In the warm seas of the Triassic, there appeared
a new kind of predator. It was a nimble hunter of fish and squid. And it was a reptile, even though it had some
features that were distinctly fish-like, such as flippers, a dorsal fin, and a tail fluke. It’s known today as Californosaurus, and
it was an ichthyosaur, a marine reptile that played a pivotal role in shaping ocean life
throughout the Mesozoic Era. Ichthyosaurs arose after the catastrophic
extinction event at the end of the Permian Period, which wiped out at least 90 percent
of life in the oceans, changing the seas forever and triggering a new evolutionary arms race
between predator and prey. And in these turbulent times, the ichthyosaurs
turned out to be true pioneers, innovators. Because they helped create roles in the oceans
that had not existed before. They got better at crunching through the shells
of ammonites, flushing out bivalves on the seafloor, and, higher up in the water column,
hunting fish, and other reptiles. And in time, their prey diversified, too,
and developed better defenses, like harder shells, spines, and more mobility. This dynamic between predator and prey marked
a revolution in marine life. A sea change, if you will. It began just after the world’s oceans had
been at the brink of extinction, and it continues to this day. And the ichthyosaurs were some of the key
players in this remarkable transformation. As it turns out, these strange marine reptiles
wouldn’t be able to see this change through to the very end. But the fact is, the ocean life that lives
among us today is a product of that time when the ichthyosaurs helped revolutionize the
seas. When fossils of ichthyosaurs were first discovered
in the early 1800s, they were, to put it mildly, baffling. The more complete specimens, like those found
by Mary Anning in England, revealed animals that were shaped much like fish or porpoises,
with streamlined profiles, fins, and powerful tails. But their bones were distinctly reptilian. Today we know that ichthyosaurs are a group
of marine reptiles that were actually descended from terrestrial ancestors, and then made
their way back to the sea. This unusual journey began right after the
biggest extinction event in Earth’s history: the Permian-Triassic extinction, also known
as the Great Dying. Because of this event, some 252 million years
ago, many of the marine animals that had defined life in the Permian Period vanished -- including
98% of crinoids, 80% of brachiopods, and all of the trilobites. Poor guys. But for the survivors, this catastrophe presented
an opportunity. It left a lot of niches, or environmental
roles, open for organisms to fill -- including roles for new predators. Now, predation itself first became a thing
some 540 million years ago, during that burst of evolutionary complexity known, of course,
as the Cambrian Explosion. But after the Great Dying, the complexity
of marine life ramped up to a new level. Some predators acquired new abilities, like
being able to crush shells, or bore into them. And some prey species responded by developing
harder, spinier shells. And, some predators even began hunting other
predators! Suddenly the food webs of the ocean became
much more complex. This restructuring of sea life came to be
known as the Mesozoic Marine Revolution, in which predators and prey radiated into new
forms and lifestyles, in a sort of evolutionary arms race. And this revolution is actually still underway
today. You could say that the world’s oceans, even
now, continue to respond to the disaster of the Great Dying. But at the start of the Mesozoic Marine Revolution,
some of the leading figures were the ichthyosaurs. The earliest ancestor of ichthyosaurs appeared
just around 4 million years after the Great Dying. It was probably a reptile that spent most
of its time in the ocean but could also haul itself onto land, kind of like a seal. And one of the oldest true ichthyosaurs appears
in the fossil record very soon after that. For example, Chaohusaurus was a fully aquatic
ichthyosaur. And, thanks to a fossil of a female found
in China with embryos still in tact, we know that it gave birth to live young, like all
ichthyosaurs did. But Chaohusaurus still looked kind of like
a finned lizard. The first ichthyosaurs to adopt more familiar
shapes -- ones that looked more like sharks or porpoises -- wouldn’t appear until later,
in the Middle Triassic Period. One of these was Phalarodon. It used its streamlined body to keep up with
prey, which had become faster and more nimble, as the Mesozoic Marine Revolution continued. And this sleek body plan made these reptiles
look a lot like fish or marine mammals, even though they’re not closely related to either
of those. Like at all. Sharks had been around for hundreds of millions
of years before the ichthyosaurs showed up. And marine mammals like porpoises wouldn’t
show up until more than 200 million years later! So each of these groups converged separately
on the same body plan and the undulating style of movement that these bodies made possible,
known as thunniform locomotion. In thunniform swimming, most of the power
is generated by the motion of the tail, while the front half the animal stays still. This makes for fast and efficient movement
through open water. These and other ichthyosaurs came in a range
of sizes, from Mixosaurus at about 1 meter long, to Shonisaurus, which was more than
20 meters long, rivaling some of today’s largest whales. And this range in size went a long way in
helping the reptiles occupy a variety of niches in the Mesozoic seas. Some ichthyosaurs became generalists and ate
cephalopods and fish, or just scavenged on whatever they could find. But others were ram feeders -- they just keep
moving forward and ate whatever was scooped up in their mouths. Which is usually what I do There were also shell-crushers that harvested
ammonites and bivalves; and there were even macropredators that hunted other ichthyosaurs. And together, all of these feeding mechanisms
actually created newer, more complex interactions between predator and prey than the oceans
had seen before, with new niches to exploit and new layers forming in marine food webs. For example, as the shells of bivalves became
stronger and more resistant to predators, the niche opened up for a new kind of hunter,
like the ichthyosaur known as Tholodus, which had bigger, rounded, blunt teeth that could
break open shells. And in time, there were so many new predators
that some ichthyosaurs started to prey on them, like Thalattoarchon, the so-called “sovereign
of the sea.” And, ichthyosaurs also occupied both shallow-water
and open-ocean ecosystems, with many genera living all over the worlde. By the Triassic Period, Earth’s oceans had
become a different place, and this would be the heyday of the ichthyosaurs, like our old
friend Californosaurus. But this golden age wouldn’t last long. Because, the end of the Triassic was marked
by yet another extinction event: the Triassic-Jurassic mass extinction. The exact trigger of this event is still being
debated. But one possibility is massive bursts of volcanic
activity that took place as the supercontinent Pangea started to split up. What we do know is that toward the end of
the Triassic, many clades of ichthyosaurs started to disappear. In the end, only the more porpoise-like reptiles
that lived in the deep, open ocean would make it into the Jurassic. And this lack of diversity might have been
the beginning of their undoing. These survivors included generalists; specialists
that ate soft-bodied prey; and some apex predators, like Temnodontosaurus, which got to be as
long as 9 meters. In fact, most of these deep-ocean ichthyosaurs
were on the big side, and what’s more, many had the biggest eyes relative to body size
of any animal in history. Some species, like Ophthalmosaurus, had eyes
the size of soccer balls, which allowed them to see hunt in dim, deep waters. But, as the Jurassic progressed, another change
was in the works: Ichthyosaurs soon faced competition from other animals, including
plesiosaurs, marine crocodiles, and sharks. Some of these competitors would even come
to prey on the ichthyosaurs themselves. Still, ichthyosaurs made it through the Jurassic
and into the Cretaceous Period, where they managed to hold on to similar niches that
they occupied in the Jurassic. One of the most common ichthyosaurs of the
Cretaceous was Platypterygius, a big hunter that included more than a half-dozen species
and whose remains have been found around the world, including Australia, the Americas,
and Europe. But in the Late Cretaceous, the diversity
of ichthyosaurs dropped dramatically. And it would never recover. The first to go were the generalists and the
soft-prey specialists, around 100 million years ago. About 5 million years later, mostly apex predators
remained, and they were unable to branch into new niches. Fossils at this time show that existing species
were going extinct faster than new ones were appearing. Ichthyosaurs had just become too specialized,
and too few in number. Finally, around 93 million years ago, the
last of the apex predators, like Platypterygius disappeared. Ichthyosaurs had vanished from the fossil
record. So what caused the demise of an animal that
had persisted for more than 150 million years? Well, as is often the case, no one knows for
sure. But there are a few possibilities. One idea is that they were outcompeted by
new ocean hunters, like predatory fish and mosasaurs. But, mosasaurs had already been around for
a long time, and the really big mosasaurs didn’t appear until after the ichthyosaurs
had vanished. So another idea has to do with what happened
to the climate at the start of the Late Cretaceous. Studies of limestone that formed in the seas
at this time show that oxygen levels dropped sharply. This is known as an anoxic event, and it’s
thought to have started with a change in ocean temperatures, which then disrupted circulation
patterns. The change in circulation may have deprived
some ocean layers of oxygen, causing marine life to suffocate. This phenomenon began about 100 million years
ago and continued for about 7 million years, until it culminated in a major anoxic event. Many marine invertebrates, including some
cephalopods that were a main source of food for the ichthyosaurs, went extinct. And their extinction coincides the last appearance
of ichthyosaurs in the fossil record. So, unlike all of those other times in the
past, the ichthyosaurs couldn’t bounce back like they had before. By the Late Cretaceous, ichthyosaurs had become
very good at what they did, but they hadn’t really expanded into new niches. And when this happens, animals can be more
vulnerable to extinction, especially if something drastic happens to the few environments that
they’re still adapted to. But the important thing to remember here is
that the niches that they formed, and filled, continued to exist and were taken over by
other animals. Mid-sized mosasaurs like Platecarpus, for
example, became the new common generalists in the world’s oceans. And narrow-toothed mosasaurs like Pluridens
took over the role of specializing on soft-bodied prey. And the title of apex predator was eventually
handed off to the mega-mosasaurs like Tylosaurus. And when all of these reptiles also went extinct,
other animals replaced them. And those roles still exist today. These days, marine generalists include many
species of sharks and dolphins. And there are still specialists in soft prey,
like beaked whales. And of course there are apex predators like
orcas and great whites, as well as shell crushers in the form of stingrays, and many more. So the ichthyosaurs didn’t see the Mesozoic
through to its end, but they did last for a staggering 157 million years, making them
one of the world’s greatest evolutionary success stories. They exploded into diversity just as Earth
was recovering from the largest mass extinction in history. And they were a key part of the revolution
that took place in marine ecosystems, making ocean life more complex than ever. So the legacy of these strangely familiar
reptiles continues to this day, in the oceans we find around us. The ichthyosaurs died out. But their revolution lives on. Thanks for joining me today in the Konstantin
Haase studio and if you want to join us for more adventures
in deep time, just go to youtube.com/eons and subscribe. Thanks to this month’s Eontologists: Patrick
Seifert, Jake Hart, Jon Davison Ng, and Steve. If you’d like to join them and our other
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Does anyone know of another Channel with similar quality to PBS Eons? I might have a bit of an addiction.
Its curious there were no great sea reptiles before the Triassic: there were the relatively diminutive claudiosaurs and mesosaurs, but not others. Surely the archaic ichthyofauna of the time would compete less well against marine tetrapods, than do our modern elasmobranchs and teleosts. The big placoderms were long gone, and most sharks (as today!) were small. The biggest "sharks" were 12 meters long, but Guadalupian(?).
There's an open space for a YT exploring such absences.