字幕表 動画を再生する 英語字幕をプリント 420 million years ago, life was definitely better down where it was wetter...under the sea. Dry land was still new and relatively empty. But in the seas, life had reached a fever pitch of diversity, especially among fish. The world's oceans teemed with sharks, primitive ray-finned fishes, hagfish, and even our very early ancestors, the lobe-finned fish. With such an abundance of life unfolding from 420 million to 360 million years ago, it's no wonder that the Devonian Period is often called the Age of Fish. But the fish that the Devonian is perhaps best known for were a little different than the ones you and I know today. Back then, some fish were more… medieval. They wore armor, sometimes made of big plates, and sometimes made of interlocking scales. And the evolution of armor may seem like an obvious adaptation for protection, especially in the Devonian's crowded seas. But that armor may actually have served a totally different purpose, one that many animals still use today -- including you and me. The earliest armored fish we have fossils of is called Sacabambaspis, a member of the subclass known as Arandaspida. This fish lived in the coastal waters of what's now Bolivia, during the Ordovician Period, some 470 million years ago. And really, it looked more like a watermelon with a tail than a fish. It had no jaws, and no dorsal or side fins. It was only marginally more fish-like than its earlier relatives, hagfish and conodonts, with which it shared a common ancestor. But what it lacked in fishiness in the front of its body it made up for with an extravagant tail, complete with a shark-like fin and a long, scaled rod extending from the tip. And the most distinctive feature of this fish was, of course, its armor -- a new adaptation probably made of a primitive bone-like tissue called aspidin, as well as dentine and enameloid, materials that are very similar to the ones that make up your teeth. This armor covered the head in big plates, with smaller, joined chevrons running along the rest of the body. So Sacabambaspis was the first fish that we know of to acquire this kind of covering, but it would soon have many imitators. The period that came next, the Silurian, saw the evolution of armor in many groups of fish, all around the same time. One such group was the Heterostracans, whose name means “different scales.” They, too, looked like small, armored ovals with thick, fleshy tails. Tolypelepis, for example, swam around Latvia 420 million years ago. It was only about 8 centimeters long, but its armor was especially beautiful, made of intricate, ridged, interlocking scales. Other Heterostracans had fused plates on their heads, with diamond-shaped or elongated scales covering their tails. But head plates took on new forms in another group of jawless fish: the Osteostracans, whose name means “bony shields.” The armor on these fish was typically just a large, one-piece shield that covered the head, made of dentine and bone, with smaller linear scales along the body. These head shields were wider than the rest of the body, and they sometimes tended toward extravagance, as in the case of Boreaspis, a fish that sported a large spike sprouting from its face. Osteostracans also boasted a brand-new feature: fins -- namely, dorsal fins and small, paddle-like side-fins that gave them greater mobility in the water. And this is where armored fish start to take an especially interesting turn. Because, osteostracans are among the few jawless fish to have a bony internal skeleton, or endoskeleton. They didn't have much of one -- their backbones, like those of heterostracans and earlier fish, was made of cartilage. But their fins and parts of their skulls were some of the first to be made of bone. And this brings us to probably the best-known armored fish, the Placoderms. Placoderm means “plate skin,” and plated they indeed were, with big chunks of bone, dentine and enameloid armor on their head and bodies. In some placoderms, the covering got up to 5 centimeters thick. But placoderms were also unique among armored fish in having a complete, mineralized internal skeleton – meaning they had both an endoskeleton, like you do, as well as an exoskeleton. And their endoskeleton and exoskeleton were completely separate! For example, the skull of a placoderm had an internal braincase made of endoskeletal bone. Then, as an additional layer outside of that, unconnected, was a thick layer of exoskeletal bone. And one more big thing that set Placoderms apart from earlier fish was that they were the first fish to have jaws, giving them the ability to bite, chew, and grind rather than just scrape or filter feed. Now, one of the most widespread placoderms was Bothriolepis, which was able to live in oceanic environments, estuaries, and freshwater settings. And while this fish had jaws, it didn't have true teeth. Instead, it had bony plates that it used to grind up its food, like decomposing plant and animal matter. Much bigger than Bothriolepis -- and more carnivorous -- was Dunkleosteus, a giant, predator that could reach up to 9 meters in length, with massive, shearing jaws. It didn't have teeth, either, but the sharpened sections of bone it used instead could have produced up to 7400 newtons of force, making it one of the strongest bites of any fish, living or extinct, and certainly the strongest bite in the world at the time. So armored fish came in a variety of shapes and sizes during the Silurian and Devonian, from tiny Heterostracans to massive Placoderms. But why did these fish evolve armor in the first place? I mean, it's possible that armor evolved as armor: to protect them from other fish and predators. And indeed, bite marks on armored bone, and remnants of armored fish found in the feces of other animals, are common in the fossil record. There are even fossils of Dunkleosteus that bear the bite marks of other Dunkleosteus. So, if armor evolved just for protection, it certainly wasn't 100% effective. But that might not have been its only purpose. Instead, the evolution of bony armor might have had a whole lot to do with storing vital minerals. See, bone and other bony tissues contain a lot of Calcium and Phosphorous. You probably know that these minerals help keep bones strong, but they're also necessary for a lot of chemical processes in your body, like muscle movement. That's right, calcium's not just for bones! You need a certain amount of calcium for your muscles to work, because it makes your muscle fibers contract. And Phosphorous is an important part of the little molecule your body uses to store and transfer energy: Adenosine Triphosphate, or ATP. So, without phosphorus and calcium, the internal workings of your body - or that of an armored fish - basically just wouldn't run. That's why modern animals -- including you -- use bone as a repository for calcium and phosphorous. We have special cells that quickly break down bone, sending calcium and phosphorous into the bloodstream, in a process called bone reabsorption. When bone tissue is broken down like this, it changes the microscopic structure of the bone. And scientists have seen signs of those changes in the armor of heterostracans, osteostracans and placoderms. So, it seems that these armored fish did use their bony armor for mineral storage. And this ability may have been one of the key reasons that these fish were so successful, allowing them to take over all kinds of watery habitats. Both calcium and phosphorus are present in water, but they occur in much lower concentrations in fresh water than they do in the ocean. And heterostracans, osteostracans and placoderms have been found in both salty AND fresh water environments. So then having big bony shields may have acted like a camel's hump for calcium, an important resource that kept their muscles working at top speed while they explored rivers and lakes. And of course, it's also possible armor evolved for both reasons -- as a convenient storage for muscle-moving minerals, and a nice piece of protection to help keep you from getting chomped on. So you can credit these ancient fish for acquiring the evolutionary breakthrough that was the mineralized suit of armor. But, do we have them to thank for our bony skeletons? Probably not. Or at least not directly. We might be able to trace them back to a common ancestor, though. The problem is that, like so many things in paleontology, the relationship of armored fish to other living things is … vague. Most of today's bony fish are ray-finned fishes. And land-dwelling vertebrates are descended from lobe-finned fishes. But it's not clear how armored fish, especially placoderms, were related to either of these groups. Some scientists think that, while both ray-finned and lobe-finned fish shared common ancestors with armored fish, they aren't their direct descendants. Others think that most placoderms specifically share common ancestors with ray-finned and lobe-finned fishes, but there are some placoderms that are more closely related to lobe-finned fishes than to other armored fish. It's confusing, I am aware, but phylogenetics is hard. And regardless of where they came from, the thick body armor of these fish didn't make it through to the other side of the Devonian. The late Devonian is characterized by one of Earth's “big 5” mass extinctions, with a loss of 50 to 60% of marine genera, including all of the armored fish. It's not really clear what caused the Devonian extinctions, which occurred in two main bursts. The end of the Devonian shows signs of a rapid swing from greenhouse to a sudden glaciation and then back again, a see-sawing of climate that could be responsible for many of the extinctions. But scientists aren't yet sure why this climate change occurred -- or whether the cause was actually something else. Whatever the cause, armor was no protection against the effects of it, and so the armored fish disappeared. In their wake, bony fish and cartilaginous fish like sharks and rays took over the oceans. And the medieval age of fish came to a close. Thanks for joining me today, and thanks to all of our patrons who help make these videos possible. And we want to thank our first two eontologists, Duncan Miller and David Rasmussen. 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