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  • 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 trilobitesPoor 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-calledsovereign 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 patrons in supporting what we do here, then

  • go to patreon.com/eons and make your pledge!

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

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When Ichthyosaurs Led a Revolution in the Seas

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    joey joey に公開 2021 年 05 月 02 日
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