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  • 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 moremedieval.

  • 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 meansdifferent 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 meansbony 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 meansplate 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

  • skeletonmeaning 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 theyre 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 isvague.

  • 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.

  • Thank you so much for your support!

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  • and nerdy rewards.

  • And, if youre interested, like I am, in learning more about how our planet works,

  • then go on over to one of our newest sister channels, Hot Mess, which explores climate

  • science, the effects of climate change, and how we can create a better future for our

  • planet and ourselves.

  • Now, what do you want to know about the history of life on Earth?

  • Let me know in the comments!

  • And if you haven’t already, be sure to go to youtube.com/eons and subscribe!

420 million years ago, life was definitely better down where it was wetter...under the

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当鱼穿盔甲(When Fish Wore Armor)

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    joey joey 發佈於 2021 年 05 月 04 日
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