Earliest tadpole discovered from over 160 million years ago – and it was a monster

Frogs and toads develop from tadpoles that emerge from the egg. Right? Not necessarily. They don't all do this.

Hundreds of species go straight from egg to frog. True, they hatch as a small frog that grows into a big frog. But it's a frog, without an aquatic stage of metamorphosis.



The squeaking frog Pristimantis of Africa is one such. The Pipa she-toad broods her eggs on her back (don't ask how the eggs get there), from where the babies emerge as tiny Pipa toads. The South American flea toad, which is the smallest-known vertebrate and could fit its whole family on your fingernail, also skips the tadpole stage.

(While miniaturizing, some flea toad species also eschewed a middle ear, apparently rendering them deaf to their own mating calls, which is sad.) In short, the anurans are a vastly diverse bunch, yet the thinking is that they – frogs and toads – are characterized by a biphasic life cycle: egg to larva (tadpole) to frog. The snag is, frogs and toads arose at least 215 million years ago, and possibly more.

Yet not even one lousy tadpole could be identified in the pre-Cretaceous fossil record, which begged dubiety about the ancestral anuran condition. Did early anurans have a biphasic life cycle, as we assume? Did they too hatch from egg as larvae and metamorphose into adults? We still don't know what the first frogs did, but now paleontologists have thrown back the proven emergence of biphasic life in the frog, reporting on the earliest tadpole discovered to date: Notobatrachus degiustoi, from the middle Jurassic period of Patagonia, about 170 million to 160 million years ago. They did not just discover any tadpole.

This is one monster baby at 15.9 centimeters (6.3 inches) long.

That's more than half a foot, reports Mariana Chuliver of Buenos Aires' Maimonides University with colleagues in Nature . Its sheer dimensions demand soaring eyebrows: We know all about Jurassic gigantism thanks to science and Steven Spielberg, but a half-foot-long tadpole? Are they confident in its identification? As much as can be. Not only are they confident in its identification as a juvenile anuran as opposed to some other extinct creature.

They believe they have nailed down its specific species and life-stage. The specimen was Notobatrachus degiustoi. None other, and it was close to maturing into a whole frog.

Could it be the paleontologists chanced upon a froggie freak in the middle Jurassic, an individual case of gigantism caused by disease? Could be, but they don't think so. First, the poor creature was close to reaching its final metamorphosis, enabling the team to conclude that the specimen wasn't a case of pathological gigantism – it was developing normally. How do we know that? Its head is there, as it should be, but so is a forelimb.

Not quite about to be a frog, but en route. Second: Crucially for the entire identification of this long-dead animal's family tree, it was found in the context of a lot of adult N. degiustoi.

And they too were not small, the team explains. "Notably, both N. degiustoi tadpole and adult reached a large size, demonstrating that tadpole gigantism occurred among stem-anurans," they write – meaning, anurans that were ancestral to today's anurans, not branches that went extinct.

Delightfully, the giant tadpole is so well preserved that they could even discern soft tissues, including the filter-feeding mechanism that modern tadpoles feature. Why hadn't any primeval tadpoles been found to date? Them's the breaks. A tadpole's existence is fleeting, a frog's is longer.

Frogs will have the better chance of being preserved by chance after death. We find what we find. That said, supporting its extraordinary identification 161 to 168 million years after it crossed over into the void, the numerous adult N.

degiustoi frogs it was found among were also huge. Which is something of an anomaly, because gigantism evolved time and time again among frogs and toads but weirdly, few of them have both giant tadpoles and giant frogs, the team observes. Take the "paradoxical frog" aka "shrinking frog" of Trinidad and northern South America, which starts life as a huge tadpole.

Turns out the thing does its growth during the baby stage and then loses its tail, so you see why it appears to lose, say, three-quarters of its body size as it matures . Was our Notobatrachus degiustoi the biggest frog ever? Not by a long shot. The charmingly named Beelzebufo ampinga of the late Cretaceous was over a foot long – as much as 41 centimeters, it is estimated – and weighed up to 4.

5 kilograms (nearly 10 pounds). Also known as the frog from hell, showing our prejudices, Beelzebufo – truly, kudos on that pun – may have had froggie horns too. Paleontologists say it was so big it likely ate baby dinosaurs, which, based on when it lived, is on the side of obvious.

The biggest anuran today is, generally, the West African goliath frog, which can be 35 centimeters long. But let us also take our hats off to Prinsen, a pet cane toad living in Sweden who reached almost 6 pounds . As his girth might indicate, cane toads are not fussy eaters.

Maybe if he had grown up, our giant tadpole might too have dined on juvenile dinosaurs, flossing his teeth on their early feathers. In other breaking news of amphibia, just last week a new species was identified : Brachycephalus dacnis, on the coast of Brazil's São Paulo. It isn't the only mini-frog or toad, and actually B.

dacnis kept all of its skull bones – unlike some other pea-sized toads that lost or fused some of theirs as they shrank. They are very cute, but are mainly of interest to people studying diminution and its effect on physiological efficiency. So it seems the loss of a tadpole stage in flea toads is a secondary development – as in, their ancestors at some stage would presumably have had the biphasic life cycle.

The new study published last week supports the contention that the biphasic life cycle, with filter-feeding larval frogs (tadpoles) living in watery environs, began – at least – early in the evolution of frogs and has persisted for, at least, 161 million years. In most frogs and toads. But not all.

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