Eons

Kallie Moore: We mammals are the result of a long and winding evolutionary journey.

[Elephant trumpets] Hundreds of millions of years of mutation, selection, and chance have brought us from humble beginnings as small and obscure lizard-like insect hunters to the diverse and spectacular modern mammals of today.

In fact, the Cenozoic Era that we currently live in, the era of recent life that covers the last 66 million years, is sometimes referred to simply as "The Age of Mammals."

Though, considering it's us mammals doing the referring, there may be a little bias involved.

Blake de Pastino: But we really do have some justified bragging rights in this era.

Because for much of the Cenozoic, from the end of the reign of the dinosaurs right up to the modern day, the largest animals in most environments have been mammals of some sort.

From bears to bison to blue whales, big, conspicuous mammals exist almost worldwide.

And we come in all kinds of shapes and sizes, filling all kinds of ecological niches, too.

So, it does seem like something of a golden age for us.

But the story of how mammals came to be so successful, diverse, and charismatic stretches back well before the Cenozoic, before the rise and fall of the dinosaurs, and even before the first true mammals themselves.

Our family tree has deep roots, and they reach back to the world of the late Permian, more than 252 million years ago.

This was the final chapter in the era of ancient life, the Paleozoic, when the synapsid ancestors of mammals-- specifically, a group of them known as therapsids-- were in a much older golden age, one that's often forgotten about.

These therapsids, our direct ancestors and cousins, are separated from us by such a long expanse of evolutionary time that, in a lot of ways, the family resemblance is often hard to see.

They laid eggs, they had no visible external ears, and they were probably hairless, leathery, and couldn't regulate their body temperatures like we do.

But scattered across the therapsid family tree, a mosaic of increasingly mammal-like traits was gradually emerging, too.

Michelle Barboza-Ramirez: From saberteeth and tusks, to versatile limbs, to impressively complex behaviors-- features that in many cases had never been seen before, but have reemerged in mammals many times since.

In fact, for quite a while in paleontology, therapsids were referred to as mammal-like reptiles, on account of their blend of seemingly ancient and modern traits.

The Late Permian Period was a critical chapter in our evolutionary story, albeit for two wildly different reasons.

On the one hand, it was a time when therapsids thrived, diversified, and, in some cases, became more mammal-like.

But, on the other hand, this first golden age of therapsids would come crashing down by the Permian's end.

Almost all of this diversity would be wiped out, along with nearly all other Permian life, in the biggest mass extinction of all time--the Great Dying.

And it's only thanks to a lucky few survivors from this event that true mammals, including us, ever had a chance to eventually shine.

[Growling] [Film reel spinning] Gabriel-Philip Santos: The fossil record of synapsid evolution is a tale written mostly in teeth.

That's partly because they fossilize better than most other body parts, and they can tell us a lot about an animal's identity and lifestyle.

But it's also because one of the most unique characteristics of us mammals and our ancient synapsid relatives is that our teeth are highly specialized for multiple roles, sometimes in pretty elaborate ways.

The story of synapsid evolution leading to true mammals is innately a story in large part about teeth and the weird things we evolve to do with them.

It might not seem like a particularly important or radical trait compared to our big brains or our complex behaviors, but from an evolutionary perspective, having a bunch of different kinds of teeth is one of the most remarkable adaptations we possess.

Christy Rettenmund: We're currently at the Animal Health Center, where we provide health care to animals at the Milwaukee County Zoo.

Today, we'll be anesthetizing a North American river otter to do an examination on it.

So, river otters have 36 teeth.

They're composed of incisors, canines, premolars, and molars.

So, they're a fairly typical carnivore species with their dentition.

Not only do our teeth differ from each other within our mouths-- think of your molars, incisors, and canines, for example-- but the mammalian dental setup as a whole can radically evolve to better specialize on certain diets.

Christy: Probably a good example of that would be if you compare giant pandas to brown bears.

So, their diets are radically different.

Giant pandas are gonna be eating bamboo, and because of that, most of their teeth look more like molars.

So, they don't have a need for very well-developed canines.

Their molars are definitely much flatter and stronger for eating that bamboo as compared to brown bears, where their teeth tend to be a little bit more ridged, so that they can grab onto their prey and kind of chew their prey a little bit easier.

Joan Stasica: Giraffes are ruminants, so they eat a bunch of food at once, and then they bring up a cud, and they chew it again and swallow it again, and they might bring it up again, in order to get all of the nutrition out of all those plants that they're eating.

And so, those big, flat molars in the back of their jaw help with the grinding of the cud every time they bring up that cud to chew.

So, just like a cow.

Exactly.

Giraffes actually have 32 teeth, just like people do, but most of a giraffe's teeth are in the back of their mouth.

They have these big, flat molars.

And then in the front, they have incisors on the bottom, but on the top, they just have kind of a hardened gum plate.

They don't actually have any front teeth on the top.

They can use the bottom incisors and then the gum plate to kind of strip bark off of branches or to strip leaves off of branches.

So, they're really good at using their tongue and their lips and those bottom incisors to do everything they need to do.

Kallie: In contrast, fish, amphibians, and reptiles mostly have sets of pretty uniform teeth that only really vary in each mouth by size.

They tend to have less different types of teeth as compared to mammals.

So, for instance, sharks, they're carnivores, so they have pretty much all canine teeth.

So, because they don't really need any molars for grinding, they're not eating plants, reptiles, amphibians, and fish tend to have maybe one type of tooth in their entire mouth, because they just don't need all of those different specialized teeth like mammals do.

Kallie: But our uniquely adaptable dentition has allowed us to become so diverse and successful by opening the door to new abilities and specializations that weren't possible before.

And our new tools couldn't have come at a better time, because Late Permian Pangea had plenty of challenges for those ancient therapsids to solve.

The supercontinent's habitats varied from patches of tropical equatorial swamps, to vast arid deserts, to towering forests of conifers, gingko relatives, and other ancient seed plants.

There were a lot of open niches and ecological opportunities for our Permian relatives to sink their teeth into-- so to speak.

And in the fossil record of Late Permian therapsids, the relatives and forerunners of true mammals, we see familiar mammalian dental setups emerging again and again.

Michelle: OK, so we are here at the Burke Museum of Natural History, which is my local museum.

I come in once a week to volunteer in the fossil prep lab.

So, we're at the Burke, because we are learning about Permian Age fauna, and some of the best Permian Age fauna are found in Russia and Africa.

We are at a place that has already gone and brought those fossils back for us to take a look at.

At the top of the Late Permian food chain was a group of therapsids called the gorgonopsians, the first known sabertoothed animals that the world ever saw.

Their fossils are mostly found in South Africa and Russia, which were both at the high latitudes on the supercontinent of Pangea at the time.

So, what are we looking at here?

This is a gorgonopsian from Africa named Lycaenops.

They're a fascinating transitional animal that's kind of halfway between a reptile and halfway between a mammal.

Michelle: Those first gorgonopsians lived and hunted in the shadows of another group of therapsids called the dinocephalians that dominated the large animal niches of the Middle Permian.

But during the chaotic transition from the Middle to the Late Permian, the dinocephalians disappeared entirely, allowing gorgonopsians to fill many of their former ecological niches.

Early on, they were a lot smaller.

Towards the end of the Permian, they get really, really big.

And by the time they get really big, they ate everything and anything.

Is this the really big?

No, this is the really small.

While some late gorgonopsian species remained small, others took the extinction of the dinocephalians as an opportunity to supersize.

Kind of like modern cats, they all kept the same basic body plan and predatory lifestyle, with only modest physical differences across species, other than their size.

By the Late Permian, some gorgonopsian species reached over 3 meters long, about the size of a large tiger, making them the biggest predators of the Late Permian.

They specialized as hypercarnivores, capable of hunting even big and powerful prey that could put up a fight.

Like the pareiasaurs, for example-- heavily armored, cow-sized reptiles that also reached up to 3 meters from head to tail.

More ancient apex predator synapsids, like Dimetrodon from the Early Permian, had jaws optimized for grasping and holding onto struggling prey-- long and lined with lots of teeth.

But the gorgonopsians of the Late Permian had evolved their own distinctive approach to predation-- maximum damage.

Reptiles walk with their legs out on the side of their body.

The sprawl, right?

Yeah.

And then their hands are also turned sideways, pointing sort of to the back of them.

Now I'm a reptile.

You're a reptile.

Meanwhile, I am a synapsid like Lycaenops, and my limbs are underneath my body, and my hands point forward when I walk.

So, we go from this to this.

To this.

And it's helpful, because when your limbs are under your body, and your feet are pointing forward, you're a lot more active and quicker than some of the animals that might be waddling around you with their legs on the side.

So, he's like... beast mode.

They were very active hypercarnivores.

They were chasing and running down each other and other animals in the environment.

Michelle: Their jaws were more squared off at the front with a chin, and they had fewer but more specialized teeth, mostly towards the front of their mouths.

This suggests that they were adapted for inflicting deep wounds and ripping chunks of flesh so that their victims were killed or subdued quickly, before they had much chance to resist.

This was probably a kind of arms race response to the species they preyed on, evolving bigger body sizes in the Middle and Late Permian.

And perhaps the most iconic adaptations of the gorgonopsians for this hypercarnivorous lifestyle were their saberteeth.

Now, earlier predators had experimented with enlarged canines too, like Dimetrodon, for example, but none had evolved ones quite this exaggerated and specialized before.

[Gasps] Based on the size, we think it's the African species of Inostrancevia, which is one of the largest gorgonopsians, typically found in Russia and Siberia.

Oh, my God.

Wow, you can really see the serrations here.

Michelle: These sabers were a first for their time, but they're a trait that would pop up over and over again in later mammal evolution.

You might have even seen them before in the famous sabertoothed cats of the Cenozoic Era.

They were also apex predators that could quickly overpower big and potentially dangerous prey.

And the gorgonopsians didn't just use their teeth to bite prey.

They may have had a social function too.

That evidence comes in the form of a fossilized gorgonopsian snout with what might be a healed bite mark and tooth fragment embedded in it.

I have another jaw of a gorgonopsian right here.

[Gasps] It's another lower jaw, different size.

Right here... That's not his tooth.

It's not his tooth.

Ew... That is probably the tip of another gorgonopsian's tooth, embedded in the side of the denary, or lower jaw.

And it's got signs of healing around it.

This wasn't a killing blow.

There's a couple different ways to interpret this.

One could be that it wasn't attacked by another gorgonopsian that was seeking to end this gorgonopsian's life and make it food.

It could have also been a social behavior, like asserting dominance.

So, is that idea because modern animals today also do that sort of social biting?

We see in a couple different groups, like canids and modern dogs, they have a dominance behavior where they will bite each other without the intent to kill.

It can get a little rough.

Yeah.

It's a rare snapshot into the complex social life of a Late Permian gorgonopsian, and a reminder that behaviors we think of as advanced can sometimes be found in surprisingly ancient places.

I think they would have looked terrifying with those huge canines coming at you.

But I think they would have looked somewhat like a mammal.

They would have had characteristics that would have connected you to them.

They might have had a little fleshy nose at the front, kind of like a dog's.

They might've had whiskers or some primitive hair-like structures on their face.

They would've started to look kind of fuzzy and lovable.

Almost there.

Almost there.

I think gorgons should be talked about more, because no one knows about them.

They get so little attention in terms of popular culture and TV.

I think they're the coolest thing you've never heard of.

Gabriel: It wasn't just the carnivorous therapsids that were specializing in increasingly mammal-like ways.

Many of the herbivores were doing it too, and alongside the first saberteeth came the first tusks.

Tusks evolved for the first time in the Late Permian, in a group of barrow-bodied herbivores called dicynodonts, which literally means "two-dog teeth."

Dicynodonts were one of the most successful therapsid groups of the time, radiating into nearly 100 species, ranging from small, gopher-sized burrowers to big and bulky grazers and browsers spread across the supercontinent of Pangea.

Like the saberteeth of the gorgonopsians who preyed on them, the tusks of dicynodonts are the first of their kind in the fossil record, but they've convergently evolved again and again in mammals.

Here you go.

I got to feed an elephant.

[Laughs] You're joining us in our Elephant Care Center.

We're home to three African savanna elephants.

What are elephant tusks?

Elephant tusks are elongated incisor teeth from an elephant.

Elephants naturally use them for debarking logs, digging holes, fighting, defense, shows of dominance, if you're a male.

Kallie: But dicynodonts were the original pioneering tuskers, probably using them for things that modern tusk animals do, like rooting around for plant material, defending against predators, and for social display.

Now, despite so many typically mammalian adaptations emerging for the first time in the Permian, therapsid evolution was not a single linear pathway towards true mammals.

And true mammals ourselves were not destined to end up with the particular combination of features that we did.

Evolution is much more branching than that.

And many ancient branches of our family tree, like the dicynodonts, also evolved in ways unlike any other mammal relatives, past or present.

Gabriel: Over the course of their evolution, many dicynodont lineages actually replaced most of their teeth, other than their tusks, with a turtle-like beak made of keratin.

It's an unusual and kind of ironic form of dental specialization, losing your teeth almost entirely.

And permanently trading teeth for a beak is a unique trait in therapsids that had never been seen before, and has never been seen again either.

And the story of the dicynodonts' beak shows us that although these ancient therapsids seem more and more like later true mammals, in a lot of ways, they were evolving in completely different directions, too.

But in the Late Permian, dicynodonts' beak and tusk combo seems to have paid off big time.

Michelle: Perhaps the single most impressive therapsid of the Late Permian was a small, cylindrical, and extremely abundant dicynodont called Diictodon.

So, this is a skull of Diictodon.

And Diictodon, in South Africa, at least, in the Karoo Basin, there's at least 3,000 of these specimens.

Most other species are known from a handful, like two or five or ten.

So, I know of a couple of localities where you'll find 100 Diictodons for every gorgonopsian, for example, that you might find.

Wow.

So, it gives you an idea of how abundant they are on the landscape.

Diictodon's success probably came down to the ecological niche it filled.

It was a gopher-like burrower that dug spiral tunnels with chambers.

Diictodon's beak and tusks were adapted for clipping tough vegetation and pulling fibrous roots and tubers from the soil.

And this would have helped them access nutrients, even in pretty harsh environments, on the increasingly hot and dry supercontinent of Pangea.

Plus, their burrows would have provided shelter from the elements, helped them regulate their temperature, escape from predators, and possibly even raise their young.

Christian: So, we haven't found the babies yet inside there, but you definitely find multiple skeletons of the same species living together.

So, they were social.

Perhaps.

I mean, if you look in cynodonts in the Triassic, you definitely see burrows where they're creating, like, a network of burrows.

Little underground city.

Little meerkats, right?

Wow.

Diictodon and dicynodonts don't seem to be doing that specifically.

They are doing individual burrows, perhaps with multiple animals living in them.

Although, I should say, when you find one burrow, you almost always find more.

So they are perhaps living in the same area.

The property value, they could still afford their own home.

They hadn't quite moved into condos yet.

Exactly.

[Laughs] Gabriel: The complex and dynamic ecosystems of Late Permian Pangea sparked an evolutionary arms race between predators and prey, pushing species to innovate to survive.

And one of the most intriguing adaptations to emerge from this coevolutionary dance came from a therapsid group known as the therocephalians, or "beast-heads."

Therocephalians were a diverse group that ranged from small insect-eaters to large predators that almost rivaled gorgonopsians in the Late Permian food chain.

But it's from the smaller side of the family that an especially impressive weapon may have emerged for the first time in vertebrates-- a venomous bite.

Now, venom is something usually more associated with the reptile side of the vertebrate tree, but it may have actually first emerged in the mouths of these very ancient relatives of mammals.

Because in the Late Permian, around 100 million years before the first snakes evolved, the earliest potentially venomous vertebrate was a therocephalian around the size of a small dog, called Euchambersia.

There are only two of these known in the world.

And we have one?

Well, we have a 3D print.

OK, so this is finally where I get to talk about my earrings.

These are a cast of an actual gorgonopsid skull.

We have technology now that is allowing us to 3D scan and CT scan fossils that are in our collections.

The fossil was CT scanned, the data was converted into a digital format, and then you can send that data to a 3D printer and print a new version of it.

I mean, for a paleontologist, this is an amazing thing, because there are only two of them in the world, and we can now print one here at my museum for teaching, for research, for whatever we want.

Right behind the canines on the upper jaws of Euchambersia's skull are a pair of huge deep hollows in the bone.

No other animal has anything like this in the fossil record.

And the question is, what fills that big gap, right?

Could it be a scent gland?

Could it be, you know, some other type of gland, maybe something for digestion?

We don't know.

And so, is it possible that this housed a venom gland?

That's a definite possibility.

But I think there's still lots of other possibilities out there.

Whenever you're talking about soft tissue in the fossil record, you have to make inferences.

And our best inferences come from animals that are alive today.

And despite being much more common on the reptile side of the family tree, venom has evolved a few times, at least in modern mammals, including the platypus, the slow loris, and the shrew-like solenodon.

But none of them house the venom glands in the snout below the eye, as Euchambersia seems to have.

It's still sort of an open question, I would say.

Is this an experiment that didn't work out?

Or are we misinterpreting this?

But if this animal did have venom, that would open up a lot of really interesting possibilities, right?

The extraordinary diversity of ways that therapsids were experimenting at this time means that there's always the possibility that the truth is actually so strange that we would never guess it in a million years, or 255 million years, in this case.

Gabriel: Somewhere in this therapsid golden age, dodging saberteeth, tusks, and possibly venom, were our direct ancestors.

We mammals trace back to just a sliver of this diversity, and those other therapsid cousins of ours-- the gorgonopsians, the dicynodonts, and the therocephalians-- would eventually disappear entirely.

But we belong to the one and only lineage of therapsids-- and synapsids as a whole-- that did survive all the way from the Paleozoic era through the Mesozoic and into the Cenozoic-- the cynodonts.

So, this is a cynodont from the Late Permian of Zambia.

And cynodonts among the Late Permian therapsids are the closest relatives of mammals.

This will give rise to the animals that cross over that major mass extinction at the end of the Permian, and then eventually give rise to mammals later on.

And so, a lot of the features that we think about mammals actually have their origins way back in the Late Permian.

Michelle: The cynodonts of the Late Permian were not the biggest, strongest, or most successful group of therapsids around.

Blake: They were just a new and relatively obscure side branch of the family tree, with early cynodonts, like this Procynosuchus, for example, being small generalists up to a meter long at most.

And their toolbox of traits was much less flashy than many other therapsids.

They evolved cusped teeth and specialized jaw muscle attachments that made their chewing more effective, which was useful for feeding on things like insects and other arthropods.

And changes in their skull structure allowed them to chew their food and breathe at the same time, a trait that we mostly take for granted today, but which was a pretty big innovation at the time.

The early cynodonts also show adaptations for a greater range of motion and an increased use of their forelimbs for capturing and manipulating their food.

This agility and dexterity meant that their range of movement and style of foraging were probably more mammal-like than any of their therapsid cousins.

The cynodonts were still very much side characters in Late Permian ecosystems compared to the sabertoothed gorgonopsians or the ridiculously abundant Diictodon, but their legacy has outlasted all of the other Permian therapsids and can still be found in the form of you, me, and all other true mammals around today.

These animals are really poorly understood, and they're a really good snapshot into the evolution of mammals and how we developed the features that we have today.

Christian: So many things that we think of that are characteristics of mammals actually evolved before dinosaurs evolved.

You can trace some features to such an ancient point in Earth history, 250-plus million years ago.

Michelle: The array of Late Permian species that thrived across the supercontinent of Pangea were the product of one of the most dynamic and ecologically complex periods of the planet's history so far.

Blake: It was a time that saw our therapsid ancestors and relatives evolve brand-new traits and behaviors that have been part of life on Earth in some form ever since.

And it's precisely because life was doing so well that the Great Dying at the end of the Permian Period was such an unparalleled evolutionary tragedy.

This proto-mammal heyday was on the cusp of a catastrophic change that would bring the other branch of the amniote family tree, the reptiles, to the forefront for an era.

Kallie: And while a lucky few therapsids would make it through the coming apocalypse unscathed, it would be another nearly 200 million years before their descendants, true mammals, regained our place in the ecological spotlight.

"Eons" "Life and Death on Pangea" is available on Amazon Prime video