Deep beneath the waves,聽 gliding over the ocean floor,聽聽 swims a bizarre creature known as the ratfish.

With its wing-like fins, whip-like聽 tail, and shell-crushing teeth,聽聽 it looks like something stitched together from聽聽 spare parts of other animals – which  is why it’s also called a chimaera.

But what really stands out about聽 the ratfish is what it’s made of:   cartilage, not bone.

Just like sharks.

And that’s because the ratfish is a  strange leftover from a once-great聽聽 evolutionary chapter in our planet’s history.

Long before the rise of the great聽 whites and hammerheads we know today,聽聽 sharks and their cartilaginous relatives ruled聽 Earth’s oceans and rivers in astonishing variety.

It was the golden age of sharks.

But this golden age wasn’t dominated by  massive killing machines like, say, megalodon.

Instead, it belonged to creatures that聽 were…well…just really, really weird.

We’re talking sharks with saws for teeth, sharks  that looked more like flying fish than fearsome聽聽 predators, and sharks with what can only be聽 described as anvils growing out of their heads.

So why did sharks get so incredibly聽 diverse and odd during this period,聽聽 only to lose most of that diversity forever?

Turns out, the same thing that gave rise聽 to the golden age eventually ended it.

Tracing the fossil record of sharks聽 and their relatives is no easy task.

And by 'sharks and their relatives,'聽 I mean the whole group of fish whose聽聽 skeletons are mostly made of cartilage.

We'll be calling them all聽 'sharks' in this episode,聽聽 mostly because many of the fossil species聽 are commonly called 'sharks,' even if they聽聽 aren't part of the taxonomic group聽 we recognize as true sharks today.

Because cartilage is softer than bone, it doesn’t  fossilize well, which means that much of the shark聽聽 fossil record is teeth, spines, and scales –  which are all made of the more durable dentine.

The first hints of the emergence of聽 sharks come from some isolated scales聽聽 from the Late Ordovician Period,聽 around 465 million years ago.

But the first definitive fossils come from the聽 Silurian Period, around 439 million years ago.

These early sharks were small聽 generalists that shared the聽聽 oceans with a number of other groups of fish.

And of those other groups, one in particular might聽聽 have limited the diversity聽 of sharks: the placoderms.

By the Devonian Period, around 419聽 million years ago, these heavily-armored聽聽 fish had radiated into many different聽 species, essentially ruling the seas.

Some, like the massive Dunkleosteus, were聽 apex predators, while others were more modest聽聽 bottom swimmers or feeders and ranged聽 from marine to freshwater environments.

Because placoderms were everywhere, some聽 paleontologists think they probably competed聽聽 with sharks or even excluded them聽 entirely from certain marine niches.

But despite the limitations imposed by placoderms,聽聽 the ancestors of chimaeras and true sharks were聽 already present – they just weren’t dominant.

Sharks were small-time players,聽 stuck on the ecological margins.

This balance of power changed,聽 though, when the Late Devonian聽聽 Mass Extinction hit marine ecosystems聽 starting around 373 million years ago.

While this mass extinction was聽 a disaster for the placoderms,聽聽 which eventually disappeared altogether,聽 for sharks, it was an opportunity… See, some sharks and their relatives聽 probably managed to survive by exploring聽聽 a wider range of feeding strategies and聽 living or venturing into open oceans,聽聽 rather than just sticking to the shallow seas聽 where the extinction effects were most severe.

With their main competitors gone,聽 sharks and other cartilaginous fish聽聽 wasted no time in filling the now-empty聽 niches that the placoderms left behind.

And during the final few聽 million years of the Devonian,聽聽 sharks didn’t just bounce back from  the extinction event, they boomed.

By the Carboniferous Period, around 359 million聽 years ago, warm, shallow seas were widespread.

And in those waters, sharks聽 began to get, well, weird.

Near the equator, sharks thrived as they evolved聽聽 into a range of species of聽 varying shapes and sizes.

It was an explosion of shark diversity聽 unlike anything ever seen before.

In fact, in some of those environments,聽聽 sharks and their relatives were up to 60%聽 to 70% of their total number of species.

One glimpse into this golden age comes from the聽 roughly 325 million-year-old Bear Gulch Limestone聽聽 in Montana, a site of exceptional preservation聽 that captured the ecosystem in stunning detail.

Here, we don’t just find teeth and scales, but  also complete skeletons and even soft tissues.

And this glimpse reveals a聽 truly wild cast of characters.

Because, unlike their generalist聽 ancestors before them,聽聽 these creatures were well-adapted聽 to very specific ways of life.

Take Thrinacodus, for example – a shark  that evolved an eel-like body, allowing聽聽 them to move through tight spaces and ambush聽 prey in crevices no other shark could manage.

There was also Janassa - which looked聽 remarkably like a modern ray with a聽聽 flattened body and teeth that were聽 specialized for crushing shells.

And perhaps the most surprising was Iniopteryx,聽 a shark that looked like a flying fish – though   it likely used its large pectoral fins to聽 move vertically in the water as it chased聽聽 after prey rather than glide above the聽 water’s surface like flying fish do today.

There were even sharks that defied all comparison.

Like the stethacanthids, a group of sharks聽 that sported what looks like an anvil聽聽 on their heads, complete with hundreds聽 of tooth-like structures on top of it.

This bizarre headgear may have been聽 used to ward off potential predators聽聽 or possibly for some sort of courtship with mates.

These golden age sharks were the result聽 of evolution getting… experimental.

And they weren’t all just small specialists.

Some, like Saivodus, grew to sizes rivaling聽 modern great whites, as much as 8 meters long.

One impressive specimen from what’s now Mammoth  Cave National Park in Kentucky reveals that聽聽 these apex predators would have even looked聽 somewhat familiar – not unlike sharks today.

Sharks and their relatives also managed to聽 conquer freshwater habitats during this time.

One shark called Xenacanthus, which聽 roughly translates to ‘alien spine’   for the long spine that emerged from聽 the top of its head, was part of a聽聽 larger group that was just as diverse聽 ecologically as the sharks in the ocean.

Some of these xenacanths ate fish, while聽 others hunted invertebrates or even filter-fed.

But what allowed sharks to diversify聽 so dramatically and fill so many niches聽聽 so quickly from the end of the聽 Devonian into the Carboniferous?

Well the answer might lie in the most iconic聽 feature of sharks even today: their teeth.

See, during the Late Devonian and聽 throughout the Carboniferous, sharks聽聽 developed a huge variety of tooth structures,聽 each specialized for different feeding styles.

For example, ratfish and their聽 shell-crushing relatives developed聽聽 super-strong tissues that hardened their tooth聽 plates, creating a hypermineralized dentine.

This allowed them to feast on prey that was聽 harder and had larger shells than they ate in the聽聽 Devonian – expanding their options to include a  greater diversity of brachiopods and crustaceans.

And they didn’t stop at a  single shell-crushing mode.

Sharks evolved an array of tooth shapes, from聽 large plates, to teeth covered in knobby bumps,聽聽 each useful for eating different聽 types of hard-shelled animals.

For sharks that preferred soft-bodied prey,聽聽 including fishes, the cladodont聽 tooth was most common.

This kind of tooth had multiple聽 cusps surrounding a large central聽聽 one - perfect for puncturing and grasping聽 slippery things like fish and squid.

The arrangement and size of these cusps聽 varied between species and even within a聽聽 single shark’s mouth, creating a more versatile  toolkit for handling and processing prey.

Teeth specialized for cutting and聽 gouging big animals were also popping up,聽聽 which would become more common in later sharks.

But possibly the most bizarre聽 of all were the “tooth whorls,”   teeth that were arranged in spirals, fused聽 together to create large cutting surfaces.

Overall, this dental diversity,聽 combined with the wide range of聽聽 body forms that sharks evolved,聽 allowed them to dominate aquatic聽聽 ecosystems in ways no single group聽 of vertebrates has managed since.

And being able to occupy different niches聽 from shallow marine basins to open oceans,聽聽 and from tropical seas to freshwater聽 rivers, meant that the Carboniferous聽聽 truly was the golden age of sharks, a time聽 with seemingly no limits on what they could do.

And as the Carboniferous gave聽 way to the Permian Period,聽聽 around 299 million years ago,聽 sharks were still going strong.

Whorl-toothed sharks like Helicoprion proliferated聽聽 and more freshwater sharks聽 emerged among the xenacanths.

But the Earth was changing, which would聽 make the very specializations that made聽聽 these sharks so successful into a vulnerability.

As the Carboniferous became the Permian,聽聽 the supercontinent of Pangaea formed,聽 changing coastlines and sea levels.

And by the Middle Permian, sharks聽 and shark-like cartilaginous fish聽聽 diversity was not as high as it聽 had been during the Carboniferous.

During this time, there’s a spike in  the number of species going extinct in聽聽 the fossil record, while fewer and fewer聽 new species appear to take their place.

But what exactly caused聽 this shift is still debated.

It might have been ocean anoxic events – periods  when oxygen levels in the waters plummeted.

Or maybe it was climate change聽 and falling sea levels caused by聽聽 the continents colliding to form聽 Pangaea, disrupting food chains.

Whatever the cause, the tides had聽 begun to turn for sharks, whose bizarre聽聽 adaptations were well-suited to specific聽 conditions that were changing dramatically.

Now, many shark groups that had evolved during the聽聽 Carboniferous did manage to press on,聽 just not speciating as fast as before.

These groups managed to adapt聽 to the changing conditions,聽聽 while other groups disappeared entirely.

It all came to a head during the聽 mother of all mass extinctions,聽聽 the End-Permian mass extinction聽 event, AKA the Great Dying.

Approximately 252 million聽 years ago, massive volcanic聽聽 eruptions pumped enormous amounts of聽 carbon dioxide into the atmosphere,聽聽 triggering global warming and ocean acidification聽 that wiped out over 80% of marine life.

And sharks and other cartilaginous聽 fish were no exception.

Because, unlike the Late Devonian聽 mass extinction that their ancestors聽聽 had soared through, many of these聽 sharks were no longer generalists.聽聽 They had traded their flexibility for聽 their bizarre, specific adaptations.

The same thing that had launched聽 sharks into their golden age of聽聽 weirdness – a mass extinction – would  now punish them for getting too weird.

Many of the strange sharks and cartilaginous聽 fish that had characterized the golden age聽聽 disappeared completely – the  incredible array of shark body聽聽 plans and feeding strategies mostly聽 erased in an evolutionary instant.

But not all was lost.

Just as sharks had survived the Late Devonian聽 Mass Extinction by venturing into open water,聽聽 some sharks were able to pull from that聽 ancient playbook to weather the Great Dying.

These survivors were less specialized聽 and less weird, but more resilient.

While we don’t have all the details worked out,   it seems like it took a one-two punch聽 of anoxia or climate change and the聽聽 greatest mass extinction event in Earth’s  history to finally end this golden age.

As the planet slowly recovered, sharks began聽 to rebound.

Some groups from the golden age聽聽 bounced back in the Triassic, though many聽 would be gone by the end of that period.

And some of the other survivors from聽 the golden age would re-diversify again聽聽 later in the Mesozoic Era and聽 continue on into the Cenozoic.

This ultimately produced familiar sharks like聽 megalodon by the Miocene epoch, as well as聽聽 the sharks and cartilaginous fish we have today,聽 like great whites, hammerheads, rays, and skates.

The rise and fall of weird sharks reminds us聽 that the aftermath of crises is when ecological聽聽 space opens up, so that’s often when we see  evolutionary experiments in radical new forms.

But it also reminds us that there's聽 a trade-off between being perfectly聽聽 adapted to a particular set of聽 conditions and being adaptable.

As fantastically bizarre as they were,聽 the most specialized sharks with the most聽聽 spectacular solutions to life's challenges聽 struggled when those challenges changed.

And, for a time, generalists inherited the Earth.

But through it all, the ratfish glides on along聽 the seafloor, using their specialized teeth聽聽 to crush the shells of their prey, looking聽 much like they did during the Carboniferous.

Today, they’re a reminder of that golden age,