Why Wasn't There A Second Age of Reptiles?

the story of the rise of mammals

is one you may have heard before

66 million years ago an asteroid

impact triggered the kpg

mass extinction wiping out the non-avian

dinosaurs and ending the so-called

age of reptiles in the post-apocalyptic

Wasteland that followed our small furry

mamalian ancestors and relatives

emerged from the shadows

and well the rest is natural history

as the planet began to recover they rapidly Diversified

and replaced the dinosaurs

as the dominant large land animals

a using in the age of mammals

that continues to this day

and led to you and I but there's a mystery at the heart of

this famous evolutionary Turning Point

why was it the mammals who triumphed

how did they manage to pull off such a stunning

ecological coup in the wreckage

of planet Earth why didn't the many other reptile species

that did make it through become the planet's dominant megaphon

instead keeping us mammals

confined to the ecological

margins where we had been for so long why didn't this simply Mark the

start of a second age of reptiles

after the first had been cut tragically short

well perhaps it might have

and even should have

if it wasn't for an unlikely

Ally that some scientists

believe may have given ancient mammals

a Competitive Edge at this pivotal

once in an era moment

a global explosion of

fungi sometimes it's tempting to see the story of Life on our planet as

an inevitable linear chain of events that was just

destined to play out the way it did mammals Rose become the dominant megap

of planet Earth after the extinction

event because well it was simply our turn and our reward for being smarter

scrappier and more adaptable

than our reptilian cousins

but as regular viewers of this show will know

Evolution doesn't work like that

at all sometimes things happen the way they did for reasons that were far

from inevitable and even downright

bizarre to uncover the factors at play scientists

often need to get a little creative

when generating hypotheses connecting

dots that no one had previously

thought to connect and in the case of asking

why mammals Rose to ecological

prominence after the kpg

instead of the surviving

reptiles an especially strange idea has emerged in recent years

it's called the fungal

infection mamalian selection hypothesis

or fims and according to its proponents

it could be the solution to the mystery

behind the age of mammals

so how exactly does the fim's

hypothesis work how could fungi

of all things have been the secret ingredient

behind the last 66

million years of mamalian

success well the Story begins

at the end of the

world when the asteroid

hit clouds of Ash

dust and soot were kicked into the atmosphere

blotting out the Sun and causing an impact winter

that lasted years or even decades

photosynthesis basically shut down worldwide

plants withered away and the food chains that relied on them collapsed like a

house of cards any big megap

from the age of reptiles

that had been lucky enough to survive the immediate

effects of the impact

like the shock waves firestorms

and tsunamis were then finished off by the ecological

collapse that followed those medium and large-sized

stragglers eventually died hungry

cold confused and in darkness

all in all more than 75%

of species went extinct

most likely including every land animal

bigger than 25 kg

But A Wasteland ain't so bad if you actually

like your land full of waste

and one Kingdom of life in particular

has a notorious fondness

for Darkness death and Decay

fun guy seeing as they don't photosynthesize

and generally prefer dark and cool environments

an impact winter would have been a pretty fun time to be a fun

guy you knew we had to do that pun

plus being decomposers that feed on dead stuff

the aftermath of a mass extinction

event was basically an all you can eat buffet of dead plant and animal

matter for a fungi to Feast on

while the impact winter was one of the worst times in history to be

pretty much any other form of complex life

if you were a fungus

this was maybe the closest to Paradise that the world had ever been

so unsurprisingly perhaps a rich layer of fungal spores

abruptly appears in the fossil record at this time

literal hard evidence of the fungal bloom in the wake of the Extinction event

now we've known about the fungal sports bike at the kpg

boundary for over 20 years but it usually isn't seen as connected to one

of the other notable changes in the fossil record that begin soon after the

extinction event like the great radiation

of mammals as they took over the medium and large siiz animal niches that

had previously been dominated

by reptiles the fungal Spike

and the dawn of the age of mammals have generally been thought of as

simply two unrelated things that happened in the early paleos scene but in 20

5 an expert in fungal Immunology

proposed that the fungal Bloom could have been

exactly what gave mammals The Edge over reptiles

at this critical time

changing the trajectory of Life on land

forever now to be clear the fims hypothesis

is not yet widely accepted or even widely known within the scientific Community

you probably won't find mention of fims in any textbooks

but it is based on a few key facts and plausible

logical arguments first off there are reasons to think that reptiles should have coped

better with and bounced back faster

from the devastation of the kpg

compared to mammals because their energy

requirements are much lower than mammals

meaning they need way less food to stay alive

a pretty handy trait when food chains have collapsed

they also have more offspring

on average than mammals do so

in theory they can repopulate

faster following an Extinction event

and their offspring require less parental care than mammals who produce generally

pretty helpless babies for a while at least

which is not ideal when times are tough the mamalian

lifestyle is just so much more expensive

and complicated than The Reptilian

one so you'd think that they would have fared better during the apocalypse

and seized ecological opportunities

much faster in its wake

the fact that the total

opposite actually happened is the first big hint that some other Factor must have

been at play one that acted as a powerful

filter that's selected for mammals

over reptiles enter the fungal Bloom

see we know from Modern evidence that reptiles and amphibians

who are coldblooded are much more vulnerable to fungal infections

than us mammals as well as Birds who are warm-blooded

our body temperatures are simply

too hot for most pathogenic fungi to cope with it's the reason why there

are actually relatively few fungal diseases

that affect mammals compared with the many

bacterial viral or parasitic

diseases that threaten us far fewer in fact than the many fungal diseases

that infect our coldblooded

cousins and in most

cases the few fungal

pathogens that do infect mammals are

minor annoyances rather than life-threatening

mammals were pretty much just as hot 66

million years ago and reptiles

just as cool meaning the fungal Bloom right after the kpg

would have been a much worse time for them than it was for us

the air was suddenly filled with fungal spores

at least some of which would have been pathogenic

infecting reptiles and their eggs on mass

while leaving us mammals

relatively unbothered in fact

all those mushrooms sprouting up all over the place would have been a great

new food source for mammals when little else was available

and while reptiles could have eaten mushrooms too

we know that when it's cold they're less able to move around

reducing their foraging ability

they also struggle to digest at lower temperatures

neither of which are issues that mammals would have had to deal with put

all these facts together

and suddenly the kpg

aftermath with its mushroom

Meadows and dense mist of spores

suddenly begins to look like a much better time for mammals than reptiles

if the fungal Bloom offered even a modest survival

Advantage for us over them

then it becomes easier to imagine how those mammals were able to out compete

reptiles at this critical juncture

mammals would have repopulated

the world in greater numbers

and had a great St over reptiles

as both groups scrambled to fill all the medium and large siiz animal niches

that had been left open by the demise

of the non-avian dinosaurs

and so a second

age of reptiles was averted

and the age of mammals

began now the fim's hypothesis

is hard to prove for sure

and scientists may never be able to but it points to something that we

do know for sure

disease must have played a massive role in the story of life on Earth

just as it plays a massive role today

seeing as it rarely leaves tangible

evidence behind in the fossil record though

we often Overlook it when reconstructing

events from Deep time

while we struggle to know exactly

what role disease played

the only definitely wrong answer is that it played

no role even though that tends to be the default

assumption and the fim's hypothesis

shows that big radical ideas about the history of our world don't

just come from people digging through dirt and discovering new fossils and new data

that no one had found before

Sometimes they come from digging through old papers

and old discoveries but making new connections

that no one had made before

and if true the fim's hypothesis

means that we don't just owe the evolutionary

success of mammals to the

innate resilience and eological streets

SMS of our Underdog

ancient ancestors instead we might have had a helping hand from a brief period

of fungal Paradise that kept our competition

down without which you and I would never have evolved

the prologue to the age of mammals

may have in fact been

an age of fungi

a question we're asked a lot

here on eons is how long could a human survive

if they were dropped into a particular

period of the geologic

past well that's exactly what we're talking about on the second season

of The eons podcast

Eon surviving deep time is available now at youtube.com/

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