Transcription
Around 300 million years ago, long before the first dinosaurs would evolve, our semi-aquatic ancestors started to make the necessary evolutionary leaps to live on the land permanently. But at this time, and for a while after this, most of the largest and most dominant animals around still breathe through their skin when it's wet and still lay their eggs in the water, meaning they couldn't spend too long on land. And one lineage still alive today descended from these animals: the amphibians.
Many of the amphibians still live in a very similar way to these ancestors, but one group took a very different evolutionary pathway, adapting to become quite adept out of the water and also evolving to leap. Frogs.
300 to 350 million years ago was during a period called the Carboniferous, which is best known for its giant insects. But the largest creatures of the day were actually amphibians. Due to amphibians being some of the most dominant animals and that they had far fewer predators, they had less evolutionary restrictions than today. And so the steamy swamps under the archaic fern trees that covered much of the Carboniferous landscape were filled by many diverse amphibians with a much larger variety of different shapes and sizes than today.
For instance, one amphibian from this time named *Diplo caulis* had a large boomerang-shaped head. And these weren't horns protruding from their head; the points were direct extensions of its skull. The reason for its unusually shaped head is not known. However, one study using a model of *Diplo caulis*'s skull in a wind tunnel found that the skull shape creates a lot of lift. So if the creature was swimming forward, the water would push under the skull, causing the skull to pull the animal up like a hydrofoil. It is still unknown why *Diplo caulis* would need such a function, but its skull is so good at doing this, it seems unlikely it is just by chance.
Another group of oddly shaped amphibians from the time were named the *Aistopoda*, that evolved in a similar way to snakes, evolving to elongate their body and losing their limbs. And in fact, the fossils of their skull and body look very much like snake skeletons at a glance, but they were actually amphibians.
One of the largest groups of ancient amphibians were called *Temnospondyli*, that had some members that were actually mostly terrestrial creatures, likely only returning to the waters to breed. With some of them developing claws and scaly-like skin, and so possibly being more adapted to life on land than any modern amphibians. And many of these ancient amphibians were much larger than today, with body lengths of 2 meters or more being commonplace. And with some behemoth amphibians being able to grow up to the size of the largest living crocodiles.
The modern amphibians, frogs, salamanders, and caecilians are named the *Lissamphibia*. And the *Lissamphibia* most likely diverged from the other amphibians sometime in the late Carboniferous period, around 300 million years ago. However, which of these ancient amphibian groups *Lissamphibia* are most closely related to is not known and is still hotly debated.
One theory is that modern amphibians are *Lepospondyli*, which is the group of ancient amphibians that *Diplo caulis* belongs to. And that *Temnospondyli* were more closely related to reptiles, birds, and mammals, and by extension us, than they are to any modern amphibians. And this seems to make sense because *Lepospondyli* are the most primitive out of the ancient amphibians. But some *Temnospondyli* had started to adapt to life on the land, so they may represent a transitional stage in amphibians giving rise to the land animals.
However, in 2008, a small amphibian was discovered in Texas called *Gerobatrachus* that had features from both salamanders and frogs, meaning it may have been closely related to their common ancestor. *Gerobatrachus* was about 11 centimeters long and so would have been a small amphibian for the time. It had a large head like a frog, but a salamander-like tail. However, there are many finer details that link them to the *Lissamphibia*, like them having a unique tooth layout that is only shared by modern amphibians.
However, *Gerobatrachus* also had many features that tied them to *Temnospondyli*, showing that modern amphibians may actually be more closely related to *Temnospondyli*, or maybe frogs and salamanders and caecilians descend from a more primitive lineage of amphibian. The problem is that as vertebrates started being able to come onto the land for at least a small amount of time, they went through a phase of very rapid changes in adaptation to take advantage of new niches, which make it very difficult to work out their family tree from their fossils.
The earliest uncontroversial *Lissamphibian* was called *Triadobatrachus* that lived about a hundred million years later, in the very early Triassic, 250 million years ago. *Triadobatrachus* was about the size of a modern-day large toad. And by this period, many amphibians had shrunk down, with the vast majority of large animals in most ecosystems around the world now being reptiles. However, some giant amphibians actually survived into the early Triassic, like *Mastodonsaurus*. They may have been able to grow to around 5 meters long and were powerful predators. And giant amphibians would persist even further, surviving up until the Cretaceous period, about a hundred million years ago.
*Triadobatrachus* lived in a part of the ancient supercontinent Pangaea that would eventually become modern-day Madagascar. And they were most closely related to frogs out of the living amphibians. However, they weren't true frogs, scientifically known as *Anura*, and had many primitive features that set them apart. Their heads and bodies were very similar to modern frogs. This is because they had large frog-like heads, but also their tail had shrunk down dramatically compared to some of its older ancestors. This meant that, like frogs, it most likely swam with its hind legs rather than with its tail, like many Carboniferous era amphibians do and salamanders still do today.
However, their legs and pelvis had noticeable differences. For instance, frogs have adapted to enlarge and fuse together the bones in their lower legs, which is what gives them their unique shape and also makes them capable of powerful leaps. Despite this, it is thought that *Triadobatrachus* was already capable of hopping due to the way its legs were attached to its pelvis, although it was certainly not as powerful of a jumper as its modern relatives. And this means that the ability to leap started to evolve very early in frog evolution and was most likely responsible for frogs evolving to lose their tail once they have reached adulthood.
However, one of the most important differences between frogs and *Triadobatrachus* was in their pelvis. Modern frogs have a strangely shaped pelvis joint that is unique to the true frogs, but *Triadobatrachus*'s pelvis was much smaller and less specialized. This unique pelvic joint isn't required for leaping over large distances, but it helps frogs control their trajectory and so allows them to leap more accurately. This is important because it shows that *Triadobatrachus* would have had less control over the direction it was travelling. This meaning that primitive frogs wouldn't have evolved to jump as a way of getting around like they do today, and instead most likely leapt to escape predators because accuracy wouldn't have mattered as much. This also shows that moving this way is not at odds with living in the water as well, because *Triadobatrachus* would have been able to quickly hop back into the safety of the nearest pool of water if it found itself in danger from predators on land. And then over time, natural selection saw frogs evolve to use hopping and leaping as a mode of travel.
The earliest true frogs first appeared by at least the early Jurassic, with a small creature named *Prosalirus* being the earliest known frog that lived about 190 million years ago. *Prosalirus* lived in what would become Arizona and Colorado and would have hopped and peeked above the water line in a world that was now heavily populated by dinosaurs.
Today, the vast majority of amphibian species, 80 to 90 percent of them, are frogs. But although frogs are almost as old as dinosaurs, it seems the massive number of species that exist today only appeared relatively recently. As throughout the Jurassic and Cretaceous, not many of their fossils are known. This may be because frogs are small and have small bones, or because they live in boggy environments where fossilization doesn't occur as easily. However, after the extinction of the non-avian dinosaurs 66 million years ago, there are many more prehistoric frog species in the fossil record. And this is also seen in their genetics as well, with many of the big frog families that live today most likely evolving around 66 million years ago. It is likely that frogs just weren't as commonplace during the Jurassic and Cretaceous periods. However, after the KPG extinction, their populations exploded, most likely because they filled the niches of lizards and other amphibians like salamanders that had gone extinct during the extinction event.
However, despite frogs being a lot more obscure in their Jurassic and Cretaceous periods than they are today, by the end of the Cretaceous period, one group of them evolved to grow into ferocious predators. During the Cretaceous, Madagascar was connected to India, that itself wasn't connected to Asia yet, so both regions made up one large island, similar to Madagascar today. Because of its isolation, this island had its own unique animals, like endemic species of dinosaur not found anywhere else. But also a giant toad made its home here called *Beelzebufo*, which means devil toad. Early study of its fossils suggested that this toad was a monster, being similar in size to a house cat. However, with the discovery of new remains and more study, it was probably more likely around 25 centimeters long, making it slightly larger than an African bullfrog, the largest living frog. However, what was impressive was that it had a very large head and mouth that was proportionately larger than that of an African bullfrog. However, more recent fossil discoveries of this creature have shown that its large skull was ornamented by armored plates, similar to those found on the bones of South African horned frogs. And so *Beelzebufo* probably looked similar and was perhaps distantly related. And scaling up the bite force of a horned frog to the size of *Beelzebufo* would mean they may have had a very powerful bite. Since it was powerfully built and possessed powerful jaws, but also just that the overwhelming majority of frogs are carnivorous, *Beelzebufo* was almost certainly predatory. Like horned frogs, it probably fed on anything big enough to fit in its mouth that it shared its habitat with, like small reptiles, mammals, and although there is no direct evidence, it certainly would have been capable of swallowing hatchling dinosaurs.
*Beelzebufo* most likely went extinct during the KPG extinction that wiped out the non-avian dinosaurs, but frogs as a whole managed to not just fully recover, but become one of the most populous groups of animals, covering almost all of the world. So in many ways, frogs have retained many features from their ancient ancestors, like keeping their soft amphibian skin and still breeding in the water. But in other ways, they have come leaps and bounds and evolved in a completely different direction, like developing complicated mechanisms for survival, like a uniquely shaped pelvis that helps them jump with pinpoint accuracy. And these adaptations help them survive for almost 200 million years.
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