Evolution of Mammals

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The Evolution of Mammals

Mammals are vertebrates.

* backbone which encloses a sheath of

nerves which leads in turn to a brain ina box or skull.

* four limbs and special pentadactyl ends to

these limbs (i.e. 5 fingers and toes).



Reptiles, amphibians, birds,as well as the dinosaurs,

are built around this same

simple plan.

Fish had the backbone and

the skull, but not thepentadactyl limbs.



Fish transformed their fins into legs,

developed lungs -legs for moving and lungs for breathing.

The invasion of the land had begun.

About 10 million years later reptiles started todifferentiate - from amphibians and diversity

Various groups evolved out of these early reptiles -

crocodiles, dinosaurs and birds,

- to a strange group of animals calledthe Mammal-like Reptiles.



Mammals evolved from a group of

reptiles called the synapsids.These reptiles arose during the

Pennsylvanian Period (310 to 275 million

years ago).

A branch of the synapsids called the

therapsids appeared by the middle of thePermian Period (275 to 225 million years

ago).



The Mammal-like Reptiles, or Therapsids

* first appeared near the beginning of

the Permian which is well before thedinosaurs.

* evolved quickly and many differentgroups arose.

* were very successful until about the

end of the Permian,



• about 245 million years ago,

something catastrophic happened -

nearly all of the species then living

died out.

New species evolved rapidly to fill this emptyhabitat, among them:

* the first dinosaurs and

* few million years later the first mammals

Early in the evolution of the therapsids arose a

group called the cynodonts.



These early mammals were small, insectivorous,

nocturnal, hairy and warm-blooded.

Warm-bloodedness- first evolved among the

cynodonts, successful group of mammal-like reptiles

from which the mammals evolved.

The cynodonts - only mammal-like reptiles to survive

to the Jurassic, nearly made it into the Cretaceous, anddefinitely coexisted with many of the major dinosaurs.



The cynodonts dominated the world of 250

million years ago. At this time, all the land

masses were joined together in one super-

continent, called Pangaea.

By about 200 million years ago, the dynasty

of the cynodonts suddenly was ended by theappearance of dinosaurs.

Early mammals played a very small role in

the world, often termed the 'Dark Ages' of

mammalian evolution. However, theevolution of the remaining mammals did not

stop.



While dinosaurs were getting larger and

developing extensive defensive and

offensive tools, evolving primarily their

physical hardware,

the evolution of the early mammals

used its energy in evolving their brain

and behavioral software.



During the Jurassic the mammals

* remained small* mainly nocturnal,

* living beneath the 'metaphorical‘

feet of the great dinosaurs.

These early mammals were more like

small monotremes and probably laid

eggs still.



These mammal-like reptiles gave rise to abranch of:

* primitive mammals (prototheria)

living descendants, the monotremes:

* duckbilled platypus and

* spiny anteater.



Towards the end of the Jurassic a group of mammals

known as 'multituberculates' appeared.* the most successful of the primitive mammal groups

* with species still alive only 30 million years ago (MYA)

* had survived as a group for 130 million years

Some of the later multituberculates possessed

* marsupial-like bone structures which indicate

that they had pouches like marsupials,

suggesting a similar life cycle involving live birth

of very premature young. (marsupials(metatheria branched off )



During this period of dinosaur

dominance, the three forms of

mammals that still exist today

appeared;

* the monotremes,

* the marsupials, and* the placentals.

All three of these groups are believedto have originated from a common

ancestor.



The end of the Cretaceous however saw another

mass extinction - known as K-T event :

* resulted in the complete extinction of the

dinosaurs

* saw the death of all the Pterosaurs, the flying

reptiles.* all these species dying out left huge niche

vacancies in the habitat.

* the mammals alone of the remaining groups of

animals diversified to take advantage of thisnew situation.



Over the next 15 million years the

remaining 10 mammal families (fivebecame extinct with the dinosaurs

during the K-T event)

expanded to become 78 families by the

early Eocene.

This sudden massive increase in species

from a single stem group is an example of

what is called 'adaptive radiation'.



By the middle of the Eocene (45

MYA) all the major groups of mammals alive today had come into

existence.

During the 2nd half of the Eocene

(12 million years) the Oligocene (23

million years) and the Miocene (18

million years) the mammals havebeen dominant.



The changes from a reptilian to a mammal's

lifestyle is significant.

Reptiles are cold-blooded.

During cold periods and at night, the reptile mustaccept substantial down time, awaiting the

warmth of the next day to get started again.

This leads to a lifestyle of relatively short bursts of energy

needed to catch its prey and long periods of inactivity whilethe body processes the food.



Mammals are warm-blooded, generating their heat

internally, retaining it by insulation of fur and/or fat.

Having a more consistent body temperature

more continuous activity level,

more fuel (food) required to supply the energy

necessary for this active lifestyle.more efficient metabolism is required which means

greater oxygen intake abilities and nutrient uptake.

Larger lungs, diaphragm, a four-chambered heart, and

improved circulatory systems were the result.



Reptiles relied on giving birth to eggs. Eggs are basically

left to hatch and fend for themselves.

Mammals evolved a system of reproduction which leaves

the young to develop more fully inside the mother. After

birth,

* the young are afforded extended parental care -

reliable provision of the mother's mammaryglands.

* the young can spend its energy in further

physiological growth.

Allowed more time for the development of a morecomplex organism and the cerebral system in the

mammalian ancestry.



The hallmarks of today's mammals

- hair,

- warm blooded and

- milk-producing glands



The earliest mammals are the ancestors of

todays' monotremes.

Monotremes resemble reptiles and differ

from all other mammals in that they lay shell-

covered eggs that are incubated and hatched

outside of the body of the mother.

Like reptiles, the ducts of the excretory

system and the genital ducts open into a

single external opening known as the cloaca

(thus, the ordinal name Monotremata )



Like mammals, they have fur,

- a four-chambered heart, but incomplete,

- warm-blooded, but the body temperatureaverages lower than that of other mammals

- nurse their young from specialized glands,but

there are no nipples; rather, the milk is

excreted through skin glands, with the

young suckling the body fur to receive

the milk.

The monotremes are represented today by two

species of echidnas and one species of platypus.



The mammalian line ceased layingeggs and began bearing live young.

These forms of mammals were the firstmarsupials - bore their young at a very

early stage in their development and

transferred them to a pouch wheremodified sweat glands secreted milk.



By the the end of the Cretaceous (65 million

years ago), Pangaea had begun to break

apart.

These early marsupials (and the limited

number of monotremes) were the soleinhabitants of the southern land mass,

Gondwanaland,

The marsupial lines radiated out to fill out

every niche available.



In Laurasia, another branch of mammals took

the marsupial reproductive strategy one step

further.

As the early mammals evolved, they continued to

expand their awareness of the environment

reflected in improved senses of sight, smell,touch and hearing.

The embryo remained in the uterus, receiving

nutrients and oxygen from the mother for anextended amount of time, enabling the

development of the brain.



The splitting of the placentals from the

marsupials date back to about 120 million

years ago.

The first living placental order is believed by

most scientists to be the Insectivores (includingshrews and moles),

while others support the Xenarthra, which

includes the armadillos, sloths, and anteatersas more primitive.



About 70 million years ago, placentals and

marsupials arrived in South America;

* the placentals being the herbivores, and

* the marsupial, the carnivores.

By the end of the Cretaceous period, some65 million years ago, as the continents

diverged, the climate became more erratic,

the vegetation changed, and the dinosaurs

disappeared, the 'Age of Mammals' finallygets under way.

Evolution of Mammals

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