Section 12.6Mammals

The Rise of Mammals• The ancestor of modern mammals appear to have

been reptiles called therapsids which lived 240 million years ago– Therapsids had teeth, a skull, and limbs similar to modern

day mammals• Mammals remained small and uncommon until the

Cretaceous extinction which saw the end of the dinosaurs

• After this extinction mammals underwent extensive differentiation– As of today there are about 4,500 living species of

mammals

Rise of Mammals

• As mammals diversified in the Cenozoic many changes occurred and as of today mammals are divided among three groups– Monotremes– Marsupials– Eutherians

Monotremes• There are only three living

species:– Duck-billed platypus– 2 species of Echindas

• One found in Australia the other in New Guinea

• Are egg laying mammals– Eggs are laid and warmed by

the parent• When hatched parent

nourishes with milk secreted by a gland into the fur– This is as they lack nipples

Marsupials

• Include animals such as– Kangaroos, opossums,

bandicoots, and koalas• Young are born in a

extremely immature condition and undergo further development in their mothers pouch– In the pouch suckle on

their mother’s nipple for milk for nourishment

Eutherian• Are placental mammals

– Young develop in mother’s uterus– Are nourished via the umbilical

cords connection to the placenta– Compared to marsupials have much

longer periods of pregnancy• Young are born with varying

degrees of maturity which dictates length of dependence on their mother– E.g. whales have to swim right

away, humans are born naked and helpless

• Most mammals are in this group– About 95%– Are divided among various orders as

compared to monotremes and marsupials who are their own order

Major Orders of Mammals*See page 468-469 of textbook*

Major Orders of Mammals

Major Order of Mammals

Major Orders of Mammals

• This diagram shows the 3 groups of mammals• Note how the eutherians are placed into four clades– This is done based on a current hypothesis for the

evolutionary relationships among the eutherian orders– This is hypothesis is based on molecular systematics

Closer Look at order Primates

• The classification of this order is complex and under constant review– Part because humans

are in this order– Also because getting

more and more new fossils that deem review

• For some more details read through section in text on page 466-467

• Diagram shows one of the present classification systems

General Characteristics of Mammals

• Here are key characteristics for an animal to be a mammal:• Warm-blooded• Are air-breathing, with diaphragm separating the lungs from the

general body cavity • 4 chambered heart • Skin is covered with hair (could be bristles, wools, scales or fur) and

most have sweat glands• Fat layers• Teeth that are fixed into sockets in the jawbone• 4 limbs, limbs often pentadactyl (with five toes), and endoskeleton• Middle ear• Well developed brain• Are viviparous (bear live young) {only exception are the monotremes}• One of most distinctive features is the presence of the mammary

gland (hence the name mammal from mammary)

Warm-Blooded

• Being warm blooded makes mammals endothermic

• To be endothermic means an active metabolism• To meat this metabolism mammals have efficient

cardiovascular systems– 4 chambered heart keeps oxygenated and

deoxygenated blood from mixing– Lungs with extensive internal folding improve surface

area, making more efficient gas exchange

Hair, Sweat Glands and Fat Layers• Hair, sweat glands and fat layers help in

maintaining body temperature– Hair and fat help insulate to keep heat– Sweat glands make sweat to cool down

(note not all mammals have sweat glands)• Hair can also help provide camouflage

depending on the colouration• Amount of hair and exact type vary

among species as some hair has become specialized– E.g. porcupine guard hair makes the quills,

one of its defense mechanisms• Hair is made of the protein keratin

– Keratin also makes other structures such as: horns, toes, hooves, fingernails, claws, and antlers

Teeth • Mammals can have various types of teeth and

arrangements of such teeth– Based on the type and their arrangement reveal

if the animal is an omnivore, carnivore, herbivore or insectivore

• Teeth types are as follows– Incisors– Canines – Premolars (bicuspids)– Molars (tricuspids)

• The types of teeth appear in the order given from the front to the back of the mouth on both sides in the upper and lower jaw (if the animal happens to have all 4 types)

• Another thing is the teeth in both jaws match up, unlike those of reptiles– This aids in chewing

Types of Teeth and their Roles

• Incisors– Used for biting and tearing– In elephants the upper incisors have developed

into tusks• Canines– Help in holding and piercing prey– In walrus are called tusks

• Premolars and Molars– Used for chopping and grinding food

Teeth and Diet• Based on the teeth an

animal has it can help us determine their diet– E.g. cows chew grass and thus

have largely incisors and molars

• Also based on animals diet will influence what their digestive system is like– Note how herbivores

digestive system larger as this is needed to break down food with high cellulose content

Showing how Teeth and Jaws Vary Based on Animals Diet

Endoskeleton

• Having an endoskeleton gives 3 advantages over animals with an exoskeleton– Can grow continuously without moulting phase– Do not have to divert metabolic energy to make

new skeleton– Do not have vulnerable period while new

exoskeleton forms

Skeleton and Limbs

• With well developed muscles and bones, particularly where they are jointed the skeleton is very flexible

• Structure and location of limbs help permit ease of motion and speed

• The pectoral and pelvic girdles have been modified to support and accommodate the location of the limbs

• Limbs have modified and adapted for many mammals based on their environment

Pentadactyl Limbs

• The variation of limbs being pentadactyl (have five toes) is due to adaptations to running– For example for some the central toe has become the most

important and the total number of toes has reduced – We will also look at some other limb modifications– Note regardless of the modification/adaptation the number and

organization of limb bones have remained relatively constant

Limb Modifications

• Modification for flight:– Elongated fingers– Between fingers a web

of membranous skin stretches that extends to the hind limb

• The only true flying mammals are bats

Limb Modification

• For aquatic mammals– Forelimbs have become

flippers– In some hind limbs also

flippers (seals, sea lions) and in others they lack hind limbs (whales, dolphins)

– Those without hind limbs have well developed tails

Limb Modification

• For upright mammals– Hind limbs are limited to

movement– Has allowed forelimbs to

be free for specialized functions such as grasping and manipulating objects

Middle Ear

• Most obvious is allows for hearing• Hearing is very vital for survival– Especially to those using echolocation (e.g. bats)

Developed Brain

• Require developed brain to enable the learning of various behaviour patterns

• Has been a key in mammals success– Most notably in humans

as it is what has made us so successful

Mammary Gland

• Produces milk that is used to nourish young

• Young get milk from their mother by suckling on a nipple

Reproduction• For breeding some

mammals have complex behaviours/courtships

• Fertilization is internal

• Mammals are viviparious as they give birth to live young– Monotremes are the

exception

Home Work

• Read pages 465-475

• Answer questions– 1-6 page 467– 1-6 page 478