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Lecture 1.4_Mammalian Radiations

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welcome Intrepid students to the fourth

installment in your first week of the

life history of mammals so today is a

short lecture I promise where we'll

focus on important mammalian adaptive

radiations during the evolution of

mammals so I'm going to be summarizing

chapter 3 which is entitled phylogeny

and diversification of mammals

specifically timing of mammalian radiations

radiations

so we're definitely not gonna start uh

here so this picture illustrates the

pleistocene mega fauna uh on a radiation

that included some really impressive

species like the giant ground sloth here

uh Mega theorem the massive glyptodont

which is an armadillo and then of course

the impressive

saber-toothed cats the Smilodon so you

can see uh these wonderful Smilodon

skulls as well as dire wolves and a lot

of these pleistocene megafauna at the LA

um La Brea Tar Pits in Los Angeles

so these enormous species they only went

extinct about 11 000 years ago which is

really a geologic blink of the eye

so our colonization of the globe ours

meaning Homo sapiens likely contributed

to many of these uh species demise but

I'm going to return to the pleistocene

at the end of this lecture

so as you'll surely recall from our last

lecture we started way back here in the

Carboniferous period with those very

first amniotes and then I spent most of

the lecture in the Permian and the

Triassic so today we are going to start

from the late Triassic here with the

Divergence of the monotrines and the

therians we're going to talk a bit about

the end Cretaceous as well as the eocene

and then we'll finish up with the

pleistocene so please note those last

two time frames the eocene and the

pleistocene those are epochs and an

Epoch is even shorter than a period and

of course considerably shorter than an

era or an eon evolutionary biologists

call the clade

including the the most recent common

ancestor from all of the living lineages

as the crown group for that taxon thus

the crown group for the class mammalia

represents that common ancestor of the

living lineages which are the monotreams

here the duckbilled Platypus the spiny

Echidna as well as the therians here so

the metatheria those are the marsupials

and the euthyrians are the placentals so

you can see when those three living

lineages extant lineages

excuse me converge that represents the

most recent common ancestor and these

are deemed the early Crown mammals so

this lineage

diverged before the evolution of the

crown group so we're going to refer to

this lineage the mammalia formas

as the stem group okay so the

recognition of a crown group at least

relative to stem group is really

essential for those molecular analyzes

that are estimating Divergence times

the oldest monotreme yet recovered from

the fossil record is steropodon here

here

that fragment right there dating at 110

million years ago and recovered from

Australia the oldest Med eatherian and

again the metatherian includes the

present-day marsupials as well as the

non-marsupial medatherians like the

amazing sporacidance

of South America and we'll get into that

um adaptive radiation soon the oldest

metatherian is here and is named Sino delphis

delphis

it dates at

125 million years ago and was actually

recovered in China and then lastly we

have Jeremiah here on the bottom center

this is the oldest you theory in and

it's actually coming in at 160 million

years ago

so we know given that the common uh

ancestor of these lineages must be at

least as old as the oldest fossil

Jeremiah here we know that the crown

group of mammals are already in

existence a minimum minimum of 160

million years ago during the Jurassic

however the relative scarcity of mammal

fossils throughout most of the Mesozoic

means that monotremes metatherians and

euthyrians they didn't occur during the

age of dinosaurs at anything like their

current diversity

so when did the great adaptive

radiations of mammals take place

there are two primary sources of

evidence the fossil record as well as

molecular clocks and they often have

produced very different answers over the

past 40 years evolutionary biologists

have been investigating the possibility

that some evolutionary change mutations

may occur in a clock-like fashion in

that these mutations are going to occur

at a regular rate over the course of

millions of years mutations May build up

in a given stretch of DNA at a reliable

rate so for example the gene that codes

for the protein Alpha globin which is a

component of hemoglobin in animal red

blood cells it experiences a base uh

change rate of

0.56 changes per base pair per billion years

years

so if this rate is reliable then this

Gene could be used as a molecular clock

looking at the different number of

mutations in this protein across species

so when a stretch of DNA does indeed

behave like a molecular clock it becomes

a powerful tool for estimating the date

of lineage splitting events like you see

here so imagine we've got two stretches

of DNA two different species represented

by the red lines that share a common

ancestor here

so we've got four bases that are

different between these two species you

can see this is a c whereas this is a t

okay this is a g where this is a t

okay we're going to hypothesize

um that the entire length of DNA is

going to change at a rate of

approximately one base pair per 25

million years

so four base pairs 25 50 75 that's a

hundred million years of evolutionary

distance that said you've got two

lineages so this represents 50 million

years and this represents 50 million

years for a total evolutionary time of

100 million years we can then infer that

the common ancestor lived 50 million

years ago so that is an accurate but

very simplified version of the methods

used to generate the phylogenetic tree

that you see before you so this was

published by Meredith and colleagues in

2011 the figure is in your book

here researchers undertook a super

Matrix analysis of 164 mammal species

representing 97 to 99

of extant uh mammal families and DNA

sequences from 26 protein coding loci

that is 35

603 bases so very impressive data set

they use maximum likelihood and Bayesian

analyzes to produce the most

parsimonious phylogenetic tree making

the least assumptions and that's the

tree that you see before you so Meredith

at all

2011 they estimate that the monotremes

diverged from the theory somewhere between

between

203 and

238 million years ago

there's considerably more Precision in

their estimates of the Divergence

between the metatheria

and the eutheria and they estimate that between

between

168 and

178 million years ago so that's very

consistent uh with Jeremiah the oldest

euthyrian fossil which dates at 160

million years ago very important to take

a moment here and stress that

phylogenetic Divergence or speciation is

driven by and maintained by reproductive

isolation and this reproductive

isolation was facilitated by the breakup

of Pangea

remember Pangea is the supercontinent

that existed right around this time at

200 million years ago

recall that at 200 million years ago the

therians likely hadn't evolved and if

they had they certainly weren't widely

distributed but the prototheria uh what

are to become the monotreams and the

metatheria are likely already in Australia

Australia

by 180 million years ago the Primitive

eutheria are in West godwana

and the monotreams are in East godwana

and then there's metatheria in both

okay by 65 million years ago the

eutheria have radiated in present-day

Africa 65 million years ago so they're

about to go gangbusters

notice that Madagascar has already split

from the African continent

and the metatheria are in Australia a

green Antarctica as well as South America

America wait

wait

metatherian marsupials in South America

you may have thought that marsupials

only lived in Australia well we're going

to talk more about this uh next week but

for now please put me on pause and I

want you to check out this amazing 10

minute video it's entitled how South

America made the marsupials the YouTube

video is embedded in canvas so take a

moment and check it out now

a really consequential evolutionary

event occurs

125 million years ago and that is the

evolution of the angiosperms the

flowering plants so this period here

denoted by light green is referred to as

the Cretaceous terrestrial Revolution

but following the evolution of flowering

plants you have the co-evolution of all

of those arthropod pollinator species

you see this incredible

explosion of beetles

bees butterflies moths straight up flies

and this is occurring from the middle

Cretaceous all the way to the lake

Cretaceous when the dinosaurs still

ruled as you would likely predict if you

have this incredible explosion this

incredible diversification of arthropods

of insects that suddenly represents lots

of new ecological niches for insectivores

insectivores

and we see at this time this simultaneous

simultaneous

diversification radiation of uh both the metatherian

metatherian

and the youth Therian insectivores

as well as these lineages we see

diversification in the

multi-tuberculates and the dryolastoids

however these lineages are bound for

eventual Extinction

my larger Point here is that the

metatherian and euthyrian lineages were already

already

diversifying during the time of the dinosaurs

dinosaurs

before that hellish day

66 million years ago which radically

Alters The evolutionary trajectory of

Life on planet Earth and without which

we wouldn't be here

one fateful day

66 million years ago the world was

changed forever

an asteroid estimated to be six to nine

miles in diameter the size of Manhattan Island

Island

traveling at an estimated 44

712 miles per hour

smashes into our planet

with an energy equivalent of 100

trillion tons of TNT that's 21 billion

Hiroshima Atomic Palms it generated a

cloud of superheated Ash and steam

killing everything for hundreds of miles

and triggering perhaps the most massive

earthquakes the world has ever seen

tsunamis with hundred foot waves around

the world

further trillions of tons of material

are ejected into the atmosphere

initially raising global temperatures

and causing raging wildfires around the globe

globe

the water air and soil were Poisoned

With Cyanide and heavy metals nickel and

Lead that composed that asteroid the

sulfur released into the atmosphere

caused acid rain and then we have a

global cooling known as a nuclear winter

that probably lasted for decades

the loss of photosynthesis

reverberated throughout the food web and

all of the large bodied animals greater

than 55 pounds with the exception of the

crocodilians and the sea turtles who

managed to fake it through somehow all

of the large-bodied animals over 55 pounds

pounds

blink out

it was hell on Earth

Carl Sagan famously said

extraordinary claims require

extraordinary evidence well here's the

evidence this is the impact site for

that asteroid it's known as the chick

zulub crater and it's hidden beneath the

Yucatan Peninsula and the Gulf of Mexico

in present-day Mexico if you've ever

been to Cancun or Cozumel it's on the

Yucatan so this buried crater is over

93 miles in diameter and it's 12 miles

deep well into the continental crust

further the in Cretaceous is marked by

this very thin dark layer of iridium

that's been detected at hundreds of

sites around the world iridium is a very

rare element in the Earth's crust in the

sedimentary rock but it's abundant in

meteorites so in this iridium is found

directly above the Cretaceous fossil so

as this meteorite struck with this

incredible velocity and impact it

vaporized sending that iridium into the

atmosphere and eventually raining down

in this sedimentary layer it's now found

at 100 times

the background rate of iridium and the

other sedimentary layers small nocturnal

nocturnal

insectivorous mammals somehow make it

through the end Cretaceous Extinction

event and then they are going to radiate

into an incredible diversity of forms so

most mammalian orders are now freed to

begin differentiating radiating in the

early cenozoic because as you may have

likely heard nature abhors a vacuum

so this mass extinction event as

devastating as it was for life on our

planet it allowed for the age of mammals

and eventually Homo sapiens most of the

extant orders orders of mammals are

recognized in the fossil record by the

eocene epoch that's 55 million years ago

and the vast majority of families of

course families uh below uh the

taxonomic rank of order most families

are recognized by the myocene epoch

which is 23 million years ago

so for the next six weeks we are going

to systematically cover 21 of these

orders and we're going to relate those

orders to significant attributes of

mammalian life history so it's going to

be an exciting ride

I promised at the outset that I would

return to the pleistocene mega fauna so

if you're interested in woolly mammoths and

and

Smilodon the Sabretooth cat and giant

ground sloths giant armadillos all these

incredible animals that were roaming

North America

I've embedded one of my Estrella

Mountain Community College YouTube

videos into your canvas page it is not

required you don't have to watch it and

you won't be assessed over it but if

you're interested uh it's about a 20

minute summary on a new debate in

conservation biology revolving around

this technique referred to as pleistocene

pleistocene

rewilding so given new advancements in

biotech is it appropriate to bring back

highly influential ecologically

influential species that we likely drove

into Extinction so more recently the

passenger pigeon and the thylacine or

even the woolly mammoth which only went

extinct about 4 000 years ago so if

you're interested check it out

I've got a couple of references that are

not in your book that I used some of

their Graphics I've got Benton is a new

one here 2022 as well as uh gross nickel

at all from 2019 I just thought I'd

Point those out uh I hope you learned

something and I'll see you next time cheers

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