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Digital Studios It’s 252 million years ago, and the world
is on the cusp of the worst disaster it would ever experience. 70 percent of all life on land is about to
vanish. But one of the very-lucky very-few that will survive is an animal called Thrinaxodon. It’s about the size of a fox and It’s a strange creature, full of contradictions. It doesn’t
stand upright like a mammal or sprawl its legs like a reptile. Its jaw is full of specialized teeth, like
a mammal’s, perfect for capturing and chewing up insects and other small creatures. And
its nose is more mammal-like than lizard-like. But it doesn’t have the soft ears that mammals
would have. It looks like a creature in transition—which
is exactly what it is. Thrinaxodon is a synapsid, a group once called
“mammal-like reptiles,” but who are now more accurately known as “stem mammals”,
and they evolved in the peculiar conditions of the Permian Period. Synapsids were the world’s first-ever terrestrial
megafauna, with carnivores that grew to the size of bears and herbivores that reached
2 tons. But the vast majority of these giants were
doomed to extinction. Almost none of the synapsid lineages survived the extinction at the end
of the Permian, known as the Great Dying, and their legacy would soon be overshadowed
by the Mesozoic Age of Reptiles. However! Thrinadoxon and its relatives rallied!
They lived on, thriving in the Mesozoic while keeping a low profile among the dinosaurs. And it’s thanks to these unassuming survivors
that the synapsids would eventually culminate in what is arguably the world’s dominant
form of life on land today — the mammals. Many of the adaptations that these forgotten
ancestors acquired way back in the Permian Period still persist today -- in you and me
both. So more than 250 million years later, our
world is the way it is, because of the time the synapsids struck back. During the Permian Period, something big was
happening on Earth. Its landmasses were forming a supercontinent called Pangea. And Pangea would become the biggest unbroken
stretch of land in the history of animal life. Its massive size would change the climate
on dry land, by reducing the moderating effects of ocean currents that kept some parts of
the land damp and cool. As a result, throughout the Permian, deserts
would spread, and the world’s climate would become much more variable. But in the Early Permian, humid tropical swamps
still straddled the equator. And in lush forests dominated by ferns and ancient plants, a new
kind of tetrapod was taking over: the amniotes. Amniotes first appeared nearly 315 million
years ago, in the Carboniferous Period. And unlike the amphibians before them, they
could likely lay their eggs on land. This made them less dependent on water and opened
up new terrestrial niches—something that would really come in handy for life on a big,
dry supercontinent. And the amniotes quickly split into two branches
around 310 million years ago: one that would give rise to all reptiles and birds, called
the Sauropsids, and the other that would eventually give rise to mammals: the Synapsids. The difference between these two groups was
initially small. Synapsids had one pair of openings in the skull, called a temporal fenestra,
while the Sauropsids had two pairs. But these two branches of land vertebrates
would soon plot very different courses. While both groups looked a lot like lizards
when they first appeared in the late Carboniferous Period, by the end of the Permian, Synapsids
had developed many traits that hinted at the mammals to come. Fossil evidence shows that early synapsids
radiated much more quickly than their sauropsid cousins. By the Early Permian, synapsids became
much more common in fossil assemblages. It’s hard to know exactly why this was the case,
since members of these two groups still seemed a lot alike. And this early group of reptile-looking synapsids,
known informally as Pelycosaurs, quickly radiated to fill many different niches on land, including
some that had never been filled before. In fact, Pelycosaurs were some of the first-ever
large land animals, with species ranging from 3 meters to 6.5 meters in length. This growth
spurt might have initially occurred because some pelycosaurs were adapting to herbivory. Pelycosaurs were some of the first terrestrial
vertebrates to start eating plants. It required them to grow large fermenting stomachs to
make the most of their high-fiber and low-nutrient meals. This rotund, barrel-like belly is on full
display in the early herbivorous pelycosaur Cotylorhynchus, whose body dwarfed its tiny
head. And these stomachs weren’t the only thing
that was changing. Plant-eating pelycosaurs were also adapting more specialized teeth
and jaws that could crush, grind, and shred vegetation. Pelycosaurs dominated the land until the end
of the Early Permian, about 271 million years ago. They probably out-competed the sauropsids
by being more adaptable and specializing more quickly to take advantage of food sources. And many of them, like Dimetrodon, had large
sails that suggest synapsids might also have been better adapted at regulating their body
temperatures. By the middle of the Permian, though, conditions
were changing, as they do. Tropical, swampy ecosystems started to shrink;
seasonal temperatures became more variable, and deserts spread. Pelycosaurs needed an environment that was
consistently warm and humid in order to survive, and they weren’t equipped to handle the
transition. But a new group of synapsids was: the therapsids. Therapsids were the second major radiation
of synapsids in the Permian, appearing around 272 million years ago. By about 265 million
years ago, they were the dominant group of terrestrial vertebrates. These synapsids had a number of traits that
suited them to a much more active and high-energy lifestyle. And these are also some of the
earliest traits that resemble those in mammals. For example, instead of having a row of teeth
that were all the same size and shape, therapsids had teeth with different shapes for different
functions. Carnivores like the massive Anteosaurus had
large front and canine teeth for ripping flesh, and smaller teeth in back for holding prey. This trend toward more specialized teeth and
jaws would continue to develop. Later carnivorous therapsids could even chew their prey rather
than ripping off chunks of meat and swallowing them whole, like what reptiles and birds still
do today. Furthermore, they developed a “secondary
palate” which redirected their nasal passage, allowing them to chew and breathe at the same
time. Which is certainly something I enjoy. Along with this secondary palate, some late
therapsids developed features known as nasal turbinates, which are specialized structures
in the nose that help animals take in oxygen more effectively. These two traits together
indicate a higher metabolic rate. The shape of therapsids’ skulls also had
space for more jaw musculature, which allowed for an increased bite-force. And therapsids
developed a more flexible neck, a longer stride, and a more upright stance. These changes indicate that therapsids had
a more versatile range of motion than other land vertebrates, which made them able to
move faster and more energetically. And being high energy is linked to another
key trait that we associate with mammals: the ability to maintain an elevated internal
body temperature. In addition to their physical adaptations,
we have other evidence that therapsids were capable of basic thermoregulation. For one thing, their fossils are found in
temperate regions that had cooler winters and hotter, drier summers. And researchers have also found hair-like
structures alongside therapsid bones in the coprolites -- or fossilized poop -- of late
Permian carnivores. This indicates that at least some late therapsids might have had
hair—a key adaption for insulation to maintain body temperature. And all of these traits show up in one particular
group of therapsids. One that’s of special interest to us mammals: the cynodonts. They weren’t the biggest or fastest, but
that’s probably what helped them get through the extinction event at the end of the Permian. These animals had the active, energetic lifestyle
of other therapsids, but their small size meant they required less energy—which is
important when you’re trying to survive a mass extinction. Cynodonts probably also survived thanks to
their ability to avoid predators and keep a low profile. One way successful cynodonts,
like Thrinaxodon, might have done this is through burrowing. The oldest known example of a cynodont burrow
is dated to around 251 million years ago — just at the boundary between the Permian Period
and the Triassic, when the Great Dying was coming to a close. And burrowing would come to be a quintessential
mammalian trait — today, at least half of mammal species are burrowers. Because of their ability to survive in more
variable conditions, to use fewer resources than other synapsids, and to avoid predators
using speed and stealth, cynodonts managed to not only survive but to remain diverse
after the greatest mass extinction of all time. And if they hadn’t, you and I would not
be here. After the vast majority of the synapsids went
extinct the sauropsids had come out on top by the mid-Triassic. The Age of Reptiles
had begun. Meanwhile, the descendants of those little
cynodonts persisted through the entire Mesozoic Era, hiding in the dinosaur’s shadows. So, to this day, the legacy of those very
first terrestrial ecosystems, and the forgotten Age of Synapsids, lives on in today’s mammals,
including you. You, for example, have one pair of temporal
fenestra in your skull, because you, my friend, are a synapsid. You also have hair, specialized teeth, and
nasal turbinates, like some of our distant cousins, the later therapsids, had. And our closest synapsid relatives, the cynodonts,
had these traits too, as well as an ability to regulate body temperature and a knack for
burrowing -- which you may or may not do, but I don’t know how you spend your free
time. So, you may not see the family resemblance
between yourself and little Thrinaxodon, but those features still connect you to that long-forgotten
animal, even after more than a quarter of a billion years. Thank you to CuriosityStream for supporting
PBS Digital Studios. CuriosityStream is a subscription streaming service that offers
documentaries and non¬fiction titles from a variety of filmmakers, including CuriosityStream
originals. For instance, you can now stream “Ancient
Earth,” a three-part series that chronicles the history of life on Earth from the Permian
to the Cretaceous. You can learn more at curiositystream.com
slash eons Thanks for joining me today in the Konstantin
Haase studio and thanks to this month’s Eontologists: Jake Hart, Jon Ivy, John Davison
Ng, and Steve. If you’d like to join them in supporting
us, go to Patreon.com/eons and make your pledge!
