Around 250 million years ago, at  the dawn of the Triassic Period, a group of small terrestrial  reptiles made a big lifestyle change.

After millions of years of living on land, they dipped their limbs into the oceans  and started on a new evolutionary journey.

These were the earliest ichthyosaurs,  which literally means ‘fish-lizards.’ And their rise was pretty remarkable.

Because,  while they started out small - with the earliest known species being around a meter or less in  length - they wouldn't stay that way for long.

Within just a few million years  of taking to the water - a blink of an eye in evolutionary time - the group  produced a giant: Cymbospondylus youngorum, a truly enormous ichthyosaur around  the size of a modern sperm whale.

Its rapid evolution marked  a new era in the oceans, one that continues to this  day: an era of ocean giants.

See, it isn't just the earliest  known giant ichthyosaur, it’s the earliest known giant animal, period.

As far as we know, no animal - either on land or  in the oceans - had ever gotten so big before.

So what drove Cymbospondylus to grow to  such unprecedented size at break-neck speed?

Well, ironically enough, the sudden rise and  revolutionary success of the giant ichthyosaurs began with a global ecological catastrophe.

Now, the origin of the ichthyosaurs  is still pretty mysterious.

Most modern analyses place them in the  larger group that includes crocodilians, snakes, lizards, birds, and turtles, but we’re not  sure exactly where in that family tree they fit.

But one thing we do know for sure is  that when they first invaded the oceans, they were entering a post-apocalyptic world.

Just a few million years earlier, around  252 million years ago, the planet had been hit by the biggest mass extinction in  its history - the one at the end of the Permian Period, often called the Great Dying.

It was probably driven by the sudden release of  greenhouse gasses from massive volcanic eruptions, creating environmental chaos around the world.

Rising temperatures, ocean acidification,  and other rapid global changes led to the loss of around 70% of terrestrial species, and  up to 96% of marine species, by some estimates.

But as devastating as the Great Dying was, it reset the planet’s ecosystems in the  way that mass extinctions often do - wiping the biological slate clean, allowing the  survivors to suddenly take center stage.

And as the planet gradually recovered,  ichthyosaurs were able to quickly establish themselves as key players in the new  Mesozoic Era, thanks to their taste in prey.

One of the fastest marine groups to bounce back  from the Great Dying seems to have been ammonoids.

Despite being hit hard initially, they  recovered so quickly that within 1 to 2 million years they were actually more diverse than they’d been before the mass extinction.

An ancient group of small, eel-like  jawless vertebrates called conodonts were also common in the waters of the early Triassic.

They seem to have been one of the few marine groups to make it through the extinction event relatively unscathed.

And the abundance of both these groups during  the planet’s initial recovery period meant that, for the earliest ichthyosaurs,  the open oceans were basically an unattended seafood buffet – one with lots of  potential prey and few surviving competitors.

And the fossil record shows that this  opportunity was huge for them - literally.

The remains of the gigantic Cymbospondylus  youngorum date to as little as just 3 million years after the appearance  of the first smaller ichthyosaurs.

Now, for ichthyosaurs to have evolved such  enormous proportions - 17 to 25 meters long and upwards of 40 metric tons in this  case - in such a short amount of time is pretty exceptional.

And  Cymbospondylus youngorum wasn’t alone.

While it was the first known giant  ichthyosaur - and the first giant animal in general - several other medium  and large-sized ichthyosaurs have also been at the Fossil Hill site in Nevada  and elsewhere, dating to around the same time.

So it looks like the shift towards large body  sizes in ichthyosaurs happened in a sudden, early burst.

On land, it wouldn’t be until the rise of sauropod  dinosaurs over 40 million years later that the animal kingdom produced giants that could match  the size and mass of Cymbospondylus youngorum.

In the ocean, no other group of amniotes  would get that big until whales appeared.

And while some parts of the evolutionary journeys  of whales and ichthyosaurs are quite similar, like their terrestrial  origins, overall body plans, and eventual gigantic size, that’s  really where the similarities end.

Because, as a group, the evolution  of large body sizes in whales didn't happen in a sudden, early burst as it did  in ichthyosaurs.

Instead, it was a slow, gradual increase over a much longer timespan.

It took nearly 50 million years after  first becoming aquatic for whales, as a group, to reach the sizes we see today.

And although at least one genus of early whale  - Basilosaurus - did manage to become huge faster than the rest, this outlier still took  much longer than the ichthyosaurs to bulk up… More like 10 to 14 million years, rather  than as little as 3 million years.

So what made the difference?

Well, the biggest whales today are the baleen  whales - a group that includes the blue whale, fin whale and humpback whale, for example.

These giants seem to have developed  their impressive proportions by becoming filter-feeding specialists, eating huge  amounts of tiny prey like krill and plankton.

They’ve adapted to modern oceans  that are highly productive even at the very lowest levels of the food chain.

And this high level of productivity also supports  some other giant whales further up the food chain, like sperm whales, which specialize in  suction feeding on squid in the deep ocean.

But we thought that the oceans of the early  and middle Triassic were not that productive… So how was there enough energy  available in the food chain to sustain a giant like Cymbospondylus youngorum?

Well, by modeling the flow of energy through  the food chain of the Fossil Hill ecosystem, researchers found that the abundance of ammonoids at its base would have provided enough  energy to anchor a stable food web.

And instead of being a specialist like modern  giant whales, the snout and teeth of the first giant ichthyosaur suggest it was more of a  generalist predator that fed on a variety of prey.

These would have included fish, squid, and  smaller marine reptiles - potentially even other ichthyosaurs!

With all this  potential prey, there would have been plenty of energy in the food chain to  fuel Cymbospondylus youngorum’s gigantism.

And its ability to feed effectively  enough to get so huge so fast may have been due to some aspects of its anatomy.

For one thing, it had large  eyes relative to its body size.

These are thought to be a pretty ancient  trait of ichthyosaurs and this probably would have made it a highly capable hunter that  could spot and zero in on prey successfully.

And while Cymbospondylus youngorum was the first  giant ichthyosaur, it certainly wasn’t the last.

The fossil record shows us  that, by the late Triassic, many other giant ichthyosaurs had appeared.

Some might have exceeded 26 meters in length - perhaps even rivaling modern blue whales,  the largest known animal to have ever existed.

But just as their rise was kicked off by  a mass extinction, it was another mass extinction that would be their downfall.

At the peak of their heyday, in the last years of the Triassic, another ecological upheaval  took place: the Triassic-Jurassic extinction.

The causes of this event are still  hotly debated, but we can tell that while it wasn’t quite as catastrophic  as the Great Dying 50 million years earlier, it still devastated life both  on land and in the oceans.

And it seems that this extinction event acted  as a kind of bottleneck that only a handful of ichthyosaur lineages managed to pass through.

Those that survived were smaller, and while some did grow to impressive sizes  in the Early Jurassic - like Temnodontosaurus, for example - ichthyosaurs never fully recovered.

And they faced competition from new giant marine predators on the block - like the plesiosaurs  that became common in the Jurassic.

They managed to persist for another 100  million years or so after the extinction, but eventually they vanished entirely by the  Late Cretaceous, around 95 million years ago.

So while they were the trendsetters of ocean  gigantism - achieving sizes no other animals had ever approached, and in record-breaking time  - their golden age ended as suddenly as it began.