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.
Gigantic thanks to this month’s Eontologists:
Become an Eonite at patreon.com/eons and you can get fun perks like submitting a joke for us to
read, like this one from Matty Dahman, which is a friend of mine, I'm so excited, alright
What did the coach say when the brachiosaurus hit a homerun?
Look at that dino SCORE!
That was good, I love it. I love baseball so this is just perfect. Thanks Matty!
And as always thank you for joining me in the Konstantin Haase studio.
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