Eons

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

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

But what really stands out about the ratfish is what its made of: cartilage, not bone.

Just like sharks.

And thats because the ratfish is a strange leftover from a once-great evolutionary chapter in our planets history.

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

It was the golden age of sharks.

But this golden age wasnt dominated by massive killing machines like, say, megalodon.

Instead, it belonged to creatures that werewelljust really, really weird.

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

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

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

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

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

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

Because cartilage is softer than bone, it doesnt fossilize well, which means that much of the shark fossil record is teeth, spines, and scales which are all made of the more durable dentine.

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

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

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

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

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

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

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

But despite the limitations imposed by placoderms, the ancestors of chimaeras and true sharks were already present they just werent dominant.

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

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

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

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

And during the final few million years of the Devonian, sharks didnt just bounce back from the extinction event, they boomed.

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

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

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

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

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

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

Here, we dont just find teeth and scales, but also complete skeletons and even soft tissues.

And this glimpse reveals a truly wild cast of characters.

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

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

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

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

There were even sharks that defied all comparison.

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

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

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

And they werent all just small specialists.

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

One impressive specimen from whats now Mammoth Cave National Park in Kentucky reveals that these apex predators would have even looked somewhat familiar not unlike sharks today.

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

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

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

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

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

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

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

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

And they didnt stop at a single shell-crushing mode.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

But what exactly caused 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 and falling sea levels caused by the continents colliding to form Pangaea, disrupting food chains.

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

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

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

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

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

And sharks and other cartilaginous fish were no exception.

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

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

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

But not all was lost.

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

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

While we dont have all the details worked out, it seems like it took a one-two punch of anoxia or climate change and the greatest mass extinction event in Earths history to finally end this golden age.

As the planet slowly recovered, sharks began to rebound.

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

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

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

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

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

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

And, for a time, generalists inherited the Earth.

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

Today, theyre a reminder of that golden age,