When Ichthyosaurs Led a Revolution in the Seas

In the warm seas of the Triassic, there appeareda new kind of predator.

It was a nimble hunter of fish and squid.

And it was a reptile, even though it had somefeatures that were distinctly fish-like, such as flippers, a dorsal fin, and a tail fluke.

It’s known today as Californosaurus, andit was an ichthyosaur, a marine reptile that played a pivotal role in shaping ocean lifethroughout the Mesozoic Era.

Ichthyosaurs arose after the catastrophicextinction event at the end of the Permian Period, which wiped out at least 90 percentof life in the oceans, changing the seas forever and triggering a new evolutionary arms racebetween predator and prey.

And in these turbulent times, the ichthyosaursturned out to be true pioneers, innovators.

Because they helped create roles in the oceansthat had not existed before.

They got better at crunching through the shellsof 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 markeda revolution in marine life.

A sea change, if you will.

It began just after the world’s oceans hadbeen at the brink of extinction, and it continues to this day.

And the ichthyosaurs were some of the keyplayers in this remarkable transformation.

As it turns out, these strange marine reptileswouldn’t be able to see this change through to the very end.

But the fact is, the ocean life that livesamong us today is a product of that time when the ichthyosaurs helped revolutionize theseas.

When fossils of ichthyosaurs were first discoveredin the early 1800s, they were, to put it mildly, baffling.

The more complete specimens, like those foundby 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 groupof marine reptiles that were actually descended from terrestrial ancestors, and then madetheir way back to the sea.

This unusual journey began right after thebiggest extinction event in Earth’s history: the Permian-Triassic extinction, also knownas the Great Dying.

Because of this event, some 252 million yearsago, many of the marine animals that had defined life in the Permian Period vanished — including98% of crinoids, 80% of brachiopods, and all of the trilobites.

 Poor guys.

But for the survivors, this catastrophe presentedan opportunity.

It left a lot of niches, or environmentalroles, open for organisms to fill — including roles for new predators.

Now, predation itself first became a thingsome 540 million years ago, during that burst of evolutionary complexity known, of course, as the Cambrian Explosion.

But after the Great Dying, the complexityof marine life ramped up to a new level.

Some predators acquired new abilities, likebeing able to crush shells, or bore into them.

And some prey species responded by developingharder, spinier shells.

And, some predators even began hunting otherpredators! Suddenly the food webs of the ocean becamemuch more complex.

This restructuring of sea life came to beknown as the Mesozoic Marine Revolution, in which predators and prey radiated into newforms and lifestyles, in a sort of evolutionary arms race.

And this revolution is actually still underwaytoday.

You could say that the world’s oceans, evennow, 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 appearedjust around 4 million years after the Great Dying.

It was probably a reptile that spent mostof its time in the ocean but could also haul itself onto land, kind of like a seal.

And one of the oldest true ichthyosaurs appearsin the fossil record very soon after that.

For example, Chaohusaurus was a fully aquaticichthyosaur.

And, thanks to a fossil of a female foundin China with embryos still in tact, we know that it gave birth to live young, like allichthyosaurs did.

But Chaohusaurus still looked kind of likea finned lizard.

The first ichthyosaurs to adopt more familiarshapes — 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 withprey, which had become faster and more nimble, as the Mesozoic Marine Revolution continued.

And this sleek body plan made these reptileslook a lot like fish or marine mammals, even though they’re not closely related to eitherof those.

Like at all.

Sharks had been around for hundreds of millionsof years before the ichthyosaurs showed up.

And marine mammals like porpoises wouldn’tshow up until more than 200 million years later! So each of these groups converged separatelyon 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 poweris generated by the motion of the tail, while the front half the animal stays still.

This makes for fast and efficient movementthrough open water.

These and other ichthyosaurs came in a rangeof sizes, from Mixosaurus at about 1 meter long, to Shonisaurus, which was more than20 meters long, rivaling some of today’s largest whales.

And this range in size went a long way inhelping the reptiles occupy a variety of niches in the Mesozoic seas.

Some ichthyosaurs became generalists and atecephalopods and fish, or just scavenged on whatever they could find.

But others were ram feeders — they just keepmoving forward and ate whatever was scooped up in their mouths.

Which is usually what I do There were also shell-crushers that harvestedammonites and bivalves; and there were even macropredators that hunted other ichthyosaurs.

And together, all of these feeding mechanismsactually created newer, more complex interactions between predator and prey than the oceanshad seen before, with new niches to exploit and new layers forming in marine food webs.

For example, as the shells of bivalves becamestronger 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 couldbreak open shells.

And in time, there were so many new predatorsthat some ichthyosaurs started to prey on them, like Thalattoarchon, the so-called “sovereignof the sea.

” And, ichthyosaurs also occupied both shallow-waterand open-ocean ecosystems, with many genera living all over the worlde.

By the Triassic Period, Earth’s oceans hadbecome a different place, and this would be the heyday of the ichthyosaurs, like our oldfriend Californosaurus.

But this golden age wouldn’t last long.

Because, the end of the Triassic was markedby yet another extinction event: the Triassic-Jurassic mass extinction.

The exact trigger of this event is still beingdebated.

But one possibility is massive bursts of volcanicactivity that took place as the supercontinent Pangea started to split up.

What we do know is that toward the end ofthe Triassic, many clades of ichthyosaurs started to disappear.

In the end, only the more porpoise-like reptilesthat lived in the deep, open ocean would make it into the Jurassic.

And this lack of diversity might have beenthe beginning of their undoing.

These survivors included generalists; specialiststhat ate soft-bodied prey; and some apex predators, like Temnodontosaurus, which got to be aslong as 9 meters.

In fact, most of these deep-ocean ichthyosaurswere on the big side, and what’s more, many had the biggest eyes relative to body sizeof any animal in history.

Some species, like Ophthalmosaurus, had eyesthe size of soccer balls, which allowed them to see hunt in dim, deep waters.

But, as the Jurassic progressed, another changewas in the works: Ichthyosaurs soon faced competition from other animals, includingplesiosaurs, marine crocodiles, and sharks.

Some of these competitors would even cometo prey on the ichthyosaurs themselves.

Still, ichthyosaurs made it through the Jurassicand into the Cretaceous Period, where they managed to hold on to similar niches thatthey occupied in the Jurassic.

One of the most common ichthyosaurs of theCretaceous was Platypterygius, a big hunter that included more than a half-dozen speciesand whose remains have been found around the world, including Australia, the Americas, and Europe.

But in the Late Cretaceous, the diversityof ichthyosaurs dropped dramatically.

And it would never recover.

The first to go were the generalists and thesoft-prey specialists, around 100 million years ago.

About 5 million years later, mostly apex predatorsremained, and they were unable to branch into new niches.

Fossils at this time show that existing specieswere 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, thelast of the apex predators, like Platypterygius disappeared.

Ichthyosaurs had vanished from the fossilrecord.

So what caused the demise of an animal thathad persisted for more than 150 million years? Well, as is often the case, no one knows forsure.

But there are a few possibilities.

One idea is that they were outcompeted bynew ocean hunters, like predatory fish and mosasaurs.

But, mosasaurs had already been around fora long time, and the really big mosasaurs didn’t appear until after the ichthyosaurshad vanished.

So another idea has to do with what happenedto the climate at the start of the Late Cretaceous.

Studies of limestone that formed in the seasat this time show that oxygen levels dropped sharply.

This is known as an anoxic event, and it’sthought to have started with a change in ocean temperatures, which then disrupted circulationpatterns.

The change in circulation may have deprivedsome ocean layers of oxygen, causing marine life to suffocate.

This phenomenon began about 100 million yearsago and continued for about 7 million years, until it culminated in a major anoxic event.

Many marine invertebrates, including somecephalopods that were a main source of food for the ichthyosaurs, went extinct.

And their extinction coincides the last appearanceof ichthyosaurs in the fossil record.

So, unlike all of those other times in thepast, the ichthyosaurs couldn’t bounce back like they had before.

By the Late Cretaceous, ichthyosaurs had becomevery good at what they did, but they hadn’t really expanded into new niches.

And when this happens, animals can be morevulnerable to extinction, especially if something drastic happens to the few environments thatthey’re still adapted to.

But the important thing to remember here isthat the niches that they formed, and filled, continued to exist and were taken over byother animals.

Mid-sized mosasaurs like Platecarpus, forexample, became the new common generalists in the world’s oceans.

And narrow-toothed mosasaurs like Pluridenstook over the role of specializing on soft-bodied prey.

And the title of apex predator was eventuallyhanded 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 manyspecies of sharks and dolphins.

And there are still specialists in soft prey, like beaked whales.

And of course there are apex predators likeorcas and great whites, as well as shell crushers in the form of stingrays, and many more.

So the ichthyosaurs didn’t see the Mesozoicthrough to its end, but they did last for a staggering 157 million years, making themone of the world’s greatest evolutionary success stories.

They exploded into diversity just as Earthwas recovering from the largest mass extinction in history.

And they were a key part of the revolutionthat took place in marine ecosystems, making ocean life more complex than ever.

So the legacy of these strangely familiarreptiles 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 KonstantinHaase studio and if you want to join us for more adventuresin deep time, just go to youtube.

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