Name: When Fish Wore Armor
Uploaded: 2021-05-04T03:24:19.000Z
Duration: 10 min 19 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

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

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

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

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

But that armor may actually have served a totally different purpose, one that many animals

The earliest armored fish we have fossils of is called Sacabambaspis, a member of the

This fish lived in the coastal waters of what's now Bolivia, during the Ordovician Period,

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,

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

So Sacabambaspis was the first fish that we know of to acquire this kind of covering, but

The period that came next, the Silurian, saw the evolution of armor in many groups of fish,

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

Other Heterostracans had fused plates on their heads, with diamond-shaped or elongated scales

But head plates took on new forms in another group of jawless fish: the Osteostracans,

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

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

They didn't have much of one -- their backbones, like those of heterostracans and earlier fish,

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

Then, as an additional layer outside of that, unconnected, was a thick layer of exoskeletal

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

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

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

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

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

And Phosphorous is an important part of the little molecule your body uses to store and

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When Fish Wore Armor

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