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Lecture 1.5_Mammalian Dentition

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congratulations you've almost made it

through module one in ABS 470. this is

the last lecture

for the week Professor Jared here and

today we're going to be focusing on

mammalian dentition so I think you're

going to find this lecture extremely

helpful for your module 1 assignment on

mammal dentition and skull structure and

I can also assure you that you're going

to have dental formula on your first

assessment at the end of next week so

this is going to help you with those

questions on your next test

in this video lecture I'm going to cover

the second part of characteristics of

modern mammals and that's entitled dentition

dentition

so you may recognize this skull from

illustrations in our last lecture when

we touched on the pleistocene megafauna

this of course is the saber-toothed cat

Smilodon and it's eight inch canines

we've thus far established that mammals

are essentially eating machines with

high metabolic rates that are necessary

for endothermia although all mammals are

going to start out their lives solely

drinking milk upon weaning they're going

to enter into one of numerous adaptive

feeding modes and we're going to take a

deep dive into modes of feeding in

module six week six but for now

recognize what I mean by an Adaptive

feeding mode is carnivory

herbivory the frugivores the

insectivores and of course the

generalist omnivores like this species

right here that's capable of subsisting

on an incredible range of foods from

cultures in the Arctic that thrive on

marine mammals to entire cultures that

are built on rice in Southeast Asia to

the sun Bushman in the Kalahari Desert

running down antelope

so these modes of feeding are going to

be reflected in the animals specialized

teeth so those teeth are going to tell

us about their mode of feeding as well

as their trophic level in the food web

with respect to tooth Anatomy what I

want you to recognize is that the

occlusal surface here the chewing

surface is covered with this very hard

crystallized mineral called enamel

enamel is not composed of cells it's

composed of calcium phosphate it's

incredibly durable structure it's

actually harder than any other

vertebrate tissue and then below the

enamel we have the living dentine that

makes up most of the tooth okay the

dentine is fed by blood vessels here in

this pulp in this pulp cavity and then I

certainly don't have to tell anybody

that's ever had a toothache that the

pulp is innervated you can see the nerve

here coming through the root canal it's

these yellow

tracks here the incisors of rodents are

deemed open rooted because dentine

growth is continual

and what's going to happen is is you're

going to have differential rates of wear

between the very hard enamel here on the

anterior surface and the dentine right

here so the dentine is going to wear

away faster and it's going to create

this sharp chisel like continually sharp

chisel like Edge that is just perfect

for gnawing on tree nuts and seeds and

even wood in the case of the beaver and

the woodchuck very quickly I want to

show you this small Beaver skull that I

have and it's not a great specimen it's

a little broken and it's actually

missing an incisor but you can see that one

one

bright orange incisor so this is how

you're going to recognize rodents uh be

it a naked mole rat or a Norway rat or

in this case a small Beaver rodents are

recognizable by their orange incisors

that orange is the enamel

okay so that's the incredible incredibly

durable substance and then if we flip it

up and over here uh you can see the dentine

dentine

and you can see how this thing has a

really nice sharp edge to it a nice

beveled Edge almost like a knife

so the rodent incisors the open rooted

teeth as most teeth mature the root

canal gradually closes off and the pulp

cavity with the blood vessels is sealed

these are now deemed close rooted teeth

as the teeth are now no longer growing

they begin to wear away with use

now there's considerable individual

variation depending upon diet but

generally speaking animals such as this

white-tailed deer can be aged by

carefully examining tooth wear

so for example by one and a half years

of age you can see here this third

premolar of the white-tailed deer is

already showing some heavy wear also

this is tooth six it has just erupted

by three and a half years of age the

dentine is really showing the dentine is

the dark part it's now wider uh in some

places than the enamel which is the

white part okay by five and a half years

of age the dentine showing is now twice

as wide as that hard white enamel

and by eight and a half years uh

numerous enamel ridges are completely

worn away and these molars are just

totally dished out okay so compare those

dished out molars to this juvenile elk

so this is a yearling elk the lower jaw

bone and I want you to see how sharp

those enamel ridges are on both the

molars and the premolars okay so this is

a young animal showing very little tooth

wear at this point a juvenile elk that

said the most accurate method for aging

mammals is to look at layers of cementum

which is a modified bony material that

is laid down annually around the root of

the tooth like rings on a tree so you

can see these layers these annual layers

of cementum

the leader in the field for cementum

annuli aging analysis is Gary Matson out

of Missoula Montana so I worked with

Gary about 20 years ago now when we were

trying to understand the age

distribution of cow elk in the Blackfoot

Valley in Montana so we asked successful

Hunters to send in the first premolar of

their cows canine is actually better for

this analysis but as any hunter will

tell you they don't want to part with

those canines the ivories as they call

them they keep their ivories numerous

lineages of mammals have independently

lost their teeth these are called the

identate mammals the toothless mammals

so important this is a secondarily

derived characteristic as evidenced by

The Observer Invasion that many embryos

of these species will begin to develop

teeth only to have them reabsorbed later

on during development

further the Eden Tate condition is not

homologous it's not shared by a common

ancestor of these different lineages

rather all of these lineages have

independently converged on this Anatomy

those lineages include the monotremes

the spiny Echidna here the order pelosa

this anteater right here the pangolins

which is the order fola DOTA so these

species are all primarily gobbling up

ants and termites and then lastly we

have the mysticides the Baleen whales

the filter feeding whales like you see

down here on the bottom left so they

have this highly specialized structure

called a baleen which is rooted in the

upper jaw and the function of the baleen

is to strain out copious amounts of

Plankton like Krill there are several

species of of mammals that do exhibit

homo daunt dentition that is to say that

their teeth do not vary and form or

function they're all essentially shaped

and sized the same so those lineages

include the odontocetes that is the

toothed whales like this bottlenosed

dolphin here on the right as well as the armadillos

armadillos

the armadillos are in the order

singulata that said the majority of

mammals exhibit

heterodontentition that is to say

different types of teeth that vary in

their morphology their shape as well as

their jobs so these teeth include the

incisors at the anterior of the upper

and lower jaw denoted in Blue on this

image of a wolf skull the single pair of

canines and green

the premolars which are in purple on

this picture and then lastly we have the

molars in light blue so let's take a

moment and focus on each of those types

of teeth incisors typically have a

single root and are most often used to

snip in the grazers and the browsers to

tear in the carnivores and then to gnaw

in the rodents like this capybara here

which is the largest rodent in the world

gnawing incisors and the rodents and the

Lego morphs the rabbits and the hairs

that said like so many structures in

nature incisors can be highly modified

to do specialized tasks so consider the

incisors of the vampire bat which are

adapted for piercing flesh so that it

can lap up the blood the incisors those

are incisors of elephants which are used

for digging as well as stripping bark

off of trees

uh and then lastly the incisor here on

the Narwhal which is most likely used uh

to attract mates and to compete with

rivals in sexual selection

please note that there is never more

than one pair of canines in the upper

and lower jaw on any modern mammal so

like most of the incisors the canines

are unicuspid they just have one point

one protrusion

incisors are single rooted they're

especially pronounced in the Carnivora

like you can clearly see here in this

tiger for piercing and tearing prey

uh let's see canines comprise the tusks

in walruses and the sewer day the pig

family surprisingly we even see

elongated canines in some deer like the

musk deer of Southeast Asia as well as

the Chinese water deer and the Yangtze

Basin and Korea moving on to the first

type of cheek teeth the premolars the

premolars are immediately posterior of

the canines

so admittedly it can be kind of tricky

distinguishing between the premolars and

the molars here you can see in the horse

they look a lot alike so in general the

premolars are going to be smaller than

the molars and they most often have two

Roots whereas molars most often are

going to have three

however the best defining characteristic

for distinguishing between premolars and

molars is that the premolars are going

to have deciduous

counterparts there are milk teeth of the

premolars so those baby teeth premolars

are going to come in and then be

replaced by adult premolars whereas the

molars there are no deciduous

counterparts and finally we have the

multi-cuspid molars which again unlike

the premolars have no deciduous counterparts

counterparts

premolars and molars are collectively

deemed the cheek teeth or the molariform

teeth the function of these large teeth

is for grinding food this grinding this

mechanical digestion is very important

to maximize nutrient absorption from foods

foods

of note here Crown height is going to

vary considerably in the mammals

herbivores which consume highly abrasive

forage that contains lots of silica so

it's really hard on the teeth like this

horse they are going to have very high

crowns they're called hexadant so

hippsadont means High crowned and you

can see that those High crowns are

deeply embedded in the horse's jaw

over here in the omnivores the crown is

considerably smaller this is referred to

as brachiodont dentition further the

cusp patterns are quite variable on the

occlusal or the chewing surfaces of

cheek teeth so brachiodont remember

brachiodont has the low Crown

brachiodont cheek teeth are most often bunodont

bunodont

which means they have rounded cusps for

crushing and grinding like we see in the

monkeys and the pigs

alternatively the cusp may form this

continuous Ridge or loaf such as occurs

in elephants in a pattern termed lopedant

lopedant

and then finally if the lobes are

isolated and Crescent shaped as in deer

like this elk or Wapiti we deem it

selenodont and I'm going to show you

some skulls here with all of these cusp

patterns in just a moment the carnassial

teeth are highly specialized teeth in

the order Carnivora they're the last

upper premolar and the first lower molar

and together those carnassial teeth are

used for shearing meat like you see here

in this African lion

so a quick look at the carnassial teeth

and a number of carnivore families a

these are the carnacials on an ersid A Bear

Bear

B is the felids the cat family like this lion

lion

C is the Cadence of course dogs wolves

coyotes and then lastly on the bottom we

have the mustelid family this is the

weasel family with Wolverine Badgers and otters

otters

another term you should be familiar with

the diastema the diastema is a gap in

the tooth row between the incisors and

the molars the cheek teeth as exhibited

by this white-tailed deer as well as in

the rodents like this gray squirrel here

again this is the diastema this Gap in

the tooth row important to note these

teeth have been lost over evolutionary

time so the rodents have no canines

they've lost those

most mammals are diphyodont meaning they

have two sets of teeth during their

lifetime dye of course means two they

have their deciduous or milk teeth as

well as their permanent adult teeth the

exception is shrews shrews only have

adult teeth but that said they do in

fact have deciduous teeth they're just

reabsorbed during fetal development so

here we can see an x-ray of a human and

you can see these permanent teeth

pushing out the milk teeth the deciduous

teeth there's a canine here and a

premolar interestingly in the elephants

and the manatees the adult teeth are

going to move from the posterior of the

jaw they're going to move horizontally

and the adult teeth are going to replace

the cheek teeth that wear away and fall

out okay so let's take a few minutes and

let me walk you through uh Dental

formula a dental formula is just a

shorthand description of the total

number and position of the teeth some

important points to start number one

dental formula are always given in this order

order

incisors first

than canines then premolars and finally

molars they're always given from front

to back from anterior to posterior

number two Dental formula are only given

for one side of the mouth okay because

mammals are bilaterally symmetric so

what's going on on this side of the

mouth is going to be the exact same

number that's going on on this side of

the mouth

the number three the upper jaw is always

on top or to the left of the backslash

and the lower jaw is always beneath okay

so here we have a wolf a canis lupus and

you can see three incisors on top three

three incisors down below three canines

one and one there's always just one pair

of canines if they're there then we have

four one two three four premolars on top

one two three four premolars on bottom

two molars on top and then three molars

on the bottom okay uh one two two is

there and three okay so that's a total

of 42 teeth on the wolf alright so I

don't have a wolf but I do have its

close cousin the coyote okay

okay

um so uh coyotes have the same Dental

formula as wolves they have 42 total

teeth same exact uh positioning in fact

all canons do they all have the same

Dental formula so we're going to start

with the upper jaw and I'm going to try

and get you a nice close look here

um we are supposed to have six incisors

across the front here but we're actually

missing one okay so if we have six total

we're only going to do one side so one

side means that we have three endcisors

we just have the one canine the large

pointy canine we have one two three four

premolars this rather large tooth is

actually a premolar and then I'm going

to turn it just like this and you can

see that there are one and two molars

okay the lower jaw is going to vary slightly

slightly

so we are again going to have three

incisors on the one side uh the canine

and then we're going to have one two

three four premolars and then we

actually have three molars one

two and three hopefully you can see

those okay lastly if I can get this jaw

to articulate again

there we go I want to show you the

carnassial teeth okay remember the

carnassial teeth that last premolar on

the top

um there we go the last premolar on the

top and the first uh molar on the bottom

right here and right here and if you can

just see that it's got this shearing

capacity right so it's going to allow it

to just sheer meat and the carnivores

have the carnassial teeth next up we

have a bobcat skull

and a bobcat skull is going to have a

dental formula of three incisors on top

three incisors below

one canine on top

and one canine below

and then it's going to have uh two

premolars on top one two premolars two

premolars on the bottom and then it's

got and I know it's kind of hard to see

it's got one tiny little molar up top

and then it's got a molar down below so

the Bobcat has a total of 28 total teeth

I want to show you those carnassial

teeth again okay that shearing action

from the carnassial teeth

but most importantly what I want to show

you on this Bobcat is I want you to

check out the very short Rostrum the

Rostrum right here in the anterior part

of the skull with the nasal cavity as

well as the very short lower jaw at

least relative to a black bear

okay so here's a black bear and I'm

gonna try to get them both in the camera

for you

okay so you can see that relative to a

black bear an omnivore this highly

carnivorous cat has a much shorter

Rostrum and lower jaw

okay this cat has a much greater bite

force pound per pound than either the

bear family the ursids or the canaans

this is the most carnivorous of the

carnivores why does it have such great

bite force well uh if you remember your

unit on simple machines from way back

probably in middle school this lower jaw

right here represents a third class

lever a third class lever has the

fulcrum on the end so the fulcrum is

here where the lower jaw articulates

with the skull

the effort is provided by the masseter

muscle the masseter muscle is going to

go through the zygomatic Arch and

connect to the lower jaw and then most

importantly the load the load is the

bite force okay this cat has a very

great uh bite force because the load is

closer to the fulcrum than this black

bear this is also a third class lever

okay but here you can see that the lever

the length of the lever arm is

considerably longer okay so the lower

jaw a third class lever the shorter the

jaw the greater the bite force that said

don't let this omnivore label fool you

we'll come back to this in module six

but one thing I learned 20 years years

ago now putting radio callers on newborn

cat felt is that black bears and

especially grizzly bears are very

effective Predators at hunting and

killing neonatal elk newborn calf elk

all right I've got two more omnivores

for you first I have a primate I have a chimpanzee

and then I have a collared peccary

which you may know as the javelina

a couple of things I want to point out

number one on the javelina

the javelina is brachiodont

brachiodont

brachiodont so what that means is it has

low low Crown Heights

the second thing that I want to point

out is the occlusal surface of a

brachiodont here is the lower jaw

from the uh chimpanzee and so this is

what we call

bunodont occlusal surfaces so if you

look really closely you can see those

rounded cusps

on the cheek teeth

contrast that with a zebra and the zebra

has hipsodont dentition or high crowned

cheek teeth

further the zebra is going to have

selenodont occlusial surfaces so those

are those Crescent shaped

crescent-shaped cusps okay that are not

continuous this is selenodont

dentition a selenodont cusp pattern

finally I wanted you to note these incisors

incisors

for cropping grasses the zebra and just

two more really quick a couple more

specimens I've collected over the years

uh first I wanted to show you this

Russian boar and the reason why I wanted

to show you this dude is because of

these modified canines

so the canines are these gnarly tusks on

the Russian boar and then I've already

shown you the beaver you remember the

beaver it's a small Beaver it's got the

orange enamel on the incisor the dentine

which is going to wear away uh much more

quickly creating this beveled Edge but

what I wanted to really show you was the

diastema that Gap in the tooth row all

right so I'd like to end with a little

bit of practice for you so I'm going to

give you about 30 seconds grab your pens

grab your journals your notebooks and I

want you to practice scrawling out the

dental formula for the white-tailed deer

notice that the teeth in the lower and

the upper jaw are labeled with letters

and numbers so it should be pretty easy

so I'll give you just a few seconds on that

drum roll the answer is

on the top

we have no incisors so a zero goes on

top on the bottom we have three incisors

we have no canine on top on the bottom

we do have one tiny little canine there

labeled c one

on the top we have one two three

premolars and one two three molars on

the bottom three premolars and three

molars three premolars uh and three

molars so that's a total of 32 teeth to

get the total of course you're going to

sum those and multiply by two because

we're just doing one side of the jaw if

you're not you're going to get some more

practice with both skull identification

and dental formula on your assignment

this week so you're going to check out

this beautiful website entitled teeth

and 4D it's Dr Evans Lab from Monash

University and it's got these amazing

digital models that allow you to

manipulate them in any direction that

you want they're in 4D because they're moving

moving

um so it's going to be really helpful

for you in uh working on Dental formula

it's the next best thing to actually

holding the skull in your hands

and as always I'll end with references I

really hope you find these videos

helpful please reach out if you have any

questions and take care I'll see you

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