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Why Wasn't There A Second Age of Reptiles?
Why Wasn't There A Second Age of Reptiles?
PBS Eons
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11:05 · Feb 25, 2025
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the
story
of
the
rise
of
mammals
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1
/252
0:00
the story of the rise of mammals
0:02
is one you may have heard before
0:03
66 million years ago an asteroid
0:05
impact triggered the kpg
0:07
mass extinction wiping out the non-avian
0:09
dinosaurs and ending the so-called
0:12
age of reptiles in the post-apocalyptic
0:14
Wasteland that followed our small furry
0:16
mamalian ancestors and relatives
0:19
emerged from the shadows
0:20
and well the rest is natural history
0:23
as the planet began to recover they rapidly Diversified
0:26
and replaced the dinosaurs
0:27
as the dominant large land animals
0:29
a using in the age of mammals
0:31
that continues to this day
0:33
and led to you and I but there's a mystery at the heart of
0:36
this famous evolutionary Turning Point
0:39
why was it the mammals who triumphed
0:42
how did they manage to pull off such a stunning
0:44
ecological coup in the wreckage
0:46
of planet Earth why didn't the many other reptile species
0:49
that did make it through become the planet's dominant megaphon
0:53
instead keeping us mammals
0:55
confined to the ecological
0:56
margins where we had been for so long why didn't this simply Mark the
1:01
start of a second age of reptiles
1:03
after the first had been cut tragically short
1:05
well perhaps it might have
1:07
and even should have
1:09
if it wasn't for an unlikely
1:11
Ally that some scientists
1:13
believe may have given ancient mammals
1:15
a Competitive Edge at this pivotal
1:17
once in an era moment
1:19
a global explosion of
1:24
fungi sometimes it's tempting to see the story of Life on our planet as
1:29
an inevitable linear chain of events that was just
1:32
destined to play out the way it did mammals Rose become the dominant megap
1:36
of planet Earth after the extinction
1:37
event because well it was simply our turn and our reward for being smarter
1:42
scrappier and more adaptable
1:44
than our reptilian cousins
1:46
but as regular viewers of this show will know
1:48
Evolution doesn't work like that
1:50
at all sometimes things happen the way they did for reasons that were far
1:55
from inevitable and even downright
1:58
bizarre to uncover the factors at play scientists
2:00
often need to get a little creative
2:02
when generating hypotheses connecting
2:05
dots that no one had previously
2:06
thought to connect and in the case of asking
2:09
why mammals Rose to ecological
2:11
prominence after the kpg
2:12
instead of the surviving
2:14
reptiles an especially strange idea has emerged in recent years
2:18
it's called the fungal
2:19
infection mamalian selection hypothesis
2:22
or fims and according to its proponents
2:25
it could be the solution to the mystery
2:27
behind the age of mammals
2:29
so how exactly does the fim's
2:31
hypothesis work how could fungi
2:34
of all things have been the secret ingredient
2:37
behind the last 66
2:38
million years of mamalian
2:40
success well the Story begins
2:42
at the end of the
2:44
world when the asteroid
2:46
hit clouds of Ash
2:48
dust and soot were kicked into the atmosphere
2:51
blotting out the Sun and causing an impact winter
2:54
that lasted years or even decades
2:56
photosynthesis basically shut down worldwide
2:59
plants withered away and the food chains that relied on them collapsed like a
3:03
house of cards any big megap
3:05
from the age of reptiles
3:06
that had been lucky enough to survive the immediate
3:09
effects of the impact
3:10
like the shock waves firestorms
3:12
and tsunamis were then finished off by the ecological
3:15
collapse that followed those medium and large-sized
3:18
stragglers eventually died hungry
3:20
cold confused and in darkness
3:23
all in all more than 75%
3:25
of species went extinct
3:27
most likely including every land animal
3:29
bigger than 25 kg
3:31
But A Wasteland ain't so bad if you actually
3:34
like your land full of waste
3:36
and one Kingdom of life in particular
3:39
has a notorious fondness
3:41
for Darkness death and Decay
3:44
fun guy seeing as they don't photosynthesize
3:47
and generally prefer dark and cool environments
3:50
an impact winter would have been a pretty fun time to be a fun
3:54
guy you knew we had to do that pun
3:56
plus being decomposers that feed on dead stuff
4:00
the aftermath of a mass extinction
4:02
event was basically an all you can eat buffet of dead plant and animal
4:06
matter for a fungi to Feast on
4:08
while the impact winter was one of the worst times in history to be
4:11
pretty much any other form of complex life
4:14
if you were a fungus
4:15
this was maybe the closest to Paradise that the world had ever been
4:19
so unsurprisingly perhaps a rich layer of fungal spores
4:23
abruptly appears in the fossil record at this time
4:26
literal hard evidence of the fungal bloom in the wake of the Extinction event
4:30
now we've known about the fungal sports bike at the kpg
4:33
boundary for over 20 years but it usually isn't seen as connected to one
4:37
of the other notable changes in the fossil record that begin soon after the
4:41
extinction event like the great radiation
4:43
of mammals as they took over the medium and large siiz animal niches that
4:47
had previously been dominated
4:49
by reptiles the fungal Spike
4:51
and the dawn of the age of mammals have generally been thought of as
4:54
simply two unrelated things that happened in the early paleos scene but in 20
4:59
5 an expert in fungal Immunology
5:02
proposed that the fungal Bloom could have been
5:04
exactly what gave mammals The Edge over reptiles
5:07
at this critical time
5:09
changing the trajectory of Life on land
5:11
forever now to be clear the fims hypothesis
5:15
is not yet widely accepted or even widely known within the scientific Community
5:19
you probably won't find mention of fims in any textbooks
5:22
but it is based on a few key facts and plausible
5:26
logical arguments first off there are reasons to think that reptiles should have coped
5:30
better with and bounced back faster
5:33
from the devastation of the kpg
5:35
compared to mammals because their energy
5:37
requirements are much lower than mammals
5:39
meaning they need way less food to stay alive
5:42
a pretty handy trait when food chains have collapsed
5:45
they also have more offspring
5:47
on average than mammals do so
5:49
in theory they can repopulate
5:51
faster following an Extinction event
5:53
and their offspring require less parental care than mammals who produce generally
5:57
pretty helpless babies for a while at least
6:00
which is not ideal when times are tough the mamalian
6:03
lifestyle is just so much more expensive
6:05
and complicated than The Reptilian
6:07
one so you'd think that they would have fared better during the apocalypse
6:11
and seized ecological opportunities
6:13
much faster in its wake
6:14
the fact that the total
6:16
opposite actually happened is the first big hint that some other Factor must have
6:20
been at play one that acted as a powerful
6:23
filter that's selected for mammals
6:25
over reptiles enter the fungal Bloom
6:28
see we know from Modern evidence that reptiles and amphibians
6:31
who are coldblooded are much more vulnerable to fungal infections
6:34
than us mammals as well as Birds who are warm-blooded
6:38
our body temperatures are simply
6:39
too hot for most pathogenic fungi to cope with it's the reason why there
6:43
are actually relatively few fungal diseases
6:45
that affect mammals compared with the many
6:48
bacterial viral or parasitic
6:50
diseases that threaten us far fewer in fact than the many fungal diseases
6:54
that infect our coldblooded
6:55
cousins and in most
6:57
cases the few fungal
6:58
pathogens that do infect mammals are
7:01
minor annoyances rather than life-threatening
7:04
mammals were pretty much just as hot 66
7:07
million years ago and reptiles
7:09
just as cool meaning the fungal Bloom right after the kpg
7:13
would have been a much worse time for them than it was for us
7:17
the air was suddenly filled with fungal spores
7:20
at least some of which would have been pathogenic
7:22
infecting reptiles and their eggs on mass
7:25
while leaving us mammals
7:27
relatively unbothered in fact
7:29
all those mushrooms sprouting up all over the place would have been a great
7:33
new food source for mammals when little else was available
7:36
and while reptiles could have eaten mushrooms too
7:39
we know that when it's cold they're less able to move around
7:41
reducing their foraging ability
7:43
they also struggle to digest at lower temperatures
7:46
neither of which are issues that mammals would have had to deal with put
7:49
all these facts together
7:51
and suddenly the kpg
7:52
aftermath with its mushroom
7:54
Meadows and dense mist of spores
7:56
suddenly begins to look like a much better time for mammals than reptiles
8:01
if the fungal Bloom offered even a modest survival
8:04
Advantage for us over them
8:06
then it becomes easier to imagine how those mammals were able to out compete
8:09
reptiles at this critical juncture
8:11
mammals would have repopulated
8:13
the world in greater numbers
8:14
and had a great St over reptiles
8:16
as both groups scrambled to fill all the medium and large siiz animal niches
8:20
that had been left open by the demise
8:22
of the non-avian dinosaurs
8:24
and so a second
8:25
age of reptiles was averted
8:27
and the age of mammals
8:29
began now the fim's hypothesis
8:31
is hard to prove for sure
8:32
and scientists may never be able to but it points to something that we
8:36
do know for sure
8:38
disease must have played a massive role in the story of life on Earth
8:42
just as it plays a massive role today
8:44
seeing as it rarely leaves tangible
8:46
evidence behind in the fossil record though
8:48
we often Overlook it when reconstructing
8:50
events from Deep time
8:52
while we struggle to know exactly
8:54
what role disease played
8:56
the only definitely wrong answer is that it played
8:58
no role even though that tends to be the default
9:02
assumption and the fim's hypothesis
9:04
shows that big radical ideas about the history of our world don't
9:08
just come from people digging through dirt and discovering new fossils and new data
9:12
that no one had found before
9:14
Sometimes they come from digging through old papers
9:16
and old discoveries but making new connections
9:19
that no one had made before
9:21
and if true the fim's hypothesis
9:23
means that we don't just owe the evolutionary
9:25
success of mammals to the
9:27
innate resilience and eological streets
9:30
SMS of our Underdog
9:31
ancient ancestors instead we might have had a helping hand from a brief period
9:36
of fungal Paradise that kept our competition
9:39
down without which you and I would never have evolved
9:42
the prologue to the age of mammals
9:44
may have in fact been
9:46
an age of fungi
9:50
a question we're asked a lot
9:52
here on eons is how long could a human survive
9:55
if they were dropped into a particular
9:57
period of the geologic
9:58
past well that's exactly what we're talking about on the second season
10:03
of The eons podcast
10:04
Eon surviving deep time is available now at youtube.com/
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eons and wherever you get your
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