The document discusses the origin and evolution of mammals. It describes two main theories for the ancestry of mammals - descent from amphibians or reptiles. While Huxley proposed an amphibian ancestry, most evidence supports a reptilian ancestry, with mammals evolving from mammal-like reptiles called therapsids. True mammals first appeared in the Jurassic period and were small, nocturnal creatures. After the extinction of dinosaurs, mammals underwent an adaptive radiation, diversifying to fill new ecological niches. Limbs and teeth evolved adaptations for different diets and modes of locomotion like burrowing, swimming and climbing. Distantly related groups sometimes converged on similar forms when occupying similar habitats.

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. 11
Cynognathus, a Cynodont

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.
19
Pantotheria
Docodonta

20. Adaptive Radiations in Mammals
20

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
34

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.)
39
Monotremata
Pholidota
Edentata
Tubulidentata