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Lecture 1.3_Evolution of Mammals

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hi there

Jared Rachel here again for the third

installment of module one we're going to

be taking a deep dive into the evolution

of mammals so we're going to be thinking

today about deep time ancient

evolutionary events that occurred

hundreds of millions of years ago we're

going to cover three sections in your

textbook all from chapter four which is

entitled Evolution and dental

characteristics and those include one

the synapsid lineage to the

interrelationship of characteristics and

increased metabolism and then finally

the summary of anatomical Trends and

organization from mammal like amniotes

to mammals we're going to devote an

entire lecture to mammalian dentition

because it's such a key adaptation

that's coming in lecture 1.5

so hold on to your seats we're going to

travel back in time to Envision

dimetrodon here from the early Permian

to the late Permian when the synodonts

were thriving and then we have those

first mammals that appeared during the

late Triassic or very early on in the Jurassic

Jurassic

so if you're like me you probably need a

refresher on geological time scale of

importance here this column right here

these are the eras the Paleozoic is the

most ancient the Mesozoic and the era

that we're in right now the cenozoic

the eras are divided up into periods you

may have heard of the Cambrian explosion

the ordovician the silurian the devonian

the Mississippian and the Pennsylvanian

collectively we often refer to them as

the Carboniferous period

uh the Carboniferous period because this

is the time of the great Fern forests

that covered planet Earth from pole to

pole and this is where our fossil fuels

come from hence the Carboniferous

important to note here that these periods

periods

are demarcated they're divided by

important Extinction events

so the greatest Extinction event of all

at least for terrestrial life forms was

the end Permian right here at

252 million years ago so this is also

called the Great dying there are all

these super volcanoes in Siberia that

radically change the chemistry of the

atmosphere and acidified the oceans

during this time 90 of life forms on

planet Earth

perished went extinct the extinction

event you're probably more familiar with

is the end Cretaceous that occurred 66

million years ago and we're going to

come back to the end Cretaceous and that

unbelievable asteroid in our next

lecture today we're going to be focused

on the Carboniferous period from approximately

approximately

340 Mya or millions of years ago

all the way to the Jurassic period here approximately

approximately

175 million years ago

to begin with we're going to go way back

to the Carboniferous period approximately

approximately

318 to 340 million years ago when

amphibian like tetrapods Tetra just

meaning for and pod is Greek for foot so

first four limbed organisms this

artist's rendition here looks kind of salamander-like

salamander-like

these organisms give rise to the amniota

the amniota are a monophyletic group or

a clade which consists of these first

amniotes as well as all of their

descendants so the reptiles birds and of

course the mammals the amniota or the

amniotes are characterized by the

amniotic egg which is an egg that's

impenetrable to water and has three

extra embryonic membranes the amnion the

allentos and the Corian

by the end of the Carboniferous period

299 million years ago the amniotes had

diverged into three different lineages

that are distinguished by their skull

structure so the anapsids here be in the

center have no temporal opening or fossa

and is Greek for without

the sin absence have one temporal fossa

and then we have the dye absence which

have an upper and a lower temporal

openings dye of course means to

So eventually the mammals will arise

from this top group here the synapses

this is really important the synapses

were actually the first dominant

terrestrial vertebrates they were the

main actors on the stage for like 70

million years before the emergence of

the Dinosaurs the temporal finasteride

finasteri is just the plural of fossa so

multiple fossa equals fenestrin and I

want you to recall the synapses

synapsids just have one temporal fossa

one on each side of the skull the

temporal fossa is located

post-orbital meaning behind the eye socket

socket

and this is the place where the

temporalis muscle attaches

this very important muscle attaches here

in the temporal fossa and then it's

going to pass through the zygomatic Arch

and it's going to connect to the lower

jaw right here at the coronoid process

so again let me emphasize the synapsida

or synapses is the phylogenetic clade

that includes all of the mammals

Turtles are an absids meaning they lack

temporal fossa but the molecular

evidence actually suggests that their

dye absence that have secondarily lost

their temporal finaster stray

and then we have down here in the bottom

the dye absence which have two temporal

finasteri on each side of the skull and

they include all of the living and

extinct reptiles and birds okay really

quickly I'd like to show you this

javelina skull we know all about

javelina's living here in the Phoenix

Valley javelina's of course are mammals

so they're going to have a sin acid

skull that is to say one

temporal fossa

temporal opening on each side of the

skull and the temporal fossa is where

the temporalis muscle is going to attach

and then it's going to pass through this

opening right here

this is the zygomatic Arch

and it's going to connect to the

coronoid process

right up in here on the lower jaw so the

synapses during the early Permian the

early synapses diverged into diverse

herbivorous and carnivorous forms the

pelicosaurs and the thoracida or

thoraxis the pelicosaura were the most

primitive of these two groups and is

known from fossil remains in North

America and South Africa the the

rhapsoda the younger and more

evolutionarily derived group were the

top carnivores in the food web as you

probably deduced from these impressive

canines here the rhapsid fossils have

been found in Russia South Africa and

China the Spina codantia are still

considered pelicosaurs but these are now

the carnivorous pelicosaurs and they

include the infamous genus

dimetrodon which you see the skull of

here on the top and I'll show you some

artist Renditions of various members of

dimetrodon on the next slide

the sphenocodons can be distinguished

from the other herbivorous pelicosaurs

in the fossil record by looking at the

angular bone here in the lower jaw and

noted with an a

the sphenicodontia have this reflected

lamina this process right here on the

angular bone which is going to denote

this group this process is actually

going to become the Bony ring that

supports the tympanic membrane the

eardrum and the mammalian middle ear

the sphenicodons like dimetrodon are

then going to give rise to the thoracids

which we talked about on the last slide

titano Phineas here is an example of a

thoracid this derived evolutionarily

derived lineage

at least 12 species from the genus

dimetrodon have been recovered and

identified mostly from the red beds of

Texas and Oklahoma but also now from

Germany the dimetrodon date from approximately

approximately

295 to

272 million years ago

these sales were originally thought to

Aid in thermoregulation so similar to

the large thin ears of African elephants

which are going to help them dissipate

heat but increasingly paleontologists

believe it's more probable that these

sales were used in a courtship displays

to show off to potential mates or to

threaten rivals

so right now I want you to put me on

pause please uh and I want you to take

six minutes and please check out the

amazing video that I've embedded into

canvas it's the series PBS eons and it's

called dimetrodon our most unlikely

ancestor but it's quite good so take a

moment and check that out all of the

pelicosaurs including dimetrodon are

replaced by the more evolutionarily

derived thoracids by the middle to late Permian

Permian

when I took mammology what feels like

eons ago now we were taught that the

thrasbins were the mammal-like reptiles

however the evidence is strong that

mammals didn't evolve directly from

reptiles so this term really isn't

appropriate we no longer use mammal-like

reptiles so the reptiles and the mammals

do share a common ancestor way back in

the Carboniferous and that ancestor is

an amniote but at this point at the time

of Divergence between the lineage that

is to produce eventually produce uh the

mammalia and eventually produce the

class reptilia

uh that that Divergence happens long

before the reptiles actually evolve

here's some fun pictures of some of the

well-known theraspids on the top left we

have eno transcevia this is a genus of

large carnivorous Gorgon options with long

long

saber-tooth-like canines the largest

specimen yet recovered was 11 feet long

and estimated to weigh

660 pounds on the top right we have the

four horned tetra ceratops

on the bottom left here we have you

limosaurus this was a thick herbivore

with a dense skull plate suggesting that

it may have been butting heads with

Rivals and then this guy over here on

the bottom right may have been using

those canines to dig up roots and tear

up vegetation so incredible diversity

that we can only Glimpse

by looking at the fossils and making

inferences connecting those fossils to

structures and behaviors that we observe

today okay so we're hurtling through

time over tens of millions of years and

a lineage of thraspants known as the

Sino dantia are going to evolve numerous

mammal-like characteristics including a

secondary palette

complex cheek teeth meaning post canine

teeth the premolars and the molars these

cheek teeth were tricuspid meaning they

have three cusps or bumps on their

occlusial their chewing surface these

cheek teeth were double rooted

they're approaching mammalian jaw

structure and masseter muscles meaning

they have increased dentry or lower jaw

bone size and a reduction in the bones

that comprise the jaw and we're going to

return to that here shortly like mammals

the cyanodons had acute hearing and they

had a modified

post-cranial skeleton with

differentiated vertebrae like the first

two vertebrae the uh Atlas and the axis

they had modified both pectoral and

pelvic girdles as well as modified

thoracic ribs and finally the

intermediate cyanodons had even involved

the same phalangeal formula as modern

mammals alright so I know I've thrown a

lot at you so let's take a moment and

quickly review

this first lineage the most basal

attacks on here these are the

herbivorous pelicosaurs

so at this point we are amniotes meaning

we have an amniotic egg as well as

synapsids meaning we have one temporal

fossa on each side of the skull

this lineage right here are the

carnivorous pelicosaurs like dimetrodon

so these guys have that process on their

jaw bone which is eventually going to

give rise to the Bony ring that supports

the tympanic membrane and the mammalian

middle ear

this lineage right here represented by

titano Phineas these are the thraspits

previously the mammal like reptiles so

the thoracids have increasingly

specialized dentition heterodont

dentition they have excuse me they have

larger brains

and they also have an erect

posture rather than a sprawling

reptilian gate and then the most derived

lineage that we've discussed thus far

are the synodonts the synodonts have

these large dentary bones as well as

well-developed cheek teeth premolars and

molars as we approach the mammalian

clade it's important to to stop and and

take a moment and recognize that from

the ancient synapses like dimetrodon

here at the top to the increasingly very mammal-like

mammal-like

cyanonephus to True mammals we see this

gradual transition and all of these key

mammalian adaptations so for example

look at how the dentary bone which is

number one is going to become

increasingly larger and heavier as these

other bones in the lower jaw are reduced

and then eventually repurposed

important to note Evolution doesn't

create things from scratch natural

selection is going to modify and

repurpose existing structures so great

example here is the articular bone which

is number seven here initially it's part

of the jaw joint on dimetrodon it's

going to be increasingly reduced and by

the time we get to mammals the articular

bone is no longer part of the lower jaw

the lower jaw is just formed by the

dentory bone instead the articular bone

has become the malleus in the middle ear

of mammals it's this dentary Bone Alone

that is going to articulate with the

squamosal bone in the skull so here is a

color-coded diagram shown from the above

perspective here on the right and we're

going to move from the Sino dantia to

the mammalia formes to the true mammals

the dentary bone is in maroon and then

the squamoso in the skull is in light

blue but what I want to point out here

is in the cyanods the dentary bone isn't

anywhere close to the squamoso and then

over time that dentery bone is going to

enlarge and here we see contact between

the dentary bone and the squamous so

it's evident here and then when we get

to the true mammals we have and this is

important we have articulation between

that heavy dentary bone and the cranial

squamousle that's going to form that

incredibly powerful mammalian jaw joint

concurrently the articular bone which is

shown here in this slightly uh it's a

slightly darker blue color the articular

bone initially part of the lower jaw in

those ancient amniotes is going to

become progressively reduced and then

when we get to mammals the articular

bone is going to detach from the lower

jaw and become the malleus in the middle

ear the amalius is going to receive

vibrations sound waves from the tympanic

membrane the eardrum

the quadrate bone here this is a lighter

blue uh again uh we're going to

transition smaller and detach again

that's going to become the incus which

is the middle ossicle here these are the

three ossicles in the mammalian middle

ear the incus is going to receive those

vibrations from the malleus and amplify

those and then pass them on to the

stapes the stapes shown in black is

again going to detach and then it's

going to form this Stirrup shape the

stapes is actually the smallest bone in

our bodies and the stapes is going to

tap on the oval window here it's opening

to the inner ear and then those Taps are

going to create waves of fluid

uh in the inner ear

here's a gif a movable image that shows

it a bit better again we've got sound

waves traveling through the ear canal uh

causing the movement of the tympanic

membrane the head of the malleus is

attached to the back of the tympanic membrane

membrane

this is the arm of the malleus it's

called the manubrium that manubrium is

going to

transfer that energy to the second

ossicle to the incus which is again

going to magnify amplify the sound pass

it on to that Stirrup shaped stapes

which is going to tap on the um uh the

oval window also called the fenestra

ovalus at this point I want you to

remember that from the mid Triassic all

the way through the end Cretaceous

dinosaurs rule the land so they have

radiated into every conceivable Niche

the pterosaurs have conquered the skies

and eventually the mosasaurs have

radiated in the oceans so during this

time period mammals are just bit players

during this act in the play of Life the

early Mesozoic mammals which may have

evolved as early as

238 million years ago they're small

they're diminutive they're they're

nocturnal and they're rather

insignificant in the ecosystems

mammals are to remain rodent sized Until

the End Cretaceous and the extinction of

the dinosaurs which we'll talk about in

our next lect this is a wonderful figure

so it's figure

4.12 in your textbook it's going to

summarize all of the evolutionary Trends

as we move from those ancient synapses

all the way to the euthyrians the

placental mammals so I'm going to

conclude this lecture by discussing

eight of these anatomical Trends but

before I do so I just wanted to point

out this figure and encourage you to

spend some time with this figure take a

good look at this make sure that you

recognize these Trends before your next

assessment Trend number one the 10

temporal opening or temporal fossa right

here is going to enlarge and it's going

to allow for the attachment of the jaw

muscle which is now going to pass

through the zygomatic Arch and connect

to the lower jaw providing mammals with

considerably greater bite force than reptiles

reptiles

so we've talked quite a bit now about

number two

mammals their lower jaw is solely

comprised of the dentary bone you can

see the dentry bone is red here that is

the only bone that makes up the

mammalian lower jaw it's become quite heavy

heavy

the other bones like the articular bone

and the quadrate those have been

repurposed as ossicles in the mammalian

middle ear so this figure demonstrates

how the maxillary bones denoted with an

M how over vast periods of time they are

going to extend

posteriorly like this and then

concurrently the Palatine bones here

denoted with a p they are going to

extend laterally like this and so the

maxillary and the Palatine bones

together are going to make the secondary

palette the roof of the mouth

also the internal nares denoted with an

N these are the holes that facilitate

breathing they're going to move to the

back of the mouth

here so this is going to allow for

constant breathing while

chewing so recall from lecture 1.1 that

we need lots of respiration we need lots

of oxygen for increased metabolism and

endothermia okay uh we're gonna go back

to our javelina skull and really quickly

I just want you to note the secondary

palette right here which is composed of

the maxillary and the Palatine bone and

then when we go to the back of the mouth

here you can see those internal nares

which is going to facilitate uh the

intake of oxygen while the javelina

chews in our last lecture for the week

I'll be discussing mammalian dentition

but let me just start by emphasizing

that mammals have hetero

heterodont dentition mammals have incisors

incisors canines

canines

premolars and molars that have

differentiated in both their structure

and their function whereas reptiles like

this boa restrictor here have relatively

uniform peg-like teeth again recall that

essentially we're eating machines so

specialized teeth arose very early on by

dimetrodon we already see enlarged

canine teeth

so here you can clearly see the

relatively uniform and peg-like Teeth of

the American alligator so the morphology

of these teeth they're all very very

similar so compare that to a raccoon here

here

and we've got incisors in the front here

and then we have these large canines

and then we have

premolars and molars okay and these

carnassial teeth here for shearing meat

but you can clearly see a

heterodont dentition

specialized teeth and so we're coming

back to teeth I promise okay moving

right along number five in the cyanodons

we observe the evolution of a second

occipital condyle these are how the

skull is going to articulate with the

first vertebrae C1 which is this

ring-shaped Atlas and here you can see

these depressions within which the

occipital condyles are going to sit and

allow that skull motion on the right I

know this is an American robin it's a

bird uh it's a theropod

um but you can see this single occipital

condyle which is comparable to what we

observe in Dimetrodon

so here we have a gorilla skull and I'm

just going to flip it around really

quickly so that you can see those two

occipital condyles which are going to

allow for articulation of the skull with

that Atlas with that first vertebrae

number six

that reptilian stance those splayed legs

and that sinusoidal gait as demonstrated

by the Komodo dragon here on the bottom

left that is going to evolve into the

mammalian stance where the legs have

become perpendicular to the ground and

are directly beneath the body as

illustrated by this horse so that's

going to allow for much faster running

we're going to talk about all of the different

different

ways in which mammals move about

patterns of locomotion later in the

semester here letter a is lysanops

this organism is from the late Permian

period so more than 252 million years

ago whereas B is an early Triassic

synodont thread axidon that lives tens

of millions of years later so with

thrinaxidon we observe a second cervical vertebrae

vertebrae

the axis we have enlarged scapula the

shoulder blades we now have lumbar

vertebrae those vertebrae in the lower

back that have no ribs and then we have

an enlarged pelvis which is going to

allow for the rotation of those legs as

well as a heel bone right here

okay next up we're going to take a

moment and check out the full skeleton

of this domestic cat and just quickly

point out those features starting with

the second

cervical vertebrae the axis

here is the Atlas

moving down here to the scapula the

enlarged scapula or shoulder blade

then we have this lumbar region here

which is going to provide for lots of

flexibility you can see there's no ribs

attaching to the lower back

and then we have this enlarged pelvis here

here

and finally we have the heel bone which

is up here on the cat and finally

anatomical Trend number eight and I know

this has been a dense lecture so thank

you so much for sticking with me

so the carpal or the wrist bones as well

as the tarsal bones in the foot and the

phalanges the fingers and the toes

those bones are the number of those

bones have all been reduced as we move

from primitive amniotes or ancient

amniotes to the more derived mammals to

today's mammals

I really do hope that you find these

lectures helpful in keying in on those

important ideas uh that may be kind of

uh hidden in the text and uh I'm

grateful for your time and I'll see you

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YouTube Transcript:
Lecture 1.3_Evolution of Mammals