may also have been a trait inherited from ancestors, but as we have no fossil evidence this hypothesis cannot be tested. We stated in our study that there are independent appearances of gigantism along the evolutionary tree of frogs. We can’t establish at the moment if it evolved at the base of the tree and then got lost, but it’s something we can explore further if we find more tadpoles.”

The oldest tadpole

The discovery of Notobatrachus degiustoi is a significant breakthrough. By uncovering the oldest-known tadpole, the researchers have provided a foundational piece in the puzzle of amphibian evolution.

For instance, the researchers report a surprising finding related to the hyobranchial skeleton, a structure in tadpoles that supports their feeding apparatus. Remarkably, the tadpole of Notobatrachus degiustoi exhibits a unique preservation of this hyobranchial skeleton which was not preserved in any other fossil tadpole described to date. Basically, this ancient tadpole used a similar mechanism to filter-feed, marking this as an ancient and stable trait in frog evolution.

But while this study establishes a baseline understanding, many questions remain unanswered. For instance, was the two-phase lifecycle an inherited trait from even earlier amphibians, or did it emerge uniquely within this anuran lineage? Further fossil discoveries could shed light on this mystery, especially if researchers can uncover fossils from other life stages or related species from the same period.

“This study is relevant because we provide the first evidence for the presence of a tadpole stage in the life cycle of anurans from the beginning of the evolutionary history of the group. This research has been carried out entirely in Argentina, but to continue making this kind of discoveries the Argentinian researchers need financial support from the government,” the researchers conclude.

The study “The oldest tadpole reveals evolutionary stability of the anuran life cycle” was published in Nature. https://doi.org/10.1038/s41586-024-08055-y