It may not surprise you to learn that some kinds of fossils are more common than others.

Most dinosaurs, for instance, are known from only one or two specimens.

Meanwhile, some trilobite species are known from hundreds or thousands of specimens.

But one of the most abundant kinds of fossils on Earth, numbering in the millions of specimens,

came from something most people have never heard of.

And for decades, their identity was a mystery to paleontologists.

But geologists figured out that these mysterious fossils could basically be used to tell time

in the deep past!

Please allow me to introduce you to the conodonts.

From the time they were first discovered in the 19th century, and right up until the 1980s,

conodonts were only known from isolated hard parts, called conodont elements.

Some of them looked like little fans, or saw blades, or even barbed wire, but most of them

looked like cone-shaped teeth.

So they were given the name conodont, which means “cone tooth.”

These elements are all verrry tiny.

Most are under one millimeter, and none greater than five millimeters long.

And for a long time, paleontologists were totally stumped about what animals these things

belonged to, and where on the Tree of Life they belonged.

Some experts thought that, although they look kind of like teeth, their similarities to

teeth are pretty superficial.

But others pointed out that the fossils were made from a mineral called hydroxylapatite.

This is the same mineral that bones and teeth are made from in vertebrates.

So, this led many scientists to think that they were teeth, maybe from some kind of weird,

extinct fish.

But while paleontologists were spending a hundred years or so debating what conodonts

were, geologists were discovering what conodonts could do.

When a species is really abundant, widespread, distinctive, and found in a restricted span

of time, their remains can be used as Index Fossils.

These are fossils that help geologists correlate and date rock layers all over the world.

And conodonts were all of those things -- abundant, widespread, and composed of lots of distinctive

species.

So they turned out to be some of the most important index fossils from the whole Paleozoic

Era.

In fact, many subdivisions of the Paleozoic are actually defined by when certain conodont

species first or last appear.

For instance, you know when the Devonian Period ends and the Carboniferous Period begins,

because that's when you start finding fossils that belong to a species of conodont known

as Siphonodella sulcata.

Likewise, in the Triassic Period, nearly every age is defined by the first appearance of

various conodonts.

The extinction of Metapolygnathus parvus marks the end of the Carnian, for example, while

the start of the Rhaetian is defined by the first appearance of the genus Misikella.

So, geologists are able to use these fossils to basically tell time!

But conodont elements can do more than just that!

They're also, essentially, geological thermometers!

It turns out that conodont elements actually change color when they're heated up.

And no matter what species they belong to, they go through the same range of color changes

at the same temperatures.

Geologists have used these fossils to devise a six-point scale of how they change from

their natural tan color, to brown, then gray, then black, and finally white, as they get

hotter and hotter.

With this scale, called the Conodont Alteration Index, geologists can use the color of a conodont

that they find, to figure out how hot the surrounding rock once was.

And this can be really important in fields like petroleum geology, because it can reveal

whether sediments ever got hot enough for organic hydrocarbons to be converted into

oil and gas.

So conodont elements quickly became one of the handiest tools in the geologist toolkit.

But meanwhile, paleontologists still had no idea what kind of animals conodonts were.

In the 1970s, fossils of some vaguely fish-like creatures were found in Montana that were

thought to be conodont animals.

But further study revealed that the conodont elements found in those fossils were actually

in the animals' guts.

So it turned out that they weren't conodont animals; they were fossils of creatures that

ate conodont animals!

The mystery lingered until 1983, when fossils were discovered in 350 million year old sediments

from Scotland that brilliantly preserved the soft-bodied animals.

And … they were kinda weird, with long eel-like bodies, tail fins, a stiff rod of tissue down

their backs, and giant bulging eyes.

Thanks to these beautiful specimens, we were able to learn that conodont animals were a

kind of fish after all!

The fossils revealed distinctive, zig-zag-shaped muscles, known as myomeres, which are still

present in fish today.

And the rod down its back was a notochord, an early precursor of the vertebral column

seen in vertebrates.

Both those features--myomeres and notochords--are found only in chordates.

And the fossils also revealed that the tiny, tooth-like conodont elements were concentrated

in the animal's mouth -- but also in its throat!

The elements were arranged in a complicated array of blades and points, like some kind

of horror movie monster.

Some experts think these spines and barbs may have been helpful in gripping and slicing

tiny prey.

But others suggest that they were used to filter plankton from the water.

Either way, the picture of these creatures was finally coming into focus!

Like modern lampreys and hagfish, conodonts were jawless fish, and they were one of the

earliest and most successful groups of vertebrates.

And they thrived all over the world throughout the Paleozoic Era, with many species schooling

in the open ocean like modern sardines, while others stayed closer to shore.

Throughout their history, conodonts were affected by several mass extinctions, including the

Great Dying at the end of the Permian Period 252 million years ago.

But it wasn't until the Triassic-Jurassic Extinction about 200 million years ago that

they were finally wiped out.

A quick rise in carbon dioxide caused the climate to warm up, while the acidity, salinity,

and oxygen levels in the oceans all began to change.

Their final extinction brought an end to 300 million years of conodonts' role as a cornerstone

of the world's ocean communities.

Sad, I know.

But their remains ended up being some of the most important fossils of the entire Paleozoic

Era.

They've helped geologists find oil and tell deep time, while allowing paleontologists

to understand a whole new type of animal life.

Thanks to their success and incredible abundance, they're among the most useful fossils in

the world.

Thanks to CuriosityStream for supporting PBS Digital Studios.

With CuriosityStream you can stream documentary films, and programs about science, nature,

and history, including exclusive originals!

For example, you could check out Planet Dinosaur, a three-part BBC series about the latest discoveries

from the days of the non-avian dinos.

CuriosityStream offers unlimited streaming, and for you Eons viewers -- because I like

your face! -- the first two months are free if you sign up at curiositystream.com/eons

and use the promo code EONS.

Thanks for joining me!

I'm always interested in what you want to learn about!

So leave me a comment below with your questions about ancient life!

And be sure to go to youtube.com/eons and subscribe.

Now I'm sure finding out how precise these tiny teeth fossils are was a bit shocking,

but what shouldn't be surprising is the preciseness of math.

Go learn the language of the universe with our sister channel Infinite Series and find

out what numbers are made of and if there's a way to divide by zero.