字幕表 動画を再生する 英語字幕をプリント 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 patrons in supporting what we do here, then go to patreon.com/eons and make your pledge!