The document discusses the origin and evolution of mammals. It describes two main theories for the ancestry of mammals - through amphibians or reptiles. While amphibian ancestry was proposed, reptilian ancestry is now widely accepted based on fossil evidence. Many characteristics of early mammal-like reptiles called therapsids were mammalian. True mammals first appeared in the Jurassic period but remained small until after the extinction of dinosaurs. When ecological niches opened up in the Cenozoic era, mammals underwent adaptive radiations into various forms through modifications of limbs, teeth and other features for different habitats like trees, ground, burrows, water and air. Convergent evolution also led to similarities between unrelated mammals adapting to the same nic

1. Origin and Evolution
of Mammals
BY- Sanju Sah
St. Xavier’s College, Maitighar, Kathmandu
Department of Microbiology
1

2. Origin of Mammals
• The earliest mammals appeared in Jurassic period of Mesozoic
era.
• They were the descendants of some of the true mammal-like
reptiles, the Therapsida.
• However, two thoughts exist about the origin and ancestry of
mammals:
1. Ancestry through Amphibia, and
2. Ancestry through Reptilia
2

3. 1. Mammalian Ancestry through Amphibia
• T. H. Huxley (1880) proposed amphibian ancestry of mammals.
• Justifications:
1. There are two occipital condyles in the skulls of both Amphibia
and Mammalia.
2. Presence of left aortic arch in mammals.
3. Highly glandular skin in both the classes.
• Criticisms on Huxley's theory:
1. The occipital condyles are derived from exoccipitals in Amphibia
but from basioccipital in Mammalia.
2. The two classes have different modes of life and display many
fundamental differences.
• Not reliable.
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4. 2. Mammalian Ancestry through Reptiles
• Paleontological evidences support reptilian ancestry of mammals.
• There is enough evidence from extinct reptiles and mammals for
this universally accepted view that mammals had a reptilian
ancestry.
• Monotreme mammals and living reptiles have much resemblance
in anatomical features.
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5. Affinities of Monotremes with Reptiles
1. Presence of cloaca.
2. Presence of ectopterygoid in
skull.
3. Vertebrae without epiphysis
and with cervical ribs.
4. Ribs are single headed
(Capitulum).
5. A median T-shaped
interclavicle present.
6. Pelvic girdle possesses
prepubic bone.
5
7. Body temperature is not perfectly
constant.
8. Cochlea of internal ear with
lagina.
9. Ureters lead into a urinogenital
sinus.
10. Corpus callosum is absent.
11. Testis abdominal.
12. Oviparous and meroblastic
segmentation.

6. 3. Ancestral Mammal-like Reptiles
• Long before the arrival of true mammals, one group of extinct
reptiles, the Synapsida, acquired several mammalian
characteristics.
• They lived throughout the Carboniferous and Permian periods,
dating back 280 million years or more.
• The more mammal—like synapsids belonged to the order
Therapsida.
• Only the last therapsid subgroup to evolve, the Cynodontia,
survived to enter the Mesozoic era.
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7. 7

8. • The Cynodonts evolved several novel
features-
• a high metabolic rate, which supported a
more active life;
• increased jaw musculature, permitting a
stronger bite;
• several skeletal changes, supporting
greater agility; and
• a secondary bony palate enabling the
animal to breathe while holding prey or
chewing food.
• One of the more advanced Cynodonts was
called Cynognathus (dog jaw).
• Cynognathus lived during the early
Triassic period.
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9. 9

10. 10

11. Cynognathus, a Cynodont
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12. Mammalian Characters in Therapsids
• Cynognathus showed many mammalian characters:
i. The presence of two occipital condyles and enlarged lateral
temporal fossa.
ii. A well-developed secondary or false palate which separates the
nasal passages from the mouth cavity.
iii. Thecodont and heterodont dentition differentiated into small
peg like incisors, elongated canines, and cusped molars.
iv. The quadrate of the skull and the articular of the lower jaw
forms a joint, but both of them are reduced in size, thus,
indicating the beginning of craniostylic jaw suspension of the
mammals.
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13. Mammalian Characters in Therapsids
v. The dentary of lower jaw is greatly enlarged and other
reptilian bones of the lower jaw reduced or altogether absent
vi. The scapula at its front border was everted or turned out
which is the beginning of the scapular spine of the mammals.
vii. The number of phalanges in the digits are reduced to two in
the thumb and great toe, and three in all others.
viii. The sacrum and consequently the ilia are elongated, the ilium
form anterior iliac blades.
ix. Typical upright mammalian limbs capable of generating
considerable speed.
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14. Reptilian Characters in Therapsids
• Therapsids retained several reptile-like features also.
i. Their skull was intermediate between that of reptiles and
mammals, having small cranium, parietal foramen, single
middle ear bone, reduced quadrate and quadratojugal, many
lower jaw bones, etc.
ii. It is also not known whether therapsids were warm-blooded,
had hairs instead of scales and nursed their young.
iii. They were not necessarily the direct ancestors of present day
mammals.
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15. 4. First True Mammals
• Very few evidences from the fossil remains, mainly teeth and jaws,
which reveal very little about the first true mammals.
• They were mostly tiny creatures not bigger than rats and mice and
were ecologically insignificant.
• They could still manage to survive by exploiting different ways of
life from those practiced by the contemporary gigantic reptilian
enemies.
• They were nocturnal, thus, avoiding direct conflict and
competition with the mostly diurnal reptiles.
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16. • They were either burrowing hunting for insects, or arboreal in
contrast to their ground-dwelling herbivorous or carnivorous
contemporaries.
• They had a regulated high body temperature (endothermic), hairy
integument, and probably cared their young in pouches for further
development after birth, and safety.
• They were provided with larger brains and greater intelligence.
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17. • By the end of Cretaceous period (Mesozoic era) the vast majority
of dominant reptiles became extinct for reasons which are still not
understood well.
• Many ecological niches (space and resources) were now left open
to mammals who started their great adaptive radiation.
• By the close of Cretaceous period, placental mammals became
distinct from marsupials.
• During Coenozoic era, there were established all the orders of
placental mammals, so that it is called the Age of Mammals.
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18. 5. Polyphyletic Origin of Mammals
• Whether mammalian evolution is monophyletic (from single
ancestor) or polyphyletic (from multiple ancestors)?
• Living mammals are divided into two different reproductive
subclasses:
1. The primitive reptile-like egg-laying (oviparous) monotremes
are in the subclass Prototheria.
2. All other mammals give birth to living young (viviparous) and
form the subclass Theria.
• The living therians are divided into marsupials or infraclass
Metatheria, and placental or infraclass Eutheria.
• Nothing is known about the origin of primitive Prototheria, for fossils
older than Pleistocene are unknown.
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19. • Origin of mammals is
polyphyletic because they
derive from at least two Triassic
reptilian stocks.
• It is generally assumed that:
1. the living Prototheria possibly
evolved from the docodonts,
2. the Metatheria and Eutheria
evolved independently from
the pantotherians, by the end
of Cretaceous period.
Pantotheria
Docodonta
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20. Adaptive Radiations in
Mammals
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21. Adaptive Radiation
• The process of rapid divergence of multiple species from a single
ancestral lineage is called adaptive radiation.
• The concept of adaptive radiation in evolution was developed by H.F.
Osborn (1898).
• Adaptive radiation is a process in which organism diversify rapidly
from an ancestral species into a multitude of new forms (Smith, 1976).
• Causes of adaptive radiation:
o Entry into an adaptive zone by evolution of a key innovation or
by invasion into a new habitat,
o Extinction of competition,
o Ecological opportunity
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22. Conditions for Adaptive Radiations
• A new habitat has opened up: E.g., a volcano, formation of a
large new lake habitat, an extinction event opening up niches that
were previously occupied by species that no longer exist, etc.
• The new habitat is relatively isolated: If a newly formed
habitat is isolated, the species that colonize it will likely be
somewhat random and uncommon arrivals.
• The new habitat has a wide availability of niche space.
The rare colonist can only adaptively radiate into as many forms
as there are niches.
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23. Adaptive Radiation in Mammals
• Mammals evolved from Synapsids in the Mesozoic era.
• They were mouse-like with quadrupedal locomotion.
• Mesozoic mammals were small, generalized and rare.
• They remained small till Cretaceous due to the presence of
Dinosaurs.
• ~65 million years ago, after the extinction of Dinosaurs, in the
early Coenozoic era the number and diversity of mammals
expanded into varied evolutionary patterns.
• During Eocene and Oligocene epochs, most of the orders of
mammals originated.
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24. Adaptive Radiation in Mammalian Limbs
• Mammalian limbs are the modifications of the pentadactyl limb.
• Primitive, ancestral mammals- short legged, pentadactyl, terrestrial,
ground dwelling.
• Adaptive radiation occurred in five different lines or habitats with
modifications in their limb structure:
i. Running (cursorial),
ii. Burrowing (fossorial),
iii. Tree-climbing (arboreal),
iv. Flying (aerial), and
v. Swimming (aquatic).
• All the mammals of different radiating lines have limbs more or less
adapted for some particular mode of locomotion.
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25. 25

26. 1. Arboreal:
• They have adapted limbs for life in trees
(e.g., squirrels, sloths, monkeys, etc.).
• They have strong chest muscles, ribs and
limb girdles.
• Pelvic and pectoral girdles are strong to
support the body weight during climbing
and hanging.
• Feet are prehensile and grasping type.
• Tail is long and prehensile, used for
swinging and balancing the body.
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27. 2. Aerial:
• They are the mammals adapted for flight (e.g., bats).
• Only bats occupy the position at the terminus of this line, since
they are the only truely flying mammals.
• Somewhere along this line we can place for gliding mammals such
as “flying squirrel. ”
• It is believed that the ancestral aerial forms were previously lived
in trees having gliding type of locomotion which later gave rise to
true flight.
• Hence, perhaps the gliding formed transitional type of locomotion
between climbing and true flight.
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28. 3. Cursorial:
• Developed limbs suitable to rapid
movements over the surface of the ground
(e.g., horses and antelopes).
• Along this line also developed other
mammals with less strongly modified limbs,
such as wolves, foxes, hyaenas, lions, etc.
• There are three forms of digit arrangement
that impact speed and affinity for cursorial
movement in mammals: Plantigrade,
Digitigrade and Unguligrade.
• Lengthening of limbs, loss of digits, fusion
of metacarpals/metatarsals etc.
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29. 4. Fossorial:
• Burrowing mammals, well adapted for digging.
• Spindle shaped body, tapered head, small eyes
and ears, short neck, shorter but robust
forelimbs, etc.
• Some are poorly adapted for locomotion on the
ground (e.g. moles).
• Some retain structures enabling them to move
readily on the surface of ground (e.g., pocket
gophers and badgers).
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30. 5. Aquatic:
• Whales and porpoises having limbs
strongly adapted for aquatic life,
but they cannot move about on
land.
• While seals, sea lions and walruses
have also strongly modified limbs
for aquatic life but they are also
able to move about on land.
• The third group includes
accomplished swimmers such as
others and polar bears which are
equally at home in water or on
land.
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31. Adaptive Radiations in Mammalian Teeth
• The mammals with few exceptions (Cetacea, ant-eaters etc.)
possess heterodont dentition- incisors for biting, canines for
grasping, tearing or for defence or offence, premolars and molars
for grinding.
• The premolars and molars show greatest structural modification
for different types of food.
• In insectivorous type, premolars and molars are low-crowned
simple with few cusps, generally sharp pointed and suitable for
crushing feeble prey.
• In carnivorous type, premolars and molars are high crowned,
trenchant, shearing structures (carnassial). Cats have no grinding
teeth, while dogs have more of grinders.
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32. • In sperm whale, Physeter, no teeth in the upper jaw, but germs of
upper teeth are present.
• In whalebone whales (Mystaceti) upper teeth are totally absent
and their place is taken by whalebone which hangs from the
palate.
• In herbivorous types, incisors are for seizing and cutting the
vegetation.
• In ruminants, they are absent in the upper jaw, but a horny pad is
present there.
• Canine teeth are of little importance for herbivores, but in musk
deer they are used for defence and in swine they are used for
uprooting the vegetation.
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33. • Grinders (premolar and molars) may be:
• Short-crowned and brachydont (low crowned teeth) adapted for
succulent leaves and twigs,
• Long-crowned and hypsodont (high-crowned teeth) adapted for
harsh grasses.
• In myrmecophagous type teeth have disappeared, jaws reduced
and mouth opens at the extreme anterior end of tubular snout
with a highly extensible and prehensile adhesive tongue for eating
ants.
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34. Adaptive Convergence
in Mammals
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35. • Distantly related animals inhabiting similar habitats, often
develop independently similar morphological features that make
them look similar. This phenomenon is termed adaptive
convergence or convergent evolution.
• The same or similar niches produce great similarities in very
different species.
• It is the opposite phenomenon of adaptive radiation.
35

36. 1. Adaptive Convergence in Placental and
Marsupial Mammals.
• During Paleocene, Eocene and Oligocene, the marsupials in Australia
and the placentals in the rest of the world underwent much adaptive
radiation, moving into habitats vacated due to extinction of dinosaurs.
• Marsupial and placental mammals are not closely related as they have
polyphyletic origin.
• But, convergent evolution has resulted in similar looking members in
both the groups occupying similar ecological niches and leading
similar ways of life.
• Thus, there are marsupial wolves, mice, cats, anteaters, moles, sloths,
flying phalangers and wombats.
• These are not related to their true counterparts in placental mammals
but resemble them due to convergent evolution.
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37. 37

38. 2. Adaptive Convergence among Anteaters
• Anteaters belong to different orders of class Mammalia (e.g.,
Monotremata, Pholidota, Edentata, Tubulidentata, etc.); not
closely related.
• They have evolved from different non-ant-eating ancestors
independently, acquiring similar features or adaptations for a diet
of ants, termites and other smaller insects.
• Thus all anteaters have teeth much reduced or absent, elongated
snout, long extensile sticky tongue and sharp stout claws on front
legs for digging into termite mounds, rotten logs, etc.
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39. Adaptive Convergence among Anteaters (contd.)
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Monotremata
Pholidota
Edentata
Tubulidentata